IGC Code IMO's "International Gas Carrier Code" is a common basis for calculations for the classification societies Implemented in DNV Rules Yield and fatigue requirements for the LNG tank structure: ”… the operating life is normally taken to correspond to 108 wave encounters on the North Atlantic". This corresponds to 20 years of operation in the North Atlantic. For membrane type tanks the Code has no specific requirement for fatigue assessment of hull structures.

LNG Carriers with Membrane Tanks Geometry of Cargo Tanks Material Selection Acceleration ellipse and C/tank pressures, Pt. 5 Ch. 5 Strength of inner hull - plates and stiffeners Cargo hold FEM model- typical results Cases Fatigue Additional Notation - PLUS-1 / PLUS-2 Critical Areas with respect to Fatigue Rev. 030611

Typical Midship Section Trunk Deck No CL Bulkhead Complete double hull ì.e. “clean” tanks Rigid double bottom grid structure High grade steel in inner hull Passage Way Upper Deck

Membrane Tanks - Tank Shapes Hull Structure Membrane Tanks - Tank Shapes H1 C1 Relationship between parameters as follows: - C1  0.3 x H1 - C2  2.5 m Appr. 135° Double hull: Height and width limited by the IGC Code C2 Double side width : min 760 mm Double bottom height : min 2 m or B/15

Membrane Tanks - Tank Shapes Hull Structure Membrane Tanks - Tank Shapes Plan view Cross section Tank nos. 2, 3 & 4 Min 2.2 m Tank no 1

Typical Tank Arrangement Hull Structure Typical Tank Arrangement For a typical 4 tank / 140000 m3 ship: - Tank 1  13% LBP - Tank 2 & 3  17% LBP - Tank 4  15% LBP

Reinforced Areas Reinforced Area Transverse corners Long. Dihedron Hull Structure Reinforced Areas Reinforced Area Transverse corners Long. Dihedron Trihedron Oblique Dihedron

Selection of Steel Grades Cofferdam: without heating: with - 61ºC - 64ºC + 5ºC 0ºC Membrane, GTT NO96 Assumptions: • LNG on secondary membrane • Air temp.: - 18°C (USCG) • Sea temp.: 0°C • LNG temp.: - 163°C USCG Alaska is not included Separate analysis for outer hull, IGC: air 5°C & sea 0°C Insulation thickness: Primary : 230 mm Secondary : 300 mm Blue: Inner hull steel temperature Red: Compartment temperature

Selection of Steel Grades DNV Rules:

Selection of Steel Grades NVE NVD Several material grades, NVA, B, D, E & SUS NVD NVD NVB

Hull Strength FAILURE MODES IN HULL STRUCTURES Yield, e.g. permanent plastic deformations/rupture of a bulkhead stiffener after a ballast tank has been subjected to overpressure. Buckling, e.g. a plate, a stiffener or a pillar subjected to compression may fail. Fatigue, e.g. a crack in way of a bracket toe due to wave loads or vibration. Brittle fracture, e.g. carbon steel will become brittle if the temperature becomes too low; hull material grade selection

IGC Code IMO's "International Gas Carrier Code" is a common basis for calculations for the classification societies Implemented in DNV Rules, Pt.5 Ch.5 Yield and fatigue requirements for the LNG tank structure, i.e. the inner hull: - ”… the operating life is normally taken to correspond to 108 wave encounters on the North Atlantic". - This corresponds to 20 years of operation in the North Atlantic.

Local Strength of Inner Hull

Liquid Pressure in Cargo Tanks - Pt. 5 Ch. 5

Acceleration Ellipse - Pt. 5 Ch. 5 DYNAMIC LIQUID PRESSURE IN CARGO TANKS Pgd (Pgd)max.   Z   Z pgd 0° 5°

Accelerations for Liquified Gas Carriers The Rule values of ax, ay and az may be replaced by accelerations calculated from direct wave load analysis

Liquid Pressure in Cargo Tanks

Liquid Pressure in Cargo Tanks

Local Strength of Inner Hull - Plates

Local Strength of Inner Hull - Stiffeners

Inner Hull - Allowable Stresses Allowable stresses given for GTT NO96 and GTT Mark III: stat + dyn  all [N/mm2] where stat = bending stress due to the maximum still water moment calculated for the severest loaded condition or ballast condition which ever are the most severest dyn = bending stress due to maximum wave corresponding to the 10-8 probability for winter north Atlantic Conditions all = allowable hull girder bending stress for inner hull, 120 N/mm2 for GTT NO96 and 175 N/mm2 for GTT Mark III

