FIXED OFFSHORE WIND
STRUCTURE DESIGN

What Sesam and Bladed can do for fixedoffshore wind turbine structure design andanalysis.
April 2022

WHITE PAPER

Table of contents
1

EXECUTIVE SUMMARY................................................................................................................................... 1

2

INTRODUCTION ............................................................................................................................................... 3

3
3.1
3.2
3.3
3.4

PRELIMINARY DESIGN ................................................................................................................................... 4
Modelling4
Natural frequency analysis
5
FLS analysis using damage equivalent loads and waves
6
ULS and SLS analysis using extreme loads
6

4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
4.12

DETAILED DESIGN .......................................................................................................................................... 7
Modelling
7
Wave load generation in time domain
7Structural analysis in time domain
8
FLS analysis in time domain
8
ULS analysis in time domain
9
Earthquake analysis in time domain
9
Post-processing of concrete structures in time domain
10
Boat impact analysis
11
Vortex induced vibrations of J-tubes
11
Cathodic protection system design assessment
11
Redesign
11
Cloud analysis and parallel computing
11

5

TURBINE CONCEPT MODELS FOR INTEGRATED LOAD ANALYSIS ........................................................ 13

6
6.1
6.2

BEYOND IN-PLACE ANALYSIS ..................................................................................................................... 14
Transportation analysis
14
Lifting analysis
14

7
7.1
7.2
7.3

BEYOND OFFSHORE WIND TURBINE SUBSTRUCTURES ........................................................................ 15
Substations
15
Installation (jack-up) and other vessels
15Power cables
15

8

OVERVIEW OF SESAM AND BLADED SOFTWARE MODULES FOR FIXED OFFSHORE WIND
STRUCTURES ................................................................................................................................................ 17
Sesam Wind Manager
17
Sesam Manager
17
GeniE
17
Wajac
17
Splice17
Sestra17
Framework
17
Sesam Insight
18
Xtract18
ShellDesign
18
Usfos
18
FNCorrosion
18
Helica18
FatFree
18

8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
8.10
8.11
8.12
8.13
8.14

DNV – www.dnv.com/digital

Page ii

8.15

Bladed99.1
9.2

INTEGRATION OF SESAM WITH OTHER SOFTWARE ............................................................................... 19
Integration between Sesam and Bladed
19
Integration between Sesam and third-party software
19

10

REFERENCES ................................................................................................................................................ 20

DNV – www.dnv.com/digital

18

Page iii

1

EXECUTIVE SUMMARY

This document aims to guide the reader through the different analyses related to fixed offshore wind turbine
support structures and how Sesam and Bladed support those.
Sesam can perform a multitude of different analyses applicable to the offshore wind turbine (OWT) support
structure industry, based on many years of proven technology from the offshore oil and gas industry, andextended with new functionalities specific for the offshore wind industry in accordance with international
standards such as IEC61400-3-1, the DNV Standards DNV-ST-0126 (Support Structures for Wind Turbines)
and DNV-ST-0437 (Loads and Site Conditions for Wind Turbines) and the DNV Recommended Practices
DNV-RP-C203 (Fatigue Design of Offshore Steel Structures) and DNV-RP-0585 (Seismic Design of Wind
Power Plants).
In preliminary design, Sesam for fixed offshore wind turbine structurescan be used for modelling and thevarious types of analysis. The support structure can be modelled in the 3D modelling environment. Benefits
during modelling include reference point modelling and parametric scripting, resulting in a powerful interface
to perform a trade-off study of several conceptual designs in a quick and efficient manner. Some of theanalyses that can be performed in the conceptual design phase are natural frequency analysis (eigenvalue
analysis), ultimate limit state (ULS) and serviceability limit state (SLS) analysis including member and jointcode checking, as well as fatigue limit state (FLS) analysis using damage equivalent loads or wave loads.
Non-linear pile-soil analysis can be performed in these static analyses, whereas equivalent linearized pilesoil spring matrices to be used in dynamic analyses can be automatically obtained by the software.
In the detailed design phase, Sesam for fixed offshore wind turbine structures can be used to perform time
domain analyses from a customized workbench, Sesam Wind Manager. Sesam Wind Manager can perform
both fatigue analysis (FLS) as well as ultimate strength analysis (ULS) and earthquake analysis in the time
domain. These analyses can be perfomed in two ways, either using a superelement approach or a fully
integrated approach:• In the superelement approach, the model and wave (and optionally, seismic) loads are converted
into a superelement and load files by Sesam. The wind turbine is simulated together with thesuperelement and loads in an aeroelastic tool (e.g. Bladed), after which the wind turbine loads are
extracted at the interface point between superelement and turbine. Wind turbine loads from any
third-party wind turbine tool can be used in Sesam, and converters are available for Bladed,
BHawC, VTS/Flex5 and HAWC2. These loads are then merged into the analysis in Sesam, followed
by a time domain analysis to obtain the stress time histories in the structure. These stresses arethen post-processed for FLS and/or ULS requirements. Superelement generation is available for
Bladed, Siemens Gamesa’s BHawC and MHI Vestas’ VTS/Flex5.
•

In the integrated approach, the model can be created in Sesam, after which it is converted to Bladedand combined with the (tower and) rotor-nacelle assembly. After the analyses are run in Bladed, allresults of the complete structure are converted from Bladed into a Sesam result file. The stress time
histories are then post-processed in Sesam for FLS and/or ULS requirements. In recent years, theintegrated approach is being used more in-house by foundation designers too, running both Bladed
and Sesam, for R&D purposes or early stage development. To support these activities, DNV offers
wind turbine concept models and/or assistance in running the software.

