G2

Liquefied gas cargo tanks and process(1974)(cont)(Rev.1 1979) pressure vessels
G2G2(Rev.2Dec 2018)(Rev.3May 2023)

G2.1

GeneralG2.1.1 The present texts give the general principles which are applied by Classification
Societies for approval and survey of the relevant items of liquefied gas tankers forclassification purpose. They do not intend to cover full details of such approval and survey
procedures which are to be found in the individual Rules of Classification Societies.
G2.1.2 Where appropriate, these Rules refer to the basic tank types which are defined
under 4.1 of IMO Resolution MSC.370(93) Amendments to the International Code for theConstruction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code). Tanks
differing from these definitions will be the subject of special consideration.
G2.1.3 Consideration of future technical advances may warrant modifications to theprinciples and details set forth in the text. IACS will accordingly review continuously theserequirements.
G2.1.4 When reference is made in this Requirement to 'Classification Society', onlymembers or associates of IACS are considered.
G2.1.5 IGC Code means the International Code for the Construction and Equipment ofShips Carrying Liquefied Gases in Bulk (as amended by IMO Resolutions MSC.370(93),MSC.411(97) and MSC.441(99)).

Note:
1.

The Rev.2 of this UR is to be uniformly implemented by IACS Societies for independent
cargo tanks type C (pressure cargo tanks) such as defined in 4.23 of the IGC Code:i) when an application for certification is dated on or after 1 January 2020; andii) which are installed in new ships for which the date of contract for construction is on orafter 1 January 2020.

2.

The “contracted for construction” date means the date on which the contract to build thevessel is signed between the prospective owner and the shipbuilder. For further details
regarding the date of “contract for construction”, refer to IACS Procedural Requirement(PR) No. 29.

3.

The Rev.3 of this UR is to be uniformly implemented by IACS Societies for independent
cargo tanks type C (pressure cargo tanks) such as defined in 4.23 of the IGC Code :i) when an application for certification is dated on or after 1 July 2024; andii) which are installed in new ships for which the date of contract for construction is on orafter 1 July 2024.

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G2G2(cont)G2.2

ScopeThe requirements here below apply to independent cargo tanks type C (pressure cargotanks) such as defined in 4.23 of the IGC Code. They may also apply to process pressure
vessels if required by the Classification Society. The words 'pressure vessels' are used in thistext to cover the two above-mentioned categories. These requirements apply to tanks andvessels made of materials defined in W1 and the IGC Code.
G2.3Calculation of thickness under internal pressureG2.3.1 General
For pressure vessels, the thickness calculated according to 4.23.2.4 of the IGC Code shall beconsidered as a minimum thickness after forming, without any negative tolerance.
Scantlings based on internal pressure shall be calculated as follows: the thickness and formof pressure containing parts of pressure vessels under internal pressure, including flanges,
are to be determined according to the Rules of the Classification Society. These calculations
are to be based in all cases on generally accepted pressure vessel design theory.
Openings in pressure containing parts of pressure vessels are to be reinforced in accordancewith the Rules of the Classification Society.
G2.3.2 Design pressure
For calculation according to G2.3.1, the design liquid pressure defined under 4.13.2 of theIGC Code is to be taken into account in the internal pressure calculations.
G2.3.3 Efficiency factor for welded joints
The welded joint efficiency factor to be used in calculation according to G2.3.1 is to be 0.95
when the inspection and nondestructive examination stated under G2.9.2 (i) are carried out.
This figure may be increased up to 1.0 taking into account other considerations, such as
materials used, type of joints, welding procedure, type of loading, etc. For process pressure
vessels, the Classification Society may accept partial nondestructive examinations, but notless than those under G2.9.2 (ii) may be allowed depending on the material used, the designtemperature, the nil ductility temperature of the material as fabricated, the type of joint,welding procedure, etc., but in this case the efficiency factor 0.85 is to be adopted.
For special materials, the above mentioned factors are to be reduced depending on thespecified mechanical properties of the welded joint.
G2.3.4 Maximum allowable stress
The maximum allowable stresses to be used in calculation according to G2.3.1 shall not
exceed the value defined in 4.23.3.1 of the IGC Code.
G2.3.5 Corrosion allowance
Corrosion allowance need not be required in addition to the thickness resulting from thestructural analysis. However, where there is no environmental control, such as inerting
around the cargo tank, or where the cargo is of a corrosive nature, the Classification Society
may require a suitable corrosion allowance.