Strength Analysis of Membrane LNG Carrier Hull structure shall generally to be designed according to Pt. 3 Ch. 1, similar to a conventional tanker Maximum hull girder stresses at inner hull to be within allowable stresses for the containment system Inner hull supporting the cargo containment system shall be designed based on dynamic loads at 10-8 level, ref. Pt. 5 Ch. 5 Material selection for hull to be according to Pt. 5 Ch. 5 based on temperature analysis

Midship Section - Section Scantlings At hand verification of: Hull girder strength Local strength and buckling ( plates/stiffeners )

NAUTICUS-Hull MODELLING Concept Model

Cargo Hold Analysis - FEM FEM Model Concept Model FEM Results

Cargo Hold Analysis - Load Cases

Cargo Hold Analysis - Strength Analysis Scantling draught: T Minimum draught with one C/tank full: TA The cargo tanks should not be operated in sea going condition with filling between 10% of tank length and 80% of tank height (sloshing). Allowable stresses and buckling control for double hull structure and cofferdam bulkheads according to main class as given in Pt.3 Ch.1

Cargo Hold Analysis - FEM Results

FEM Results - Outer Shell Buckling, in the middle of empty hold

FEM Results - Inner Hull Buckling, in the middle of empty hold

FEM Results - Transverse Bulkhead Buckling, in way of full hold, compression both horizontally and vertically

FEM Results - Girders High shear stress

LNG Carriers with Spherical Tanks Design for spherical tanks and hull tanks Wave load analysis Hull structural design - Temperature analysis - Selection of material - Cargo hold analysis - Fatigue analysis Edit in Veiw > Header and footer Edit in Veiw > Header and footer

MOSS Type Containment System

Design for Spherical Cargo Tanks BACKGROUND DNV developed the first set of design criteria in connection with the introduction of the Spherical LNG Containment system in the early 1970’ies Keywords: Leak-before-Failure, fracture mechanics, direct load and strength analysis, buckling and fatigue 1

Design for Spherical Cargo Tanks DEVELOPMENTS Based on extensive experimental and analytical research on the buckling strength criteria of the cylindrical skirt foundation and the spherical tanks DNV introduced improved buckling design criteria in the late 1970’ies (CN30.3) 1979 : A design acceptance programme for the spherical shell part was made in based on the current set of criteria (NVKULE). 1987 : The criteria were issued as Class Note 30.3 covering spherical shells only 2

Design for Spherical Cargo Tanks, cont. NEW CRITERIA 1995: An updated PC version of NVKULE with new spherical tank criteria and extended membrane stress combinations 1996: A new PC design acceptance programme NVSKIRT for the cylindrical skirt foundation available 1997: Class Note 30.3 with new design criteria issued 4

Design for Spherical Cargo Tanks NEW DEVELOPMENTS The structural reliability and the buckling criteria were in the period 1989-1996 re-examined through a series of projects A new set of buckling criteria for both the spheres and skirts were developed and formulated in a modern Limit State format 3

Structural Analysis Spherical Tank LNG Carrier 1

Spherical Tank - frame and girder models 4 3 2 1 Include hull, skirt, cargo tanks and covers Interaction forces in tank shell Tank foundation flexibility Coarse overall stress flow FEM MODEL REQUIRED FOR CLASS APPROVAL 4

FEM Analysis of Hull and Tank Structure In this case a global FEM model from bow to end of tank 3 shall has a sufficiently fine mesh to analyse deformation and stresses in: Skirt Cargo tanks Hull girder/framing system Tank foundation deck

FEM Analysis of Hull and Tank Structure No filling restrictions due to sloshing.

