CLASS GUIDELINE
DNVGL-CG-0136
Edition October 2015
Liquefied gas carriers with membrane tanks
The electronic pdf version of this document, available free of charge
from http://www.dnvgl.com, is the officially binding version.
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FOREWORD
DNV GL class guidelines contain methods, technical requirements, principles and acceptance
criteria related to classed objects as referred to from the rules.
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Changes - current
CURRENT CHANGES
This is a new document.
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 3
Current changes......................................................................................................... 3
Section 1 General....................................................................................................... 7
1 Introduction............................................................................................7
1.1 Objective.............................................................................................7
1.2 US coast guard requirement..................................................................8
Section 2 Material Selection....................................................................................... 9
1 Temperature calculation......................................................................... 9
1.1 General............................................................................................... 9
1.2 Ambient temperature............................................................................9
1.3 Calculation of the steel significant temperatures.......................................9
1.4 Connecting members............................................................................ 9
1.5 At supports......................................................................................... 9
2 Hull structures...................................................................................... 10
3 Corrosion additions...............................................................................10
Section 3 Design loads............................................................................................. 12
1 Introduction..........................................................................................12
1.1 General............................................................................................. 12
1.2 Loading conditions..............................................................................12
2 Internal pressure in cargo tanks.......................................................... 12
2.1 Rule load...........................................................................................12
2.2 Direct wave load analysis.................................................................... 12
3 Sloshing and liquid impact................................................................... 13
3.1 Sloshing............................................................................................ 13
3.2 Liquid impact..................................................................................... 13
3.3 Partial filling...................................................................................... 13
3.4 Application.........................................................................................13
4 Stern slamming.................................................................................... 13
Section 4 Ultimate strength assessment.................................................................. 14
1 General................................................................................................. 14
2 Inner hull stress limits......................................................................... 14
2.1 Allowable stress of inner hull............................................................... 14
2.2 Vessels with double corrugated stainless steel membrane........................ 14
2.3 Vessels with plane invar membrane(s).................................................. 14
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 4
Contents
CONTENTS
1 General................................................................................................. 15
2 Modelling.............................................................................................. 15
2.1 General............................................................................................. 15
2.2 Model extent......................................................................................15
3 Design application of loading conditions and load cases....................... 17
3.1 General............................................................................................. 17
3.2 Loading conditions..............................................................................18
3.3 Load cases........................................................................................ 18
4 Acceptance criteria............................................................................... 25
4.1 Yielding............................................................................................. 25
4.2 Buckling............................................................................................ 25
Section 6 Local structural strength assessment....................................................... 26
1 General................................................................................................. 26
2 Locations to be checked....................................................................... 26
2.1 General............................................................................................. 26
2.2 Double hull longitudinals subjected to large deformations........................ 26
3 Load cases............................................................................................ 26
4 Acceptance criteria............................................................................... 26
Section 7 Fatigue Assessment.................................................................................. 27
1 General................................................................................................. 27
2 Locations to be checked....................................................................... 27
3 Loads.................................................................................................... 30
3.1 Loading conditions..............................................................................30
3.2 Dynamic load cases............................................................................ 30
4 Fatigue evaluation................................................................................ 30
4.1 General............................................................................................. 30
4.2 Parameters to be used........................................................................31
4.3 Fatigue due to sloshing load................................................................ 31
Section 8 Welding.................................................................................................... 32
1 Weld improvement................................................................................32
1.1 General............................................................................................. 32
1.2 Weld toe grinding...............................................................................32
1.3 Weld profiling.....................................................................................32
2 Recommended weld details for inner hull.............................................33
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 5
Contents
Section 5 Cargo hold strength assessment...............................................................15
1 References............................................................................................ 34
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 6
Contents
Section 9 References................................................................................................ 34
Section 1
SECTION 1 GENERAL
1 Introduction
1.1 Objective
This Class Guideline describes procedures for strength assessment of liquefied gas carriers with membrane
tanks in compliance with the rules RU SHIP Pt.5 Ch.7.
