CLASS GUIDELINE
DNVGL-CG-0135
Edition February 2016
Liquefied gas carriers with independent
cylindrical tanks of type C
The electronic pdf version of this document, available free of charge
from http://www.dnvgl.com, is the officially binding version.
DNV GL AS
FOREWORD
DNV GL class guidelines contain methods, technical requirements, principles and acceptance
criteria related to classed objects as referred to from the rules.
©
DNV GL AS February 2016
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employees, agents and any other acting on behalf of DNV GL.
Changes - current
CHANGES – CURRENT
This is a new document.
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Changes – current.................................................................................................. 3
Section 1 General.................................................................................................... 5
1 Introduction.........................................................................................5
2 Ship application................................................................................... 6
3 Design basis........................................................................................ 8
4 Design loads........................................................................................ 9
Section 2 Ultimate strength assessment of tank...................................................14
1 General.............................................................................................. 14
2 Scantling due to internal pressure.....................................................15
3 Scantling due to external pressure.................................................... 16
4 Scantling of swash bulkhead............................................................. 16
5 Reinforcement of openings and attachments.....................................17
6 Evaluation of saddles and supporting structure................................. 17
7 Stress categories and acceptance criteria..........................................17
Section 3 Cargo hold strength assessment........................................................... 20
1 General.............................................................................................. 20
2 Modelling........................................................................................... 20
3 Design application of loading conditions and load cases.................... 22
4 Acceptance criteria for hull structure................................................ 26
Section 4 Local structural strength assessment....................................................27
1 General.............................................................................................. 27
2 Locations to be checked.................................................................... 27
3 Load cases......................................................................................... 27
4 Acceptance criteria............................................................................ 27
Section 5 Special considerations for other tank designs....................................... 28
1 Introduction.......................................................................................28
2 Bi-lobe tanks......................................................................................28
3 Deck Cargo Tanks.............................................................................. 28
Section 6 References.............................................................................................30
1 Reference list.....................................................................................30
Changes – historic................................................................................................ 31
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Contents
CONTENTS
Section 1
SECTION 1 GENERAL
1 Introduction
1.1 Application
This class guideline gives scope, methods and design criteria required for strength analysis of the tank
system for gas carriers with independent tanks of type C. This class guideline is also applicable for tank
systems with independent tanks of type C on other ship types than gas carriers.
Structural analysis carried out in accordance with the procedures outlined in this class guideline will normally
be accepted for plan approval.
Attention should be given to additional requirements by flag or port authorities, e.g. ref. Sec.6 /2/ and /3/.
1.2 Definition of independent tank type C
A type C independent tank is defined as follows:
— simple geometric shape of tank system, carrying loads mainly as membrane stresses
— mainly static pressure - fatigue and crack propagation is in general considered not to be critical
— strength of tank can be documented by simple formulas/methods.
1.3 Objective
This class guideline provides additional information not covered in RU SHIP Pt.5 Ch.7 Sec.22, Design with
cylindrical tanks of type C. Design and assessment procedures are given for hull structures and cargo tanks
of cylindrical tanks of type C in accordance with the rules. The objective is to give a general description of
how to carry out relevant calculations and analyses. In case of discrepancy between the rules and this class
guideline, the rule prevails.
This class guideline covers the main structural aspects for the ship hull and typical type C tanks consisting
of a cylindrical midbody with hemispherical, elliptical or torispherical end caps supported by two saddles. In
addition some considerations for bi-lobe tanks are included. Associated gas system related aspects are not
covered.
1.4 Scope of documentation
Structure-related documents required for approval of independent tank types C are given in RU SHIP Pt.5
Ch.7 Sec.1 [4.1] Table 6.
The submitted documentation should among others contain the following descriptions:
—
—
—
—
—
—
—
Maximum allowable relief valve setting (MARVS)
Maximum vacuum pressure
Maximum gas pressure
Maximum external pressure
List of products carried
Product density and temperature
Tank locations.
1.5 Definitions of symbols and abbreviations
For symbols not defined in this class guideline, refer to RU SHIP Pt.3 Ch.1 Sec.4 and RU SHIP Pt.5 Ch.7 Sec.1
[3.2].
