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Recommended Practice DNV-RP-F301, April 2007
This Recommended Practice (RP) provides general requirements for the design-, manufacture-, testing and certification
processes for subsea gravity separators intended used for deepwater applications. In this context deepwater may be defined
as water depths where the governing load is the external, rather
than the internal pressure.
to provide an internationally acceptable standard for the
structural integrity of Subsea Separators
to provide more exact design criteria when the external
pressure is governing for required thicknesses of the
to serve as a technical reference document in contractual
to serve as a guideline for the designers, suppliers, purchasers and regulators reflecting 'state-of-the art' as well as
consensus on accepted industry practice
to specify procedures and requirements for certification
(or classification) of Subsea Separators intended used on
deepwater installations.
GENERAL .............................................................. 7
General .....................................................................7
Objectives ........................................................................... 7
Application and scope......................................................... 7
PED, particular compliance issues ..................................... 7
How to use the RP ...................................................7
Users of the RP ................................................................... 7
Structure of this RP............................................................. 7
Normative references ..............................................8
Offshore Standards ............................................................. 8
Recommended Practices..................................................... 8
Other references.................................................................. 8
Definitions ................................................................9
Abbreviations and symbols.....................................9
DESIGN PHILOSOPHY ..................................... 10
General ...................................................................10
Objective........................................................................... 10
Applicability ..................................................................... 10
Safety philosophy...................................................10
Safety objective ................................................................ 10
Systematic review............................................................. 10
Fundamental requirements................................................ 10
Installation and operational considerations ...................... 11
Design principles .............................................................. 11
Quality assurance.............................................................. 11
Design format.........................................................11
Basic considerations ......................................................... 11
Safety class methodology ................................................. 12
Design by LRFD-method ................................................. 12
Working Stress Design (WSD)......................................... 12
Reliability based design.................................................... 12
Safety Class Concept and PED.............................13
DESIGN................................................................. 14
General ...................................................................14
Material selection ..................................................14
Loads and load effects ...........................................14
Resistance ...............................................................14
Limit states and failure modes .............................14
Calculation methods..............................................14
Design criteria........................................................14
General.............................................................................. 14
Guidance for EN 13445-3, Annex B ................................ 15
Design details .........................................................16
MATERIALS........................................................ 16
Application .............................................................16
Normative references ............................................16
General requirements ...........................................16
Type of materials .............................................................. 16
C-Mn steel with SMYS > 555 MPa ................................. 16
Corrosion .......................................................................... 16
Material manufacturing........................................16
Manufacturing Procedure Specification (MPS)................ 16
General requirements........................................................ 16
Material requirements ..........................................17
Steelmaking ...................................................................... 17
Chemical composition ...................................................... 17
Mechanical properties....................................................... 17
Material testing......................................................19
Chemical analysis ............................................................. 19
Mechanical testing............................................................ 19
Hardness test..................................................................... 19
SSC test............................................................................. 19
Pitting corrosion testing.................................................... 20
Metallographic examination ............................................ 20
Re-testing.......................................................................... 20
Non-destructive testing and workmanship ......... 20
General.............................................................................. 20
Visual examination and workmanship ............................. 20
Ultrasonic examination..................................................... 20
Repair of defects............................................................... 20
Material certification ............................................ 20
FABRICATION, TESTING AND
INSPECTION OF CLAD STEEL PLATES ..... 20
Application............................................................. 20
Normative references ............................................ 20
Manufacturing of clad steel materials................. 20
Manufacturing Procedure Specification (MPS) ............... 20
General requirements........................................................ 20
Qualification of cladding procedure ................................. 20
Fabrication testing ................................................ 20
Tensile test........................................................................ 21
Impact testing ................................................................... 21
Hardness testing................................................................ 21
Metallographic examination ............................................. 21
Bend tests of cladding....................................................... 21
Shear strength of cladding ................................................ 21
Pitting corrosion test......................................................... 21
Re-testing.......................................................................... 21
Non-destructive testing and workmanship ......... 21
General.............................................................................. 21
Inspection and tolerances.................................................. 21
Surface crack examination................................................ 21
Ultrasonic examination..................................................... 21
Repair of defects............................................................... 21
Personnel qualifications.................................................... 22
Inspection document ............................................. 22
FABRICATION, TESTING
AND INSPECTION OF SEPARATOR............. 22
Application............................................................. 22
Normative references ............................................ 22
Resistance to external corrosion and HISC ........ 22
Manufacture of separator .................................... 22
Manufacturing Procedure Specification for separator
fabrication (MPS) ............................................................. 22
Manufacturing Procedure Qualification Test for separator
fabrication (MPQT) ......................................................... 23
Plate forming .................................................................... 23
Welding ............................................................................ 23
Heat treatment................................................................... 23
Non-destructive testing ......................................... 23
General.............................................................................. 23
Visual inspection .............................................................. 23
Magnetic particle inspection and ultrasonic examination 23
Correction of defects ........................................................ 23
Personnel qualifications.................................................... 23
Fabrication testing ................................................ 24
General.............................................................................. 24
Type of tests...................................................................... 24
Sampling and extent of fabrication tests........................... 25
Dimensions............................................................. 25
Pressure testing ..................................................... 26
External over-pressure ......................................................26
Internal over-pressure .......................................................26
Conclusion pressure testing ...........................................26
AND PERIODIC INSPECTION........................ 27
Inspection documents ........................................... 26
CERTIFICATION PROCESS............................ 26
Introduction ........................................................... 26
Certification procedures....................................... 26
Documentation requirements .............................. 27
REFERENCES .................................................... 28
Codes and standards............................................. 28
Papers and publications ....................................... 28
APP. A SAFETY CLASS, CALIBRATION ................. 29
APP. B DESIGN OF SUBSEA SEPARATOR
ACCORDING TO EN 13445-3 ANNEX B ..................... 31
This Recommended Practice (RP) provides general requirements for the design, manufacture, testing and certification
processes for subsea gravity separators intended used for deepwater applications. In this context deepwater may be defied as
water depths where the governing load is the external rather
This document provides recommended practice to achieve an
acceptable overall safety level regarding the structural strength
of the separators.
This RP has been developed for general world-wide application. Governmental legislation may include requirement in
excess of the provisions of this standard depending on the
Extracts from the requirement in the EU Directive Pressure
Equipment, PED, EU Council Directive No. 97/23/EC are
partly included, which need to be considered on subsea separators to be installed on one of the Continental Shelves within
the EEA (European Economic Area).
The functionality of the separator is not covered by this Recommended Practice.
The main benefits of using this RP comprise:
provision of subsea separator solutions for deepwater
applications that are safe and feasible for construction
specific guidance and requirements for efficient design
analysis based on EN 13445, that satisfy the Pressure
Equipment Directive (Applicable within EEA)
application of a risk based approach where the magnitudes
of the safety factors depend on consequence of failure
(safety class methodology).
This objective of this document is to:
provide an internationally acceptable RP for the structural
integrity of subsea separators
provide more exact design criteria when external pressure
is governing for the required thicknesses
and regulators reflecting state-of-the art and consensus on
specify procedures and requirements for certification or
classification of Subsea separators intended used on deepwater installations.
1.1.3 Application and scope
This standard applies primarily to subsea production separators at deepwater installations within the petroleum and natural
gas industries. At more ordinary water depths, existing practice, e.g. using the design by formulae (DBF) methodology in
EN 13445-3, may provide feasible solutions. For deep water
locations the design by analysis (DBA) approach provides
consistent means to achieve more optimal designs with acceptable reliability. The design philosophy as focused on in this RP
may also be utilised for ordinary water depths.
Connecting piping, foundation, anchoring and skids used for
transportation, installation, etc. is considered outside the scope
for this standard.
For others applications, special considerations may need to be
agreed with the parties involved and according to the statutory
1.1.4 PED, particular compliance issues
This RP is essentially based on application of EN 13445,
which is a harmonised standard and gives presumption of conformity with PED. However, this RP covers designs that were
not in focus when PED was developed. In particular two issues
have been addressed in this RP where PED does not provide a
clear guidance, and additional considerations have been made
in order to ensure that the essential safety requirements (Annex
I of PED) are met. These issues relate to:
application of safety class methodology
proof test (pressure testing).
This RP provide explicit guidance on these issues as further
described in Subsection 2.4 and in 6.8 respectively. Clarification of these issues may be of common interest within EU.
Questions together with proposed answers (as reflected in
these subsections) have been formulated and will be sent to the
National Authorities for potential further processing in EU. A
possible outcome is that this may end up as guidelines to PED.
1.2 How to use the RP
1.2.1 Users of the RP
The client (or purchaser) is understood to be the party ultimately responsible for the system as installed and its indented
use in accordance with the prevailing laws, statutory rules and
The contractor is understood to be the party contracted by the
client to perform all or part of the necessary work required to
bring the system to an installed and operable condition.
The designer is understood to be the party contracted by the
contractor to fulfil all or part of the activities associated with
the design, and provides the main contribution to the design
The manufacturer is understood to be the party contracted by
the contractor to manufacture all of part of the system.
The certification body is usually appointed by the client to perform independent certification.
1.2.2 Structure of this RP
The documents is organised as illustrated in the flowchart in
Section 1 contains the objectives and scope of the Recommended Practice. It further introduces essential concepts, definitions and abbreviations.
Section 2 provides the design philosophy which includes the
safety philosophy and design format. In particular the concept
of safety class is given and discussed in relationship to PED
and the fully harmonised standard EN 13445.
Section 3 deals with the design criteria. Here the relevant load
effects and material properties to be applied in the analysis are
given together with a detailed description on how to carry out
the design analysis.
Section 4 covers requirements to the base material, and coves
aspects of manufacturing, chemical composition, properties,
testing and resistance towards corrosion and Hydrogen
Induced Stress Cracking (HISC) with particular focus on
important parameters regarding use of clad and duplex steel
and for the manufacturing of thick plates.
Section 5 contains requirements for the fabrication, testing and
inspection of clad and duplex steel plates, whereas Section 6
covers such requirements for the separator.
Section 7 gives the certification process in terms of certification activities to be carried out by the certification body during
design and fabrication. It also includes a list of documentation
to be submitted by the manufacturer and designer for review
Section 8 on operation, maintenance and inspection addresses
important issues to be addressed in preceding activities since
the vessel is likely never to be seen again once it is installed.
Note that installation aspects are not covered by this RP.
All users should go through Section 1 and 2 describing the
scope of the RP and the design principles. The design analysis
should be carried out by the designer according to Section 3,
taking into account the design premises that are to be specified
by the client and contractor. The contractor, manufacturer and
certification body should consider Sections 5, 6 and 7, covering fabrication and certification.
(cert. body )
Flow chart of RP
The following standards below include requirements that
through reference in the text constitute provisions of this standard. Last revision of the references shall be used unless otherwise agreed. Other recognised standards may be used provided
it can be demonstrated that these meet or exceed the requirements of the standards referred to herein and accepted by the
involved parties as supplier, contractor, field operator, any
third party or certifying authority/notified body.
Any deviations, exceptions or modifications to the codes and
standards shall be documented for agreement or approval need
to be given by the parties involved.
1.3.1 Offshore Standards
1.3.2 Recommended Practices
1.3.3 Other references
ISO/FDIS 2394 General Principles on Reliability of Structures
PED, Pressure Equipment Directive, Directive 97/23/EC of the
PD 5500 Specification for Unfired fusion welded pressure
EN-13445-1, Unfired pressure vessels Part 1: General
EN-13445-2, Unfired pressure vessels Part 2: Materials
EN-13445-3, Unfired pressure vessels Part 3: Design
EN-13445-4, Unfired pressure vessels Part 4: Fabrication
EN-13445-5, Unfired pressure vessels Part 5: Inspection and
ISO 15156-1, Petroleum and natural gas industries Materials
for use in H2S-containing environments in oil and gas production Part1: General principles for selection of cracking resistant materials
ISO 15156-2, Petroleum and natural gas industries Materials
for use in H2S-containing environments in oil and gas production Part 2: Cracking resistant carbon and low alloy steels,
and the use of cast iron.
ISO 15156-3, Petroleum and natural gas industries Materials
for use in H2S-containing environments in oil and gas production Part 3: Cracking resistant CRAs (corrosion resistant
alloys) and other alloys
EN 10028-1, Flat products made of steels for pressure purposes - Part 1: General requirements.
EN 10028-6, Flat products made of steels for pressure purposes - Part 6: Weldable fine grain steels, quenched and tempered.
EN 288-3:1992+A1, Specification and approval of welding
procedures for metallic materials Part 3: Welding procedure
tests for the arc welding of steels (Amendment A1:1997
EN 1043-1, Destructive tests on welds in metallic materials.
Hardness testing Part 1: Hardness test on arc welded joints.
Clad component: component with internal liner where the
bond between base and cladding material is metallurgical. This
includes corrosion resistant layer applied by weld overlay, hot
rolling and explosion bonded plates.
Corrosion allowance: The amount of thickness added to the
thickness of the component to allow for corrosion/erosion/
Deepwater separator: Subsea separators for deepwater applications. In this context deepwater may be defined as water
depths where the governing load is the external rather than the
Environmental loads: Loads due to the environment, such as
waves and current, wind.
Failure: An event causing an undesirable condition, e.g. loss
of component or system function, or deterioration of functional
capability to such an extent that the safety of the unit, personnel or environment is significantly reduced.
Fatigue: Cyclic loading causing degradation of the material.
Fatigue Limit State (FLS): Related to the possibility of failure
due to the effect of cyclic loading.
Fracture Analysis: Analysis where critical initial defect sizes
under design loads are identified to determine the crack growth
life to failure, i.e. leak or unstable fracture.
Inspection: Activities such as measuring, examination, testing,
gauging one or more characteristics of an object or service and
comparing the results with specified requirements to determine
Installation: The operation related to installing the separator,
including tie-in.
Limit State: The state beyond which the separator or part of the
separator no longer satisfies the requirements laid down to its
performance or operation. Examples are structural failure or
Load: The term load refers to physical influences which cause
for example stress, strain or deformation in the separator.
Load Effect: Response or effect of a single load or combination
of loads on the structure, such as stress, strain and deformation.
Load and Resistance Factor Design (LRFD): Design format
based upon a limit state and partial safety factor methodology.
The partial safety factor methodology is an approach where
separate factors are applied for each load effect (response) and
resistance term.
Location class: A geographic area classified according to the
distance from locations with regular human activities.
Lot: A number of plates from the same heat, the same heat
treatment batch and with the same thickness.
Non-destructive testing (NDT): Structural tests and inspection
of welds or parent material with radiography, ultrasonic, magnetic particle or eddy current testing.
Offshore Standard (OS): Offshore Standard: The DNV offshore standards are documents which presents the principles
and technical requirements for design of offshore structures.
The standards are offered as DNVs interpretation of engineering practice for general use by the offshore industry for achieving safe structures.
Operation, Normal Operation: Conditions that are part of rou-
tine (normal) operation of the separator.
Out of roundness: The deviation of the perimeter from a circle.
