DNVGL-CG-0136 Liquefied gas carriers with membrane tanks

The electronic pdf version of this document, available free of chargefrom http://www.dnvgl.com, is the officially binding version.

DNV GL AS

CLASS GUIDELINE

DNVGL-CG-0136 Edition October 2015Amended April 2016

Liquefied gas carriers with membrane tanks

FOREWORD

DNV GL class guidelines contain methods, technical requirements, principles and acceptancecriteria related to classed objects as referred to from the rules.

© DNV GL AS October 2015

Any comments may be sent by e-mail to [email protected]

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In this provision "DNV GL" shall mean DNV GL AS, its direct and indirect owners as well as all its affiliates, subsidiaries, directors, officers,employees, agents and any other acting on behalf of DNV GL.

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Class guideline — DNVGL-CG-0136. Edition October 2015, amended April 2016 Page 3Liquefied gas carriers with membrane tanks

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CHANGES - CURRENT

Changes in this document are highlighted in red colour. However, if the changes involve a whole chapter,section or sub-section, normally only the title will be in red colour.

Amendments April 2016

• General— Only editorial corrections have been made.

Editorial correctionsIn addition to the above stated changes, editorial corrections may have been made.

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CONTENTS

CHANGES - CURRENT..............................................................................................3

Section 1 General....................................................................................................61 Introduction.........................................................................................6

Section 2 Material selection.................................................................................... 71 Temperature calculation...................................................................... 72 Hull structures.....................................................................................83 Corrosion additions..............................................................................8

Section 3 Design loads..........................................................................................101 Introduction.......................................................................................102 Internal pressure in cargo tanks....................................................... 103 Sloshing and liquid impact................................................................ 114 Stern slamming..................................................................................11

Section 4 Ultimate strength assessment...............................................................121 General.............................................................................................. 122 Inner hull stress limits...................................................................... 12

Section 5 Cargo hold strength assessment........................................................... 131 General.............................................................................................. 132 Modelling........................................................................................... 133 Design application of loading conditions and load cases....................154 Acceptance criteria............................................................................ 23

Section 6 Local structural strength assessment....................................................241 General.............................................................................................. 242 Locations to be checked.................................................................... 243 Load cases......................................................................................... 244 Acceptance criteria............................................................................ 24

Section 7 Fatigue assessment...............................................................................251 General.............................................................................................. 252 Locations to be checked.................................................................... 253 Loads................................................................................................. 284 Fatigue evaluation............................................................................. 28

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Section 8 Welding................................................................................................. 301 Weld improvement.............................................................................302 Recommended weld details for inner hull..........................................31

Section 9 References.............................................................................................321 References......................................................................................... 32

Changes – historic................................................................................................33

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SECTION 1 GENERAL

1 Introduction

1.1 ObjectiveThis class guideline describes procedures for strength assessment of liquefied gas carriers with membranetanks in compliance with the rules RU SHIP Pt.5 Ch.7.In case of discrepancy between the rule RU SHIP Pt.5 Ch.7 and this class guideline, the rule shall prevail.In general liquefied gas carriers with membrane tanks shall satisfy the strength criteria to main class asgiven in RU SHIP Pt.3 of the rules. In addition, the criteria for classification notation Tanker for LiquefiedGas as given in RU SHIP Pt.5 Ch.7 apply for the inner hull acting as support for the cargo tank insulation andthe membranes. An overview of the ultimate limit state (ULS) and fatigue limit state (FLS) assessment isshown in Figure 1.

Figure 1 Flow chart of strength assessment

1.2 US coast guard requirementRequirements given by the USCG, ref. Sec.9 [1].2, need to be considered for LNG vessels trading to US portsor operating under US flag.

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SECTION 2 MATERIAL SELECTION

1 Temperature calculation

1.1 GeneralPresence of cold cargo will cause lower temperatures for parts of the hull steel structures. Therefore the steeltemperatures for all hull structures, and the parts of the containment structure welded to the hull, have tobe calculated. The calculation is normally to be based on empty ballast tanks since this assumption gives thelowest steel temperature.

1.2 Ambient temperatureAmbient temperature for material selection is according to RU SHIP Pt.5 Ch.7 Sec.4 [5.1.1] and Table 1below.For ships intended for trading in cold areas, other ambient temperature may be required by the class, portauthorities or flag states.