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G2G2(cont)G2.3.6 Minimum thickness of shell and heads
The thickness, including corrosion allowance, after forming of any shell and head is not to beless than 5mm for C-Mn steels and Ni steels, 3 mm for austenitic steel or 7 mm for aluminium
alloy.
G2.4

Buckling criteria

G2.4.1 General
Buckling criteria shall be as follows: the thickness and form of pressure vessels subject toexternal pressure and other loads causing compressive stresses are to be calculated
according to the Rules of the Classification Society. These calculations in all cases are to bebased on generally accepted pressure vessel buckling theory and are to adequately accountfor the difference in theoretical and actual buckling stress as a result of plate edge
misalignment, ovality and deviation from true circular form over a specified arc or chord
length.
G2.4.2 Design external pressure
The design external pressure Pe to be used for verifying the buckling of the pressure vessels
is given by the following formula:
𝑃𝑃𝑒𝑒 = 𝑃𝑃1 + 𝑃𝑃2 + 𝑃𝑃3 + 𝑃𝑃4 (𝑀𝑀𝑀𝑀𝑀𝑀)
Where
P1 = setting value of vacuum relief valves. For vessels not fitted with vacuum relief
valves, P1 is to be specially considered, but is, in general, not to be taken lessthan 0.025 MPa.
P2 = for pressure vessels or parts of pressure vessels in completely closed spaces: theset pressure of the pressure relief valves for these spaces.
Elsewhere P2 = 0.
P3 = compressive actions in the shell due to the weight and contraction of insulation,weight of shell, including corrosion allowance, and other miscellaneous externalpressure loads to which the pressure vessel may be subjected. These include but
are not limited to weight of domes, weight of towers and piping, effect of product in
the partially filled condition, accelerations and hull deflection. The local effect ofexternal and/or internal pressure is also to be taken into account.
P4 = external pressure due to head of water for pressure vessels or part of pressure
vessels on exposed decks.
Elsewhere P4 = 0.
G2.5

Stress analysis in respect of static and dynamic loads

G2.5.1 Pressure vessel scantlings are to be determined in accordance with G2.3 and G2.4.
G2.5.2 Calculations of the loads and stresses in way of the supports and the shellattachment of the support are to be made. Loads as applicable, from 4.12 to 4.15 of the IGC

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G2G2(cont)

Code, are to be used. Stresses in way of the supports are to be according to a recognized
standard acceptable to the Classification Society.
G2.5.3 Furthermore, when required by the Classification Society, secondary stresses andthermal stresses are to be specially considered.
G2.5.4 In special cases, a fatigue analysis may be required by the Classification Society.
G2.6

Accident design condition

G2.6.1 The tanks and the tank supporting structures shall be designed for the accidentalloads and design conditions specified in 4.3.4.3 and 4.15 of the IGC Code, as applicable.
G2.6.2 When subjected to the accidental loads specified in 4.15 of the IGC Code, the stressshall comply with the acceptance criteria specified in 4.23.3.1 of the IGC Code, modified as
appropriate taking into account their lower probability of occurrence.
G2.7

Welding joints details

G2.7.1 All longitudinal and circumferential joints of pressure vessels are to be of butt
welded, full penetration, double vee or single vee type. Full penetration butt welds are to beobtained by double welding or by the use of backing rings. If used, backing rings are to be
removed except from very small process pressure vessels. Other edge preparations may bepermitted depending on the results of the tests carried out at the approval of the welding
procedure.
G2.7.2 The bevel preparation of the joints between the pressure vessel body and domes
and between domes and relevant fittings are to be designed according to a standard
acceptable to the Classification Society. All welds connecting nozzles, domes or otherpenetrations to the vessel and all welds connecting flanges to the vessel or nozzles, are to be
full penetration welds.
G2.8

