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IGC Code IMO's "International Gas Carrier Code" is a common basis for calculations for the classification societies Implemented in DNV Rules Yield and.

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Presentation on theme: "IGC Code IMO's "International Gas Carrier Code" is a common basis for calculations for the classification societies Implemented in DNV Rules Yield and."— Presentation transcript:

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2 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 10 8 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.

3 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 LNG Carriers with Membrane Tanks Rev. 030611

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

5 Hull Structure C2C2 C1C1 Double side width: min 760 mm Double bottom height: min 2 m or B/15 H1H1 Relationship between parameters as follows: - C 1 0.3 x H 1 - C 2 2.5 m Double hull: Height and width limited by the IGC Code Appr. 135° Membrane Tanks - Tank Shapes

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

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

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

9 Selection of Steel Grades -23ºC -22ºC -2 ºC -3ºC -15ºC -7ºC -5ºC -19ºC -27ºC Cofferdam: without heating: with heating: - 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

10 DNV Rules: Selection of Steel Grades

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

12 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

13 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 10 8 wave encounters on the North Atlantic". - This corresponds to 20 years of operation in the North Atlantic.

14 Local Strength of Inner Hull

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

16 Acceleration Ellipse - Pt. 5 Ch. 5 DYNAMIC LIQUID PRESSURE IN CARGO TANKS Pgd (Pgd)max. Z p gd 0° 5° Z

17 Accelerations for Liquified Gas Carriers The Rule values of a x, a y and a z may be replaced by accelerations calculated from direct wave load analysis

18 Liquid Pressure in Cargo Tanks

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20 Local Strength of Inner Hull - Plates

21 Local Strength of Inner Hull - Stiffeners

22 Inner Hull - Allowable Stresses stat + dyn all [N/mm 2 ] 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/mm 2 for GTT NO96 and 175 N/mm 2 for GTT Mark III Allowable stresses given for GTT NO96 and GTT Mark III:

23 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 Strength Analysis of Membrane LNG Carrier

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

25 NAUTICUS-Hull MODELLING Concept Model

26 Cargo Hold Analysis - FEM FEM Results FEM Model Concept Model

27 Cargo Hold Analysis - Load Cases

28 Scantling draught: T Minimum draught with one C/tank full: T A 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 - Strength Analysis

29 Cargo Hold Analysis - FEM Results

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

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

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

33 High shear stress FEM Results - Girders

34 Edit in Veiw > Header and footer Slide 33 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

35 MOSS Type Containment System

36 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 1970ies Keywords: Leak-before-Failure, fracture mechanics, direct load and strength analysis, buckling and fatigue

37 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 1970ies (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 Design for Spherical Cargo Tanks

38 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 Design for Spherical Cargo Tanks, cont.

39 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 Design for Spherical Cargo Tanks

40 Structural Analysis Spherical Tank LNG Carrier

41 Spherical Tank - frame and girder models 123 4 FEM MODEL REQUIRED FOR CLASS APPROVAL Include hull, skirt, cargo tanks and covers Interaction forces in tank shell and covers Tank foundation flexibility Coarse overall stress flow

42 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

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

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

45 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

46 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 Structural Analysis -2

47 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.

48 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 Structural Analysis - 4

49 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. Structural Analysis - 5

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

51 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)

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

53 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

54 Loading Conditions Normal ballast condition (LC06)

55 Loading Conditions Departure - full load (LC11)

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

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

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

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

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

61 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) Wave Load Analysis

62 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 Wave Load Analysis

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

64 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 Stochastic Fatigue Analysis Full stochastic analysis

65 Midship section Tank Weather cover Pipe Tower

66 Midship section Cylindrical skirt Supporting girder

67 Steady - state temperature distribution in tanks Temperatures: LNG = - 162 o C Below tank inside skirt = 20 o C Outside tank skirt = 28 o C

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

69 Steady - state temperature distribution in tanks

70 The Equator Profile

71 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.

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

73 Selection of Materials - Temperature Analysis 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 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:

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

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

76 Selection of Materials - Temperature Analysis DNV Rules:

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

78 3DGM - Inner Structure

79 Double Bottom Foundation deck

80 Double side Passage way

81 Transverse bulkhead Single skin trv. bhd Upper stool

82 3D Global Model

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

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

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

86 Fatigue Strength

87 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 Hull Structure Fatigue

88 Fatigue Crack

89

90

91 Fatigue Requirements

92 Fatigue - Higher Tensile Steel

93 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

94 Hull Structure Fatigue damages are caused by dynamic loading Fatigue

95 Fatigue and Corrosion Thinning Effect Fatigue Level Fully protected 51015202530 Years Bare Steel, Corroding Unacceptable Damage Zone 5 yr. Paint Spec. 10 yr. Paint Spec. 15 yr. Paint Spec. World Wide Trading

96 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 Operation Route Reduction Factor, f e f e = 1,0 for North Atlantic operation = 0,8 for world-wide operation

97 Trading Route Fatigue Level 51015202530 Years Unacceptable Damage Zone Years of Operation Fully protected, World Wide Fully Protected, North Atlantic 10 yr. Paint Spec. Fully protected, PG- Japan Unacceptable Damage Zone

98 Part Time at Sea, Assumptions

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

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

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

102 Hopper Knuckle High Stress Concentration

103 Critical Areas - Lower Hopper Knuckle Hull Structure Inner Bottom

104 Fatigue Calculations L/Gir. Local FEM

105 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

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

107 PLUS - Local FEM models Local models in D/B

108 PLUS - Local FEM models Standard lugNew lugNo lug

109 Hull Structure Fatigue: PLUS-2 High stress concentration

110 Fatigue: PLUS-2 Hull Structure Deck Opening

111 Critical Areas against fatigue Hull Structure 3 2 1 4 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 5

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

113 Critical Areas – Tank boundary Weld joint in tank boundary

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

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

116 Fatigue Transverse bulkhead Longitudinal bulkhead Critical Areas - TBHD & LBHD

117 Critical Areas – Lower Hopper Corner Important: alignment & grinding

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

119 Yield & Fatigue Shear Critical Areas – Vertical girder in TBHD

120 Critical Areas – deck opening Opening edge grinding

121 Critical Areas - Trans. BHD Fatigue Weld toe grinding

122 Material grade of hull structures Hull Structure D B B B D E E E E A

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

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


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