Table 1 Ambient temperature for temperature calculation

Regulations Still sea watertemperature, °C Air temperature, °C Wind speed,

knots Applicable areas

RU SHIP Pt.5 Ch.7Sec.4 [5.1.1] 0.0 +5,0 0.0 All hull structure in cargo area

USCG requirements,except Alaskan water 0.0 -18.0 5.0 Inner hull and members connected

to inner hull in cargo area

USCG requirements,Alaskan water -2.0 -29.0 5.0 Inner hull and members connected

to inner hull in cargo area

1.3 Calculation of the steel significant temperaturesThe calculation of the steel significant temperatures shall be based on ambient temperatures as described in[1.2].The load condition giving the lowest draft among load conditions of two tanks empty and the other tanks fullmay be used for the temperature calculations.

1.4 Connecting membersFor members connecting inner and outer hulls, the mean temperature may be taken for determining the steelgrade.

1.5 At supportsAt supports (e.g. at upper and lower pump tower supports, anchoring bars and anchoring pillar) where coldspots will occur, a local thermal analysis may be deemed necessary in order to establish the steel significanttemperature.

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2 Hull structuresThe material of the hull structure is to be in accordance with RU SHIP Pt.3 Ch.3 Sec.1, unless the calculatedtemperature of the material in the design condition is below -5°C due to the effect of low temperature cargo.In which case the material is to be in accordance with the rules RU SHIP Pt.5 Ch.7 Sec.4 [5.1].Additional USCG requirements ref. Sec.9 [1].2 apply to hull plating along the length of the cargo area asfollows:

— Deck stringer and sheer strake is to be at least Grade E steel— Bilge strake at the turn of the bilge is to be of Grade D or Grade E.

3 Corrosion additionsCorrosion additions of hull structures are given in the rules RU SHIP Pt.3 Ch.3 Sec.3. Corrosion additions ofmembrane system are in accordance with RU SHIP Pt.5 Ch.7 Sec.4 [2.1.5].The following figures show corrosion additions in way of upper deck and cofferdam bulkhead.

Figure 1 Corrosion additions in way of upper deck and trunk deck

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Figure 2 Corrosion additions in way of transverse cofferdam bulkhead

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SECTION 3 DESIGN LOADS

1 Introduction

1.1 GeneralDesign loads for local strength assessment of inner hull structures such as plates and stiffeners supportingthe membrane tanks are given in the rules RU SHIP Pt.5 Ch.7 Sec.23 [1.4].Design loads and load cases for strength analysis of the cargo hold are described separately in Sec.5 [3].

1.2 Loading conditionsLoading conditions described in the rules RU SHIP Pt.5 Ch.7 Sec.23 [3.1.2] are taken into consideration.The following design parameters are to be given in the loading manual:

— Design loading pattern of cargo tanks— Minimum design draft in ballast, at FP, AP and at L/2— Maximum design draft with any cargo tank(s) empty— Minimum design draft with any cargo tank full— Filling condition of double hull ballast tanks under the loaded/empty cargo tank. This should be noted as

an operational limitation— Filling limitations of cargo tanks. This should be noted as an operational limitation— Maximum design GM for calculating design accelerations for each tank, normally based on single tank

filling.

For fatigue assessment, loading conditions described in the rules RU SHIP Pt.5 Ch.7 Sec.23 [4.2.3] are to beused.

2 Internal pressure in cargo tanks

2.1 Rule loadInternal cargo tank pressures, based on a 10-8 probability level for the North Atlantic, are given in the rulesRU SHIP Pt.5 Ch.7 Sec.4 [3.3.2].The acceleration, aβ , is calculated by combining the three component accelerations ax, ay and az valuesaccording to an ellipsoid surface.For different directions of aβ in the ellipsoid, the pressure at different corner locations in the cargo tank iscalculated. Between corner points the pressure may be found by linear interpolation.

2.2 Direct wave load analysisThe rule values of ax, ay and az may be replaced by accelerations calculated from a direct wave loadanalysis. The procedure for direct wave load analysis is given in DNVGL CG 0130, Wave load analysis.These accelerations shall be on a 10-8 probability level for the North Atlantic and calculated for the loadingconditions in the loading manual that give the highest accelerations. As a guidance, the loading conditionswith only one tank full, while other tanks are empty are normally considered to produce the largesttransverse accelerations.