This can be an ovalisation, i.e. an elliptic cross section, or a
local out of roundness, e.g. flattening. The numerical definition of out of roundness and ovalisation is the same.
Ovalisation: The deviation of the perimeter from a circle
resulting in an elliptic cross section.
Prior Service Life: The duration that a component has been in
service, since its installation. Duration is computed from the
time of installation or production if relevant.
Recommended Practice (RP): The publications cover proven
technology and solutions which have been found by DNV to
Residual Service Life: The duration that a component will be
in service, from this point forward in time (from now). Duration is computed from now until the component is taken out of
Safety Class: A concept adopted herein to classify the criticality
of the subsea separator with respect to consequence of failure.
Safety Class Resistance Factor: Partial safety factor which
transforms the lower fractile resistance to a design resistance
reflecting the safety class.
Service Life: The length of time assumed in design that a component will be in service.
Uncertainty: In general the uncertainty can be described by a
probability distribution function. In the context of this Recommended Practice, the probability distribution function is
described in terms of bias and standard deviation of the variable.
Design By Formulae
Norwegian Deepwater Program
Pressure Equipment Directive (applicable within
Fabrication process for welded pipes
2.2.1 Safety objective
An overall safety shall be established, planned and implemented by company covering all phases from conceptual
development until retrieval or abandonment.
Mid plane diameter
Failure probabilities (annual)
Nominal (specified) wall thickness
Design life time in years
Safety class factor accounting for the failure
Fluid (water) density
Standard deviation; Nominal stress
The purpose of this section is to present the safety philosophy
and corresponding limit state design format applied in this RP.
This section applies to subsea separators that are to be built in
accordance with this RP. Note that the focus for this RP is the
overall structural integrity of subsea separators in deep water
where the static external pressure is the governing load condition. No design practice has yet been established for such conditions. At more shallow water depths, existing design practice
governed by external overpressure according to existing rules
and regulations applies, where also the design by analysis
approach as described here may be an attractive and applicable
The integrity of a subsea separator in deep water constructed to
this standard is ensured through a safety philosophy integrating the different aspects illustrated in Figure 2-1.
All companies have policy regarding human aspects, environmental and financial issues. These are typically on an overall
level, but more detailed objectives and requirements in specific
areas may follow them. Typical statements regarding safety
objectives for a subsea separator may be:
All work during the construction period shall be such as to ensure
that no single failure will lead to dangerous situations for any
person or to unacceptable damage to material or the environment.
The impact on the environment shall be reduced to as low as reasonably possible.
Statements such as those above may have implications for all or
individual phases only. They are typically most relevant for the
work execution (i.e. how the contractor executes the job) and for
specific design solutions. Having defined the Safety Objective, it
can be a point of discussion as to whether this is being accomplished in the actual project. It is therefore recommended that the
overall Safety Objective be followed up by more specific, measurable requirements.
If no policy is available, or if it is difficult to define the safety
objective, one could also start with a risk assessment. The risk
assessment could identify all hazards and their consequences,
and then enable back-extrapolation to define acceptance criteria,
testing regime and areas that need to be followed up more
In this Recommended Practice, the structural failure probability
is reflected in the choice of safety class. The choice of safety
class should also include consideration of the expressed safety
2.2.2 Systematic review
A systematic review or analysis shall be carried out at all
phases in order to identify and evaluate the consequences of
failure of the subsea separator, such that necessary remedial
measures can be taken. The consequences include consequences of such events for people, for the environment and for
assets and financial interests.
A methodology for such a systematic review is quantitative risk
analysis (QRA). This may provide an estimation of the overall
risk to human health and safety, environment and asses and comprises:
- assessment of probabilities of failure events
- accident developments
- consequence and risk assessment.
It should be noted that legislation in some countries requires risk
analysis to be performed, at least at an overall level to identify
critical scenarios that might jeopardise the safety and reliability
of the separator system. Other methodologies for identification
of potential hazards are Failure Mode and Effect Analysis
(FMEA) and Hazard and Operability studies (HAZOP).
2.2.3 Fundamental requirements
A separator shall be designed, manufactured, fabricated, operated and maintained in such a way that:
with acceptable probability, it will remain fit for the use
for which it is intended, having due regard to its service
life and its cost, and
with appropriate degree of reliability, it will sustain all
foreseeable load effects, degradation and other influences
likely to occur during the service life and have adequate
durability in relation to maintenance costs.
Sub-sea separators in this RP are based on the conditions that
the separators are built as cylindrical vessels with dished heads
at each ends. If other type of separators are selected, they will
be subject to special considerations.
The number of nozzles and penetrations through the vessel
wall should be kept as low as possible in order to minimise the
areas for potential leaks. If possible, flanged joints should be
replaced by permanent welding or similar safe joining in order
to avoid any leaks.
Dished heads should be of hemi spherical type in order to give
a smooth area between shell and heads and also to reduce possibility for buckling in any transition areas due the external
pressure. Elliptical and torispherical heads should not be used
for deep water separators.
In the case of shell and spherical head plates with different
thicknesses, the design for the joint needs special considerations. Generally, centrelines in the middle of the shell- and
spherical heads plates should merge theoretically together
without any offset. For unequal thicknesses of the plates, the
transition section should be machined with a minimum internal
and external angle/slope. The thicker part of the shell or the
heads should preferably be machined with a cylindrical part for
performing required non-destructive testing of the final weld
Horizontal vessels should be supported on two symmetrically
located saddle supports equipped with stiffening rings continuously the whole circumference of the separator in order to
reduce any local stress concentrations in the shell cased by supporting loads. Those stiffening rings will also give protection
for any externally objects which might hit the separator during
the installation- and/or operating phases. Any lifting pads/lugs
should also be integrated into the stiffening rings or saddle
supports if possible on order to reduce unnecessary welding on
separator shell.
Note that in service, inspection will be impossible (or at least
very limited) in very deep water.
In order to maintain the required safety level, the following
The design shall be in compliance with this RP.
Separators shall be designed by appropriate qualified and
The materials and products shall be used as specified in
Adequate supervision and quality control shall be provided during design, manufacture and fabrication.
Manufacture, fabrication, handling, transportation, installation and operation shall be carried out by personnel having the appropriate skill and experience. Reference is
made to recognised standards for personnel qualifications.
The separator shall be maintained and inspected in accordance with the design assumptions.
The separator shall be operated in accordance with the
design basis and the installation and operating manuals.
Relevant information between personnel involved in the
design, manufacture, fabrication and operation shall be
communicated in an understandable manner to avoid misunderstandings.
Design reviews shall be carried out where all contributing
and affected disciplines are included to identify and solve
Verification shall be performed to check compliance with
provisions contained herein in addition to national and
2.2.4 Installation and operational considerations
Operational requirements are system capabilities needed to
meet the functional requirements. Operational considerations
include matters which designers should address in order to
obtain a design that is safe and efficient to install, operate and
maintain. Operational requirements include operational philosophy, environmental limits, installation and retrieval, inservice operations, inspection and maintenance philosophy.
Safe operation of a separator requires that:
The designer shall take into account all conditions which
the separator will be subjected to during installation and
The operations personnel shall be aware of, and comply
with, limits for safe operations.
2.2.5 Design principles
In this RP, structural safety of the separator is ensured by use
of a safety class methodology, with the use of EN 13445 as a
basis. The separator including interfaces, details and components, shall be designed according to the following basic principles:
Since no (or very limited) in service inspection is possible,
particular focus on robust design is essential; e.g. weld
design (with focus on enabling proper NDT), nozzle
design, material specifications, inspection and testing
The separator shall satisfy functional and operational
requirements as given in the design basis.
In addition to the use of comprehensive and detailed
installation procedures, soft landing devices should be
specially designed to accommodate installation forces
The separator shall be designed such that an unintended
event does not escalate into an accident of significantly
greater extent than the original event.
Permit simple and reliable installation, retrieval, and be
robust with respect to use.
Provide adequate access for subsea (ROV) inspection and
replacement (and maintenance/repair as applicable)
Nozzles and components shall be made such that fabrication and adequate inspection can be accomplished in
accordance with relevant recognised techniques and practice.
Design of structural details and use of materials shall be
done with the objective to minimise the effect of corrosion, erosion and wear.
The design should facilitate monitoring of its behaviour in
terms of vibrations, fatigue, cracks, wear, erosion, corrosion, etc.
The design format within this RP requires that the possibility
of gross errors (human errors) shall be prevented by requirements to the organisation of the work, competence of personnel performing the work and verification activities during the
design, manufacture and fabrication phases and quality assurance during all relevant phases.
A quality system shall be established and applied to the design,
manufacturing, fabrication, testing, operation and maintenance
activities to assist compliance with the requirements of this
2.3 Design format
2.3.1 Basic considerations
The design procedure and its format ensure that the safety
objective is met. This is to keep the risk and the failure probability (i.e. probability of exceeding a limit state) below a certain level. Note that gross errors have to be prevented by a
quality system that ensures proper organisation of the work
and use of personnel with appropriate competence and verification.
The following design methods may be applied:
Load and Resistance Factor Design (LRFD) method
quences before a safety class of medium or low is assigned.
2.3.2 Safety class methodology
This RP gives the possibility to design with different safety
requirements, depending on the safety class to which the separator belongs. The separator shall be classified into a safety
class based on the consequences of failure. The safety class
the hazard potential of the fluid in the separator; i.e. fluid
the location of separator
whether the separator is in operating or temporary state.
Fluids are divided into two groups in accordance with the classification given in Article 9 of PED.
Group 1 comprises dangerous fluids defined as:
flammable (where the maximum allowable temperature is
above flashpoint)
Group 2 comprises all other fluids not referred to above.
For a subsea production separator, the normal operating fluid
contents are produced hydrocarbons, hence fluid group 1 applies.
Location is classified two areas:
Location 1 is where no frequent human activity is anticipated.
Location 2 is near human activity; e.g. within the platform
safety zone. A horizontal distance of 500 m from the platform is suggested at shallow water depths, whereas a
larger distance should be considered in deeper waters.
Risk analysis considering release of hydrocarbons may be used
to establish the location class. For a deepwater separator normally location category 2 applies.
The concept of safety class links acceptance criterion for the
separator design with the potential consequences of failure
defined in Table 2-1:
Table 2-1 Classification of safety classes
low risk of human injury and
minor environmental and economic consequences.
For conditions where failure implies risk of human
injury, significant environmental pollution or high
economic or political consequences
For operating conditions where failure implies high
risk of human injury, significant environmental pollution or very high economic or political consequences
For normal use, the safety classes in Table 2-2 apply. Other classification may exist depending on the conditions and criticality
of the separator. The operator shall specify the safety class to
which the separator shall be designed, and although the consequences to life and environment may be low/medium, particular
consideration should be made regarding the economic conse-
Table 2-2 Normal classification of safety classes
The concept of safety classes is not explicitly addressed in
PED or in the EN 13445 code. The safety class concept is discussed in relationship to PED in 2.4.
2.3.3 Design by LRFD-method
This is a flexible format where each partial safety factor is
intended to reflect the uncertainty in the parameter it is multiplied with. Typically different magnitudes of the partial safety
factors for different types of loading associated with different
degree of uncertainty applies. Typically load effects with associated partial safety factors are split into:
pressure load effect
functional load effect
environmental load effect
accidental load effect.
Similarly, several partial safety factors on the capacity side
may be defined, reflecting uncertainty in the material properties and capacity calculation. The factor to distinguish between
the different safety classes applies to the resistance, and is
defined as a safety class resistance factor.
2.3.4 Working Stress Design (WSD)
The Working (allowable) Stress Design method is a design format where the structural safety margin is expressed by one central safety factor or usage factor for each limiting state.
In the present RP, with focus on deep water, the dominating
uncertainty is related to the capacity of the separator, whereas the
loading governing for the main dimensions of the separator is
practically deterministic (based on the static head). A single
safety factor on the capacity is defined, whereas the load is
applied with a safety factor of unity due to its deterministic
nature. In this particular application the LRFD-method may
therefore be equivalent to the WSD method
The usage factor may be interpreted as an inverted weighted
product of partial safety factors. The usage factor is also named
Allowable Stress factor or Design Factor in some WSD codes
2.3.5 Reliability based design
As an alternative to the design formats specified in this standard, a probabilistic design approach based on a recognised
structural reliability analysis may be applied provided that:
The method complies with DNV Classification Note no.
30.6 or ISO 2394.
The approach is demonstrated to provide adequate safety
for familiar cases, as indicated by this standard.
The target reliability level complies with the acceptance
criteria defined herein; confer discussion in 2.4 on the
equivalent overall level of safety.
The reasoning for pursuing a probabilistic design approach
It is used for calibration of explicit limit states outside the
Physical properties for governing variables are know to be
different from what was applied in the calibration performed herein.
The adequate probabilistic model is know to be different
from what was applied in the calibration performed herein.
Detailed analysis, inspection, testing, application of improved
material quality, may reduce statistical uncertainty, model uncertainty and measurement uncertainty. This improved state of
knowledge may then be utilized in the design process.
Risk is defined as the product of probability of a hazardous situation (here structural failure) and its associated consequences. This is illustrated in Figure 2-2.
Suitably competent and qualified personnel shall perform the
structural reliability analysis, and extension into new areas of
application shall be supported by technical verification. As far
as possible, target reliability levels shall be calibrated against
identical of similar subsea separator designs that are known to
have adequate safety based on this standard. If this is not feasible, the target safety levels shall be based on the failure type
and safety class as given in Table 2-3.
For subsea separators in very deep water, the annual probability of
failure considering ULS is close to the probability of failure for the
entire lifetime, if material degradation and corrosion is accounted
for in the analysis. This is because the time dependent load is
insignificant compared to the static. The application of Table 2-3
is therefore somewhat more conservative than for designs where
e.g. time variant environmental loading is dominating.
Table 2-3 Acceptable failure probabilities1) vs. safety class
SLS3)
FLS4)
The failure probability from a structural reliability analysis is a
nominal value and cannot be interpreted as an expected frequency of failure.
The probability basis is failures per year for permanent condition,
or for the actual period of operation for temporary conditions.
The failure probabilities for SLS are not mandatory. SLS are
used to select operational and installation limitations and can be
defined according to the operators preference. Note that
exceeding a SLS condition requires a subsequent ALS design
The annual failure probability is usually considered in the last
year of service life or last year before inspection.
2.4 Safety Class Concept and PED
The concept of safety class is not addressed in PED nor part of
the EN 13445 code. The Essential Safety Requirements of
PED (Annex I, Section 7) allows for alternatives to the provisions given, provided that it can be demonstrated that appropriate measures have been taken to achieve an equivalent overall
level of safety. Guideline 8/6 of PED states that adequate
safety margins and deviations from a particular value can be
justified by reduced risk in the respective failure mode, or by
additional means to ensure no increase of the risk.
The safety class concept as defined here should not be confused
with the class of vessel (I, II, II and IV) based on pressure and volume as used in PED. This RP deals with large volumes and high
pressures; i.e. class IV vessels. The safety class concept introduced
in this RP is based on risk evaluations for this type of vessels.