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= combined dynamic horizontal longitudinal acceleration according to RU SHIP Pt.5 Ch.7 Sec.4
[6.1.2]
ay
= combined dynamic horizontal transverse acceleration according to RU SHIP Pt.5 Ch.7 Sec.4
[6.1.2]
az
= combined dynamic vertical acceleration according to RU SHIP Pt.5 Ch.7 Sec.4 [6.1.2]
σm
= design primary membrane stress, in N/mm
LEG
= liquefied ethylene gas
LNG
= liquefied natural gas
LPG
= liquefied petroleum gas
MARVS
= maximum allowable relieve valve setting
NMA
= Norwegian Maritime Authority.
2
2 Ship application
2.1 General
Ships with independent tank type C are applicable for carrying a wide range of liquefied gases, all having
particular properties and design requirements related to general safety. Types of cargoes with their special
requirements are given in RU SHIP Pt.5 Ch.7 Sec.19. Gas carriers with type C tanks have typically been
transport of cargoes such as LPG, LNG, ethylene, ammonia, including the more recent use for storage of LNG
Fuel and CO2 transportation.
A selection of some typical type C tank applications is listed below:
2.2 LPG carriers
Typically cargoes like Propane, Butane, Propylene etc. are transported on LPG carriers. These cargoes
have moderate density and design temperature limited to -48°C. Carbon manganese steel, e.g. VL4-4 or
equivalent is normally used as material for the cargo tanks. The ship can be both fully pressurized, semi
pressurized or fully refrigerated. LPG can also carry a number of other cargoes listed in RU SHIP Pt.5 Ch.7
3
3
Sec.19 like VCM (high density) etc. Typical sizes range from 4 000 m to 22 000 m .
2.3 LNG carriers (feeders)
Lately it has been an increased demand for small LNG Carriers for coastal service where the independent
tank type C has shown to be competitive and flexible for the operators. The LNG tanks need to be designed
for the low temperature of LNG of -163°C. Tank material is typically 9% Nickel steel or austenitic steel, with
3
typical ship sizes ranging from 1100 – 3000 m . An example of a typical coastal LNG carrier is shown in
Figure 1.
2.4 Ethylene carriers (LEG)
Ethylene carriers are normally LPG Carriers applying tank material suited for the low temperature of ethylene
of -104°C including ethane of -88°C. Tank material is normally 5% Nickel steel. Typical sizes are the same as
LPG Carriers.
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Section 1
ax
Ammonia is carried on both LPG and LEG vessels. Particular attention should be paid to the selection of
material and heat treatment to prevent possible stress corrosive cracking from the ammonia. Typical sizes
are the in the same range as for LPG/LEG Carriers.
2.6 1G ships carrying special cargo (typically Chlorine)
1G ships are required for special cargoes like Chlorine, Ethylene oxide etc., where special considerations with
regard to location of tanks, ship arrangement, damage stability and special requirements as given in RU SHIP
Pt.5 Ch.7 Sec.19 must be met. Due to the special requirements mentioned above, independent tank types C
are the only containment system which can carry such cargoes.
The strict requirements for such tanks will limit the sizes of 1G ships.
2.7 CO2 carriers
CO2 differs from most other gases in the way that it can only be liquefied by pressurization. Consequently,
tank type C is currently the only realistic containment system for transportation. For CO2 to be liquefied, the
pressure has to be above 5.18 bars.
Most of the particular design requirements for CO2 carriers are system related. Taking into account the
specific design pressures, temperatures and the density of CO2 (heavier than water), no special structural
tank design considerations are necessary beyond what normally are required for traditional type C tank.
Considerations with regard to contamination of CO2 with water or other foreign substances causing corrosion
should however be made.
3
Existing ships have typically sizes below 1500 m , however larger ships may be expected in the future.
2.8 Vessels designed to carry combination of cargoes e.g. CO2 and LPG
Ships intended to carry both normal LPG cargoes and CO2 must comply with all the requirements for both
LPG carriage and CO2. Special considerations with regard to cleaning procedures must be considered to
prevent e.g. contamination of LPG.
Figure 1 Coastal LNG carrier with independent cylindrical tank type C
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Section 1
2.5 Ammonia carriers
3.1 Hull structure
Hull girder strength and local strength of the ship shall comply with the requirements given in RU SHIP Pt.3.
Table 1 gives an overview of strength assessment for hull structure including supporting structures.