Reports for FLS and ULS results are generated automatically. For FLS the report includes the combined
fatigue damage over all included design load cases, giving a good insight into the critical locations in the
structure and the critical design load cases, and can take into account changes of the SN curve over the
lifetime of the structure. For ULS the report includes an overview over the worst load case and utilization
factor for each member and joint in the structure, taking into account load factors. Detailed results per
position in the model per design load case can be reported as well. The results can be visualized with colour
coding on the 3D model using Sesam Insight as well.
After the analyses have been performed, redesign can be performed in an efficient manner, without theneed to re-run the full analysis for intermediate checks. During redesign the code check results are basedon changes in the section or material properties using the initial analysis results. This will give results thatare sufficient during redesign. When all design changes have been performed, consistency between model

DNV – www.dnv.com/digital

Page 1and analysis results is established and updated code check results are obtained.
Sesam contains the latest versions of all main offshore standards, such as API, AISC, Danish Standard,
Eurocode, ISO and Norsok for ultimate strength analysis. The fatigue analysis in the time domain is basedon rainflow counting and can automatically compute the stress concentration factors (SCFs) for every
hotspot in the structure based on the applied loads, both for tubular joints and for butt-welds. A library of SN
curves is included and the splash zone limits can be entered so that corresponding SN curves are applied
automatically if desired.
All time domain analyses can be run in parallel, either locally or in the Sesam Cloud, thereby significantly
reducing the analysis time required. When using Sesam Cloud, multiple design iterations can be run within asingle day. The reduced analysis time results not only in time and cost savings, but also allows for further
structural optimization and thereby further cost reductions.
Conversion tools to/from other software are included. Wind turbine loads can be extracted from results from
Bladed (although Bladed can output these directly in Sesam format if desired too), BHawC, VTS/Flex5 and
HAWC2, and loads or displacements from any (other) wind turbine tool can be used when outputted as asimple text file. Conversions from Sesam to a Bladed foundation model, a Bladed superelement model, a
BHawC (Siemens Gamesa) superelement model and a VTS/Flex5 (MHI Vestas) superelement model are
possible. Structural models can be imported from SACS, Ansys, Staad, Nastran, SOLIDWORKS (Acis SAT
or DXF format), etc.
Besides primary steel design, Sesam can be used for secondary steel design of typically boat landings andJ-tubes, including (operational and accidental) boat impact analysis and vortex-induced vibration analysis of
J-tubes. Other analysis during the life-cycle, such as transportation, lifting and corrosion protection, can be
performed as well. All Sesam modules use the same model, thereby easing the process of running multiple
analyses. Sesam can also be used for designing ofsfhore substations, installation (jack-up) and othervessels, as well as to perform fatigue and strength analysis of power cables.

DNV – www.dnv.com/digital

Page 2

2

INTRODUCTION

Sesam software has been used to analyse marine and offshore structures since 1969. The long experienceand constant improvements of the software have made it one of the most used software applications in theoffshore industry. Its strong history with offshore oil and gas, combined with hands-on experience with windturbines, has made DNV a global leader in risk management of offshore wind projects. Sesam for fixedoffshore wind turbine (OWT) structures and Bladed are based on this knowledge and use the proventechnology from the offshore industry as well as functionalities developed based on requirements specificfor the offshore wind industry.
Sesam for fixed OWT structures is a tailor-made solution for structural strength analysis of fixed offshore
wind turbine structures, addressing the industry’s need to account for the combined effect of wind turbine,hydrodynamic and seismic loads. The analysis functionality offered is in accordance with international
standards such as IEC61400-3-1, the DNV Standards DNV-ST-0126 (Support Structures for Wind Turbines)
and DNV-ST-0437 (Loads and Site Conditions for Wind Turbines) and the DNV Recommended Practices
DNV-RP-C203 (Fatigue Design of Offshore Steel Structures) and DNV-RP-0585 (Seismic Design of Wind
Power Plants).
Bladed is the industry-leading wind turbine design package and is well-integrated with Sesam. It is used not
only by wind turbine manufacturers, but can also be used by foundation designers and other parties
involved in the design of offshore wind turbine structures, and is therefore also referenced at multiple placesin the document.
This document aims to guide the reader through the different analyses related to fixed offshore wind turbine
support structures and how Sesam and Bladed support those. Floating wind is not part of this document,
and the reader is instead referred to the separate whitepaper for that, availabe through www.dnv.com oravailable upon request via digital@dnv.com.
For more information, questions or to learn best practice in using Sesam and Bladed for offshore windturbine support structure analysis, please contact us via digital@dnv.com.