FEM Analysis of Hull and Tank Structure Aftship FE-model Foreship FE-model Midship FE-model

Structural Analysis -1 Structural Analyses of Hull and Cargo Tank DNV uses the SESAM suite of analysis programs, which includes Wave load analysis programs Automatic load transfer to structural analysis part Structural response (FEM) Post-processing & plotting Strength checks (yield, buckling, fatigue) Special tank shell analyses with (BOSOR4/5) or NISA for spherical tank systems 2

Structural Analysis -2 Wave Load Analysis Environmental conditions North Atlantic (Extreme loads - ULS) Word-wide operation (Fatigue - FLS) Six loading condition have been considered full load, ballast plus 4 part load conditions Calculation of transfer functions Linear strip theory program (WAVESHIP), alternatively 3D- sink source program (WADAM) and SWAN responses in irregular short crested seas 2 forward speeds have been calculated to allow for speed reduction in heavy weather (WAVESHIP, 0, 12 & 20 knots), SWAN (0 & 16 knots), WADAM (0 knots) Statistical processing for long term (extreme) loads Automatic load transfer to structural FEM model 3

A global model (full width) extending over the total hull. Structural Analysis -3 Fem Models - 1 A global model (full width) extending over the total hull. to analyze the hull girder stress response and the overall deformation response of main hull structural members The wave loads derived from the wave load analysis will be automatically transferred to the model thus ensuring equilibrium. 5

Structural Analysis - 4 Fem Models - 2 Two frame and girder models - one for tank no. 1 and one for tank 2 & 3 OBJECTIVE: To analyze deformations as well as stresses in the framing/girder system including the tank foundation deck. the model were used as a stand-alone models for a rule based midship area analysis The frame and girder models were included in the global model 6

Local finite element Models Structural Analysis - 5 Fem Models - 3 Local finite element Models Calculation of local stresses for determination of Stress Concentration Factors (SCF) in fatigue sensitive areas These models were inserted into the global model or analysed separately using the sub-modeller technique available in SESAM. 7

Ship Hull Analysis (cont..) Structural Analysis - 6 Ship Hull Analysis (cont..) Structural strength evaluation Yield and buckling checks Fatigue life evaluation Hull girder strength 8

Structural Analysis - 7 Cargo Containment System Detailed stress analyses of tanks and skirts (NISA) Detailed stress concentration analyses of tanks and skirts (FEM) equator profile and tower connections to upper and lower hemisphere Strength evaluation of tanks and skirts Strength margins of spheres and skirts (buckling, allow. stress) fatigue and fracture/crack analyses - ”leak-before-failure” Temperature distributions in cargo tanks, skirts and void spaces Steady-State temperature distributions (design) transient temperature distributions (optimisation of loading procedure) 9

Wave Load Analysis - Spectral Fatigue Analysis D irect wave load and response analysis Wave load analysis A utomatic transfer of dynamic internal/external pressures and inertia loads Pressure distribution

Loading Conditions The following six loading conditions will normally be applied: LC06: Normal ballast condition LC11: Departure - full load LC13: Departure - tank no. 1 full LC14: Departure - tank no. 2 full LC15: Departure - tank no. 3 full LC18: Departure - tanks no. 2 + 4 full 1

Loading Conditions Normal ballast condition (LC06) 2

Loading Conditions Departure - full load (LC11) 3

Loading Conditions Departure - tank no. 1 full (LC013) 4

Loading Conditions Departure - tank no. 2 full (LC14) 5

Loading Conditions Departure - tank no. 3 full (LC15) 6

Loading Conditions Departure - tanks no. 2 + 4 full (LC18) 7

Load Components - LNG Carriers Wave Load Analysis Load Components - LNG Carriers Hull girder bending and torsion external and internal pressure loads inertia loads from hull, equipment and cargo 8

Calculation Procedure Wave Load Analysis Calculation Procedure Hydrodynamic modeling and calculation of transfer functions for 6 d.o.f. at selected sections Prediction of long term values for ULS (20 year) and FLS (probability 10-4) Determine design waves (heading, height and period) Calculate pressure distribution and accelerations for design waves and transfer to structural model Determine non-linear correction factors (if any) 9

Hydrodynamic Analysis Options Wave Load Analysis Hydrodynamic Analysis Options DNV ENVIRONMENTAL LOAD PROGRAMMES WADAM FASTSEA STRIP THEORY 3-D 2-D ZERO LOW MODERATE HIGH SPEED SWAN WAVESHIP:Linear strip theory, frequency domain NV1418: Non-linear strip theory, time domain WADAM: 3-D linear diffraction theory, zero forward speed FASTSEA: 2.5-D high speed theory, valid for Fn above 0.4 SWAN: Linear and non-linear, frequency domain with forward speed, time domain with zero and forward speed 10