The risk may be reduced either by reducing the probability of
structural failure or by reducing its consequences. A hazard
with a high probability of occurrence in combination with
high consequences is associated with a high risk, which is
not tolerable. On the other hand, unlikely hazards with low
consequences may be ignored. In between is the ALARP (As
Low As Reasonably Practicable) region, where cost effective
risk control options should be implemented.
The fully harmonised standard EN 13445 forms the basis for
the design criteria in this RP. Since consequences of failure (or
the concept of safety classes) are not explicitly addressed in
EN 13445, it is reasonable to assume that the design criteria of
EN 13445 also cover cases where the consequences associated
with failure are high. In order to have an acceptable risk level
the corresponding annual probability of occurrence must in
these cases be low. Following Table 2-3, such a probability
is likely to be in the order of 10-5, and EN 13445 may be associated with the upper left corner of the risk matrix in Figure 22. The probability of failure is a controlled by the design criterion, and a change in the magnitude(s) of the (partial) safety
factor(s) implies a change in the probability of failure.
The consequences of structural failure of a subsea separator
installed in deep water are to be evaluated with respect to life,
environment and property, see also 2.3.2. Some comments are
- consequences to life are likely to be low; i.e. no injuries of
- consequences to environment are also likely to be relatively
low, provided that the separator can be isolated so that limited
or no releases from the connected pipelines are ensured. (Failure is most likely to occur in a near vacuum condition, and the
spill of content will therefore be limited due to low filling).
However, appropriate consideration of these consequences
must be made in each individual case.
- consequences to property; i.e. costs related to loss of the separator itself, and costs related to the operation interruption and
replacement. Appropriate considerations should be made by
the operator. If the consequences to life and environmental are
low, a cost benefit calculation may be performed to check if
the safety factor corresponding to safety class low is cost
effective or if a higher safety factor should be applied.
The risk matrix in Figure 2-2 Risk matrix includes a combined
consequence on the vertical axis; i.e. a combination of consequences to life, environment and property.
The risk level inherent in the EN 13445 code is assumed to fit
into the risk matrix at high consequences and low probability.
Failure of a subsea separator is likely to be associated with
lower consequences, and this allows maintenance of the same
risk level for an increased probability of failure, see Figure 22. An increased probability of failure effectively corresponds
to a reduction in the safety factor(s) of the design criterion.
The safety class methodology is in terms of a safety class
resistance factor that is to be multiplied with the partial safety
factor for the material strength. The magnitude of the safety
class resistance factor was calibrated using structural reliability analysis for different target safety levels. Background for
these calculations may be found in Appendix A. Focus for the
calculations is collapse due to external pressure, but the philosophy and the factors are also valid for designs in shallow water
governed by the internal pressure, provided that a departure
from high consequences can be justified. The value of the
safety class resistance factor for the different safety classes are:
ally for subsea separators in very deep waters, these limit states
are unlikely to be governing. However, an evaluation of potential dropped objects or other rare events should be evaluated
related to ALS. Fatigue due to environmental loading is normally not an issue, however, potential fatigue of particular
structural components and interfaces to pipes due to variation
in operational loads or VIV should be considered if relevant.
Several failure modes may be relevant for the ULS. The
Design by Analysis Direct Route of EN 13445-3 Appendix B
1.0 for safety class high (This corresponds to EN 13445)
0.93 for safety class medium
0.86 for safety class low.
The most critical design check for structures covered by the
present RP is identified to be gross plastic deformation or
instability, which in both cases leads to collapse of the subsea
separator. It is the ability to sustain the external (static) pressure load that is governing, and this depends essentially on the
actual compressive yield strength in the hoop direction and the
initial ovality of the separator. (Ref. OMAE 2003-37219) If the
separator is built from plate, an evaluation of the actual compressive yield strength in the circumferential direction of the
separator is recommended. This must be carried out through
uni-axial compressive tests with round bar specimen (i.e. not
flattened).
Progressive plastic deformation and fatigue do normally not
need to be evaluated since the load is completely dominated by
the static head. Static equilibrium relates to stability of the separator as a unit when installed. In this context the support conditions of the separator are relevant, which is outside scope of
the present RP. However, a compatible and proper interface
with the support structure design must be ensured. These
design checks are not further considered in this RP.
The use of safety class methodology effectively maintains the
safety level without increasing the risk, and complies with the
essential safety requirements of PED.
It follows according to the ALARP principle that application
of the resistance factor for a lower safety class, and hence
acceptance of a higher failure probability, should be based on
cost efficiency arguments. Possible causes may be fabrication
limitations or installation aspects, making the more reliable
design unduly costly or even unfeasible. Oppositely, if the
potential savings for the more unsafe design are marginal, the
safety factor for the higher safety class should be applied.
This section provides procedures for limit state design checks
of relevant failure modes for subsea separators in very deep
Reference is made to EN 13445 and Section 4 of this RP.
The differential pressure due to the static external pressure and
the internal design pressure or a vacuum condition is normally
the dominating and governing load. Self-weight and support
conditions should also be considered.
Evaluation of potential accidental loading, such as dropped
object or fishing gear snagging, should be made.
Gross Plastic Deformation (GPD)
Progressive Plastic Deformation (PD)
Fatigue failure (F)
Static equilibrium (SE).
3.6 Calculation methods
Several calculation methods may be applicable according to
EN 13445-3; i.e. design by formulae, design by analyses,
design by fracture analysis or design by experimental methods.
The present RP focuses on the use of design by analysis as an
alternative, or as a complement to design by formulae.
At more ordinary water depths, existing practice, e.g. using the
design by formulae (DBF) methodology in EN 13445-3, may
provide feasible solutions. For deep water locations the design
by analysis (DBA) approach provides consistent means to
achieve more optimal designs with acceptable reliability. The
design philosophy as focused on in this RP may also be utilised
for ordinary water depths.
3.7 Design criteria
Reference is made to Section 3.7 Design criteria of this RP.
3.5 Limit states and failure modes
The following limit state categories are defined:
Serviceability Limit State (SLS) corresponding to criteria
limiting or governing for the normal operation (functionality) of the separator.
Ultimate Limit State (ULS) corresponds to the maximum
structural resistance before failure.
Accidental Limit State (ALS) is an ULS, but consider
infrequent (accidental) load.
Fatigue Limit State (FLS) is an ULS condition accounting
for accumulated cyclic load effects.
The present RP covers ULS only, which is defined as the limit
state corresponding to the maximum load carrying capacity. In
principle, SLS, ALS and FLS also need to be checked. Gener-
The vessel shall be designed according to a recognised design
code. This RP focuses on use of the PED harmonised standard
EN 13445 Unfired Pressure Vessels.
EN 13445 Part 3 describes three different design methods; one
method for design by formulae (DBF), and two methods for
design by analysis (DBA).
EN 13445 Part 3 Annex B Design By Analyses Direct route is
the method that has been in focus during the development of this
RP. A non-linear analysis is carried out, taking into account the
geometrically imperfections and a non-linear material model
used. The relevant failure mode is simulated in the analysis.
This Annex B describes a method for design which is an alternative to DBF or DBA-Annex C of EN 13445-3. Annex B may
also be a compliment to DBF for load cases that is not covered
by that route, for load combinations not covered by DBF and for
cases where the manufacturing tolerances are exceeded.
EN 13445-3 Annex C, Design by Analysis/Stress Categorisation covers the same principles for design as the principles
found in ASME VIII Div. 2 Appendix 4. A linear analysis is
basis for this approach, and the actual failure mode is therefore
not physically simulated as accurately as using Annex B. The
stress categorisation route can also be rather cumbersome in
some cases. This RP does not give any guidance on how to
apply Annex C.
It is found that for shells subjected to external load, the use of
Annex B will result in less required shell wall thickness compared to other design methods or codes. One reason is that the
partial safety factor when using DBA for pressure loading P may
be set equal 1.0, since external pressure is an action with a natural
limit. The effective safety factor following the DBF route is
3.7.2 Guidance for EN 13445-3, Annex B
Finite element analysis shall be carried out using recognised
and well documented software for analysis of non-linear
response with the capability of including the possible weakening effect of geometrically non-linearity, ref. Chapter B.7.1.
Different element types may be used. The use of 8-noded shell
elements is a recommended alternative, modelled in the midplane of the wall thickness. The number of elements around the
circumference may typically be in the order of 40. Assumptions
of symmetry may be employed if relevant.
The designer shall perform sensitivity studies to ensure that the
choice of element type and mesh density is appropriate. The
analysis thickness should be used (ref. EN 13445-3 Figure 31; i.e. defined as the nominal thickness minus the tolerance and
corrosion (or erosion) allowance. In general no structural
strength shall be attributed to the cladding, however, exemption to this may be made in special cases as indicated in EN
13445-3 Annex B.7.3.
The minimum required wall thickness for a vessel subjected to
external pressure is normally found by the design checks Gross
Plastic Deformation (B 8.2) or Instability (B 8.4). The applicable constitutive material law depends on the design check. For
Gross Plastic Deformation and Instability, a linear-elastic
ideal-plastic law applies.
An initial out of roundness of 0.5% is considered to be a realistic
value for use in design; however, a lower (or higher) value may
be used depending on the precision of the manufacturing process.
In any case it needs to be verified that the final product is within
the applied value.
EN13445-3 Annex D and E may be used to determine out of
roundness measures. Local deviations from the design shape
does not need to be modelled, but should satisfy the requirements
of Annex D. Ovality (departure from the true circle of cylinders)
shall be included in the finite element model along the full length
of the separator. The modelled ovality shall be equal to, or
greater than, the value obtained from measurements made
according to Annex E.
Usually the applied pressure on a shell finite element model
refers to the mid plane diameter. The results in terms of pressure capacity from the finite element analysis shall then be corrected for the difference in diameter of the finite element
model (mid plane diameter) and the real vessel outside diameter. A scaling factor equal to MD/OD ((Mid plane Diameter)/
(Outer Diameter)) applies to the FE result. The external pressure acts on the outer surface which is greater than the modelled mid plane surface.
Most finite element software utilise von Mises yield criterion
and this is accepted as long the difference between Tresca and
von Mises yield condition is taken into account. Tresca yield
condition, ref. Chapter B.7.4, shall be accounted for by reducing the material strength parameter by a factor 3 2 in the
failure mode Gross Plastic Deformation Design Check (GPDDC).
Although the EN code states that this factor is to be applied to the
material strength parameter, it is considered conservative and
appropriate to apply this factor to the capacity itself. In cases
when the elastic buckling capacity is important for the ultimate
capacity, a certain reduction in the yield stress (material strength
parameter) will not lead to a comparable reduction in the capacity, hence the difference between Tresca and von Mises may be
ness is to be calculated as the difference between the largest
and the smallest diameter divided by the mean diameter, which
is equivalent to the deviation from the mean radius divided by
the mean radius. In case the vessel is found to have out of
roundness greater than this after manufacturing, the analysis
shall be re-calculated with the new geometry input.
Linear elastic ideal plastic material model without strain
The finite element model shall include initial out-of roundness,
implemented as a perfectly elliptic shape. The out-of round-
It is possible to adopt the partial safety factors for testing also
for operation/design, ref. EN 13445-3 Annex B.7.5.1. However, due to the limited experience from design and operation
of subsea separators at great depths, the partial safety factors in
EN 13445-3 Annex B Table B.8-1 and Table B.8-2 should be
The partial safety factor P may be set at 1.0 for external pressure, as found in Table B.8-1. Internal overpressure shall have
a partial safety factor P equal 1.2. The design strength parameter RMd is calculated by RM divided by R as found in Table
B.8-2.
The design check Instability (EN 13445 Annex B.8.4) may
become the dimensioning design check for wall thickness, at
least if the separator is not subjected to tests as called for in
EN13445-5 with external pressure. The partial safety factor R
shall be 1.5 if the tests can not be performed, with tests a value
of 1.25 applies.
The design shall, in principle also be subjected to the three
remaining design checks Progressive Plastic Deformation
(B.8.3), Fatigue (B.8.5) and Static Equilibrium (B.8.6). These
may not be relevant or dimensioning, see also discussion in
According to EN 13445-3 Annex B, the partial safety factor R
shall be combined with the material strength parameter RM and
the design strength parameter RMd shall be used as input for the
material model in the finite element analysis. However, a more
appropriate and conservative result is obtained if the partial
safety factor R is applied on the finite element analysis result
(capacity) instead. For elastic buckling, where the capacity of the
structure is less dependent on the yield stress (but more dependent on the Youngs modulus) the difference between the two
methods can be significant, and the designer should therefore use
the conservative method in order to arrive at a robust and safe
It is also noted that the partial safety factor R as given in Table
B8-2 depends on the ratio between the tensile strength and the
yield strength. When the tensile strength is much higher than the
yield strength, a lower R is required. This fact indicates that failure due to internal overpressure (e.g. burst) has been in focus for
the development of EN 13445. In the case of collapse due to
external overpressure, the tensile strength is of minor importance
since collapse actually occurs for relatively low strains and the
tensile strength does not come into effect. The definition of R as
given in Table B8-2 is therefore not logical based on the physical
behaviour. However, it is observed that with the minimum value
of R = 1.25, the GPD-DC is almost as critical as the instability
check for the case of no pressure test:
GPD-DC:
= 1.25 3 2 = 1.44 (Von Mises)
(Von Mises, no pressure test)
Physically, the instability check is most relevant for collapse due
to external overpressure.
Stiffeners must be attached in such a way that they add/
increase capacity for critical failure modes. When designing
for the failure mode instability/collapse due to external pressure, outside stiffeners should be properly welded to the shell
to effectively increase the resistance.
For fabrication processes which introduce cold deformations
giving different strength in tension and compression, a fabrication factor, fab, shall be determined. If no other information
exists, maximum fabrication factors for a separator manufactured by the UOE or UO processes are given in Table 3-1.
These factors also apply to other fabrication processes which
introduce similar cold deformations such as three roll bending
(TRB). The factor shall be applied to the yield strength in compression.
The fabrication factor may be improved through heat treatment, if documented.
3.8 Design details
Reference is made to EN 13445-3 Chapter 9: Openings in
shells, EN 13445-4 Fabrication and EN 1708-1 Recommended Weld Details.
Guidance on specific details for separators:
The requirements in this section are applicable for the base
material only. For manufacturing of the clad steel material,
consisting of the backing material and a thinner layer cladding
layer of cladding metal, reference is given in Section 5, Fabrication, testing and inspection of clad steel plates.
The requirements in this section are supplementary to EN
13445. In case of conflict between EN 13445 and the requirements stated in this section, the most stringent shall apply.
4.3.1 Type of materials
The base material shall be carbon-manganese (C-Mn) steel
with maximum SMYS of 555 MPa, or ferritic-austenitic
(duplex) stainless steel type 22 Cr or type 25 Cr. The selected
base material shall be intended for pressure vessel applications. When possible, it is recommended to use one of the
steels in EN 10028 modified as per this document.