DNV – www.dnv.com/digital

Page 3

3

PRELIMINARY DESIGNThe following is a selection of analyses involved in the preliminary design of a support structure that can be
performed in Sesam.

3.1

Modelling

Modelling of the support structure is done in the 3D modelling environment of Sesam GeniE. Any kind of
structure can be modelled, from monopile to tripod and jacket type structures, including secondary steel,such as boat landings and J-tubes, if desired.
Beams and joints can be modelled as concepts. For example, a leg can bemodelled as one (segmented) member, instead of multiple members spanningfrom joint to joint. GeniE will take care of meshing this concept member into
multiple mesh elements automatically, taking into account connections to otherstructure and changes in section and material. Joint modelling includes automatic
creation of cans and stubs, as well as options to flush braces, add gaps and add
local joint flexibilities.

Figure 3.1: The leg
is modelled as abeam concept.

GeniE offers the capability to model both beam andshell models, as well as a combination of these. Thismeans that complex transition pieces and complex
joints can be modelled as local shell models within abeam model. Tubular joints can easily be converted
from beam to shell elements. This allows the user toobtain stress concentration factors for these complex
local shell models. Besides modelling parts explicitly, it
is possible to include external matrices representing
parts of the model, such as the transition piece, suction
buckets or joints.
Figure 3.2: A joint modelled with shellelements within a beam model.

Figure 3.3: Shelltransition piece
connected to thejacket and tower
beam structures.

DNV – www.dnv.com/digital

Figure 3.4: Results
showing mesh with thestress distribution over
the transition piece.

Figure 3.5: A joint
designed withshells together
with the jacket
beam model.

Figure 3.6: Results
showing mesh with thestress distribution over
the joint.

Page 4

Environmental conditions, such as hydrodynamic
effects (wave, current, flooding, marine growth) canbe applied to the model, as well as non-linear pilesoil interaction. Automatic pile-soil linearization is
included, as natural frequency analysis, dynamic
analyses and superelement generation for the
aeroelastic tool require a linear model.

Figure 3.7: A jacket including wave and soil
environment in GeniE.

Benefits during modelling include reference point modelling and parametric scripting, resulting in a powerfulinterface to perform a trade-off study of several conceptual designs in a quick and efficient manner. Codechecking, including redesign options, are included in GeniE as well.
GeniE and other Sesam modules can be used within Sesam Manager, providing further possibilities ofautomating workflows.

3.2

Natural frequency analysis

Natural frequency analysis is easily run
from within GeniE.
The effects of the tower and rotor-nacelleassembly as well as hydrodynamic added
mass can be included in the analysis.
In the eigenvalue analysis options, the user
can indicate the desired number of modeshapes to be calculated. If required, modal
participation factors can be obtained too, todetermine the importance of each mode.
Visualization options are included in both
GeniE and Xtract, with animation of themode shapes for easy identification of themodes being available in Xtract.
Figure 3.8: An example mode shape of a jacket
structure visualized in Xtract.

DNV – www.dnv.com/digital

Page 5

3.3

FLS analysis using damage equivalent loads and waves

By applying cyclic loads onto the structure, it
is possible to obtain the fatigue damage fordamage equivalent loads (DELs). The DELs
can be applied to the structure through point
loads (e.g. representing the wind turbine
loads). The number of occurrences per load
cycle is indicated in the fatigue analysis.
Fatigue due to hydrodynamic loads
corresponding to wave conditions can also
be simulated. The combined fatigue damage
from DEL and wave loads can then be
calculated.

Figure 3.9: A damage equivalentmodelled in GeniE as a load case.

load

can

be

Stress concentration factors (SCFs) for joints and butt welds can be computed automatically (e.g. throughEfthymiou equations) and/or assigned manually. A library of SN curves is included and subtypes (e.g. in air,free corrosion, or with corrosion protection) can be assigned automatically based on defined splash zone
limits. SN curves can also be specified and/or assigned manually. Miner’s sum is used to find the total
damage over all load cycles included.

3.4

ULS and SLS analysis using extreme loads

Extreme checks are included in GeniE forthe main offshore codes, including API,AISC, Danish Standard, Eurocode, ISO andNorsok.
Both member checks and tubular joint
checksareavailable.
Code
check
parameters can be assigned both globallyand locally.
Redesign
capabilities
areincluded,
calculating the effect of property changes on
the code check results using the initial
analysis results, without the need of rerunning the complete analysis at each stepof the redesign.
Figure 3.10: Member check results in the GeniE userinterface tabulated and visualized in the 3D model.

Results (as well as model, loads, etc.) can be written to a report in tabulated form and including graphics.

DNV – www.dnv.com/digital

Page 6

4

DETAILED DESIGNThe following is a selection of steps involved in the detailed design of a support structure. Most of theseanalyses are performed using dynamic analysis in the time domain.