In case of discrepancy between the rule and 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 as
given in RU SHIP Pt.3 of the rules. In addition, the criteria for classification notation Tanker for Liquefied
Gas as given in RU SHIP Pt.5 Ch.7 apply for the inner hull acting as support for the cargo tank insulation and
the membranes. An overview of the ultimate limit state (ULS) and fatigue limit state (FLS) assessment is
shown in Figure 1.
Figure 1 Flow chart of strength assessment
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 7
Requirements given by the USCG, ref. Sec.9 [1].2, need to be considered for LNG vessels trading to US ports
or operating under US flag.
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 8
Section 1
1.2 US coast guard requirement
Section 2
SECTION 2 MATERIAL SELECTION
1 Temperature calculation
1.1 General
Presence of cold cargo will cause lower temperatures for parts of the hull steel structures. Therefore the steel
temperatures for all hull structures, and the parts of the containment structure welded to the hull, have to
be calculated. The calculation is normally to be based on empty ballast tanks since this assumption gives the
lowest steel temperature.
1.2 Ambient temperature
Ambient temperature for material selection is according to RU SHIP Pt.5 Ch.7 Sec.4 [5.1.1] and Table 1
below.
For ships intended for trading in cold areas, other ambient temperature may be required by the class, port
authorities or flag states.
Table 1 Ambient temperature for temperature calculation
Still sea water
temperature, °C
Air temperature, °C
Wind speed,
knots
RU SHIP Pt.5 Ch.7
Sec.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
Regulations
Applicable areas
1.3 Calculation of the steel significant temperatures
The 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 full
may be used for the temperature calculations.
1.4 Connecting members
For members connecting inner and outer hulls, the mean temperature may be taken for determining the steel
grade.
1.5 At supports
At supports (e.g. at upper and lower pump tower supports, anchoring bars and anchoring pillar) where cold
spots will occur, a local thermal analysis may be deemed necessary in order to establish the steel significant
temperature.
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 9
The material of the hull structure is to be in accordance with RU SHIP Pt.3 Ch.3 Sec.1, unless the calculated
temperature 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 as
follows:
— 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 additions
Corrosion additions of hull structures are given in the rules RU SHIP Pt.3 Ch.3 Sec.3. Corrosion additions of
membrane 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
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 10
Section 2
2 Hull structures
Section 2
Figure 2 Corrosion additions in way of transverse cofferdam bulkhead
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 11
Section 3
SECTION 3 DESIGN LOADS
1 Introduction
1.1 General
Design loads for local strength assessment of inner hull structures such as plates and stiffeners supporting
the 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 conditions
Loading 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 be
used.
2 Internal pressure in cargo tanks
2.1 Rule load
Internal cargo tank pressures, based on a 10
RU SHIP Pt.5 Ch.7 Sec.4 [3.3.2].
-8
probability level for the North Atlantic, are given in the rules
The acceleration, aβ , is calculated by combining the three component accelerations ax, ay and az values
according to an ellipsoid surface.
For different directions of aβ in the ellipsoid, the pressure at different corner locations in the cargo tank is
calculated. Between corner points the pressure may be found by linear interpolation.
2.2 Direct wave load analysis
The rule values of ax, ay and az may be replaced by accelerations calculated from a direct wave load
analysis. The procedure for direct wave load analysis is given in DNVGL-CG-0130 Wave load analysis.
-8
These accelerations shall be on a 10 probability level for the North Atlantic and calculated for the loading
conditions in the loading manual that give the highest accelerations. As a guidance, the loading conditions
with only one tank full, while other tanks are empty are normally considered to produce the largest
transverse accelerations.
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 12
3.1 Sloshing
As a minimum, rule inertia sloshing load given in the rules RU SHIP Pt.3 Ch.10 Sec.4 need to be considered
for the hull structure.
3.2 Liquid impact
Sloshing analysis is normally required for vessels with unconventional tank size, typically vessels larger than
155 000 cubic metres, or tank design with no or limited experience. The following locations in no.2 cargo
tank should be checked as representative locations for sloshing and liquid impact.