Table 1 Overview of strength assessment for hull structures
3.2 Cargo tank
The independent type C cargo tanks are covered by the requirements given in RU SHIP Pt.5 Ch.7 Sec.22 of
the rules.
Table 2 gives an overview of the different load components and conditions to be used as basis for the
strength assessment of the tank structure. Allowable stress and buckling acceptance criteria to be used for
the different structural members are specified.
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Section 1
3 Design basis
Section 1
Table 2 Overview of strength assessment for cargo tanks
4 Design loads
4.1 General
The design loads which shall be considered for strength evaluation of independent cylindrical tanks are in
general given in the rules RU SHIP Pt.5 Ch.7 Sec.22 [1.2] and listed in Table 3 below.
Table 3 Design loads for independent tank type C
Static loads
Dynamic loads
— Cargo weight (static pressure)
— Tank system self weight
— tank shell
— insulation
Other loads
— Vertical, transverse and longitudinal — Stationary temperature distribution
accelerations acting on the system
— Transient temperature distribution
(dynamic pressure)
of initial cool down
— Sloshing loads
— Vibration
— Dynamic interaction forces from
*)
wave loads
— domes and piping
— Internal and external overpressure
*)
— Still water interaction forces
*)
Interaction forces may normally not be relevant, see RU SHIP Pt.5 Ch.7 Sec.22 [1.2.7]
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1)
2)
30° static inclination of tank
Collision force acting on the tank corresponding to half of the mass of the tank and cargo in the forward
1)
direction
Collision force acting on the tank corresponding to a quarter of the mass of the tank and cargo in the aft
1), 2)
direction
Flooding of cargo hold
Tank test condition.
3)
4)
5)
The different load components described in RU SHIP Pt.5 Ch.7 Sec.22 [1.2] and in the following shall be
combined as described in the rules RU SHIP Pt.5 Ch.7 Sec.22 [2.2].
4.2 Cargo weight and tank system self weight
For design purposes the specific density of the intended cargo shall be used. A minimum density is generally
3
to be taken as ρ = 500 kg/m .
The static weight of the cargo tank, including piping, insulation etc. should be included in the assessment. If
the weight of the additional piping etc. is not known, this may conservatively be taken as a small percentage
of the cargo tank weight. In case of external loads see RU SHIP Pt.5 Ch.7 Sec.22 [1.2.4].
The static weight of the cargo is in the calculation to be combined with the dynamic cargo loads as described
in RU SHIP Pt.5 Ch.7 Sec.4 [6.1.1].
4.3 Calculation of maximum cargo tank pressure
The maximum simultaneous combination of the acceleration components is however not considered to act
over a larger area of the tank. Hence, for design purposes of cylindrical shell and tank ends, it is normally
considered sufficient to calculate the accelerations aβ at transverse and longitudinal direction at the angle of
βmax X and βmax Y according to the rules RU SHIP Pt.5 Ch.7 Sec.4 Figure 1. The design pressure Peq in MPa at
any location of the tank can be determined from the following formula:
over the range 0 <
β ≤ βmax
where
ρ
d
= specific density of the cargo, in kg/m
3
= horizontal distance to pressure point, in m
= x longitudinal distance, in m, from the end of the cylindrical part, Figure 2
z
R
βx, βy
βmax
1
2
= y transverse distance, in m, from the tank centre, Figure 2
= vertical distance, in m, from tank centre to pressure calculation point (positive downwards)
= tank radius, in m
= angles of resulting acceleration vector in relative to the vertical plane as defined in RU SHIP Pt.5
Ch.7 Sec.4 Figure 3 and Figure 2 below, longitudinal and transverse direction respective.
= maximum angle as defined in RU SHIP Pt.5 Ch.7 Sec.4 Figure 1
Stricter requirements issued by other authorities may apply (e.g. Norwegian Maritime Authority (NMA)
requirements for fuel tanks, ref. Sec.6 /4/)
Normally covered by item 2) for symmetrical tanks.
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Section 1
In addition to the design loads above, the following load scenarios shall be considered:
= resulting acceleration vector in fraction of gravity (g) for inclination angle
RU SHIP Pt.5 Ch.7 Sec.4 Figure 3.