Figure 4.1: Industry workflows are supported – integrated and superelement.

The analyses can be performed in two ways:• Integrated analysis: The modelling is done in Sesam. The model is then imported/ converted and linked
to a wind turbine in a program such as Bladed, after which the resulting forces and moments are
extracted for every beam in the structure. These results are then converted into Sesam format, where
fatigue and ultimate strength analysis is performed.
•

4.1

Superelement (or sequential) analysis: The modelling is done in Sesam. The model and the wave loads
and (optionally) seismic effects are converted to a superelement and linked to a wind turbine in a
program such as Bladed, BHawC, VTS/Flex5, HAWC2, etc. Structural analysis is then run in the windturbine tool after which the forces and moments are extracted at an interface point. These loads arethen applied to the model in Sesam, together with the wave loads and seismic effects, and the structural
analysis is run. Fatigue and ultimate strength analysis is subsequently performed in Sesam.

Modelling

The same modelling capabilities are available for detailed design as for preliminary design, based on
Sesam’s GeniE modelling environment.

4.2

Wave load generation in time domain

Sea states can be modelled in time domain according to thestandards. Hydrodynamic loads are computed by Wajac
based on he waves, current, marine growth, flooding, andadded mass, using Morison theory and optionally the
MacCamy-Fuchs correction for large diameter monopiles.
Wheeler stretching can be applied if desired.

Figure 4.2: Irregular, regular andconstrained waves can be used.

DNV – www.dnv.com/digital

Page 7

Multiple wave types are available, such as:• irregular waves (Airy, random seeds) either using a single-peaked (PM, Jonswap) or double-peaked
spectrum (Ochi-Hubble, Torsethaugen full/simplified)• regular waves (Airy, Stokes, Stream, Cnoidal)• constrained waves (irregular combined with NewWave and/or Stream function)
If desired, it is possible to import/export waves (surface elevation or wave components), e.g. to use
measured waves or to use the same wave in another program.

4.3

Structural analysis in time domain

Structural analysis is performed using Sesam’s finite element solver Sestra. Time domain analyses can be
dynamic (either using direct time integration or modal superposition) or quasi-static.

4.4

FLS analysis in time domain

The fatigue limit state (FLS) analysis isperformed
using
Sesam
Wind
Manager. The program is developedwith the requirements of the offshore
wind standards in mind. All design loadcases (DLCs) can be set up according
to their wind and sea state
combinations, after which the total
fatigue damage for each hotspot oneach beam is summed and reported
over the included DLCs, considering
the relative occurrence of each DLC
over the lifetime. Partial fatigue damage
results per hotspot in the model per
design load case can be reported aswell. Changes in SN curves over the
lifetime of the structure can beconsidered as well.

Figure 4.3: Sesam Wind Manager includes a grid to easily
enter all design load cases for the analysis.

The fatigue analysis itself uses Framework and is based on rainflow counting of the stress time histories. It
includes automatic computation of stress concentration factors (SCFs) for tubular joints, based on geometry
or loadpath of the tubular joints in the model, as well as for butt welds.
SN curves can be assigned per hotspot around thecross-section and along each member. A library of
SN curves is included and subtypes (e.g. in air, freecorrosion, or with corrosion protection) can be
assigned automatically based on defined splash
zone limits. SN curves and SCFs can also be
specified and/or assigned manually.
Figure 4.4: Hotspots around a tubular crosssection are used in the fatigue analysis.

Visualization and animation of the stress time histories for the model are available through Xtract.
Visualization of the FLS results on the 3D model is available through Sesam Insight.

DNV – www.dnv.com/digital

Page 8

4.5

ULS analysis in time domain

The ultimate limit state (ULS) analysis isperformed
through
Sesam
WindManager, like the FLS analysis.
Both random sea states, regular sea
states and constrained sea states can be
set up from Sesam Wind Manager. Loadfactorsforenvironmentalloads,
buoyancy and gravity can be included if
desired.
The ultimate strength checks are
performed through Framework, whichallows for fast and efficient running of allcode checks for members and tubular
joints. Standards available include: API,AISC, Danish Standard, Eurocode, ISO
and Norsok.

Figure 4.5: Code check results (worst utilization
factors) reported in Sesam Wind Manager over all
design load cases.

All time steps of all DLCs can be checked for ultimate strength checks, or a selection of time steps can bechecked if desired.
ULS result reporting is included through Sesam Wind Manager. The report includes an overview over the
worst load case and utilization factor for each member and joint in the structure, taking into account loadfactors, over all design load cases. Detailed results per position in the model per design load case can bereported as well. For each DLC, graphical and textual output can be generated. Visualization of the ULS
results on the 3D model is available through Sesam Insight.

4.6

Earthquake analysis in time domain

Earthquake analysis can be performed in two ways in Sesam, one being in the frequency domain and one
being in the time domain. For wind turbines, the time domain method allows for adding the wind turbine loadtime series and wave load time series.
The earthquake is applied to the structure
through prescribed accelerations and/or
displacements, based on a time history
defined by the user. The applied seismic
time histories can be defined individually per
point in the model if desired, for example totake into account different seismic waves atdifferent depths.