Wave Climate Description Wave Load Analysis Wave Climate Description Traditional Scatter diagram for sea area - conditional Weibull Distribution of Hs and Tz Long term distribution derived from short term responses Present approach Uses actual scatter diagram of Hs and Tz for the sea area considered Actual contribution from each Hs and Tz taken into account Result can be used for both Ultimate Strength (ULS) and Fatigue (FLS) evaluations 11

Stochastic Fatigue Analysis Full stochastic analysis Mesh size in the order of the plate thickness All local and global load effects included 8 headings times 22 wave periods per heading => 176 load cases for each loading condition 16

Midship section Tank Weather cover Pipe Tower

Midship section Cylindrical skirt Supporting girder

Steady - state temperature distribution in tanks Temperatures: LNG = - 162 oC Below tank inside skirt = 20 oC Outside tank skirt = 28 oC 18

Steady - state temperature distribution in tanks Equator Temperatures: Sea = 32 0C Air = 45 0C LNG = -162 0C 19

Steady - state temperature distribution in tanks 18

The Equator Profile

Hull Structures Generally hull structural analysis according to Pt.3 Ch.1: Local Scantlings of Plates and Stiffeners Longitudinal Strength Fatigue, NAUTICUS(Newbuilding), PLUS-1/2 USCG material grade for deck corner and bilge strake: USCG: Deck corner to be of grade NVE, NV32E or NV36E and bilge plate to be of grade NVD, NV32D or NV36D.

Midship Section - Section Scantlings A t hand verification of: Hull girder strength Local strength and buckling capacity of plates/stiffeners Typical hull girder section i.w.o. centre of cargo tank

Selection of Materials - Temperature Analysis Temperature analysis for selection of material grade to be based on a hypothetical outflow of gas (leak before failure). Steel grade according to Pt.5 Ch.5 Sec. 2 for the following conditions: IGC: Air temperature 5°C and sea temperature 0°C, applicable for all hull structure in cargo area USCG: Air temperature -18°C and sea temperature 0°C, applicable for inner hull and members connected to inner hull USCG Alaska: Air temperature -29°C and sea temperature -2°C, applicable for inner hull and members connected to inner hull

Temperature Analysis Results -6ºC -3ºC IGC temperature: Air: 5ºC Sea: 0ºC -3ºC 1ºC -25ºC -8ºC -10ºC

Temperature Analysis Results -31ºC -27ºC USCG temperature: Air: -18ºC Sea: 0ºC -19ºC -15ºC -26ºC -8ºC -10ºC

Selection of Materials - Temperature Analysis DNV Rules:

Local Stresses applying net Scantling Corrosion additions, tk, in DNV Rules: Cargo Hold Model to be based on net Scantlings, t - tk:

3DGM - Inner Structure

Double Bottom Foundation deck

Double side Passage way

Transverse bulkhead Upper stool Single skin trv. bhd

3D Global Model

Midship block - Plate thickness map Trv. Bhd Double side Double bottom

Midship block - Material class map NV-NS NV-36 NV-32

Cargo Hold Analysis - Long. Stresses Empty Hold Bi/axial buckling of bottom plate, Shear stress in DB floors/gir.

Fatigue Strength

Fatigue Why focus on fatigue? • Most common hull damage Hull Structure Fatigue Why focus on fatigue? • Most common hull damage • May cause water ingress to insulation spaces • High cost and time consuming repairs • LNG vessels often designed for extended life time

Fatigue Crack

Fatigue Crack

Fatigue Crack

Fatigue Requirements

Fatigue - Higher Tensile Steel

Fatigue in General In a simplified way the fatigue life can be expressed: were N = fatigue life in years C = constant including the environment  = nominal stress k = stress concentration factor (notch factor) 10% uncertainty in stresses gives 30% uncertainty in fatigue life

Hull Structure Fatigue Fatigue damages are caused by dynamic loading

Fatigue and Corrosion Fatigue Unacceptable Damage Zone Level 5 10 15 Thinning Effect 15 yr. Paint Spec. 10 yr. Paint Spec. Fully protected 5 yr. Paint Spec. Bare Steel, Corroding 5 10 15 20 25 30 Years World Wide Trading