βx and βy as shown in
Figure 2 Coordinates for calculation of pressure height at a given point in the cylindrical tank
For parts of the structure for which a localized pressure is dimensioning, the maximum pressure calculated
from the ellipsoid (aβ) should be used.
The governing design pressure in the transversal or longitudinal direction is used as input to the minimum
scantling check which is described in Sec.2 of this document.
The design accelerations used for dimensioning of tanks and supports are given in the rules RU SHIP Pt.5
Ch.7 Sec.4 [6.1.2]. These formulas are considered to give conservative design accelerations for traditional
type C gas carriers. As an alternative, the design accelerations may be determined by direct calculations
and used in connection with the acceleration ellipse, but this has not been the general practice for past and
current designs. This is mainly because the dynamic contribution on the scantling is normally significantly
less compared to the static contribution, and an optimization with regard to the dynamic pressure has not
been considered necessary.
If direct calculations are carried out (i.e. a wave load analysis) to determine the accelerations, the following
assumptions should be made for ULS design:
—
—
—
—
Ship speed of 5 knots
A known wave spectrum with short crested waves based on North Atlantic wave environment to be used
All headings considered equally probable and the average load values over all headings shall be applied
-8
Loads shall be taken at probability of exceedance equal to 10 . This corresponds to the most probable
8
largest load the ship will experience during 10 wave encounters in the North Atlantic and is normally
interpreted as being equivalent to a service life of 25 years.
Design accelerations shall be calculated at the tank centre for all tanks. The loading conditions assumed in
the hydrodynamic analysis shall reflect the vessels loading manual with full load and part loading conditions
as relevant.
4.4 Sloshing loads
Evaluation of the sloshing loads on the tank shell, supports and the internal structure (except for swash
bulkheads) may be disregarded provided that the free surface liquid length ℓslh < 0.13 L and/or bslh < 0.56
B, where the parameters L and B are the ship length and breadth, respectively.
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Section 1
aβ
L have been
The rationale for introducing a restriction on the free liquid surface length is to avoid exposure to large
amplitude ship motions at motion periods coinciding with the liquid motion resonance periods in the tank.
The situation is illustrated in Figure 3. The left hand diagram represents the surge motion response amplitude
operator (RAO) for a vessel with length 110 m, and the right hand side diagram the same RAO for a vessel
with length 220 m. The hatched regions represent the sloshing resonance period range for a cylindrical tank
with length 35 m for two tank filling levels. For the small vessel this tank length corresponds to 0.32 L. For
the larger vessel it corresponds to 0.16
L.
If the free liquid length remains within the given limit, it is seen that the amplitude of vessel motion with
periods coinciding with the sloshing resonance periods of the tank is small, and is hence not likely to excite
violent and potentially harmful liquid motions in the tank. Increasing tank/ship length ratios will increase the
amplitude of motions at tank resonance, and hence the severity of the sloshing in the tank.
Figure 3 Surge excitations for a smaller ship (L = 110 m, left) and larger ship (L = 220 m, right),
and typical sloshing resonance period ranges for a tank with length of 35 m
Large sloshing loads are potentially critical for tank shell ends, tank internals (riser pipe from cargo pump,
other internal piping or access ladders) and supports. Studies and information available so far indicate that
sloshing loads for tanks with free liquid surface lengths up to 0.35 L are moderate in terms of the strength of
the tank shell and the tank supports. Violent sloshing is, however, expected to occur, and particular attention
should be given to the lateral impact pressure on the ring stiffeners and the routing of tank internals (piping
etc.).
Based on the above, the following steps should be taken to ensure that tanks are designed to sustain
sloshing loads:
1)
In case of tank length 0.13L < ℓslh ≤ 0.16L
a)
b)
2)
Tank may be designed without internal swash bulkheads
Sloshing evaluation is not required
In case of tank length 0.16L < ℓslh ≤ 0.35L
a)
Designed with swash bulkheads (free liquid length ℓslh ≤ 0.16L)
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Section 1
Based on the current practice and past experience, cargo tanks with lengths exceeding 0.1
arranged with swash bulkheads to reduce the free liquid surface length.
b)
Designed without swash bulkheads (free liquid length ℓslh > 0.16L)
— Sloshing evaluation of tank shell and supports not required
— Sloshing evaluation of ring stiffeners to be considered
— Particular attention should be given to tank internals
3)
In case of tank length ℓslh > 0.35L
a)
Designed with swash bulkheads (free liquid length ℓslh < 0.16L)
— Same as item 2) a)
b)
Designed without swash bulkheads (free liquid length ℓslh > 0.35L)
— An evaluation of potential sloshing loads on tank ends, supports and internals should be carried
out.