—
—
—
—
—
Lower
Lower
Upper
Upper
Upper
chamfer
chamfer
chamfer
chamfer
chamfer
connection
connection
connection
connection
connection
at middle of a cargo tank due to roll dominant motion.
at transverse bulkhead due to pitch and surge dominant motion.
at middle of a cargo tank due to roll dominant motion.
at transverse bulkhead due to pitch and surge dominant motion.
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 filling
Partially filled membrane tanks may be vulnerable with respect to sloshing and liquid impact loads. These
tanks may therefore be subject to filling level restrictions in order to avoid seagoing operation at the most
critical filling levels.
3.4 Application
Results of the analysis shall be applied to the other cargo tanks including no.1 cargo tank, unless sloshing
analysis 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 slamming
For ships where the lower part of the shell has large flare angle, e.g. twin skeg vessels, the impact pressure
on the stern shall be considered in accordance with the rule slamming requirements, RU SHIP Pt.3 Ch.10
Sec.3. The impact pressures may be obtained by model tests or direct calculations, if applicable.
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
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Section 3
3 Sloshing and liquid impact
1 General
The 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 hull
The criteria of inner hull stress as given below and others, if relevant, shall be confirmed by the designer of
the cargo containment system for each project.
2.2 Vessels with double corrugated stainless steel membrane
With the primary barrier of stainless steel being double corrugated, the in plane stiffness is very low. Thus
this type of membrane is less sensitive to hull deformations than plane membranes. The following design
limitation is applicable with respect to acceptable longitudinal elongation of inner hull structure due to hull
girder bending ref. Sec.9 [1].4.
σst + σdyn + σloc ≤ 185
where
2
σst
= hull girder bending stress, in N/mm , due to maximum still water bending moment calculated for
the most severe loaded condition or ballast seagoing condition, based on gross scantling
σdyn = hull-8 girder bending stress, in N/mm2, due to maximum wave bending moment corresponding to
10 probability in North Atlantic, based on gross scantling
σloc = Maximum bending stress, in N/mm2, of inner hull due to double hull deflection when considering
alternate 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 plane
plate rigidly connected to the cofferdam bulkhead structure. In order to keep the total stress level in the
membrane at an acceptable level, the cargo containment system designer has given restrictions to be
complied 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/mm
2
as defined in [2.2].
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 14
Section 4
SECTION 4 ULTIMATE STRENGTH ASSESSMENT
1 General
The following describes acceptable methods for the strength analysis, with focus on finite element models of
the cargo hold area. The analysis shall confirm that the stress levels are acceptable when the structures are
loaded in accordance with the described design conditions.
2 Modelling
2.1 General
Modelling of hull and tank structure shall unless defined otherwise follow RU SHIP Pt.3 Ch.7 of the rules and
the guidance in DNVGL-CG-0127 Finite element analysis. This covers, but is not limited to, the following
items:
—
—
—
—
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 are
directly applied to the inner hull.
2.2 Model extent
Longitudinal 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 cargo
hold 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.
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 15
Section 5
SECTION 5 CARGO HOLD STRENGTH ASSESSMENT
Section 5
Figure 1 Example of a cargo hold model (Midship)
Figure 2 Example of a cargo hold model (Aft)
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 16
Section 5
Figure 3 Example of a cargo hold model (Forward)
3 Design application of loading conditions and load cases
3.1 General
The 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 0.5
3
2
t/m ) and the design overpressure (P0) shall not be less than 25 kN/m and shall be applied for all loaded
cargo tanks.
The cargo density used in the FE model should be corrected for the difference between the volume inside
tank and the volume of hold space at inner hull boundary so that static pressure on inner bottom is correct in
the 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
VC
VH
3
= design cargo density of LNG, in t/m
3
= volume of cargo tank measured in way of primary barrier, in m
3
= volume of cargo hold measured in way of moulded dimensions of inner hull, in m .
This is also applicable for fatigue analysis.