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3 Sloshing and liquid impact

3.1 SloshingAs a minimum, rule inertia sloshing load given in the rules RU SHIP Pt.3 Ch.10 Sec.4 need to be consideredfor the hull structure.

3.2 Liquid impactSloshing analysis is normally required for vessels with unconventional tank size, typically vessels larger than155 000 cubic metres, or tank design with no or limited experience. The following locations in no.2 cargotank should be checked as representative locations for sloshing and liquid impact.

— Lower chamfer connection at middle of a cargo tank due to roll dominant motion.— Lower chamfer connection at transverse bulkhead due to pitch and surge dominant motion.— Upper chamfer connection at middle of a cargo tank due to roll dominant motion.— Upper chamfer connection at transverse bulkhead due to pitch and surge dominant motion.— Upper chamfer connection in way if inner deck due to pitch and surge dominant motion.

For location of lower and upper chamfer connections see Sec.8 Figure 3.

3.3 Partial fillingPartially filled membrane tanks may be vulnerable with respect to sloshing and liquid impact loads. Thesetanks may therefore be subject to filling level restrictions in order to avoid seagoing operation at the mostcritical filling levels.

3.4 ApplicationResults of the analysis shall be applied to the other cargo tanks including no.1 cargo tank, unless sloshinganalysis and liquid impact analysis for the other cargo tanks are carried out.For detail procedure of sloshing analysis, see DNVGL CG 0158, Sloshing analysis of LNG membrane tanks.

4 Stern slammingFor ships where the lower part of the shell has large flare angle, e.g. twin skeg vessels, the impact pressureon the stern shall be considered in accordance with the rule slamming requirements, RU SHIP Pt.3 Ch.10Sec.3. The impact pressures may be obtained by model tests or direct calculations, if applicable.

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SECTION 4 ULTIMATE STRENGTH ASSESSMENT

1 GeneralThe local scantling of inner hull structure shall be in accordance with the rules RU SHIP Pt.5 Ch.7 Sec.23[2.2]. In addition the criteria given in this section need to be considered.

2 Inner hull stress limits

2.1 Allowable stress of inner hullThe criteria of inner hull stress as given below and others, if relevant, shall be confirmed by the designer ofthe cargo containment system for each project.

2.2 Vessels with double corrugated stainless steel membraneWith the primary barrier of stainless steel being double corrugated, the in plane stiffness is very low. Thusthis type of membrane is less sensitive to hull deformations than plane membranes. The following designlimitation is applicable with respect to acceptable longitudinal elongation of inner hull structure due to hullgirder bending ref. Sec.9 [1].4.

σst + σdyn + σloc ≤ 185

where:

σst = hull girder bending stress, in N/mm2, due to maximum still water bending moment calculated forthe most severe loaded condition or ballast seagoing condition, based on gross scantling

σdyn = hull girder bending stress, in N/mm2, due to maximum wave bending moment corresponding to10-8 probability in North Atlantic, based on gross scantling

σloc = Maximum bending stress, in N/mm2, of inner hull due to double hull deflection when consideringalternate loading cases. The bending stress may be taken in the middle between the floors/transverse frames, based on gross scantling.

2.3 Vessels with plane invar membrane(s)Invar membrane, 36% Ni steel, may in the longitudinal direction of the ship be considered as a planeplate rigidly connected to the cofferdam bulkhead structure. In order to keep the total stress level in themembrane at an acceptable level, the cargo containment system designer has given restrictions to becomplied with when evaluating the necessary section modulus for the hull girder in the cargo area, ref. Sec.9[1].5.

σst + σdyn ≤ 120

σst and σdyn in N/mm2 as defined in [2.2].

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SECTION 5 CARGO HOLD STRENGTH ASSESSMENT

1 GeneralThe following describes acceptable methods for the strength analysis, with focus on finite element models ofthe cargo hold area. The analysis shall confirm that the stress levels are acceptable when the structures areloaded in accordance with the described design conditions.

2 Modelling

2.1 GeneralModelling of hull and tank structure shall unless defined otherwise follow RU SHIP Pt.3 Ch.7 of the rules andthe guidance in DNVGL CG 0127, Finite element analysis. This covers, but is not limited to, the followingitems:

— Geometric modelling of hull and tank structure in general— Element types and mesh size— Boundary conditions for 3 hold models— Load application.