C-Mn steels with SMYS > 555 MPa are not covered by this
document. If applicable, qualification according to DNV-RPA203, Qualification Procedures for New Technology, is recommended. The qualification testing should be based on fracture mechanics testing under simulated operational conditions.
4.3.3 Corrosion
Resistance towards external corrosion and Hydrogen Induced
Stress Cracking (HISC) is covered in 6.3.
4.4 Material manufacturing
4.4.1 Manufacturing Procedure Specification (MPS)
It is required that the Contractor/manufacturer of the vessel
prepares a Manufacturing Procedure Specification (MPS), see
6.4.1. This MPS shall address the important factors influencing
on the quality and reliability of the production. The material
manufacturer shall ensure that all relevant requirements in this
MPS are fully complied with.
All manufacturing of plate shall be performed following the
sequence of activities and within the agreed allowable variations of the qualified MPS. The manufacturing practice and the
instrumentation used to ensure proper control of the manufacturing process variables and their tolerances shall be described
The following requirements shall apply for the manufacturing:
Table 3-1 Maximum fabrication factor, fab
UO & TRB
4.3.2 C-Mn steel with SMYS > 555 MPa
- nozzle to shell/head connections
- connections for internal and external stiffening rings and
- joining of lined components
- access openings: number and sizes
- lugs for lifting and transportation.
the mill shall have proper control of start and finish rolling
temperature, rolling reduction and post-rolling cooling
rate (i.e. accelerated cooling)
plate thickness shall be controlled by continuously operating devices
heat treatment shall be controlled by calibrated temperature measuring devices
plate edges shall be cut back sufficiently after rolling, to
ensure freedom from defects.
4.5 Material requirements
4.5.1 Steelmaking
4.5.1.1 C-Mn steel
All steels shall be made by an electric- or one of the basic oxygen processes. C-Mn steel shall be fully killed and made to a
fine grain practice. Details and follow-up of limiting macro, as
well as micro, segregation shall be given in the MPS.
For steel to be used for sour service, special attention to impurities and inclusion shape control shall be required. Details of
the inclusion shape control treatment shall be given in the
4.5.1.2 Ferritic-austenitic stainless steel
As specified in 4.5.1.1. Additionally, ferritic-austenitic stainless steels shall be refined by argon oxygen or vacuum oxygen
decarburization before casting.
4.5.2 Chemical composition
4.5.2.1 C-Mn steel
The chemical composition shall be agreed prior to start of production.
The chemical composition shall ensure the intended heat treatment response, and that the required mechanical properties are
The following general requirements with respect to chemical
composition shall apply:
sulphur 0.010% on cast analysis
phosphorous 0.020% on cast analysis
max. Carbon Equivalent, i.e. CE, shall be as specified in
The carbon equivalents shall be calculated according to the
Cr + M0 + V
Mn + Cu + Cr
It is recommended to use the latter formulae, i.e. Pcm, for carbon-manganese steels with carbon content < 0.18%.
If sour service applies, the required modifications in ISO
15156 shall be fulfilled.
Table 4-1 Max. Carbon Equivalent (CE)
Local brittle zones (LBZs) can be formed in the HAZ of C-Mn
micro alloyed steels. These areas tend to exhibit very low
cleavage resistance, resulting in low CTOD values. The LBZs
are associated with the sections of the HAZs that are experiencing grain coarsening. These zones have a predominantly
bainitic structure, with a large amount of martensite/austenite
(M/A) constituents (BI-microstructure). The M/A constituents, as opposed to ferrite/carbide aggregate such as pearlite,
may have a detrimental affect on the material's toughness. This
should particularly be kept in mind when selecting the chemical composition for steels with SMYS > 450 MPa. In order to
improve HAZ toughness, it is essential to refine the grain size
and suppress the formation of bainite with M/A constituents.
For material to be quenched and tempered, the content of hardening elements Cr, Mo, Cu and Ni shall be sufficient to obtain
the desired microstructure in the centre of the component. The
selected chemical composition shall have adequate hardening
ability to ensure through thickness hardening of the respective
4.5.2.2 Ferritic-austenitic stainless steel
If not otherwise agreed the types 22 Cr and 25 Cr duplex stainless steels shall comply with the chemical compositions specified in EN 10028-7, as applicable, with the following
sulphur 0.020% on cast analysis
phosphorous 0.03% on cast analysis
PRE = %Cr + 3.3%Mo + 16%N 40 for type 25 Cr.
The material selected shall have appropriate properties for all
operating conditions which are reasonable foreseeable.
If the selected material specification does not specify appropriate properties, the minimum values shall be agreed with the
material manufacturer and included in the MPS, see
4.5.3.1 Strength and ductility
The selected materials should have mechanical strength vs.
ductility as specified in Table 4-2 and Table 4-3.
Attention is made to the relation between yield- and tensile
strength in both longitudinal and transverse direction.
Table 4-2 Mechanical properties for carbon-manganese steels 1)
YS (Rt0.5)
(MPa) 2)
(MPa)3)
energy (KVT)
minimum J 5) 6)
T = transverse direction, L = longitudinal direction.
The actual yield strength in longitudinal direction shall not exceed SMYS by more than 120 MPa.
SMTS in the longitudinal direction can be 5% less than the required values in transverse direction.
The YS/UTS ratio in the longitudinal direction shall not exceed the maximum specified value in the transverse direction by more than
0.020 for standard material, and more than 0.030 for sour service material.
The KVL values (when tested) shall be 50% higher than the required KVT values.
For thickness 40 mm the Charpy-V impact test temperature shall be T = TMDT-20oC (MDT = minimum design temperature).
For thickness > 40 mm the Charpy-V impact test temperature shall be agreed upon.
Table 4-3 Mechanical properties for ferritic-austenitic stainless steels 1)
minimum J 4)5)
For thickness 40 mm the Charpy-V impact test temperature shall be T = TMDT-20oC (MDT = minimum design temperature.).
4.5.3.2 Properties at elevated temperatures
If elevated temperature properties for the steel is not included
in the applicable material specification the minimum values
shall be agreed with the material manufacturer and included in
the MPS, see Section 6.4.1.
The proposed de-rating effects of the yield stress, in Figure 41 below, may be used as guidance for establishing elevated
For test temperature, see Tables 4-2 and 4-3.
NOTE: These de-rating curves are conservative compared to
EN 10028.
4.5.3.3 Toughness
Minimum toughness requirements should be based on one of
Toughness values specified in Tables 4-1 and 4-2
Using Method 2 in EN 13445-2, Annex B
The required toughness is specified as a function of the
Proposed de-rating values for yield stress
NOTE: For thickness above 40 mm the impact test temperature and/or the required impact toughness should be based on
agreement. Increasing thickness requires higher toughness
Method 2 in EN 13445-2, Annex B, concerns technical
requirements developed from fracture mechanics and operating experience. The method is valid for carbon-, carbon-manganese- and low alloy steels with SMYS 460 MPa, and for
duplex stainless steels with thickness 30 mm.
Fracture mechanic analyses should be performed as specified
in EN 13445-2, Annex B, in method 3. This method will ensure
that the acceptance criteria used for Non Destructive Testing
(NDT) are adequate for a higher material utilisation. Possible
problems related to NDT of thick plate will be easy to solve
since the maximum allowable defect size will be determined
by the analysis. Ultrasonic testing must be specified since Xray testing cannot be used to determine defect height. The
uncertainty in sizing and probability of detection must be
established (this may be time consuming and costly). Method
3 is applicable for all steels covered by this document.
Method 3 will include fracture toughness Crack Tip Opening
Displacement (CTOD) testing (that takes through thickness
variations into account) to provide data that can be used to
determine calculate NDT acceptance criteria and impact test
CTOD testing shall be carried out at minimum design temperature.
4.6 Material testing
The mechanical and corrosion testing shall include the testing
shown in Table 4-5 as applicable.
All tests shall be performed as specified in the selected material specification if not otherwise specified in the MPS.
If neither the material specification nor this document or the
MPS, specify sampling, test methods, the testing should be
based on recognised standards, e.g. ISO-, EN, ASTM standards.
NOTE: The tensile test, impact tests, hardness tests, pitting
corrosion test and metallographic examination shall be performed by the material manufacturer. The remaining tests shall
be performed by the material manufacturer or by the contractor.. If not otherwise agreed it is in the responsibility of the
4.6.1 Chemical analysis
The steel shall be subject to both heat analysis and product
heat analysis shall be carried out on each melt
product analysis shall be performed on one randomly
selected plate from each test unit.
The chemical analyses shall be performed as specified in the
selected material specification, if not otherwise specified in the
The content of the following elements shall be determined and
reported: C, Mn, Si, P, S, Cu, Ni, Mo, Cr, Al, Nb, V, Ti, N, B.
Other elements for controlling the material properties may be
added, subject to agreement. When scrap material is being
used for production of C-Mn steel, the content of the elements
As, Sb, Sn, Pb, Bi and Ca shall be checked once during MPQT,
see 6.4.2, and reported. Limitations on amount of scrap metal
shall be stated in the MPS.
If the value of any elements at the product analysis, or combination of elements fails to meet the requirements specified by
the MPS, a re-test consisting of two specimens shall be made.
The re-test specimens shall be sampled from two additional
plates from the same heat. If one or both re-tests still fail to
meet the requirements, the heat should be rejected.
4.6.2 Mechanical testing
The mechanical testing shall be carried out on each separately
rolled plate and shall as minimum contain:
tensile testing transverse to rolling direction
one set of Charpy V impact testing in transverse to rolling
direction with notch perpendicular to the surface.
The mechanical testing shall be performed as specified in the
NOTE: In 4.3.1 it is specified that steels selected shall be
intended for pressure vessel applications. It is therefore
assumed that the test sampling is convenient. If any doubt, the
test sampling should be especially checked and considered for
each material specification / material data sheet.
Table 4-4 Mechanical- and corrosion testing
Type of test 1)
Mandatory 2)
Strain ageing test
Mandatory 7)
All testing shall be performed in accordance with 4.6, if not
otherwise specified in the MPS.
Shall be performed under the responsibility of the material
manufacturer and included in Type 3.2 Material Certificate.
The contractor. shall specify whether this test (CTOD-testing)
is relevant or not, see 6.6.2.8. If relevant, the contractor. shall
specify whether the test shall be carried out under the responsibility of the material manufacturer or the contractor..
The contractor. shall specify whether this test is relevant or not,
see 6.6.2.9. If relevant, the contractor. shall specify whether the
test shall be carried out under the responsibility of the material
manufacturer or the contractor.. The test shall be carried out as
specified in 6.6.2.9.
see 6.5.2.7. If relevant, the contractor. shall specify whether the
specified in 6.6.2.7.
see 4.6.4. If relevant, the contractor. shall specify whether the
manufacturer or the contractor..
Mandatory for duplex stainless steel, type 25 Cr. If applicable
for type 22 Cr, the test conditions shall be agreed upon.
4.6.3 Hardness test
Hardness testing is required.
Unless sour service is specified, the hardness shall comply
with Table 4-1 and Table 4-2, as applicable.
If sour service is relevant the acceptance criteria shall be as
specified in ISO 15156.
4.6.4 SSC test
If sour service is applicable, Stress Sulphide Cracking (SSC)
test is required unless the material is listed in ISO 15156.
When applicable, the test should be carried out according to
4.6.5 Pitting corrosion testing
Corrosion testing according to ASTM G48, method A, shall be
performed in order to confirm adequate manufacturing procedures affecting the microstructure of ferritic-austenitic stainless steel, type 25 Cr.
The maximum allowable weight loss is 4.0 g/m2 for solution
annealed material tested 24 hours at 50oC.
4.6.6 Metallographic examination
documents according to EN 10204.
NOTE: Type 3.1 inspection document according to EN
10204:2004 may be accepted provided there are no doubt that
the applicable requirements for inspection documents in the
Directive 97/23/EC are fulfilled, ref. the Directive 97/23/EC
Annex 1 Ch. 4.3 and PED Working Group Pressure Guideline No. 7/2.
5. Fabrication, Testing and Inspection
of Clad Steel Plates
Metallographic examination shall be conducted at 400X magnification for ferritic-austenitic (duplex) stainless steels. The
material shall be essentially free from grain boundary carbides,
nitrides and inter-metallic phases. The ferrite content shall be
measured according to ASTM E562. The ferrite content shall
be within the range 35-55%.
4.6.7 Re-testing
If one of the tests fails to meet the requirements, two additional
re-tests shall be performed on samples taken from the same test
unit. Both re-tests shall meet the specified requirements. The
test unit shall be rejected if one or both of the re-tests do not
4.7 Non-destructive testing and workmanship
Non-destructive testing shall be performed as specified in
4.7.2 Visual examination and workmanship
Full visual testing, i.e. 100%, on both sides of the plates is
The visual inspection shall be carried out as specified in the
selected material specification.
The acceptance criteria specified in the selected material specification applies, if not otherwise restricted in the MPS.
4.7.3 Ultrasonic examination
Full ultrasonic testing, i.e. 100%, of plates for laminar imperfections is required.
4.7.4 Repair of defects
Surface defects may be repaired as specified in the selected
Repair welding is not permitted.
NOTE: Surface grinding may introduce cold working and harnesses incompatible with the service requirements, i.e. sour
service. In such cases, hardness testing may be required in
order to permit grinding.
Table 4-5 Non-destructive testing
The requirements in this section are applicable for fabrication
of clad steel plates when carbon-manganese steel is the base
5.3 Manufacturing of clad steel materials
5.3.1 Manufacturing Procedure Specification (MPS)
It is required that the contractor. of the vessel is preparing a
Manufacturing Procedure Specification (MPS), see 6.4.1. This
MPS shall address all factors which are influencing on the
quality and reliability of the production. The clad steel plate
Clad steel materials can be manufactured by any manufacturing process which guarantees a metallurgical bond between the
base metal and the cladding.
The cladding material shall be selected based on the corrosion
resistance required by the internal environment. Materials
selection for cladding, the associated hardness criteria, and
requirements to manufacturing and fabrication shall comply
with NACE MR0175/ISO 15156 (latest edition). The same
applies to welding consumables for weldments exposed to the
internal fluid.
Overlay welding should be carried out in minimum two passes
to control substrate dilution and total cover of the backing
The cladding thickness shall not be less than 2.5 mm.
5.3.3 Qualification of cladding procedure
Before cladding commences the cladding procedure shall be
qualified. The procedure should be qualified according to EN
13445-2, Annex C. Additionally, one extra tensile test of the
clad metal is required to prove an elongation after fracture A5
of at least 12%.
The required tests are specified in Table 5-1.
All testing shall be performed in accordance with 4.7, if not otherwise specified in the MPS.
Alternative cladding procedure qualification tests may be used
provided equivalency.
5.4 Fabrication testing
4.8 Material certification
Methods and procedures for mechanical- and corrosion testing
shall be according to recognised industry standards, if not otherwise specified in 5.4.2 to 5.4.8 or in the MPS.
The base materials shall be delivered with type 3.2 inspection
The mechanical- and corrosion testing shall include the testing
shown in Table 5-1 as applicable.