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 17
The basic loading conditions as described in Sec.3 [1.2] shall normally be considered. The loading patterns
given in Table 1 are based on these conditions, and are regarded as the minimum conditions. Other
conditions may be considered when relevant.
Based on operational limitations, e.g. if surrounding ballast tanks in way of an empty cargo tank are always
filled, the standard load cases shown in Table 1 may be modified.
3.3 Load cases
Load cases that can be considered in the cargo hold analysis are shown in Table 1 for mid hold, Table 2 for
aftmost hold and Table 3 for foremost hold.
Table 1 Typical design load combinations for midship cargo region for FE analysis of membrane
LNG tanker
No.
Loading pattern
Aft Mid Fore
Draught
% of perm.
SWBM
% of perm.
SWSF
Dynamic load case
Seagoing conditions
100% (hog.)
L1
TSC
HSM-2, FSM-2
100%
4)
Max SFLC
HSM-2, FSM-2
≤ 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,
8)
OST-2S, OSA-1S, OSA-2S
100% Max
5)
SFLC
HSM-1, FSM-1
100% Max
3)
SFLC
HSM-1, FSM-1
100%
4)
Max SFLC
HSM-1, FSM-1
≤ 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 8)
100% Max
5)
SFLC
HSM-2, FSM-2
1)
0% (sag.)
100% (sag.)
L2
100% Max
3)
SFLC
TA
0% (hog.)
Harbour conditions
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 18
Section 5
3.2 Loading conditions
TA
L3
1)
2)
TSC
L4
% of perm.
SWBM
Draught
1)
100% (sag.)
100% (hog.)
% of perm.
SWSF
Section 5
Loading pattern
Aft Mid Fore
No.
Dynamic load case
100%
6)
Max SFLC
N/A
100%
7)
Max SFLC
N/A
100%
6)
Max SFLC
N/A
100%
7)
Max SFLC
N/A
Maximum draft with one cargo tank empty may be used instead of scantling draft TSC, if this is stated as an
operational condition in the loading manual.
2)
Draught not to be taken greater than minimum of 2 + 0.02 L and the minimum ballast draught
3)
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 sea
and 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.
9)
Ballast is assumed in way of empty hold
Where:
TA
TSC
= Minimum relevant seagoing draught in m, may be taken as 0.35 D if not known.
= Scantling draught in m.
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 19
No.
Loading pattern
Aft Mid Fore
Draught
% of perm.
SWBM
% of perm.
SWSF
Dynamic load case
Seagoing conditions
L1
TSC
100% (sag.)
100%
7)
(hog.)
L2
TSC
100%
Max SFLC
HSM-1, FSM-1
100% Max
SFLC
HSM-2, FSM-2
≤ 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,
5)
OST-2S, OSA-1S, OSA-2S
100% Max
SFLC
HSM-1, FSM-1
100% Max
SFLC
HSM-1, FSM-1
≤ 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,
5)
OST-2S, OSA-1S, OSA-2S
100% Max
SFLC
HSM-2, FSM-2
1)
0% (sag.)
0% (sag.)
L3
TA
50%
7)
(hog.)
Harbour conditions
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 20
Section 5
Table 2 Typical design load combinations for aft hold cargo region FE analysis of membrane LNG
tanker
L4
L5
7)
L6
7)
Loading pattern
Aft Mid Fore
Draught
TSC
TA
2)
TSC
% of perm.
SWBM
100% (sag.)
50% (hog.)
100% (hog.)
% of perm.
SWSF
Dynamic load case
100%
3)
Max SFLC
N/A
100%
3)
Max SFLC
N/A
100%
4)
Max SFLC
N/A
100%
3)
Max SFLC
N/A
100%
4)
Max SFLC
N/A
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Section 5
No.
Page 21
Draught
% of perm.
SWBM
% of perm.
SWSF
Section 5
Loading pattern
Aft Mid Fore
No.
Dynamic load case
1)
Maximum draft with one cargo tank empty may be used instead of scantling draft TSC, if this is stated as an
operational limitation in the loading manual.