The stiffness of the tank system is normally not included in the structural FE model. Pressure loads aredirectly applied to the inner hull.

2.2 Model extentLongitudinal extent of the model is to be over three cargo tank lengths (1+1+1), where the middle tank/hold of the model is used to assess the yield and buckling strength. For fore and aft cargo hold assessment,fore end and engine room structures shall be included in the model and the foremost and aftmost cargohold including cofferdam need to be located in the middle of the model respectively as far as possible.Transversely, the model shall cover the full breadth of the ship.In addition, following areas are to be included in fore and aft cargo hold assessment.

— longitudinal members in deck house between trunk deck and upper deck— fore end of the trunk deck for vessels equipped with double corrugated stainless steel membrane, i.e. GTT

Mark III cargo containment system— vertical girders in fore end cofferdam bulkhead between trunk deck and upper deck for vessels equipped

with the GTT Mark III cargo containment system— transition between longitudinal bulkhead in tank no. 1 and tank no. 2 for vessels equipped with the GTT

Mark III cargo containment system.

Examples of models are shown in Figure 1, Figure 2 and Figure 3.

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Figure 1 Example of a cargo hold model (Midship)

Figure 2 Example of a cargo hold model (Aft)

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Figure 3 Example of a cargo hold model (Forward)

3 Design application of loading conditions and load cases

3.1 GeneralThe loads given in RU SHIP Pt.3 Ch.4 are to be applied, unless otherwise defined below.

The design cargo density shall not be taken less than the maximum acceptable cargo density (usually 0.5t/m3) and the design overpressure (P0) shall not be less than 25 kN/m2 and shall be applied for all loadedcargo tanks.

The cargo density used in the FE model should be corrected for the difference between the volume insidetank and the volume of hold space at inner hull boundary so that static pressure on inner bottom is correct inthe FE model.

Self weight of hull structures shall be taken into account.

To take into account of the insulation, the cargo density, ρh, for cargo loads may be adjusted as follows.

where:

ρc = design cargo density of LNG, in t/m3

VC = volume of cargo tank measured in way of primary barrier, in m3

VH = volume of cargo hold measured in way of moulded dimensions of inner hull, in m3.

This is also applicable for fatigue analysis.

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3.2 Loading conditionsThe basic loading conditions as described in Sec.3 [1.2] shall normally be considered. The loading patternsgiven in Table 1 are based on these conditions, and are regarded as the minimum conditions. Otherconditions may be considered when relevant.Based on operational limitations, e.g. if surrounding ballast tanks in way of an empty cargo tank are alwaysfilled, the standard load cases shown in Table 1, Table 2 and Table 3 may be modified.

3.3 Load casesLoad cases that can be considered in the cargo hold analysis are shown in Table 1 for mid hold, Table 2 foraftmost hold and Table 3 for foremost hold.

Table 1 Typical design load combinations for midship cargo region for FE analysis of membraneLNG tanker

No.Loading pattern

Aft Mid ForeDraught % of perm.

SWBM% of perm.

SWSF Dynamic load case

Seagoing conditions

100% MaxSFLC3) HSM-2, FSM-2

100%Max SFLC4) HSM-2, FSM-2

100% (hog.)

≤ 100%

HSA-2, BSR-1P, BSR-2P, BSP-1P, BSP-2P,OST-1P, OST-2P, OSA-1P, OSA2P, BSR-1S,BSR-2S, BSP-1S, BSP-2S, OST-1S,OST-2S, OSA-1S, OSA-2S 8)

L1

TSC 1)

0% (sag.) 100% MaxSFLC5) HSM-1, FSM-1

100% MaxSFLC3) HSM-1, FSM-1

100%Max SFLC4) HSM-1, FSM-1

100% (sag.)

≤ 100%


L2

TA

0% (hog.) 100% MaxSFLC5) HSM-2, FSM-2

Harbour conditions

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No.Loading pattern


SWBM% of perm.


100%Max SFLC6) N/A

L3

TA 2) 100% (sag.)100%Max SFLC7) N/A

100%Max SFLC6) N/A

L4

TSC 1) 100% (hog.)

100%Max SFLC7) N/A

1) Maximum draft with one cargo tank empty may be used instead of scantling draft TSC, if this is stated as anoperational condition in the loading manual.