Operation Route Reduction Factor, fe fe = 1,0 for North Atlantic operation = 0,8 for world-wide operation All Rule requirements except fatigue are based on north Atlantic trading. This applies for all classification societies. Hence it is very important to take this into consideration. Critical areas except from the North Atlantic is the north sea and also the Alaska trade. Standard Rule Requirement is Assuming World Wide Trading 20 years world wide corresponds to 10 years North Atlantic Wave environment for fatigue needs to be specified by owner if increased fatigue strength is requested

Trading Route Fatigue Level Unacceptable Damage Zone Fully protected, World Wide Fully Protected, North Atlantic 10 yr. Paint Spec. Fully protected, PG-Japan Fatigue Level Unacceptable Damage Zone Unacceptable Damage Zone 5 10 15 20 25 30 Years Years of Operation

Part Time at Sea, Assumptions

Fatigue Satisfactory Fatigue Life Depends on:  Intended trade area Hull Structure Fatigue Satisfactory Fatigue Life Depends on: Design / Approval:  Intended trade area  Paint Specification  Workmanship  Appropriate Class Notations

NAUTICUS(Newbuilding) - Fatigue Analysis CLASSIFICATION NOTE 30.7 F atigue life assessment based on SN-curves R ule dynamic loads for identification of posproblem areas End connections

NAUTICUS(Newbuilding) Most critical area w.r.t. fatigue of longitudinals

Hopper Knuckle High Stress Concentration

Critical Areas - Lower Hopper Knuckle Hull Structure Critical Areas - Lower Hopper Knuckle Inner Bottom

Fatigue Calculations L/Gir. Local FEM

Additional Notation - PLUS-1 / PLUS-2 Additional Fatigue requirements compared to  1A1 and NAUTICUS(Newbuilding): Increased design lifetime, 20years  30 years / 40 years Additional details, e.g. stiffener on top, cut out and collar plate

PLUS - Location of hotspots Hotspots for slit type Hotspots for lug type

PLUS - Local FEM models Local models in D/B

PLUS - Local FEM models Standard lug New lug No lug

High stress concentration Hull Structure Fatigue: PLUS-2 High stress concentration

Hull Structure Fatigue: PLUS-2 Deck Opening

Critical Areas against fatigue Hull Structure Critical Areas against fatigue 5 Details to pay particular attention to: 1. Hopper tank, lower knuckle 2. Hopper tank, upper knuckle 3. Side longitudinals 4. Alignment, bulkhead - bottom structure 5. Deck opening 3 4 2 1

Critical Areas - Typical Web Frame PLUS-1/ PLUS-2 Shear Stress Fatigue Shear Stress PLUS-1 / PLUS 2

Critical Areas – Tank boundary Weld joint in tank boundary

Material & Welding Control Ensuring weld quality and tightness Upper hopper, lower joint: - Deep penetration - 100% MPI Lower hopper, upper joint: - Full penetration 100% MPI 100% UT Lower hopper, lower joint: - Full penetration 100% MPI 100% UT

Weld profiling and weld toe grinding SCF (Kw) =1,19 SCF (Kw) =1,09 Weld Toe Grinding Weld profiling (dressed weld)

Critical Areas - TBHD & LBHD Fatigue Longitudinal bulkhead Transverse bulkhead

Critical Areas – Lower Hopper Corner Important: alignment & grinding

Critical Areas – Upper Hopper Corner Important: alignment & grinding Important: alignment & grinding

Critical Areas – Vertical girder in TBHD Shear Yield & Fatigue

Critical Areas – deck opening Opening edge grinding

Critical Areas - Trans. BHD Fatigue Weld toe grinding

Material grade of hull structures B B B D

Wave Load Analysis - Spectral Fatigue Analysis D irect wave load and response analysis Wave load analysis A utomatic transfer of dynamic internal/external pressures and inertia loads Pressure distribution

Critical areas with respect to transverse stresses Wave Load Analysis - Spectral Fatigue Analysis Fatigue analysis of anchoring bar to be carried out in case of invar membrane Critical areas with respect to transverse stresses Hotspot positions for upper hopper knuckle Hotspot positions for lower hopper knuckle