2)
Draught not to be taken greater than minimum of 2 + 0.02 L and the minimum ballast draught
3)
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 sea
and oblique sea dynamic load cases calculated for P may be applied only to the model and the dynamic load cases
for S may be omitted provided the results (maximum stress and buckling) are mirrored.
6)
Ballast is assumed in way of empty hold
7)
Tanks in Engine room to be 100% full.
where:
TA
TSC
= Minimum relevant seagoing draught in m, may be taken as 0.35 D if not known.
= Scantling draught in m.
Table 3 Typical design load combinations for forward cargo region for FE analysis of membrane
LNG tanker
No.
Loading pattern
Aft Mid Fore
Draught
% of perm.
SWBM
% of perm.
SWSF
Dynamic load case
Seagoing conditions
L1
TSC
100%
Max SFLC
HSM-1, FSM-1
≤ 100%
HSA-1
100% (sag.)
Class guideline — DNVGL-CG-0136. Edition October 2015
Liquefied gas carriers with membrane tanks
DNV GL AS
Page 22
L2
L3
7)
Loading pattern
Aft Mid Fore
Draught
TSC
TSC
1)
1)
% of perm.
SWBM
100% (sag.)
% of perm.
SWSF
Dynamic load case
100%
Max SFLC
HSM-1, FSM-1
100% Max
SFLC
HSM-2, FSM-2
≤ 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,
5)
OST-2S, OSA-1S, OSA-2S
100% Max
SFLC
HSM-1, FSM-1
≤ 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,
5)
OST-2S, OSA-1S, OSA-2S
100% Max
SFLC
HSM-2, FSM-2
100% (hog.)
100% (sag.)
L4
Section 5
No.
TA
100% (hog.)
Harbour conditions
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L5
L6
L7
7)
Loading pattern
Aft Mid Fore
Draught
TSC
TA
100% (sag.)
2)
TSC
% of perm.
SWBM
1)
100% (sag.)
100% (hog.)
% of perm.
SWSF
Dynamic load case
100%
4)
Max SFLC
N/A
100%
3)
Max SFLC
N/A
100%
4)
Max SFLC
N/A
100%
3)
Max SFLC
N/A
100%
4)
Max SFLC
N/A
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Liquefied gas carriers with membrane tanks
DNV GL AS
Section 5
No.
Page 24
Draught
% of perm.
SWBM
% of perm.
SWSF
Section 5
Loading pattern
Aft Mid Fore
No.
Dynamic load case
1)
Maximum draft with one cargo tank empty may be used instead of scantling draft TSC, if this is stated as an
operational limitation in the loading manual.
2)
Draught not to be taken greater than minimum of 2 + 0.02 L and the minimum ballast draught
3)
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 sea
and 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)
Ballast is assumed in way of empty hold
7)
All Tanks forward cargo tank No.1 to be 100% full.
where:
TA
TSC
= Minimum relevant seagoing draught in m, may be taken as 0.35 D if not known.
= Scantling draught in m.
4 Acceptance criteria
4.1 Yielding
Acceptance criteria for yielding is given in the rules RU SHIP Pt.3 Ch.7 Sec.3 [4.2].
4.2 Buckling
Acceptance criteria for buckling is given in the rules RU SHIP Pt.3 Ch.8 Sec.1 [3.3] and method description is
given in DNVGL-CG-128 Buckling analysis.
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1 General
Local structural analyses with fine mesh finite element models are to be carried out in accordance with the
rules RU SHIP Pt.3 Ch.7 Sec.4 and the DNVGL-CG-0127 Finite element analysis.
2 Locations to be checked
2.1 General
The areas to be considered are according to the rules RU SHIP Pt.5 Ch.7 Sec.23 [3.2.1]. Based on screening
results from the cargo hold analysis the most critical location can be selected. The screening method is
according to DNVGL-CG-0127 Finite element analysis.
2.2 Double hull longitudinals subjected to large deformations
Relative 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 for
both sides from the area under investigation
— the width of the model shall be at least 2 + 2 longitudinal stiffener spacing.