Sloshing evaluations may be based on model testing, CFD analyses or similar. Simple assessments of the
tank natural periods can be used as basis for further evaluations. The following formula may be used for
estimating the liquid sloshing natural period in seconds in the longitudinal tank direction:
where
Ccyl
=
ℓ
g
h
R
= cargo tank length, in m
2
= 9.81 m/s
= filling height, in m
= tank radius, in m.
4.5 Vibration analysis
Separate vibration analysis may in special cases be required for type C tank designs, RU SHIP Pt.5 Ch.7
Sec.4 [3.3.5].
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Section 1
— Sloshing evaluation not required, except dimensioning of swash bulkhead. Sloshing loads
specified in the rules RU SHIP Pt.3 Ch.10 Sec.4 [2] should be used for the assessment of swash
4
bulkheads. The sloshing pressure values are assumed to be a return period equivalent to 10
wave encounters.
Section 2
SECTION 2 ULTIMATE STRENGTH ASSESSMENT OF TANK
1 General
1.1 Introduction
In the following, scantling requirements for the tank structure are described in detail. The requirements
include allowable stress and buckling assessment of the tank shell, internal structure and support, and
dimensioning control of opening and attachments, as described in RU SHIP Pt.5 Ch.7 Sec.22 [2] and RU
SHIP Pt.4 Ch.7 Sec.4, as applicable. Some of the formulations given are re-produced from the rules for
convenience, otherwise specific references to the rules are given.
1.2 Division of tank
A typical C-tank consists among others of the following parts:
Figure 1 Elements of typical type C tanks
The procedures and references for the design of the different parts are given in the following.
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2.1 Shell plate thickness calculation based on allowable stress
1)
The plate thickness for the locations defined in Figure 1 is calculated as defined in the rules RU SHIP Pt.4
Ch.7 Sec.4. The thickness formulas are summarized in Table 1 below.
Table 1 Minimum thickness calculation for the different tank sections
No
Location
1
Cylindrical tank body
2
Dished end/ spherical shells
3
Dished end /transition
Tank Part
Formula
— Cylindrical Shells
RU SHIP Pt.4 Ch.7 Sec.4 [3.2.1]
— Hemispherical ends
RU SHIP Pt.4 Ch.7 Sec.4 [3.3.1]
— Spherical shells
— Hemispherical ends
RU SHIP Pt.4 Ch.7 Sec.4 [4.1.1]
— Elliptical ends
— Torispherical ends
The nominal thickness after forming of any shell or head including corrosion allowance shall not be less than:
1)
5 mm for carbon-manganese steels and nickel steels, 3 mm for austenitic steels or 7 mm for aluminium alloys
2)
, (mm)
where
Di
2)
3)
= Inside diameter in mm of shell or inside diameter of cylindrical shirt for dished ends.
It should be noted that the thickness formula for the dished ends is taking into account the increased
stress level at the conjunction with the cylindrical tank body, and no extra considerations in the transition
zone is normally necessary. As an example, the shape factor β according to RU SHIP Pt.4 Ch.7 Sec.4
[4.1.3] Figure 13 for a hemispherical end (Hd/D0 = 0.5) is given as 0.55, which is 1.1 times the
thickness requirement for a sphere.
If the end cap has a complete hemispherical shape, if may be beneficial to divide the end cap into two
parts. The part adjacent to the cylindrical shell body (transition zone) may be dimensioned according to
formula 3 in Table 1, while the outer end cap may be dimensioned according to formula 2.
2.2 Evaluation of global forces in the tank and in way of supports
1)
The basic requirements for evaluation of the global response of the tank and in way of supports are
described in RU SHIP Pt.5 Ch.7 Sec.22 [2.5]. For design, the calculation procedures as given in PD5500
Annex G.3, ref. Sec.6 /5/, may be followed. Alternative equivalent procedures specified in other
recognized standards may be used.