Table 5-1 Fabrication tests of clad steel plates
All testing shall be performed in accordance with 5.4 if not otherwise specified in the MPS.
The contractor shall specify whether this test is relevant or not.
If relevant, the contractor shall specify whether the test shall be
carried out under the responsibility of the material manufacturer or the contractor.
5.4.2 Tensile test
One set of tensile tests is required for each plate. One set of tensile tests consists of two tensile tests as follows:
One test from the full clad plate which is to have a tensile
strength Rm not less than derived from the following formulae:
S1R m1 + S 2 R m2
min. tensile strength of base material
min. tensile strength of cladding metal
nominal thickness of the clad plate (S1+S2)
nominal thickness of the base metal
nominal thickness of the cladding metal
One test of the base metal after removal of the cladding
metal. The test is to satisfy the requirements for the base
Tensile test pieces are to be of the flat type. The test pieces are
normally to have the full thickness of the plate. Where the
thickness of the plate is more than 50 mm, or if necessary for
the capacity of the testing machine, the thickness of the test
piece may be reduced by machining. On single clad plates,
both sides of the test piece are to be machined to maintain the
same ratio of cladding metal to base steel as in the plate, but the
cladding metal does not need to be reduced to less than 3 mm.
NOTE: In the case of clad steels where the cladding has lower
ductility than that of the base metal, a tensile test on the cladding after the base has been removed should show an elongation after fracture A5 of at least 12%.
5.4.3 Impact testing
Impact testing is required. The testing shall be carried out
according to EN 13445-2, Annex C.
The impact test results shall comply with the requirements for
5.4.4 Hardness testing
For both qualification testing and production testing the hardness measurements shall be performed as indicated in
NOTE: The hardness testing in the root area indicated on this
Figure is not relevant.
specified in ISO 15156. Otherwise, the hardness requirements
in Table 4-1 apply.
5.4.5 Metallographic examination
Metallographic examination of the weld metal and the HAZ of
the cladding material shall be performed at a magnification of
400X. The microstructure shall be essentially free from grain
boundary carbides, nitrides and inter-metallic phases.
5.4.6 Bend tests of cladding
The bend test pieces are to be bent 180oC round a former without showing signs of cracking or loosening of cladding metal
from the base material. The diameter of the former is to be
twice the plate thickness when the tensile strength of the plate
is less than 490 MPa, and three times the thickness of the plate
when the tensile strength is more than 490 MPa. Two bend
tests are to be taken from each plate. On single clad plates, one
test is to be bent with the cladding in tension and the other with
the cladding in compression. On double clad plates, the test
pieces are to be bent, so that both cladding metals are tested
5.4.7 Shear strength of cladding
One shear strength test is required from each plate. The test
shall be carried out according to ASTM A 264, or another recognised standard. The shear strength shall be at least 140 MPa.
5.4.8 Pitting corrosion test
Pitting corrosion testing may be considered to confirm that the
cladding process or a subsequent heat treatment has not
affected the corrosion resistance of the cladding material. For
this purpose, ASTM G48 method A, e.g. 24 hours at 50oC, of
specimen machined from the cladding is adequate.
The test piece shall be machined to remove the carbon steel
portion and are to contain the full weld and any heat affected
zone in the corrosion resistant alloy.
5.4.9 Re-testing
5.5 Non-destructive testing and workmanship
5.5.2 Inspection and tolerances
EN 13445, Annex C, applies.
5.5.3 Surface crack examination
Full surface crack examination, i.e. 100%, of the cladding is
Crack like indications are not allowed.
5.5.4 Ultrasonic examination
Full ultrasonic testing, i.e. 100%, of clad plates to check for
laminar imperfections and lack of bonding is required.
Laminar imperfections are not allowed.
Accept criteria for lack of bonding shall be based on EN
13445-2, Annex C, Ch. C.3.
5.5.5 Repair of defects
Minor surface defects and bonding defects may be repaired
welded. However, the plate will be rejected without repair if:
a repair will cause a weakening of the plate
a bonding defect exceeds 8 dm2, or several boding defects
amounting to more than 5% of the surface of the plate
5.5.6 Personnel qualifications
The personnel qualification requirements in EN 13445 applies.
NOTE: If the Directive 97/23/EC applies, the operator qualifications must also satisfy these criteria, i.e. qualified by a
third-party organisation recognised in one of the Member State
(ref. the Directive 97/23/EC Annex 1 Ch. 3.1.3).
Table 5-2 Non-destructive testing
Mandatory2)
All testing shall be performed in accordance with 4.7, if not
Magnetic particle examination for magnetic cladding, dye penetrant testing for non-magnetic cladding.
5.6 Inspection document
In order to prove the conformity to the applicable requirements, the manufacturer shall provide sufficient documentation. This should include, as relevant:
material certificates of the plates
cladding procedure
welder/welding operator approvals
non destructive testing operator qualifications
heat treatment information.
This information may be in the form of a component certificate.
6. Fabrication, Testing and Inspection
The requirements in this section are applicable for fabrication,
testing and inspection of subsea separators.
6.3 Resistance to external corrosion and HISC
The need for application of Cathodic Protection (CP) shall be
evaluated. This evaluation shall as a minimum take into consideration:
the anticipated corrosion rate
the maximum allowable wall-thickness reduction
impact of connection to other systems.
If the outcome of such an evaluation is that CP is redundant,
then CP may be omitted.
In case CP is required for corrosion control, the CP system
shall be designed according to DNV RP B401, Cathodic Protection Design (latest revision).
HISC (Hydrogen Induced Stress Cracking)
CP will cause discharge of hydrogen atoms at the metal surface
when this surface is in contact with seawater. Some of this
hydrogen will become absorbed into the metal matrix. In combination with high tensile loads and/or high internal stresses,
this hydrogen can cause Hydrogen Induced Stress Cracking
(HISC) of susceptible materials. The risk for HISC can be
reduced by application of a higher, i.e. less negative, anode
potential than that produced by zinc-aluminium anodes. Omitting CP will eliminate the risk for HISC caused by discharged
If CP is not required, then no limitations on material strength
are required with respect to HISC.
If CP is required, then the limitations of DNV RP B401 shall
apply, i.e. maximum SMYS up to 550 MPa. This shall be
required both for the welding metal and base metal. It is also
proposed to include an upper limit on AYS, i.e. 650 MPa.
NOTE: Specific requirements to ferritic-austenitic (duplex)
materials with respect to HISC are found in DNV RP F112.
The selection of the C-Mn steel backing steel is, in ambient
seawater conditions, normally not subject to any special sour
service requirements. If, however, the external conditions are
sour according to NACE MR0175/ISO 15156, the requirements of NACE MR0175/ISO 15156 and the supplementary
requirements S of DNV OS-F101, shall apply for the backing steel.
NOTE: ISO 15156 addresses all mechanisms for cracking that
can be caused by H2S and related environments, including sulphide stress cracking, stress corrosion cracking, hydrogen
induced cracking and stepwise cracking, stress-oriented hydrogen induced cracking, soft zone cracking and galvanically
induced stress cracking (these terms are defined in the standard.
6.4 Manufacture of separator
6.4.1 Manufacturing Procedure Specification for separator fabrication (MPS)
Before production commences, the Manufacturer shall prepare
an MPS. The MPS shall demonstrate how the specified properties may be achieved and verified through the proposed manufacturing route. The MPS shall address all factors which
influence the quality and reliability of production. All main
manufacturing steps from control of received raw material to
shipment of finished product, including all examination and
check points, shall be covered in detail. References to the procedures established for the execution of all steps shall be
The MPS shall be subject to agreement. The MPS should as a
minimum contain the following information:
plan(s) and process flow description/diagram
manufacturer and manufacturing location of raw material
and/or plate
raw material scrap content including allowable variation
steelmaking process, casting process, alloying practice,
rolling or working condition and heat treatment, including
target values and proposed allowable variation in process
target values for chemical composition, including a critical
combination of intended elements and proposed allowable
variation from target values
elevated temperature properties, if applicable
alignment and joint design for welding and production
final heat treatment condition
list of specified mechanical and corrosion testing
marking, coating and protection procedures
handling, loading and shipping procedures.
For ordering of plates, and/or clad steel plates, the manufacturer must specify the relevant requirements and target values
6.4.2 Manufacturing Procedure Qualification Test for
separator fabrication (MPQT)
Each MPQT shall include full qualification of two plates from
two different heats. The plates shall be subject to relevant cold
deformation and heat treatment as specified in the MPS.
The type and extent of inspections and tests are specified in
Table 6-1 and Table 6-2. The acceptance criteria for qualification
shall comply with 6.5 and 6.6, and shall be specified in the MPS.
All tests shall be performed according to relevant ISO, EN or
ASTM standards, unless otherwise specified in 6.4 and 6.5, or
NOTE: Additional testing may be required (e.g. weldability
testing, analysis for trace elements for steel made from scrap,
etc.) as part of the qualification of the MPS.
The validity of the qualification of the MPS shall be limited to:
grain refining practice
forming procedure (of vessel)
fabrication facilities (for the vessel).
6.4.5 Heat treatment
It is assumed that subsea separators fabricated from C-Mn
steels will be heat treated after forming and welding.
If clad plates are used, the heat treatment procedure shall be
suitable for the base material and the clad material.
6.5 Non-destructive testing
The separator shall be non-destructively tested according to
EN 13445. The required inspections are summarised in
See also 6.5.2 for further details.
6.5.2 Visual inspection
The separator shall be subject to 100% visual inspection of the
outside and the inside in final condition.
Conditions and acceptance criteria in EN 13445 apply.
6.5.3 Magnetic particle inspection and ultrasonic examination
The finished product shall be subjected to non-destructive testing (NDT). Requirements for personnel, methods, equipment,
procedures and acceptance criteria for NDT shall be in compliance with EN 13445.
When automated NDT equipment is used for the plates, a short
area at the edges may not be tested. The untested areas shall be
subjected to NDT during control of the finished separator. The
extent of untested areas and description of the technique, sensitivity and parameters used for testing of the pipe ends shall
be included in the MPS.
6.5.4 Correction of defects
The conditions and requirements in EN 13445 apply.
If one or more tests in the qualification of the MPS fail to meet
the requirements, the MPS shall be reviewed and modified as
necessary, and a complete re-qualification performed.
6.4.3 Plate forming
Relevant parts of EN 13445 apply.
The level of cold deformation shall be calculated and reported
6.4.4 Welding
Welding procedures, welding personnel, handling of welding
consumables and the execution and quality assurance of welding, shall meet the requirements of EN 13445-4 and EN 13445-5.
Welding procedure qualification testing shall comply with EN
ISO 15614-1, plus additional testing as specified in EN 134454, Ch.7.3 (Edition 2002).
Unless sour service applies the qualified welding procedure
shall be limited to maximum hardness according to Ch.4,
Table 4.2, for carbon-manganese steels. For duplex steels the
maximum hardness shall be limited to 350HV10.
If sour service applies, maximum hardness shall comply with
NOTE: For deep water applications the high water pressure
may affect the formation and absorption of hydrogen. This
may imply that using existing experience from more shallow
water application of C-Mn steels in seawater under CP is nonconservative, and that additional margins in the specified maximum yield strength and hardness should be included.
6.5.5 Personnel qualifications
The personnel qualifications stated in EN 13445 apply.
Table 6-1 Type and extent of non-destructive testing in
connection with manufacturing procedure, qualification testing
and fabrication testing 1)
Imperfections in untested areas
Longitudinal imperfections in weld
Transverse imperfections in weld
Laminar imperfections in plate body
and in area adjacent to weld seam 3)
External surface imperfections in
All testing shall be performed in accordance with the requirements of EN 13445.
ST = Surface imperfection testing
RT = Radiographic testing.
Unless tested on plate prior to forming and assembly.
6.6 Fabrication testing
Fabrication testing is required for both the formed materials
and for the production welds.
6.6.2 Type of tests
The fabrication tests to be carried out are specified in Table 62 and are further described in 6.6.2.1 to 6.6.2.11.
The sampling and the extent of the fabrication tests are specified in 6.6.3. See also Table 6-2.
6.6.2.1 Tensile testing
Tensile testing is required for both the formed material and for
the weld. For tensile testing of the weld, both cross-weld tensile test and longitudinal tensile test are required.
The tensile testing shall cover the full thickness of the formed
material and of the weld.
The tensile tests shall prove compliancy with the base material
specification (as modified in the MPS when relevant).
Location for hardness testing
6.6.2.2 Impact testing
Impact testing is required for both the formed material and for
the weld. For impact testing of the weld impact testing shall be
carried out both in the weld metal, fusion line and in the heat
For the formed materials two sets of Charpy-V impact testing
For the welds four sets of Charpy-V impact tests are required,
i.e. two sets 2 mm below the surface and 2 sets just below the
mid-thickness (alternatively, in the root area).
The impact testing shall prove compliancy with the base material specification (as modified in the MPS when relevant).
6.6.2.3 Hardness test
Hardness testing is required for both the formed material and
for the weld. For hardness testing of the weld the tests shall be
carried out at location as specified in Figure 6-1.
Unless sour service is relevant, maximum hardness shall be in
accordance with Ch.4, Table 4.1, for carbon-manganese steels.
For ferritic-austenitic steels, maximum hardness shall be in
accordance with Table 4.2 for the base material, and maximum
350 HV10 for the weld metal and HAZ.
6.6.2.4 Metallographic examination
For carbon-manganese steel, metallographic examination of
the weld metal and the HAZ in the root area of the cladding
material, shall be performed at a magnification of 400X. The
microstructure shall be essentially free from grain boundary
carbides, nitrides and inter-metallic phases.
For ferritic-austenitic stainless steel, metallographic examination of the weld metal root, weld metal cap, and the HAZ in the
root area of the cladding material, shall be performed at a magnification of 400X. The microstructure shall be essentially free
from grain boundary carbides, nitrides and inter-metallic
phases after solution treatment. The ferrite content shall be
be in the range 35-65% in the weld and HAZ.
6.6.2.5 Macro examination
Macro examination applies for fabrication testing of welds.
The testing and accept criteria shall conform to EN 15614-1.
6.6.2.6 Bend test
Bend testing applies for fabrication testing of welds.
6.6.2.7 Weldability testing
Weldability testing is required if not otherwise agreed. The
testing shall be carried out in compliance with EN ISO 156141. The weldability testing may be carried out by the material
manufacturer or by the contractor.. If carried out by the material manufacturer this shall be specified in the MPS and the
For carbon-manganese steels with SMYS 415 MPa, the
weldability testing /documentation shall, as minimum, include
weld on bead Y-groove, and also fracture toughness tests of
base material and HAZ. In addition, for carbon-manganese
steels with SMYS 450 MPa, metallographic examination
should be conducted to establish the presence of LBZs (Local
brittle zones, see Ch. 4.5.2). The maximum and minimum heat
inputs giving acceptable properties in the weld zones, with corresponding preheat temperature and working temperatures,
For ferritic-austenitic stainless steel, the weldability testing /
documentation shall determine the effect of thermal cycles on
the mechanical properties, hardness and microstructure. The
maximum and minimum heat inputs giving an acceptable ferrite/austenite ratio and a material essential free from intermetallic phases shall be determined. Allowances for repair
welding shall be included.