2) Draught not to be taken greater than minimum of 2 + 0.02 L and the minimum ballast draught3) For the mid-hold where xb-aft < 0.5 L and xb-fwd > 0.5 L, the shear force is to be adjusted to target value at aft

bulkhead of the midhold.4) If the mid-hold is located xb-aft < 0.5 L and xb-fwd > 0.5 L, the shear force is to be adjusted to target value at

forward bulkhead of the mid-hold. Otherwise this load combination may be omitted.5) This load combination is to be considered only for the mid-hold where xb-aft > 0.5 L or xb-fwd < 0.5 L.6) The shear force is to be adjusted to target value at aft bulkhead of the mid-hold.7) The shear force is to be adjusted to target value at forward bulkhead of the mid-hold.8) The beam sea and oblique sea dynamic load cases calculated for P and S are to be applied on the model to obtain

the results for both model sides. Alternatively, for ship structure symmetrical about the centreline, the beam seaand oblique sea dynamic load cases calculated for P may be applied only to the model (i.e. S may be omitted)provided the results (maximum stress and buckling) are mirrored.

where:

TA = Minimum relevant seagoing draught in m, may be taken as 0.35 D if not known

TSC = Scantling draught in m.

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Table 2 Typical design load combinations for aft hold cargo region FE analysis of membrane LNGtanker

No.Loading pattern


SWBM% of perm.


Seagoing conditions

L1

TSC 100% (sag.)100%Max SFLC

HSM-1, FSM-1

100% MaxSFLC HSM-2, FSM-2

100%(hog.)6)

≤ 100%

HSA-2, BSR-1P, BSR-2P, BSP-1P, BSP-2P,OST-1P, OST-2P, OSA-1P, OSA2P, BSR-1S,BSR-2S, BSP-1S, BSP-2S, OST-1S,OST-2S, OSA-1S, OSA-2S 5)L2

TSC1)

0% (sag.) 100% MaxSFLC HSM-1, FSM-1


0% (sag.)

≤ 100%

HSA-1, BSR-1P, BSR-2P, BSP-1P, BSP-2P,OST-1P, OST-2P, OSA-1P, OSA2P, BSR-1S,BSR-2S, BSP-1S, BSP-2S, OST-1S,OST-2S, OSA-1S, OSA-2S 5)L3

TA

50%(hog.)6)


Harbour conditions

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No.Loading pattern


SWBM% of perm.


L4

TSC 100% (sag.)100%Max SFLC3) N/A

100%Max SFLC3) N/A

L56)

TA 2) 50% (hog.)

100%Max SFLC4) N/A

100%Max SFLC3) N/A

L66)

TSC 100% (hog.)100%Max SFLC4) N/A

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No.Loading pattern


SWBM% of perm.


1) Maximum draft with one cargo tank empty may be used instead of scantling draft TSC, if this is stated as anoperational limitation in the loading manual.

2) Draught not to be taken greater than minimum of 2 + 0.02 L and the minimum ballast draught3) The shear force is to be adjusted to target value at aft bulkhead of the aftmost hold.4) The shear force is to be adjusted to target value at forward bulkhead of the aftmost hold.5) The beam sea and oblique sea dynamic load cases calculated for P and S are to be applied on the model to obtain

the results for both model sides. Alternatively, for ship structure symmetrical about the centreline, the beam seaand oblique sea dynamic load cases calculated for P may be applied only to the model and the dynamic load casesfor S may be omitted provided the results (maximum stress and buckling) are mirrored.

6) Tanks in Engine room to be 100% full.

where:



Table 3 Typical design load combinations for forward cargo region for FE analysis of membraneLNG tanker

No.Loading pattern


SWBM% of perm.


Seagoing conditions

100%Max SFLC

HSM-1, FSM-1

L1

TSC 100% (sag.)

≤ 100% HSA-1

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No.Loading pattern


SWBM% of perm.


L2

TSC1) 100% (sag.)

100%Max SFLC

HSM-1, FSM-1


L36)

TSC 1) 100% (hog.)

≤ 100%



100% (sag.)

≤ 100%


L4

TA

100%(hog.)6)


Harbour conditions

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No.Loading pattern


SWBM% of perm.


L5

TSC 100% (sag.)100%Max SFLC4) N/A

100%Max SFLC3) N/A

L6

TA 2) 100% (sag.)

