1. 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.

2. 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

3. 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

4. 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

5. 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

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

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

8. 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

9. Selection of Steel Grades
DNV Rules:

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

11. 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

12. 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.

13. Local Strength of Inner Hull

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

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

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

17. Liquid Pressure in Cargo Tanks

18. Liquid Pressure in Cargo Tanks

19. Local Strength of Inner Hull - Plates

20. Local Strength of Inner Hull - Stiffeners

21. 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

22. 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

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

24. NAUTICUS-Hull MODELLING
Concept Model

25. Cargo Hold Analysis - FEM
FEM Model
Concept Model
FEM Results

26. Cargo Hold Analysis - Load Cases

27. 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

28. Cargo Hold Analysis - FEM Results

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

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

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

32. FEM Results - Girders
High shear stress

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
Edit in Veiw > Header and footer
Edit in Veiw > Header and footer

34. MOSS Type Containment System

35. 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

36. 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

37. 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

38. Design for Spherical Cargo Tanks
NEW DEVELOPMENTS
The structural reliability and the buckling criteria were in the period 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

39. Structural Analysis Spherical Tank LNG Carrier1

40. Spherical Tank - frame and girder models4 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

41. 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

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

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

44. 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

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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

51. 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

52. 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 full 1

53. Loading Conditions
Normal ballast condition (LC06) 2

54. Loading Conditions
Departure - full load (LC11) 3

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

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

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

58. Loading Conditions
Departure - tanks no full (LC18) 7

59. 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

60. 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

61. 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: D linear diffraction
theory, zero forward
speed
FASTSEA: 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

62. 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

63. 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

64. Midship section
Tank
Weather cover
Pipe Tower

65. Midship section
Cylindrical skirt
Supporting girder

66. Steady - state temperature distribution in tanks
Temperatures:
LNG = oC
Below tank
inside skirt = oC
Outside tank skirt = oC 18

67. Steady - state temperature distribution in tanks
Equator
Temperatures:
Sea = C
Air = C
LNG = C 19

68. Steady - state temperature distribution in tanks18

69. The Equator Profile

70. 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.

71. 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

72. 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

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

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

75. Selection of Materials - Temperature Analysis
DNV Rules:

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

77. 3DGM - Inner Structure

78. Double Bottom
Foundation deck

79. Double side
Passage way

80. Transverse bulkhead
Upper stool
Single skin trv. bhd

81. 3D Global Model

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

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

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

85. Fatigue Strength

86. 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

87. Fatigue Crack

88. Fatigue Crack

89. Fatigue Crack

90. Fatigue Requirements

91. Fatigue - Higher Tensile Steel

92. 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

93. Hull Structure
Fatigue
Fatigue damages are caused by dynamic loading

94. 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

95. 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

96. 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

97. Part Time at Sea, Assumptions

98. 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

99. 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

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

101. Hopper Knuckle
High Stress Concentration

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

103. Fatigue Calculations L/Gir. Local FEM

104. 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

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

106. PLUS - Local FEM models
Local models in D/B

107. PLUS - Local FEM models
Standard lug
New lug
No lug

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

109. Hull Structure
Fatigue: PLUS-2
Deck Opening

110. 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

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

112. Critical Areas – Tank boundary
Weld joint in tank boundary

113. 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

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

115. Critical Areas - TBHD & LBHD
Fatigue
Longitudinal bulkhead
Transverse bulkhead

116. Critical Areas – Lower Hopper Corner
Important: alignment & grinding

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

118. Critical Areas – Vertical girder in TBHD
Shear
Yield & Fatigue

119. Critical Areas – deck opening
Opening edge grinding

120. Critical Areas - Trans. BHD
Fatigue
Weld toe grinding

121. Material grade of hull structuresB B B D

122. 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

123. 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