3 Load cases
Fine 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 criteria
Acceptance criteria for stress results from local structure analysis are given in the rules RU SHIP Pt.3 Ch.7
Sec.4 [4.2].
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DNV GL AS
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Section 6
SECTION 6 LOCAL STRUCTURAL STRENGTH ASSESSMENT
1 General
The fatigue assessment is limited to selected steel structures in the cargo area, excluding the cargo
containment system and its components.
Unless otherwise described, details of the fatigue strength assessment are given in the rules RU SHIP Pt.3
Ch.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. Details
are given in RU SHIP Pt.6 Ch.1 Sec.7.
2 Locations to be checked
The fatigue strength calculations shall be carried out for the locations given in the rules RU SHIP Pt.5 Ch.7
Sec.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 upper
hopper knuckle
connections forming
boundary of inner skin
amidships
Lower Hopper Knuckle
Upper hopper Knuckle
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DNV GL AS
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Section 7
SECTION 7 FATIGUE ASSESSMENT
F.E model
Hot spots
Section 7
Location
Inner bottom
connection to
transverse cofferdam
bulkhead
Double side stringer
connection to
transverse cofferdam
bulkhead
Refer to above location of Inner bottom
connection to transverse cofferdam bulkhead
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F.E model
Hot spots
Section 7
Location
Figure to be added
Liquid dome opening
and coaming
connection to deck, if
applicable
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DNV GL AS
Page 29
F.E model
Hot spots
Section 7
Location
Termination of aft
end of no.1 inner
longitudinal bulkhead,
if applicable
3 Loads
3.1 Loading conditions
Loading conditions described in the rule RU SHIP Pt.5 Ch.7 Sec.23 [4.2.3] shall be considered.
3.2 Dynamic load cases
The dynamic load cases to be considered for fatigue assessment are according to the rules RU SHIP Pt.3 Ch.4
Sec.2 [3].
4 Fatigue evaluation
4.1 General
Fatigue strength, unless otherwise described below, is to be evaluated according to the rules RU SHIP Pt.5
Ch.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
North Atlantic
1.0
Inner hull structures. Hot spots where cracks don’t propagate directly
through 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
Inner hull structures. Hot spots where cracks can propagate through the
inner hull plates, e.g. plates boundary between cargo and ballast tanks.
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It is recommended to avoid welded permanent backing in fatigue sensitive areas. If a welded steel backing
strip is applied as shown in Figure 1, the location of welding spots to be kept well away from areas with high
stresses (away from the areas with supporting structure below inner bottom).
Figure 1 Pump tower base support
4.2 Parameters to be used
Standard 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 loading
manual can be used instead when those are available.
4.3 Fatigue due to sloshing load
Sloshing pressures may normally be neglected in fatigue strength assessment of hull structures except for
pump 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 7
For pump tower base supports, a crack will propagate in the thickness direction inside the containment
system. Such fatigue cracks may lead to rupture of both the primary and secondary barrier. Cracks on the
secondary barrier cannot be detected before failure is effective, leading to no redundancy of the system. The
hotspots shall therefore satisfy a fatigue damage of 0.1 in North Atlantic operation, i.e. Cw = 0.1.
Section 8
SECTION 8 WELDING
1 Weld improvement
1.1 General
Improvement 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 grinding
Weld 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 weld
toe should be removed. It should be noted that the final grinding direction should go across the weld in order
to avoid additional notch due to the grinding.
Figure 1 Example of weld profiling at lower hopper knuckle
1.3 Weld profiling
Weld 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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Inner 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 weld
treatment.
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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Section 8
2 Recommended weld details for inner hull
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 of Federal
Register), Part 154.170/172/176
.3
Lindemark, T. et al: CSA-2 Analysis of a 216k LNG Membrane Carrier, The Royal Institution of Naval Architects
(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, GTT document N0-DG-33
Rev. M, May 2005
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Liquefied gas carriers with membrane tanks
DNV GL AS
Page 34
Section 9
SECTION 9 REFERENCES
DNV GL
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