100%Max SFLC4) N/A

100%Max SFLC3) N/A

L76)

TSC 1) 100% (hog.)

100%Max SFLC4) N/A

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No.Loading pattern


SWBM% of perm.


1) Maximum draft with one cargo tank empty may be used instead of scantling draft TSC, if this is stated as anoperational limitation in the loading manual.

2) Draught not to be taken greater than minimum of 2 + 0.02 L and the minimum ballast draught3) The shear force is to be adjusted to target value at aft bulkhead of the mid-hold.4) The shear force is to be adjusted to target value at forward bulkhead of the mid-hold.5) The beam sea and oblique sea dynamic load cases calculated for P and S are to be applied on the model to obtain

the results for both model sides. Alternatively, for ship structure symmetrical about the centreline, the beam seaand oblique sea dynamic load cases calculated for P may be applied only to the model (i.e. S may be omitted)provided the results (maximum stress and buckling) are mirrored.

6) All Tanks forward cargo tank No.1 to be 100% full.

where:



4 Acceptance criteria

4.1 YieldingAcceptance criteria for yielding is given in the rules RU SHIP Pt.3 Ch.7 Sec.3 [4.2].

4.2 BucklingAcceptance criteria for buckling is given in the rules RU SHIP Pt.3 Ch.8 Sec.1 [3.3] and method description isgiven in DNVGL CG 0128, Buckling.

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SECTION 6 LOCAL STRUCTURAL STRENGTH ASSESSMENT

1 GeneralLocal structural analyses with fine mesh finite element models are to be carried out in accordance with therules RU SHIP Pt.3 Ch.7 Sec.4 and the DNVGL CG 0127, Finite element analysis.

2 Locations to be checked

2.1 GeneralThe areas to be considered are according to the rules RU SHIP Pt.5 Ch.7 Sec.23 [3.2.1]. Based on screeningresults from the cargo hold analysis the most critical location can be selected. The screening method isaccording to DNVGL CG 0127, Finite element analysis.

2.2 Double hull longitudinals subjected to large deformationsRelative deformations between longitudinal stiffener supports may give rise to high stresses in local areas.Typical areas to be considered are:

— longitudinals in double bottom and adjoining vertical bulkhead members— double side longitudinals and adjoining horizontal bulkhead members.

The model is recommended to have the following extent:

— the stiffener model shall extend longitudinally to a stiffener support at least two web frame spacing forboth sides from the area under investigation

— the width of the model shall be at least 2 + 2 longitudinal stiffener spacing.

3 Load casesFine mesh analysis is to be carried out for the load cases specified in Sec.5 [3.3].All local loads, including any vertical loads applied for hull girder shear force correction in cargo hold analysis,are to be applied to the model when separate sub-modelling is used.

4 Acceptance criteriaAcceptance criteria for stress results from local structure analysis are given in the rules RU SHIP Pt.3 Ch.7Sec.4 [4.2].

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SECTION 7 FATIGUE ASSESSMENT

1 GeneralThe fatigue assessment is limited to selected steel structures in the cargo area, excluding the cargocontainment system and its components.Unless otherwise described, details of the fatigue strength assessment are given in the rules RU SHIP Pt.3Ch.9 and DNVGL CG 0129, Fatigue assessment of ship structures.Direct fatigue analysis by using wave loads may be necessary for LNG carriers with membrane tanks. Detailsare given in RU SHIP Pt.6 Ch.1 Sec.7.

2 Locations to be checkedThe fatigue strength calculations shall be carried out for the locations given in the rules RU SHIP Pt.5 Ch.7Sec.23 [4.2.4]. The detail hot spot locations are as shown in Table 1 below.

Table 1 Locations to be checked for FE fatigue assessment

Location F.E model Hot spots

Lower and upperhopper knuckleconnections formingboundary of inner skinamidships

Lower Hopper Knuckle

Upper hopper Knuckle

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Inner bottomconnection totransverse cofferdambulkhead

Double side stringerconnection totransverse cofferdambulkhead

Refer to above location of Inner bottomconnection to transverse cofferdam bulkhead

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Liquid dome openingand coamingconnection to deck, ifapplicable

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Termination of aftend of no.1 innerlongitudinal bulkhead,if applicable

Refer to above location of Inner bottomconnection to transverse cofferdam bulkhead

3 Loads

3.1 Loading conditionsLoading conditions described in the rule RU SHIP Pt.5 Ch.7 Sec.23 [4.2.3] shall be considered.

3.2 Dynamic load casesThe dynamic load cases to be considered for fatigue assessment are according to the rules RU SHIP Pt.3 Ch.4Sec.2 [3].