In connection with the evaluation of the global response of the tank structure, the following aspects need
to be considered:
— Longitudinal stresses at midspan and support
— Tangential shear stress at support and dished ends, if applicable
— Circumferential stresses in tank at supports.
2)
Longitudinal stresses in the tank
The calculated longitudinal stress according to the rules RU SHIP Pt.5 Ch.7 Sec.22 [2.3.2] consists of
a combination of the global axial forces created in the tank due to the design pressure (hydrostatic
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Section 2
2 Scantling due to internal pressure
First term
: Sectional axial stress in the shell due to internal overpressure
Second term
: Sectional axial stress in the shell due to static and dynamic weight of cargo
Third term
: Axial stress due bending of tank.
The detailed formulas for the different applications are given in PD5500 Annex G.3.3.2.3 and 3.3.2.4.
The design liquid pressure is generally to be taken as the maximum pressure obtained from the
acceleration ellipse (yz-plane), i.e. 0 ≤ βy ≤ βmax and/or the 30° static heel condition. In addition, the
tank testing load case shall be assessed with relevant acceptance criteria as applicable.
The first term of the formula is generally straight forward. For yield assessment the maximum internal
overpressure should be used. For buckling assessment the internal overpressure should generally be
disregarded.
The second term includes the average sectional axial stress due to static and dynamic cargo loading. The
axial force is found by integrating the pressure over the end cap area.
The third term includes the bending moment created by the static and dynamic loading of the tank, and
may be determined according to PD5500 Annex G.3.3.2.2.
The longitudinal stresses are normally to be checked at tank mid-span and at the saddles.
Acceptance criteria are given in RU SHIP Pt.5 Ch.7 Sec.22 [2.8.4].
3)
Tangential shearing stresses in the tank
The calculation of the tangential shearing stresses
q around the saddle supports may be calculated
K-factors.
according to BS5500 Annex G3.3.2.5, using the relevant
4)
Circumferential stresses at support
The circumferential stresses at support may be calculated according to BS5500 Annex G.3.3.2.6,
depending on the actual design configuration.
3 Scantling due to external pressure
Buckling requirements due to external pressure are given in RU SHIP Pt.5 Ch.7 Sec.22 [2.4.1] for the
cylindrical shells, [2.4.2] for spherical shells and hemispherical ends, [2.4.3] for torispherical or ellipsoidal
ends and [2.4.4] for stiffening rings. The various members exposed to external pressure should be checked
against elastic instability and yielding in the shell, and are similar to those given in other equivalent codes.
Safety factors against yielding and buckling are given in RU SHIP Pt.5 Ch.7 Sec.22 [2.8.2].
4 Scantling of swash bulkhead
Swash bulkhead shall be designed in accordance with RU SHIP Pt.5 Ch.7 Sec.22 [2.6] with consideration of
the sloshing load given in Sec.1 [4.4].
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Section 2
and overpressure) and the global bending of the tank between the supports. The longitudinal stress
formulation consists of the following three terms:
5.1 General
The main requirements with comments are included in the below. Please refer to the rules for applicability
and limitations of the below requirements.
5.2 Reinforcement plates
The strength requirements for the reinforcements around openings and penetrations are in the rules
formulated as an area requirement in the following general format:
Aact ≥ Areq
where the actual and required areas Aact and Areq are defined in RU SHIP Pt.4 Ch.7 Sec.4 [6.3].
No reinforcements are necessary for isolated openings complying with RU SHIP Pt.4 Ch.7 Sec.4 [6.2.1].
5.3 Minimum plate thickness of attachments
Scantling control of attachments such as domes and sumps should in principle follow the same calculation
procedures as for the main shell of the tank, taking into account the actual dimensions. Effects of openings or
penetrations should be included.
5.4 Requirements for manhole covers
The requirements for thickness of manhole covers are given in Pt.4 Ch.7 Sec.4 [5]. Standard dimensioning
according to e.g. ASME standards is accepted. The same applies to flanges and standard fittings.
6 Evaluation of saddles and supporting structure
6.1 General
The connection of the tank to the saddle, the saddle structure and its connection to the hull, and the
supporting hull structure should be evaluated based on the requirements in RU SHIP Pt.5 Ch.7 Sec.22 [2.5].
The supporting structure includes the part of the hull (e.g. double bottom) which supports the tank.