Stress relieving

G2.8.1 For pressure vessels made of carbon and carbon-manganese steel, post-weld heat
treatment is to be performed after welding if the design temperature is below -10°C. Postweld treatment in all other cases and for materials other than those mentioned above shall beto recognized standards acceptable to the Classification Society. The soaking temperatureand holding time are to be according to the recognized standards acceptable to theClassification Society.
G2.8.2 In the case of large cargo pressure vessels of carbon or carbon-manganese steel forwhich it is difficult to perform the heat treatment, mechanical stress relieving by pressurizing
may be carried out as an alternative to the heat treatment if agreed by the Classification
Society and subject to the conditions of 6.6.2.3 of the IGC Code.
G2.9

Inspection and nondestructive examination

G2.9.1 Manufacture and workmanship
The tolerances relating to manufacture and workmanship (i.e. out-of-roundness, local
deviations from the true form, welded joints alignment, tapering of plates having different
thicknesses, etc.) are to comply with recognized standards acceptable to the Classification
Society. The tolerances are also to be related to the buckling analysis (see G2.4).
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G2G2(cont)G2.9.2 Nondestructive examination
The extent of nondestructive testing shall be total or partial according to recognized
standards acceptable to the Classification Society, but the controls to be carried out shall not
be less than the following:(i) Total nondestructive examination (see G2.3.3)
Radiography
butt welds: 100%
Surface crack detection
all welds: 10%
reinforcement rings around holes, nozzles, etc: 100%
Ultrasonic testing
Ultrasonic testing may be accepted for replacing partially the radiographic
examination, if so specially allowed by the Classification Society. In addition theSociety may require a total ultrasonic examination on welding of reinforcement ringsand holes, nozzles, etc.
(ii) Partial nondestructive examination (see G2.3.3)
Radiography
butt welds: all welded joints crossing and at least 10% of the full length at selected
positions uniformly distributed
Surface crack detection
reinforcement rings around holes, nozzles, etc 100%
Ultrasonic testing
as may be required by the Classification Society in each instanceG2.10

Pressure testingG2.10.1 Each pressure vessel is to be subjected to a hydrostatic test according to the Rulesof the Classification Society, at a pressure, measured at the top of the tanks, of not less than
1.5 P0, In no case during the pressure test is the calculated primary membrane stress at anypoint to exceed 90% of the yield stress of material (for definition of P0, see 4.1.2 of the IGC
Code). To ensure that this condition is satisfied where calculations indicate that this stress will
exceed 0.75 times the yield strength, the prototype test is to be monitored by the use of strain
gauges or other suitable equipment in pressure vessels except simple cylindrical andspherical pressure vessels.
G2.10.2 The temperature of the water used for test is to be at least 30°C above the nil
ductility transition temperature of the material as fabricated.
G2.10.3 The pressure is to be held for two hours per 25 mm of thickness but in no case lessthan two hours.
G2.10.4 Where necessary for cargo pressure vessels, there may be carried out with specificapproval of the Classification Society, a hydropneumatic test in the conditions prescribedunder G2.10.1, G2.10.2 and G2.10.3.

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G2G2(cont)G2.10.5 Special consideration will be given to testing of tanks in which higher allowablestresses are used depending on service temperature. However, the requirements of G2.10.1
are to be fully complied with.
G2.10.6 After completion and assembly, each pressure vessel and relative fittings are to be
subjected to an adequate tightness test which may be performed in combination with thepressure testing referred to in G2.10.1.
G2.10.7 Pneumatic testing of pressure vessels other than cargo tanks will be considered on
an individual case basis by the Classification Society. Such testing will be permitted only for
those vessels which are so designed and/or supported that they cannot be safely filled withwater, or for those vessels which cannot be dried and are to be used in a service wheretraces of the testing medium cannot be tolerated.

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