4 Fatigue evaluation

4.1 GeneralFatigue strength, unless otherwise described below, is to be evaluated according to the rules RU SHIP Pt.5Ch.7 Sec.23 [4]. The maximum allowable usage factor is given in the following table.

Table 2 Usage factor, CW

Location Environment Usage factor, CW

Inner hull structures. Hot spots where cracks can propagate through theinner hull plates, e.g. plates boundary between cargo and ballast tanks. North Atlantic 1.0

Inner hull structures. Hot spots where cracks don’t propagate directlythrough inner hull plates, e.g. longitudinals end connection. World Wide 1.0

Outer hull structures. World Wide 1.0

Pump tower support. North Atlantic see Figure 1

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For pump tower base supports, a crack will propagate in the thickness direction inside the containmentsystem. Such fatigue cracks may lead to rupture of both the primary and secondary barrier. Cracks on thesecondary barrier cannot be detected before failure is effective, leading to no redundancy of the system. Thehotspots shall therefore satisfy a fatigue damage of 0.1 in North Atlantic operation, i.e. Cw = 0.1.It is recommended to avoid welded permanent backing in fatigue sensitive areas. If a welded steel backingstrip is applied as shown in Figure 1, the location of welding spots to be kept well away from areas with highstresses (away from the areas with supporting structure below inner bottom).

Figure 1 Pump tower base support

4.2 Parameters to be usedStandard values to be used for fatigue calculation such as fraction of the time in each loading condition,draft, GM and so on are given in RU SHIP Pt.3 Ch.9 Sec.4 [4.3] Table 2. Actual values from the loadingmanual can be used instead when those are available.

4.3 Fatigue due to sloshing loadSloshing pressures may normally be neglected in fatigue strength assessment of hull structures except forpump tower supports. For calculations of fatigue strength of pump tower supports, see DNVGL CG 0158,Sloshing analysis of LNG membrane tanks.

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SECTION 8 WELDING

1 Weld improvement

1.1 GeneralImprovement of fatigue life due to post weld treatment shall be according to RU SHIP Pt.3 Ch.9 Sec.4 [4.4].

1.2 Weld toe grindingWeld toe grinding as described in DNVGL CG 0129, Fatigue assessment of ship structures, is acceptable.Figure 1 shows an example of weld profiling. The weld bead should be ground, and the undercut at the weldtoe should be removed. It should be noted that the final grinding direction should go across the weld in orderto avoid additional notch due to the grinding.

Undercut to be removedat weld toe, min. 0.5 mm

Figure 1 Example of weld profiling at lower hopper knuckle

1.3 Weld profilingWeld profiling as described in the Recommended Practice DNVGL RP 0005 [7.2] is acceptable.

Figure 2 Geometric parameters for weld profiling

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2 Recommended weld details for inner hullInner hull weld details are not possible to inspect as they are covered by the membrane insulation system.The following weld details are therefore recommended to be subject to weld improvement by post weldtreatment.

Figure 3 Welding details at inner hull, within +/- 150 mm from a web frame

Different weld details may be considered depending on the stress level at the details.

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DNV GL AS

SECTION 9 REFERENCES

1 References.1 IMO: International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk

(IGC Code), Res. MSC.370(93)

.2 USCG: Safety Standards for self-propelled Vessels carrying Bulk Liquefied Gases, 46 CFR (Code ofFederal Register), Part 154.170/172/176

.3 Lindemark, T. et al: CSA-2 Analysis of a 216k LNG Membrane Carrier, The Royal Institution of NavalArchitects (RINA), ISCOT Busan, Korea, 2006.

.4 GTT: Hull design and tank dimensioning, Note MARK III 235, Rev. 12, Nov. 2005

.5 GTT: Cargo Tanks Arrangement Dimensions and Filling Ratios Hull Scantling Requirement, GTTdocument N0-DG-33 Rev. M, May 2005

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DNVGL-CG-0136 Liquefied gas carriers with membrane tanks