Figure 4.6: Seismic time histories can be included asprescribed accelerations and/or displacements.
Optionally, a zero-length spring and/or damper canbe included to represent the pile-soil spring stiffness
and/or damping respectively.

Optionally, a zero-length spring and/or damper can be included to represent the equivalent linear pile-soil
spring stiffness and/or damping matrix.
After the analysis, similar post-processing capabilities exist as for the ULS analysis in the time domain.

DNV – www.dnv.com/digital

Page 9Seismic effects can also be used ina superelement analysis. Two
options are available:
1. The superelement contains
additional interface points,
so that the seismic signal
can be applied to thesuperelement
intheaeroelastic tool. No seismic
effects are included in the
superelement loads file.
2. The seismic effects areincludedinthesuperelement loads file. No
additional interface nodes
areincludedinthesuperelement.
Figure 4.7: Various options for using seismic effects in asuperelement analysis.

4.7

Post-processing of concrete structures in time domain

In addition to post-processing of steel structures, it is possible to perform
design calculations of reinforced concrete shell structures using ShellDesign.
ShellDesign is developed by Dr. techn. Olav Olsen and based on theirextensive development and project experience. It uses Sesam results as input
and supports ULS, ALS, SLS and FLS post-processing.
ShellDesign is based on plate and shell theory. It is applicable for both solidand shell FE models, for parts of structures where in-plane dimensions aremuch larger than out of plane. Supported design codes are NS3473, NSEN1992-1-1 and DNV-OS-C502. It accounts for non-linear behaviour ofreinforced concrete in sectional response. It also includes the option to
calculate the non-linear structural response (using the Consistent Stiffness
Method) and triaxial response (using the Modified Compression Field Theory).

Figure 4.8: Concrete
GBS support structure.

DNV – www.dnv.com/digital

Page 10

4.8

Boat impact analysis

Boat impact, either operational or accidental (ALS),
requires non-linear analysis capabilities. Sesam’s
Usfos is able to run these kinds of analyses basedon a GeniE model. The boat impact can be definedon a boat landing or on any other beam in the
structure. The plastic utilization of the model can bechecked and push-over analysis can be performed
to check the remaining capacity of the structure.
See also [1] for more information.

Figure 4.9: Usfos can perform non-linear
analyses, such as boat impact analysis andpush-over analysis.

4.9

Vortex induced vibrations of J-tubes

Using Framework’s wind-induced fatigue functionality, it is possible to perform vortex induced vibration
analysis of J-tubes. The wind fatigue module evaluates fatigue damage of frame structures subjected towind loading. Buffeting loads due to wind gusts and the vortex shedding effects due to steady state windcan be considered.

4.10 Cathodic protection system design assessment
To identify areas of under and over-protection of thecorrosion protection throughout the asset lifecycle,
Sesam’s FNCorrosion tool for managing the risk ofcorrosion can be used. The software sets itself apart in themarket as an integrated part of a structural design system.

Figure 4.10: Surface corrosion potentials
on a jacket and monopile.

4.11 Redesign
One of the strengths of Sesam is that redesign can be performed in an easy and efficient manner.
After running the analyses in Sesam Wind Manager, it is possible to make changes to the structuralgeometry, without the need to re-run the meshing and structural analysis, to see the updated (approximate)result of the FLS and ULS analysis. This is done by re-using the analysis results while using updated
structural properties as part of the FLS or ULS post-processing. This allows for quick design iterations. Once
all design changes have been made, the complete analysis can be re-run once to obtain the final results.
Redesign is available both when performing local runs as well as when performing cloud runs. In bothcases, Sesam Wind Manager can pick up the structural analysis results from a previously run job to re-run
the post-processing.

4.12 Cloud analysis and parallel computing
Sesam Wind Manager offers parallel computing, thereby significantly speeding up analyses of a large set ofdesign load cases (DLCs).

DNV – www.dnv.com/digital

Page 11In addition to local parallel computing, Sesam Wind Manager is
integrated with Sesam’s Cloud Compute Services. This enables
users to analyse all DLCs in DNV’s OneCompute cloud analysis
platform, allowing for fast, simultaneous running of many DLCs. The
status of the analyses can be monitored online and results aredownloaded automatically once the runs are finished.

Figure 4.11: Sesam Wind
Manager can run analyses inparallel locally or in the Sesam
Cloud, speeding up analyses.

Significant time savings can be achieved by analysing the design load cases in parallel runs in the cloud,thereby enabling the performance of multiple design iterations in a single day 1. The reduced analysis time
not only results in time and cost savings, but also allows for further structural optimization and thereby
further cost reductions.

1 Based on measurements of FLS analysis for a selection of wind turbine jacket foundation models in detailed design projects run by Sesam

customers. Analysing 2000 design load cases in parallel in the cloud took 45min-1hr (depending on jacket complexity, number of time
steps and other settings). For a superelement analysis run one also needs to convert the Sesam model and wave loads to superelement
format. The time required for this superelement file generation is not included in the run time here and would add some additional time to
the Sesam part of the workflow.