For clad steel plates, the weldability testing / documentation
shall determine the dilution effects and the effect of thermal
cycles on the mechanical properties, hardness and microstructure. The range of heat inputs giving acceptable properties shall
be determined. Allowances for repair welding shall be included.
6.6.2.8 Fracture toughness test (CTOD)
Fracture toughness testing is required of the base material and
The fracture toughness test should be carried out in general
compliance with EN 13445-2 (2002) Annex B, or an equivalent method.
The measured fracture toughness of the base material and the
weld metal, shall as minimum have a CTOD value of 0.20 mm
when tested at the minimum design temperature.
The fracture toughness testing of the base material may be carried out by the material manufacturer or by the contractor.. If
carried out by the material manufacturer this shall be specified
in the MPS and the Purchase Order.
NOTE: For clad steel plates, the testing shall be carried out by
the contractor.. The cladding material shall be removed prior
6.6.2.9 Strain ageing test
Strain ageing test is required for carbon-manganese steel if
cold forming during subsequent manufacture exceeds 5%.
A test coupon shall be machined from the material. The orientation of the coupon shall be transverse to the rolling direction.
The test coupon shall be of either full or reduced wall thickness. The reduced (parallel) section of the coupon shall have a
width and thickness sufficient to produce the required number
of standard (full size) Charpy V-notch specimens needed for
The test coupon shall be subjected to cold forming representative for that experienced by the plate during fabrication of the
separator. After preparation the test coupon shall be aged at
250C for one hour. Thereafter, the specified number of
Charpy V-notch specimens shall be machined from the middle
of the coupon. The orientation of the specimens shall be longitudinal to the coupon centreline, with the notch perpendicular
to the surface of the test coupon.
Acceptance criteria: as specified for impact testing above.
Samples intended for strain age testing, if relevant, shall be
taken from plates that have been subjected to the maximum
cold deformation allowed.
The strain ageing test may be carried out by the material manufacturer or the contractor.. If carried out by the material manufacturer this shall be specified in the MPS and the Purchase
NOTE: If the order consists of plates from less than three
heats, testing of one plate from each heat is sufficient.
6.6.2.10 HPIC test
Hydrogen Pressure Induced Cracking (HPIC) test is required
during production and qualification for sour service.
When applicable, HPIC testing during manufacturing shall be
performed on one randomly selected plate from each of the
three (3) first heats, or until three consecutive heats have
shown acceptable test results. After three consecutive heats
have shown acceptable test results, the testing frequency for
the subsequent production may be reduced to one per casting
sequence. The Ca/S ratio shall be greater than 1.5.
DNV-OS-F101, Appendix B300.
NOTE: For a material clad with an austenitic Corrosion
Resistant Alloy (CRA), this testing is not considered relevant.
6.6.2.11 SSC test
6.6.2.12 Re-testing
If the HPIC tests during the subsequent testing fail (one test per
casting sequence), three plates from three different heats of the
last ten heats, selecting the heats with the lowest Ca/S ratio,
shall be tested. Providing these three tests show acceptable
results, the ten heats are acceptable. However, if any of these
three tests fail, then all the ten heats shall be tested. Further,
one plate from every heat following the initially failed heat
shall be tested until the test results from three consecutive heats
have been found acceptable. After three consecutive heats
have shown acceptable test results, the testing frequency may
again be reduced to one test per casting sequence.
The Manufacturer shall investigate and report the reason for
failure and shall change the manufacturing process if required.
Re-qualification of the MPS is required if the agreed allowed
variation of any parameter is exceeded.
Table 6-2 Type of tests in connection with manufacturing
procedure, qualification testing and fabrication testing
HPIC test
Applies for the MPQT (qualification of the Manufacturing Procedure Specification)
Applies for fabrication testing of the production welds
Applies for fabrication testing of the formed plates
The test required may be carried out at the contractor. or at the
manufacturer of the plate / clad plate. If carried out at the manufacturer of the plate / clad plate repetition of the test is not
required. The contractor. should specify in his purchase order
and MPS if this test should be carried out at by the material
manufacturer or by the contractor..
See 5.4.8. If relevant, scope of tests to be agreed.
Applies for sour service if the selected material is not listed in
6.6.3 Sampling and extent of fabrication tests
If not otherwise specified herein, the sampling and extent of
fabrication tests shall be as described in EN 13445.
shall be according to recognised industry standards, and be
referred to in the MPS.
The dimensional inspection shall be carried out as specified in
EN 13445-5.
The acceptance criteria shall be defined in the design documentation.
6.8 Pressure testing
6.8.2 Internal over-pressure
By pressure testing in this context is meant hydrostatic and / or
pneumatic testing. Traditionally the purpose of such testing
has been to verify structural integrity and leak-tightness of the
pressure vessel. Additional effects from the load condition
imposed by the hydrostatic pressure test have been redistribution of stresses at welded connections and corresponding beneficial effects on the mechanical properties for applicable load
6.8.2.1 Structural failure
This should be tested on land, and be dependant on the design
pressure versus the external pressure as criteria for defining the
different levels that may be used. Basically this should constitute a pressure giving the same hoop stress utilisation as for a
design condition with a certain safety margin
Subsea separators for deepwater application are characterised
by external pressure, governing the design, and heavy wall
thicknesses. Furthermore, it is recognised that external pressure testing may not be practically feasible to carry out (due to
size, or pressure rating for example).
It is considered possible and reasonable to confirm required
capacity through comprehensive and detailed design analysis
with a high degree of confidence. This should provide the substantiation and basis for omitting the external hydro-test. Such
an approach requires particular focus on the manufacturing
processes, i.e. welding procedure specifications to address an
optimal thermal process including potential pre/post heat treatment of weldments to compensate for mechanical redistribution of stresses (normally obtained during hydro-test). In
addition required efforts should be placed on defining and
implementing state-of-art NDT processes to verify, in a transparent and traceable manner, that the welded joints do not contain unacceptable defects.
Some specific aspects related to this are briefly addressed
6.8.1 External over-pressure
6.8.1.1 Structural collapse
The anticipated two major uncertainties in this prediction will
be the stress stage of the thick wall, particularly at welded
joints / penetrations, and the compliance with out of roundness
criteria. The out of roundness is directly taken into account in
the non-linear finite element analysis, whereas the stress stage
of the thick wall depends on the fabrication process. Potential
negative effects are to be limited by proper heat treatment and,
if relevant, also with the application of a fabrication factor to
reduce the applied compressive yield strength in design.
6.8.2.2 Leakage
Essentially same as for 6.8.1.2 Leakage. In this case the test
pressure from EN 13445-5 should be applied directly; i.e. 1.43
(or other depending on temperature) times the maximum
allowable (internal) pressure.
6.8.3 Conclusion pressure testing
It should be possible to avoid an external pressure test of a
separator given that a capacity model is developed considering, in particular, the influence of out of roundness and
residual stresses in very thick pipe wall.
Any leakage from exterior will be better tested by an internal overpressure test which will tend to open defects rather
than close them.
The internal pressure test can be performed on land to a
stress stage a certain factor higher than during design condition in place.
Metal and other seals needs to be designed for the dominating external pressure, and adequate means to qualify
such items, and confirm their adequacy for a particular
application will be required. It is envisaged that this can be
accomplished through relevant scaled testing.
For water depths where it is not obvious if the internal or the
external pressure is governing, the highest test pressure from
either 6.8.1.2 Leakage or 6.8.2.2 Leakage shall be applied.
6.9 Inspection documents
The following documentation is required for formed and
welded products which form part of the pressurised part of the
subsea separator:
test report from testing of production welds
the original material certificates
formed product test coupon results
type and record of heat treatment
6.8.1.2 Leakage
It is considered reasonable to establish required confidence for
leak-tightness through dedicated attention to welding procedures combined with state-of-the-art NDT as stipulated above.
However, an internal pressure test (on land) to the same pressure
as the external pressure during operation, alternatively a pressure rating of 1.10 times the external pressure, should be considered as means to obtain sound quality control. Additional safety
factors (i.e. higher pressures) are, however, not expected to be
required since the external pressure will tend to close defects
while the internal pressure will tend to open the same.
The value of 1.10 is selected in accordance with the EN 13445-5
section 10.2.3.3. The departure from the conventional standard
hydrostatic test with a factor of 1.43 on the maximum allowable
pressure is justified based on the last two paragraphs of EN
13445-5 section 10.2.3.3.4, where a value between 1.0 and 1.25
is indicated. Note that the external pressure is practically a deterministic value, and deserves a lower safety factor than e.g. the
internal pressure for operational conditions. Note also that vacuum is the lower bound of internal pressure, which is not realistic
in practice at any circumstances for a subsea separator in very
Subsea separators are generally considered as important and
critical components due to the consequences in cases of any
Subsea separators will therefore be subject to the highest certification category.
Independent certification body will be required.
The Certification Body should normally be appointed by the
7.2 Certification procedures
Certification procedure by the certification body should
include the following activities as a minimum:
design approval based on specified design condition, performed risk analyses and procedures for manufacturing
the design approval normally to be documented by a
Design Verification Report.
Fabrication activities:
pre-production meeting prior to start of fabrication in
order to review the quality plan for fabrication and testing
review and approval of fabrication and testing procedures
follow-up visits during fabrication and testing
review of fabrication and test records
witness the final testing (internal testing and/or any external pressure testing) and load testing, if applicable.
Extent of survey may be decided on the basis of manufacturers
QA/QC system, manufacturing survey arrangement and type
of fabrication methods. The hold- and check-points during fabrication and testing should be agreed when the quality plan
established by the manufacturer is reviewed in the pre-production meeting.
However, the independent survey shall not be accepted based
on the QA/QC system established by the manufacturer alone.
After final inspection and testing, the Certification Body shall
issue a Certificate of Compliance for the separator covering
the design, fabrication and testing.
National Authorities might have specific requirements regarding
the entity that is accepted used as independent Certification
Authorities, and for certification processes etc. Extract from PED
concerning the applicable certification procedure which will be
applicable for Subsea Separators to be installed on an European
Continental Shelf is outlined in Appendix B to this RP.
7.3 Documentation requirements
The following documents reflect what typically should be submitted to the Certification Body by the manufacturer:
Design Documentation (for review and approval):
technical data sheets for the separator
project specification, pressure vessel class/category/module
general arrangement drawing of the separator
hazard analyses performed by the manufacturer for
items to be taken into account for the design, manufacture
design pressure (internal and external) and max/min
design temperature external nozzle loads
environmental loading cased by as e.g. forces due to current on the sea bead, load during operation and accelerations during transport as applicable
accidental loading cased by e.g. earthquake-, explosion-,
blow-out, anchor handling and any other objects which
might hit the separator externally as applicable.
Dimensional drawings (as far as applicable, the following
items should be stated).
fabrication drawings of pressure retaining parts showing weld
details and attachments welded to the pressure retaining parts
bill of material with reference to material specifications
for the various parts in the vessel, stating material standard, grade of material, type of material documentation
information on heat treatment and testing of welds, extend
hydraulic test pressure.
Structural strength calculations report which should include
Structural strength calculations of subsea separator. In particular the calculation should include:
calculation of pressure retaining parts of the separator
in accordance with applied standards/codes for theseparator
calculation of nozzles reinforcement
local stresses on the shell at nozzle connection due to
local stresses on the shell at saddles due to weight of
the vessel and environmental loads
strength calculation of saddles
structural strength of lifting lugs and local stresses on
the shell due to lifting loads
strength calculation of foundation bolts
fatigue assessments of the critical parts as applicable.
Fabrication Documentation:
details regarding procedures for fabrication and forming
details regarding procedures for welding, Welding Specification Procedure (WPS) and Welding Procedure Qualification Report (WPQR)
details regarding production weld testing
details regarding heat treatment due to forming or welding
details regarding final strength testing of the vessel.
The following should be checked out by the surveyor during
review and approval of the quality plan prepared by the
fabrication and forming according to approved documents
status for approval of welding procedure qualification
status for approval of welders and welding operators
material according to approved documents and available
heat treatment (if performed)
dimensional/out-of-roundness check
qualifications of NDE operators
qualification of NDE procedures
witnessing of final strength tests or load test (if applicable).
When the separator is subject to CE-marking, the Notified Body
in charge might need additional documentation from the manufacturer for the Conformity Assessment.
8. Operation, Maintenance and Periodic
The design basis for separators typically requires that the vessel will operate for the design life without intervention or
inspection (except for potential external ROV surveys including cathodic protection status).
The performance of the separator is, however, continuously
monitored in terms of pressure, temperature, level control and
to some extent quality of output - such as oil in water and water
in oil for oil/water and gas separation. Another issue, which is
field dependant, will be sand or solids build up, ultimately
impacting the separation performance and having the potential
of introducing wear mechanisms in the vessel.
These are all vital aspects that need to be carefully addressed
and accounted for in design as well as fabrication and testing.
Remedies under operation could, in addition to the above mentioned, typically be hydrocarbon leakage detection at vessel,
and reasonably accurate means to measure solid accumulation
and distribution in the vessel.
PED, Pressure Equipment Directive, Directive 97/23/EC of
EN-13445-5, Unfired pressure vessels Part 5: Inspection
ISO 15156-1, Petroleum and natural gas industries Materials for use in H2S-containing environments in oil and gas production
Part1: General principles for selection of cracking resistant
ISO 15156-2, Petroleum and natural gas industries Materials for use in H2S-containing environments in oil and gas production
Part 2: Cracking resistant carbon and low alloy steels, and the
ISO 2394, General principles on reliability for structures
Part 3: Cracking resistant CRAs (corrosion resistant alloys)
EN 10028-1, Flat products made of steels for pressure purposes Part 1: General requirements.
EN 10028-1, Flat products made of steels for pressure purposes Part 6: Weldable fine grain steels, quenched and tempered.
EN 288-3 + A1, Specification and approval of welding procedures for metallic materials
Part 3: Welding procedure tests for the arc welding of steels
(Amendment A1 included).
Part 1: Hardness test on arc welded joints.
ASME VIII div 2 Pressure vessels, alternative rules.
9.2 Papers and publications
Torselletti, E., Bruschi, R., Luigino, V., Collberg, L., Minimum Wall Thickness Requirements for Ultra Deep-Water
Pipelines, OMAE 2003-37219, June 8-13, 2003, Cancun,
Institute de Soundre, Advanced design methodologies to
exploit high strength steels for pressure equipment manufacture, Report No. 38234, dated 2001-09-19.
Kvrner Oilfield Products Report No. 59-ND0099-00, Fabrication Limitations of a Subsea Separator for Deepwater
Applications, dated 2004-03-19.
P. Andrews et al., Variation of the fracture Toughness of a
High-Strength Pipeline Steel Under Cathodic Protection,
Corrosion, Vol. 57, No. 8, 2001.