DNV – www.dnv.com/digital

Page 12

5

TURBINE CONCEPT MODELS FOR INTEGRATED LOAD ANALYSISIn recent years, foundation designers have increasingly been
performing in-house analysis of the complete system of support
structure and wind turbine. Reasons for this include foundation design
initialization, sensitivity studies, additional structural design loops(outside of the turbine OEM) and learning process of turbine
foundation design. This can allow the foundation designers to achievea more optimized design from the early design phases in a project.
Such analyses are possible using Bladed and Sesam combined, andcan use either the superelement or integrated approach (see chapter
4).
In order to perform this process, the foundation designer will need awind turbine model. Foundation designers often either do not have therequired data to model a turbine and/or controller, or they do notpossess the competence to set up and run such wind turbine models
and controllers. DNV can assist in such cases by offering wind turbine
concept models. This empowers the non-turbine OEM stakeholders
like foundation designers to perform integrated offshore wind turbine
loads analyses.
Several wind turbine concept models are available upon request. This
includes models representative of existing/coming commercial windturbine models as well as turbine sizes beyond those available on themarket today. The models are created by DNV’s Turbine Engineering
department (with a track record of 21 full turbine concept designs forturbine OEMs) and are based on public information and their many
years of experience in turbine design. A closed loop and supervisory
controller designed by DNV control experts can be included and tuned
to the support structure frequency, enabling simulations for power
production and other operating modes.

Figure 5.1: Combined turbine
and support structure modelin Bladed.

A wide range of turbine models are available off-the-shelf. At the time of writing, the following models areavailable:o Concept 12MW 218• Ready off-the-shelf:o Concept 14MW 222
o Concept 6MW 154
o Concept 15MW 240
o Concept 7MW 154
o Concept 16MW 240
o Concept 8MW 167
o Concept 20MW 252
o Concept 9.5MW 164
o Concept 9.5MW 174• Future plans:o Concept 10MW 178
o Concept 20+MW
If desired, DNV can also run representative turbine models, acting as a wind turbine manufacturer andexchanging data with the foundation designer.

DNV – www.dnv.com/digital

Page 13

6

BEYOND IN-PLACE ANALYSISThe analyses described above are all considering in-place analysis of the support structure. In addition tothis, other analyses in the lifetime of the structure can be performed in Sesam. The benefit of this is that thesame models can be used throughout the different analyses in Sesam.

6.1

Transportation analysis

Structural
analysisoftransportation of structures canbe performed using GeniE and
Sestra. Static loads due to shipaccelerations(translational
and/or rotational) can beconsidered and code checks
can be performed on the
structure.

Figure 6.1: Transportation analysis ofa large jacket with sea-fasteners.

Figure 6.2: A large
topside on a barge.

Sesam also offers tools (HydroD, Sima) to perform seakeeping and hydrodynamics analyses, hydrostatics
and stability analyses and to simulate transport and other marine operations.

6.2

Lifting analysis

Structural analysis of lifting ofstructures can be performed
using GeniE and Sestra.
Static loads due to the lifting
can be considered and code
checks can be performed on
the structure.
Figure 6.3: Lifting
analysis of a topside.

Figure 6.4: Lifting operations in Sima.

Sesam also offers tools (HydroD, Sima) to perform lifting operations, including through-surface effects, andother marine operations.

DNV – www.dnv.com/digital

Page 14

7

BEYOND OFFSHORE WIND TURBINE SUBSTRUCTURES

In addition to performing analysis on the wind turbine structures, Sesam can be used for other structures
related to offshore wind.

7.1

Substations

Sesam can be used for thedesign and analysis of fixedand floating substations,such as jackets, monopiles
or semi-submersibles.

Figure 7.1 Hohe See offshore wind farm substation. Source: DNV’s
S&LP customer story.

7.2

Installation (jack-up) and other vessels

Sesam can be used for the design and analysis of jack-up
vessels used for offshore wind farm installation, as well as
for the marine operations performed with such vessels.
Other vessels can be designed and analysed in Sesam aswell, such as those used for crew transfer or O&M.

Figure 7.2 Lifting operations in Sima.

7.3

Power cables

Sesam’s tools Helica and FatFree are applicable for the assessment of power cables, such as those inside
a wind farm or connecting the wind farm to shore, both for fixed and floating wind farms.
Power cables are characterized by a multi-layered cross-section containing
helix-shaped components. It makes the cable flexible in bending, but
calculating mechanical properties and component stresses is notstraightforward. Finite element analyses may be used. However, modellingof contact and friction between layers using finite element software iscomplex and time consuming. Consequently, simplifications and/or
conservative assumptions are often made, causing either too conservativeor unconservative results.

Figure 7.3 Powercross-section.

DNV – www.dnv.com/digital

cable

Page 15

Helica software is a stress analysis tool that is tailor made for cross-section
analysis of flexible pipes, umbilicals and power cables. Helica calculatesmechanical properties, including stick and slip bending stiffness, load sharebetween cross-section components, angle of twist as a function of tension, andallowable tension versus curvature. Most importantly, Helica calculates stresses
in the cross-section due to combined loading, accounting for inter-layer contact
due to tension and pressure, radial deformation, and stick/slip effects. A stresscalculation in Helica is completed in just a fraction of a second, enabling
efficient fatigue calculations, consistent with the response from the global
analyses.
Helica performs short-term fatigue analyses based on histograms or time series imported directly fromglobal analyses. Long-term fatigue damage is calculated by accumulating damage over all load conditions
and directions. Helica also performs extreme load capacity checks in accordance with the ISO umbilicalstandard, and can calculate fatigue damage due to vortex-induced vibrations.
Helica can be used in combination with Sesam’s FatFree to perform free span analyses of flexible pipes,
umbilicals and power cables. FatFree is relevant for the seabed part of the cables before they are buried, orfor the parts not being buried, for both bottom fixed and floating wind cable systems.

DNV – www.dnv.com/digital

Page 16

8

OVERVIEW OF SESAM AND BLADED SOFTWARE MODULES FOR FIXEDOFFSHORE WIND STRUCTURES

This chapter gives a short introduction of the different Sesam programs and Bladed relevant for fixedoffshore wind projects. For more information, also see the Sesam Feature Description available on
https://sesam.dnv.com.

8.1

Sesam Wind Manager

Sesam Wind Manager is a cloud-enabled tool for time domain fatigue and ultimate strength analysis of fixedand floating offshore wind turbine support structures subjected to wave and wind turbine interface loads ordisplacements (and optionally seismic loading). Examples of for fixed offshore wind support structures arejackets, tripods and monopiles. After the analyses the results are combined over all design load cases and
FLS/ULS results can be reported. It is also used to generate superelements and load files for wind turbine
tools and for converting Sesam models to Bladed and results back. It is intended for use in the detailed
design phase of offshore wind projects.

8.2

Sesam Manager

Sesam Manager is free for all Sesam users. It helps the user create and execute an analysis workflow
based on previous best practices. It provides users with a single, shared environment for different Sesam
modules and other applications – comprising all kinds of analyses needed. It supports any Sesam analysis,
from simple to very complex, including the flexibility of Javascript. It is therefore very suitable to automate
workflows in the various stages of an offshore wind project.

8.3

GeniE

GeniE is used for modelling of beams, plates and curved surfaces with stiffeners. Load modelling includes
equipment with automatic load transfer, explicit loads, wave and wind loads and compartments. The modelis transferred to Sestra for linear structural analysis, to Wajac for hydrodynamic analysis, to Splice for pilesoil analysis and to Usfos for non-linear structural analysis. GeniE includes predefined analysis set-ups(workflows) involving Wajac, Splice and Sestra. General basic results presentation can be carried out aswell as code checking of members and tubular joints.

8.4

Wajac

Wajac calculates wind, wave and current loads on fixed and rigid frame structures. Typical examples ofsuch structures are offshore jacket platforms and jack-up rigs. The loads are calculated according toMorison’s equation (plus optionally MacCamy-Fuchs) deterministically, in the frequency domain or in thetime domain simulation. Time domain simulation allows calculation of hydrodynamic loads due to irregular(random) waves, regular waves and constrained waves. Added mass due to marine growth, flooding, etc.
can be included. Loads are transferred to structural analysis in Sestra.

8.5

SpliceSplice is a program for non-linear analysis of the structure-pile-soil interaction problem for structures
supported by piles driven into the seabed. The non-linear soil stiffnesses are generated by Gensod (part of
Splice) based on soil modelling performed in GeniE. Splice is also able to compute equivalent linear spring
stiffness matrices to replace the non-linear pile-soil interaction, e.g. for application in a dynamic analyses.

8.6

SestraSestra is the static and dynamic structural analysis program within the Sesam suite of programs. It is basedon the displacement formulation of the finite element method. In addition to linear structural analysis, Sestra
can analyse gap/contact problems as well as tension/compression-only members. Moreover, linear
buckling, stress stiffening and inertia relief analyses may be performed.

8.7

Framework

Framework is a postprocessor for frame structures with the ability of performing fatigue analysis due to wave
and/or wind, earthquake analysis and ultimate strength code checking.

DNV – www.dnv.com/digital

Page 17

8.8

Sesam InsightSesam Insight is an online tool that lets you easily share and collaborate on engineering models throughany browser and across devices, such as computer, tablet and smartphone. There is no need for preinstalled desktop applications. It includes viewing of model and results data (such as FLS and ULS results)
as well as revision and collaboration capabilities.

8.9

XtractXtract is the model and results visualization program of Sesam. It offers general-purpose features for
extracting, further processing, displaying, tabulating and animating results from static and dynamic structural
analysis as well as results from various types of hydrodynamic analysis.

8.10 ShellDesign
ShellDesign is owned and developed by Dr.techn. Olav Olsen and sold and supported by DNV. ShellDesign
is a design program and post-processor for reinforced concrete shell structures subjected to stresses inplane and out-of-plane.