Jiao, G., T. Sotberg, R. Bruschi, R. Verley, and K. Mrk, The
SUPERB Project: Wall Thickness Design Guideline for Pressure Containment of Offshore Pipelines, Proceedings, 15th
Engineering, Florence, Italy, 1996.
http://www.exxonmobil.co.im/Corporate/Newsroom/Newsreleases/xom_nr_040510.asp
DNV Classification Note no. 30.6 Structural Reliability Analysis of Marine Structures
Hagen, ., Mrk, K., Wall Thickness Design of Subsea Separator, Pilot Structural Reliability Analysis, DNV Report no.
2003-1113, Hvik 2003-12-30
SAFETY CLASS, CALIBRATION
A.1 Calibration procedure:
Calibration of partial safety factors for the collapse limit state
has been performed by comparing results from a design analysis with results from a structural reliability analysis. The principle is illustrated in Figure A-1, and enables a quantification
of the safety class resistance factors applicable for the different
safety classes as described in 2.3.2.
These factors apply to the capacity obtained using the direct
route of EN 13445-3, Annex B.
incl. ovality:
Material, given:
Incl. ovality,
Design analysis (DBA):
characteristic pressure capacity, pc
Reliability analysis (DBA):
(Range of , range of wall
pe ? pc/
Principle applied for calibration of safety class resistance factors.
A description for Figure A-1 is given in the following:
select the geometry, use nominal dimensions with analysis
thickness, ref. EN 13445-3 Figure 3-1
material strength, assume yield strength (=SMYS) and Emodulus.
perform analysis to compute the characteristic pressure
capacity, pc
loop over a set of safety factors to obtain the design external pressure, pe.
The reliability analysis model is based on DNV Report no.
2003-1113. Reference is also made to the DNV Classification
Note 30.6 on structural reliability analysis:
Start with the external design pressure pe from the design
analysis. Add an uncertainty factor (minor uncertainty for
static pressure due to density and depth).
Use the same geometry as in the design analysis, but with
a probability distribution for the wall thickness. This
uncertainty is rather low, with a mean value slightly higher
than the nominal thickness.
Use the yield strength distribution; assume the yield
strength in the design analysis corresponds to the lower
5% fractile of this distribution. Apply uncertainty in the E-
modulus. The uncertainty in E-modulus only affects the
results where the elastic buckling capacity is important.
For low D/t rations, this uncertainty has no effect.
Apply a model uncertainty for the capacity calculations.
Use PROBAN to compute the probability of failure.
Display result in terms of probability of failure as a function of safety factor. Perform the design and reliability
analysis for various geometries; i.e. D/t.
In the ideal situation the DBA result using non-linear finite
element analysis (FEA) (e.g. ABAQUS) should be applied
both in the design analysis and within the reliability analysis. This is not possible due to excessive computational
and programming effort, and a simplification is required.
The second option would be to use non-linear FEA for the
design analysis, and apply the response surface technique
in the reliability analysis. That is to calculate the capacity
by non-linear FEA for a representative grid of the random
variables involved, and interpolate on these results in the
reliability analysis. This would require a number of analyses per design, and is neither considered a feasible option
within the project budget/scope.
The third alternative is to replace the non-linear FEA both
in the design analysis and in the reliability analysis with at
simplified model that accounts for the effect of critical
parameters and the defined uncertainties in an adequate
way. In this case it may be acceptable that the actual
capacity calculated for a particular design differ somewhat
from a comparable non-linear result as long as the change
in the capacity as a consequence of a change in the uncer-
tainty parameters is reasonably well represented. Note that
the calibration represents a comparison of results from a
design and a reliability analysis, in which the same capacity calculation model is applied. For this reason the deviation from the actual non-linear result will be of
approximately the same magnitude in both these analyses,
and this deviation cancels out in the calibration of safety
factors. In the present calibration this simplification has
been employed, and the collapse equation for pipelines
given in DNV-OS-F101 has been used. This formulation
effectively accounts for the various uncertainties involved.
A.2 Limit state function:
The limit state formulation is collapse of a subsea separator
Table A-1 Uncertainty modelling for the collapse limit state
Xp,e
Pe,s/pe
ts/t1
sy,s/SMYS
Es/SMYS
(Ds-ts)/(D-t)
A.3 Summary of uncertainties
A brief summary of the uncertainties used in the structural reliability analysis is given in Table A-1. Subscript s indicates stochastic. For further details, reference is made to the pilot study
by Hagen and Mrk (2003)
A.4 Calibration results
Based on the procedure as illustrated in Figure A-1 with the
uncertainties as given in Table A-1 Uncertainty modelling for
the collapse limit state, the probability of failure is calculated as
a function of the safety factor. The results are given in Figure A2, and different designs in terms of different D/t ratios have been
analysed. Note that the difference in the results for different D/t
ratios is due to the effect of uncertainty in the E-modulus which
comes into effect only for D/t ratios greater than 20.
Here the main purpose is to quantify the difference in safety
factor between safety classes, where the difference in failure
probability between each safety class is assumed to be an order
due to external pressure. The formulation is given by:
g(X) = Xmodpc,s-pe,s
denotes the vector of stochastic variables
is the model uncertainty; i.e. reflecting the ratio
between the true capacity and the predicted capacity
is the stochastic collapse pressure capacity
is the (generally stochastic) external pressure
The probability of failure is calculated by integrating over the
failure domain (g(X)< 0) using SORM (Second Order Reliability Method).
1/(1-2CoV)
of magnitude. For this purpose the accuracy in the absolute
value of calculated failure probability is not so critical. Using
the recommended targets of 10-3, 10-4 and 10-5 for consequence class low medium and high respectively, the corresponding partial safety factors from Figure A-1 are
approximately 1.25, 1.35 and 1.45. Assume safety class high
correspond to EN 13445-3 Annex B, the safety class resistance
factors become:
0.86 for safety class low are obtained.
Note that the reference period for the probability of failure usually is one year, e.g. the tabulated values as given in DNV CN
30.6. In the present application the major uncertainty is practically time invariant, hence the annual probability of failure is
close to the lifetime probability. For this reason it may be
somewhat conservative to use the target levels as indicated in
DNV CN 30.6 when considering lifetime probabilities.
D/t 28.25
D/t 23.00
D/t 19.50
D/t 17.00
D/t 15.13
D/t 13.67
Safety factor, 0
Probability of failure versus safety factor.
DESIGN OF SUBSEA SEPARATOR
ACCORDING TO EN 13445-3 ANNEX B
The work presented in this appendix is collected from DNV
report Subsea Separator Structural Design Draft Recommended Practice, which was commissioned by AS Norske
Shell. The reason for including these results in this RP is to
provide the reader with an illustrative example on how to use
the Design by Analysis (DBA) methodology. Nevertheless,
results from this analysis cannot be used directly by the
designer. It is essential that a finite element analysis be performed for each specific design case.
The scope of this study was to investigate if the required wall
thickness obtained from DBA is lower compared to design by
other codes. European Standard EN 13445-3 Annex B Design
by Analysis - Direct Route has been the reference for DBA.
This code opens for the use of finite element analysis as an
alternative to design by formulae (DBF), and also DBA
according to EN 13445 Annex C. EN 13445-3 Annex B may
also be a compliment to DBF for load combinations not covered by DBF, and for cases where the manufacturing tolerances are exceeded. EN 13445-3 Annex C Design by
Analysis/Stress Categorisation covers the same principles for
design as the principles found in ASME VIII Div. 2 Appendix
4. The capacity of the separator vessel subjected to external
pressure has been documented for Gross Plastic Deformation
(GPD). It has been assumed that the final separator design,
regardless of size, can be tested according to the requirements
in EN13445-5 with external pressure. If this is not the case, the
Instability design check (ID) may, depending on the material
properties, become the most critical design check. With material properties as assumed in the present work the required wall
thickness will be slightly higher with ID than that found by the
GPD design check.
A number of parameters that influence the capacity of the vessel have been evaluated. These are wall thickness, Youngs
modulus, yield stress, initial ovality, length/diameter (L/D) ratio, head thickness and openings/nozzles. Furthermore, the
geometry of the vessel has been varied. This was done to facilitate a comparison with other design studies, as for instance the
design of a composite vessel.
It should be noted that corrosion allowance and manufacturing
tolerances have not been included in this work.
B.2 Finite element model
The report describes three separator sizes; 50 000, 75 000 and
100 000 BPD. Due to the large depths at which the separator is
to function, only a vessel of the smallest size is likely to meet
the production and installation demands. Therefore, only the
smallest size has been used in this study. For larger separator
volumes very thick walls will most likely be required, preventing good control of material properties during the manufacturing process.
B.2.1 Separator geometry
The smallest vessel dimension found in /1/ (50 000 BPD) has
been used as basis the calculations, with an inner diameter of
ID = 2100 mm and length of L = 15600 mm.
Typical separator geometry.
All finite element models in this report have been generated
with an initial ovality of 1%. This is twice the allowable tolerance when designing according to DBF. The rationale for use
of 1% in this report is that there is little experience in manufacturing vessels with a very high wall thickness. Hence it is
assumed that the normal manufacturing tolerances will be
exceeded when producing a separator to be installed at 3 000 m
In this study initial ovality has been calculated as the difference
between the largest and smallest diameter divided by the nominal diameter. The finite element geometry model was constructed with the largest/smallest diameter being 0.5% larger/
smaller than the nominal diameter. In this way the initial ovality is a perfectly shaped elliptic shell. The finite element model
thus has a predefined and natural way of buckling when subjected to external pressure. In order for the finite element
model to produce reliable results, it is an essential requirement
that it has a defined way of buckling. If the separator is modelled as a perfect (circular) cylinder, the capacity for external
pressure may be over-estimated, thus leading to un-conservative results.
In a real case, the separator will not have a perfect circular
shape after welding. In order to measure and analyse the
welded structure, the procedures in EN13445-3 Appendix E
Procedure for calculating the departure from the true circle of
cylinders and cones and Appendix F Allowable external
pressure for vessels outside circularity tolerance shall be
applied. Special attention should be paid to the shape of vessels
with high wall thicknesses after welding. All deviations from
the nominal diameter of the production drawing should be
evaluated, and in order to evaluate the possible reduction of
capacity for external pressure, the finite element models
should be updated with the real vessel shape.
The effect of cradles was not included in most of the analyses
presented in this report. Cradles will increase the stiffness
locally as long as they are fully welded to the shell. However,
the steel weight of the separator will result in an unsymmetrical
loading condition outside the cradles at the lower part of the
shell. The effect of this is studied in B.4.9.
B.2.2 Material model
Material data for steel of quality P500QL2 (EN100286:[1996]) was used in most of the analyses presented in this
report, except when the effects of Youngs modulus (B.4.2)
and yield stress (B.4.3) were evaluated. The choice of material
in separator design situation is of course not limited to this
The specified minimum yield strength (SMYS) for P500QL2
is a function of plate thickness, see Table B-1. In the analyses
the properties found for a plate thickness of 150 mm were used.
The effect of temperature on the mechanical properties was not
taken into account in any of the analyses. However, the de-rating effect of temperature must be included in a design situation.
590-770 MPa
540-720 MPa
According to EN13445-3 Annex B the material curve to be
used should be linear elastic ideal plastic, which excludes
strain work hardening. Excluding strain hardening will for
most applications be an assumption that gives a conservative
result. However, when the failure mode is plastic collapse,
ignoring strain hardening does not introduce any conservatism
to the result. The plastic strain at the outer fibre at collapse is
typically less than 3%. In such a case, the application of a linear elastic ideal plastic material curve is absolutely necessary
and can not be regarded as adding additional safety.
The Abaqus material input used is found in Table B-2.
Table B-2 Material parameters used in the analyses
Yield stress (SMYS)
Plastic strain at SMYS
0 (Abaqus input)
Table B-1 Mechanical properties of P500 QL2
Linear elastic ideal plastic material model without strain hardening.
Von Mises yield criterion is implemented in most finite element software, and it is accepted to use as long as the difference between the Tresca and von Mises yield criteria is taken
into account when calculating the allowable load. The difference between the two criteria is at maximum;
3 2 = 1.155.
Since the plastic strain at collapse is low for a vessel under high
external pressure, strain hardening of the material will have
only marginal effect on collapse resistance. A strain of a few
percent at the outer fibre is typical. Thus, compared with a
material curve that includes strain hardening, the linear elasticideal plastic material curve is just slightly conservative when
designing against external pressure.
B.2.3 Element model
Mesh of 1/8 of separator
Abaqus element type S8R (8 node shell elements with reduced
integration) were used for all analyses models.
approximately 40, which was found to be a sufficiently fine
element size. Refining the element density was found only to
have marginal effect on the result, except for the increased
solver time.
Cradles were not included in the FE models in this study, but
must be included in a design situation.
The number of elements around the circumference was
B.2.4 Loads and boundary conditions
B.3.1 Partial safety factors
The finite element model was exposed to an evenly distributed
external pressure load, and the load was incremented in steps
of 0.001 MPa. The external pressure at the last converged
increment was defined as the external pressure capacity (without safety factor).
Partial safety factors were applied on loads (external pressure)
and on resistance (SMYS).
An internal pressure of 0 bar (vacuum) was assumed for all
cases with external over-pressure. The internal pressure will,
however, probably never become lower than 10 bar (1 MPa).
To find the necessary wall thickness for internal over-pressure,
analyses were carried out with an internal design pressure of
207 bar (20.7 MPa).
A three-plane symmetry was applied; hence only 1/8 of a
model as illustrated in Figure B-3. Symmetry boundary conditions were assigned to nodes at the dividing lines between
symmetry sections; i.e. no rotation about in-plane axes of the
symmetry plane was allowed and neither was translation normal to the symmetry plane that defines the dividing line.
The local loads from the mass of the separator at the position
of the cradles were not included in this study.
B.2.5 Software
The finite element software Abaqus 6.5-1 were used for all calculations. The models were created and meshed in Abaqus
CAE and solved with Abaqus Standard static solver.
B.3 Calculation of allowable pressure
The finite element model can identify the failure mode due to
elastic buckling (for high D/t) or plastic collapse (for low D/t)
or a mixture of both. The load at the last increment at which
convergence is achieved was used to calculate the capacity of
the structure for external pressure. In this study, no solution
method was implemented in order to continue the analyses past
the point of collapse. If the behaviour of the shell past onset of
buckling is to be studied in detail, the Riks method is one alternative.
The partial safety factor P for external pressure is found in
EN13445-3 Annex B Table B.8-1. For actions with a natural
limit (like water depth):
The partial safety factor for resistance R is found in EN134453 Annex B Table B.8-2. For SMYS/Rm20 = 440 MPa / 540
MPa = 0.81, giving:
R = 1.5625 0.81 = 1.27
B.3.2 Calculation procedure
In a finite element model the mid plane diameter (MD) is one
wall thickness larger than the inner diameter (ID) of the vessel.
The external pressure is always applied to the mid plane, which
is the plane of nodes of the shell elements. Subsequently, in
this study the shell elements were positioned at the mid plane
surface in the centre of the vessel wall. However, when a vessel is submerged in water the pressure will act at the outside
diameter (OD). Because in an FE model the pressure acts on
the plane of nodes, it is important to take the diameter difference into account. The allowable external pressure was thus
reduced by the ratio between the two diameters, i.e. OD/MD.
There are other methods used to include the effect of the difference in diameter. Another way is to position the nodes of the
finite elements at the outside surface of the vessel. In this case
the loading surface will be correctly modelled, but the stiffness of the vessel is incorrectly represented because the diameter is larger than the nominal diameter. This would lead to
very conservative estimates of the capacity.
Another option is offset modelling. If the software supports
offset shell elements the nodes could be positioned at one
side of the shell element (at OD) while the material of the element is positioned at correct ID and OD.
Table B-3 Example of procedure for calculating allowable external pressure for a wall thickness of 100 mm and vessel inner
diameter ID = 2100 mm. Partial safety factors were applied on analysis results.
SMYS input in Abaqus.
Collapse pressure (from Abaqus) at last converged increment
Partial safety factor Table B.8-2 R
Partial safety factor Table B.8-2 P
Von Mises to Tresca conversion
Factor to reduce allowable pressure at outer diameter (OD) as a function of OD and
mid plane diameter (MD) (From Table 16, initial OD and MD diameter for 100 mm
Allowable pressure (27.50.957) / (1.271.01.155) MPa
Water depth (assuming water density 1 025 kg/m3)
It was assumed in this study that the separator can be tested
according to EN13445-5, and that the partial safety factor R
may be set to 1.25 for the Instability design check. Instability
therefore did not become the dimensioning design check, and
all results presented were thus a result of Gross plastic deformation (GPD). However, if the external pressure test can not
be performed, the partial safety factor should be set to 1.5.
Hence, Instability will be the most critical design check. The
equivalent GPD partial safety factor is 1.27 3 2 = 1.47 ;
i.e. slightly lower than 1.5.
Note that the procedure described in Table B-3, differs somewhat from the procedure described in EN13445-3 Annex B, B
7.5.1. The code specifies that the partial safety factors R shall
be the applied on the material strength parameter, RM, in this
case the yield stress/SMYS. The resulting RMd is to be used as
input/yield stress in the finite element analysis. However, the
3 2 = 1.155
MD/OD = 2 200 mm/2 300 mm
procedure described in Table B-3 applies the safety factors and
the Tresca/Mises factor on the finite element analysis result
For elastic buckling it was found that if the factors were
applied on the results, more conservative values for allowable
external pressure or wall thickness were achieved. It is the
opinion of the authors of this report that the conservative procedure described in Table B-3; should be used.
Hand calculations for pure plastic collapse of an infinitely long
pipe were compared with the DBA results. For a very high
thickness, i.e. a D/t of 13, almost the same capacity was
obtained. This indicates that the plastic capacity was strongly
dominating. It should be noted that the effect of the end domes
was not accounted for in the hand calculations, whereas it may
have a positive effect on the DBA result.
Separator geometry and material properties are given in
Table B-6. The results are given in Table B-5.
B.4 Results
Corrosion allowance and manufacturing tolerances were not
included in any of the analyses.
Table B-4 Geometry and material properties for the model
used to analyse the effect of wall thickness.
B.4.1 Effect of wall thickness
Initial ovality of vessel
In the capacity for external pressure is given as a function of
wall thickness. The figure includes the results for DBA (green
line) and DBF (blue line). The short black line shows the
capacity from DBA with 4 external stiffeners. As can be seen
from the results, wall thicknesses less than 120 mm are obtainable even at 3 000 m depth.
Collapse pressure vs. Shell thickness
{ ID=2100mm, L(T/T)=15600mm, SMYS= 440MPa, E=200000MPa, fo=1% }
EN13445-3 DBA Annex B
EN13445-3 DBF with SMYS/1.5
EN13445-3 DBA with 4 stiffeners
Shell thickness [mm]
Allowable external pressure as a function of wall thickness.
All other parameters are kept constant.
Table B-5 Summary of results for allowable external pressure as a function of wall thickness. The density of sea water is taken as 1
025 kg/m3
Length (T/T)
Allowable ext. pressure
Equivalent pressure in
B.4.2 Effect of Youngs modulus
The effect of Youngs modulus when designing by analysis
according to EN 13445-3 Annex B is illustrated in Figure B-5.
Separator geometry and material properties are given
in Table B-6.
Table B-6 Geometry and material properties for the model
used to analyse the effect of the E-modulus
Collapse Pressure vs. Young's modulus
{ ID=2000mm, L(T/T)=12000mm, T=120mm, SMYS=440MPa }
DBA acc. EN13445-3 Appendix B
DBF acc. EN13445-3
Effect of E-modulus on capacity against external pressure.
Results from DBF are presented for comparison.
As expected the collapse capacity increased with increasing Emodulus. In general, the E-modulus becomes increasingly
important with increasing D/t ratio and less important when the
collapse is governed by plastic behaviour.
The capacity found by DBA is approximately 15% higher than
the capacity calculated by DBF. The difference between DBF
and DBA with a Youngs modulus of 200 GPa is 3.6 MPa,
which corresponds to 36 bar, or approximately 350 m water
B.4.3 Effect of SMYS
The effect of SMYS when designing by analysis according to
EN 13445-3 Annex B is illustrated in Figure B-6.
Table B-7 Geometry and material properties for the model
used to analyse the effect of SMYS.
Collapse Pressure vs. SMYS
{ ID=2000mm, L(T/T)=12000mm, T= 120mm, fo=1%, E= 200000MPa}
Yield Strength/SM YS [M Pa]
in Table B-8.
Effect of SMYS on collapse capacity.
As expected the collapse capacity increased with increasing
yield stress. In the present example a 25% increase in SMYS
from 440 MPa gave an increase of 17% on the capacity.
B.4.4 Effect of initial ovality
The effect of initial ovality when designing by analysis according to EN 13445-3 Annex B is illustrated in Figure B-7.
Table B-8 Geometry and material properties for the model
used to analyse the effect of initial ovality.
Collapse pressure vs. initial ovality
{ ID=2000mm, L(T/T)=12000mm, T =120mm, SMYS=440MPa, E=200000MPa }
DBA acc. EN13445-3 Annex B
Initial ovality [%]
Effect of initial vessel shape on collapse capacity.
The collapse capacity decreased with increasing initial ovality.
This was expected. The decrease was most significant at a relatively low ovality, e.g. less than 1%. When the ovality was
increased from 0.5% to 1.0% a capacity reduction of 12%
resulted. A 12% reduction of capacity approximately corresponds to a 7% increase in wall thickness.
B.4.5 Effect of vessel length
The effect of vessel length when designing by analysis according to EN 13445-3 Annex B is illustrated in Figure B-8.
Separator geometry and material properties were as given in
Table B-9 Geometry and material properties for the model
used to analyse the effect of vessel length.
Collapse pressure vs. Length and ID
{ T=120mm, SMYS=440MPa, E=200000MPa, fo=1% }
DBA ID=2000mm
DBA ID=3000mm
DBA ID=4000mm
DBF ID=2000mm
Effect of vessel diameter and length on collapse capacity. The red
line represents results for DBF according to EN13445-3.
The collapse capacity decreased with increasing length. When
L/D became greater than 4, the decrease was relatively small.
The difference between DBA and DBF can be seen when comparing the two upper lines. Note that the initial vessel shape for
the finite elements model used for DBA was set to 1% out of
roundness, whereas the tolerance on initial ovality when
using DBF is 0.5%.
B.4.6 Effect of vessel diameter and stiffeners
A study of the effect of different vessel shapes with constant
inner volume was carried out. The optimal shape with regards
to the separating process is a long cylinder. However, a shorter
vessel is optimal to reduce the holding time. A vessel with
length 13.6 m and an internal diameter of 3.2 m is illustrated in
Figure B-9. Three other geometries with a constant volume of
63 m3 are given in Table B-10.
Table B-10 Diameter and length for constant volume 50 000
BPD vessel.
Drawing of 50 000 BPD vessel (complete drawing in Appendix A).
Excluding cradles, supports and attachments an almost constant weight of 125 tons was found for the three vessel
geometries given in Table B-10 at 3 000 m. At 1 500 m the
weight was 86 tons.
Subsea separator study - Constant volume 63 m (50 000 BPD)
ID=2300mm L(T/T)=13600mm,
ID=2100mm, L(T/T)=16800mm
ID=1900mm, L(T/T)=20900mm,
SMYS= 440MPa, E=200000MPa, fo=1%
External Pressure [MPa]
ID=2300 L=13600
IP 207 bar ID 2300
ID=2100 L=16800
IP 207 bar ID2100
ID=1900 L=20900
IP 207 bar ID 1900
ID 2300 4 stiffeners
is dimensioning
External pressure capacity as a function of wall thickness for a
constant volume of 50 000 BPD (63 m3).
The vertical part of the lines in Figure B-10 represents the burst
limit state based on a inner pressure rating of 207 bar. These
values were calculated by the DBF approach. Given a SMYS
of 440 MPa and SMTS of 540 MPa the thicknesses for the
three design geometries were found to be 92 mm, 101 mm and
111 mm. However, since these values were calculated with
internal pressure as the dimensioning load, the DBF method
was used. A consequence of this is that the wall thickness is
governed by the SMTS. For a very high SMTS, e.g. 700 MPa,
the corresponding required thicknesses will be reduced to 69
mm, 77 mm and 84 mm respectively. This effect will not be
obtained with the use of DBA, since a linear-elastic ideal plastic material description is used. For this reason the DBF result
may yield lower wall thicknesses than DBA for high SMTS.
By adding 4 outside stiffeners the shell thickness was reduced
by 30-35 mm to approximately 112 mm, see Figure B-11. It
therefore seems feasible to achieve wall thicknesses below
120 mm even for separators that are to be installed at 3 000 m
depth. However, the benefit on weight due to reduced thickness was practically eliminated by the weight of the stiffeners.
No attempt was made to optimise the stiffener design. Nevertheless, a more optimum stiffener design may exist, potentially
providing a slight weight reduction.
Model of separator with totally four stiffeners (I-shaped). Only
two stiffeners are visible in the figure, since there is symmetry
about the centre of the model.
B.4.8 Effect of openings
An opening will reduce the capacity of the structure unless the
material that has been removed is replaced with material giving equivalent strength. In this study it was found that man
ways and nozzles did not influence the buckling capacity if the
man ways and nozzles had thick enough walls. However, as
long as the ratio nozzle diameter / shell diameter is less than
0.3, ref. EN13445-3 Design by formulae, the thickness of a
nozzle cannot be more than twice the thickness of the shell.
B.4.7 Effect of head thickness
The wall thickness for a semi-spherical head shape may be
close to half that of the main shell when dimensioning for inner
pressure. For other shapes the head walls will generally be
thicker and may, depending on shell shape, be as thick as the
The same is valid for external pressure. When dimensioning
for external pressure, the capacity was not significantly influenced by a reduction of head wall thickness, see Table B-11
and Table B-12 below. Other head shapes were not evaluated.
Table B-11 Capacity as a function of head wall thickness
for D/t = 29
Collapse pressure as function Head t = 71 mm Head t = 35 mm
of head thickness
ID = 1 900 mm (OD = 2 042)
10.247 MPa
10.242 MPa
L = 20 900 mm, t = 71 mm
Table B-12 Capacity as a function of head wall thickness
for D/t = 17
Collapse pressure as function Head t = 150 mm Head t = 75 mm
ID = 2 300 mm (OD = 2 600)
31.118 MPa
31.053 MPa
L = 13 600 mm, t = 150
Table B-13 Dimensions and wall thickness of openings
In order to include the effects of end cap forces from external
pressure, the man way was modelled with capped end. As long
as the openings are modelled as described in Table B-13, the
capacity was found to increase when openings were added.
The model with openings is illustrated in Figure B-12.
Table B-14 Capacity as a function of openings for a shell with
ID 2300 and shell thickness t = 150 mm
Without any openings
With 4nozzles and man way
With 4 and 10 nozzles and man way
wall thickness as the separator shell, and was assumed to be
fully welded to the shell.
Model with openings
B.4.9 Effect of gravity and cradle support
The finite element models used to produce the results in the
previous chapters were modelled without taking mass or cradle
support into account. The two factors have opposite effects on
capacity for external pressure. Reaction forces from the mass
will act locally at the cradle supports, and thereby reduce the
capacity. The stiffness of the cradles, on the other hand, will
increase the stiffness of the shell, leading to an increase in the
capacity. In order to study the combined effect of the two factors, two different finite element models were compared, see
It was found that the collapse capacity was slightly increased
when the model included cradle support, even when gravity
was included. The cradle support was modelled with the same
Table B-15 Comparison of models with and without gravity
forces and cradle supports. Safety factors for material, Mises to
Tresca conversion and the OD/MD - ratio were included.
Separator model.
at last conCollapse load /
ID = 2 100 mm
verged load
(1.271.1551.043)
L = 16 800 mm
and T = 95 mm.
Model 1. Without mass
and without cradle sup23.9 MPa
Model 2. Including gravity forces and double
thickness at cradle
Geometry model of separator including gravity forces
and cradle support.
B.5 Typical shape of vessel at time of collapse
When subjected to external pressure the thin walled structure
deflected more than the thick shell before they collapse. The
displacements at the time of collapse were between 6.1 mm
(t = 185 mm) and 23.2 mm (t = 80 mm) in the inwards direction
of the vessel wall. At the same time the largest diameter was
increased by between 1.3 mm (t = 185 mm) to 19.9 mm
(t = 185 mm), see Table B-16.
Table B-16 Displacements of vessel wall at time of collapse as a
function of wall thickness. The initial mid plane radii are
included in the table (for an initial ovality equal 1%).
Initial mid
1 084 1 094 1 104 1 119 1 137
radius, smallest
1 095 1 105 1 115 1 130 1 148
radius, largest
Initial OD/t
(OD = outer
erty, lower consequences can be documented at deep waters, a
safety class resistance factor may be applied. This reduces
the safety factor without a subsequent increase in risk, see Figure B-14.
The value of the safety class resistance factor for the different
safety classes are:
B.6 Effect of safety class concept
The concept of safety class was proposed in the draft Recommended Practice. It has been demonstrated that this concept
complies with the PED requirements. However, a formal
acceptance has not yet been obtained.
The concept is based on risk evaluation, where the basic
assumption is that design according to the harmonised standard EN 13445-3 also covers design where the consequences of
failure are high. If, while regarding life, environment and prop-
The effect on design is illustrated in Figure B-15. It is seen that
a reduction in wall thickness of approximately 5% or 7 mm
was obtained while going from consequence class high to
Collapse pressure vs. Thickness
High (EN 13345)
Effect of the safety class concept on design
In this work it was found that DBA led to a reduction in wall
thickness of at least 15 mm compared to DBF at a depth of
3 000 m. Furthermore, at this depth it is necessary to add stiffeners in order to the keep shell wall thickness of the separator
below 120 mm. 120 mm is regarded as an upper limit for wall
thickness in plates by some steel manufacturers, ref. DNVReport 2003-1113.
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