8.11 Usfos
The Usfos software module is a special purpose non-linear program for progressive collapse and accident
analysis of jackets, topsides, floaters and other frame type structures. Accidental damage caused by
explosion, fire, dropped objects, extreme environmental events and ship collision poses a major threat to thesafety and operation of offshore structures.

8.12 FNCorrosion
Sesam’s FNCorrosion tool lets you visually simulate, test and evaluate cathodic protection systems
throughout the asset lifecycle. The software provides the ability to visualize the surface potentials andcurrent density in 3D and run multiple ‘what-if’ scenarios showing the levels of protection around thesubmerged structure. It provides assurance that the selected cathodic protection system will protect fixedand floating structures and any subsea equipment.

8.13 Helica
Sesam’s Helica software is a stress analysis tool that is tailor-made for cross-section analysis of flexible
pipes, umbilicals and power cables. Helica facilitates extreme load capacity checks and highly efficient(short-term and long-term) fatigue analyses consistent with the global response determined throughdynamic analysis.

8.14 FatFree
FatFree software is used for the design, assessment and reassessment of submarine pipeline spans incompliance with the DNV Recommended Practice DNVGL-RP-F105. The software enables the user todesign fatigue lifetime for new free spanning pipelines due to Vortex Induced Vibrations and direct wave
loading and re-assess fatigue lifetime of pipelines in operation.

8.15 Bladed
DNV's wind turbine design tool Bladed is renowned as the industry leading device modelling tool. For over
30 years, Bladed has been the industry standard aero-elastic wind turbine design tool, providing criticalinsight into turbine dynamics and optimization. It is closely integrated with Sesam for both integrated designand superelement analysis.

DNV – www.dnv.com/digital

Page 18

9

INTEGRATION OF SESAM WITH OTHER SOFTWARE

Besides the import and export formats that are available for model exchange in GeniE, Sesam Wind
Manager has the capability to exchange data with some wind turbine-specific analysis tools.

9.1

Integration between Sesam and Bladed

Models can be taken from Sesam to Bladed by the provided converter in Sesam Wind Manager, either as acomplete foundation model or as a superelement model and load files (see [2] for more information on bothmethods).
Automatic conversion into Sesam format is included from Bladed. Two options are available, either
importing the results for the full structure in an integrated analysis (i.e. the complete load time history ofeach beam) for post-processing in Sesam, or importing the wind turbine loads time series at the interface(the latter can also be exported directly from Bladed in Sesam format, see [2] for more information on bothmethods).

9.2

Integration between Sesam and third-party software

Conversion of Sesam models and loads into Siemens Gamesa’s BHawC (superelement) format (see [3] formore information) and MHI Vestas’ VTS/Flex5 (superelement) format is available through Sesam Wind
Manager. Automatic conversion of turbine interface loads from BHawC (see [3] for more information),
VTS/Flex5 and HAWC2 into Sesam format is available in Sesam Wind Manager too.
It should be noted that wind turbine load or displacement time series from any third-party tool can easily be
used in Sesam. The loads (or displacements) are read as a simple text file with seven columns, which
include time and forces/moments (or translations/rotations) in 6 degrees of freedom.
For the foundation model, many import formats exist in Sesam for model import from other tools, such as
SACS, Ansys, Staad, Nastran, SOLIDWORKS (ACIS SAT or DXF format), etc. Sesam also supports avariety of model export options. For example, it allows geometry to be taken into programs as
SOLIDWORKS (ACIS SAT or DXF format) to create fabrication drawings or into Ansys for detailed bolted
flange connection analyses.

DNV – www.dnv.com/digital

Page 19

10 REFERENCES[1] J. Cunha, “Boat impact in fixed offshore structures,” DNV GL, Høvik, Norway, 2018.
[2] L. M. Alblas, “Verification report of Sesam’s Bladed interface (report no. 2016-0866, rev.
3),” DNV GL, Høvik, Norway, 2019.
[3] L. M. Alblas, “Verification report of Sesam’s BHawC interface (report no. 2016-0681, rev.
1),” DNV GL, Høvik, Norway, 2016.

DNV – www.dnv.com/digital

Page 20

About DNV
We are the independent expert in risk management and quality assurance. Driven by our purpose, to safeguardlife, property and the environment, we empower our customers and their stakeholders with facts and reliableinsights so that critical decisions can be made with confidence. As a trusted voice for many of the world’s mostsuccessful organizations, we use our knowledge to advance safety and performance, set industry benchmarks,
and inspire and invent solutions to tackle global transformations.

Digital Solutions
DNV is a world-leading provider of digital solutions and software applications with focus on the energy, maritimeand healthcare markets. Our solutions are used worldwide to manage risk and performance for wind turbines,
electric grids, pipelines, processing plants, offshore structures, ships, and more. Supported by our domain
knowledge and Veracity assurance platform, we enable companies to digitize and manage business criticalactivities in a sustainable, cost-efficient, safe and secure way.