The calculation procedure may be carried out following recognized pressure vessel standards, taking into
account the relevant requirements in RU SHIP Pt.5 Ch.7 Sec.22 [2.5]. Fatigue assessment of the saddle and
relevant parts of the supporting structure should be considered if large dynamic stresses are present.
6.2 Material selection
Material selection for saddles and supporting structures shall be made based on the resulting temperature of
stationary temperature analysis. The temperature for saddle and supporting structures shall be calculated in
accordance with RU SHIP Pt.5 Ch.7 Sec.6 [8].
7 Stress categories and acceptance criteria
7.1 Stress categories
The calculated stresses are in general divided into different stress categories depending on their type of
nature and their criticality for the safety of the system. For independent tank types C consisting mainly of a
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Section 2
5 Reinforcement of openings and attachments
The different stress categories are generally defined as follows:
Stress category
Symbol
Definition
Membrane stress
σt
the component of a normal stress which is uniformly distributed and equal to the average
value of the stress across the thickness of the section under consideration
Primary membrane
stress
σm
membrane stress which is so distributed in the structure that no redistribution of loads
occurs as the result of yielding
Primary local
membrane stress
σL
membrane stress produced by pressure or other mechanical loading and associated with
a primary and/or a discontinuity effect produces excessive distortion in the transfer of
loads to other portions of the structure
Bending stress
σb
the variable stress across the thickness of the section under consideration, after the
subtraction of the membrane stress
Secondary stress
σg
a normal stress or shear stress developed by the constraint of adjacent parts or by selfconstraint of a structure. The basic characteristic of a secondary stress is that it is selflimiting.
7.2 Acceptance criteria
Acceptance criteria for each requirement are according to the rules RU SHIP Pt.5 Ch.7 Sec.22 [2.8] with
consideration of the following practical application:
1)
Acceptance criteria for buckling strength assessment
The increased safety factor for buckling of cylindrical shells is included to reflect that deviations from
a perfect circular cylindrical form will reduce the critical load compared to the formulation. It is found
that practical out-of-roundness deviations reduce the capacity of up to 25%, which is compensated by
increasing the safety factor from 3 to 4.
2)
In addition there are separate requirements for the moment of inertia of stiffening rings, RU SHIP Pt.5
Ch.7 Sec.22 [2.4.4].
Acceptance criteria for evaluation of swash bulkhead
4
The load and strength formulation for sloshing is based on a return period equivalent to 10 wave
-4
encounters, i.e. probability level 10 . Acceptance criteria according to RU SHIP Pt.3 Ch.10 Sec.4 [3]
shall be used with a reduction factor of 0.9 on the applicable stress factors C, i.e. to be applied on Ca,
Cs
Ct.
If the bulkhead is designed such that the prescriptive formulations in the rules do not apply, equivalent
formulations may be derived. The plate formulation may be derived allowing development of plastic
hinges (mechanisms). Supporting members (stiffeners, girders etc.) should be designed based on an
elastic approach. Allowable stresses equal to 0.9∙C∙ReH should be used.
and
If the swash bulkhead is part of the strength of the tank, the ULS condition should be checked against
-8
-4
the tank design loads defined at 10 probability level in Sec.1 [4] (exclusive the frequent loads at a 10
probability level for sloshing) and in RU SHIP Pt.5 Ch.7 Sec.22 [1.2]. In this case acceptance criteria
as specified for the tank in RU SHIP Pt.5 Ch.7 Sec.22 [2.8] apply. Considerations with regard to the
possibility of developing cracks in the shell should be made if applicable.
3)
Supporting springs etc. should be designed using acceptance criteria similar to those above, taking into
account the assumed probability level of the loads.
Acceptance criteria for accidental load cases
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Section 2
cylindrical body and dished ends, the main stress component will be membrane stresses in the shell, and the
stress can easily be calculated based on classical formulas. In special areas or designs, where other stress
components or categories are dominating, the stress evaluation may be carried out using other relevant
criteria as specified below.
For the flooding case, it has normally only been required to evaluate the strength of the anti-flotation
supports. The tank strength itself (e.g. buckling of empty/heeled tank) has normally not been
considered.
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Section 2
The collision load case is normally not dimensioning for the tank structure itself, but should be
considered in the evaluation of the supports.
Section 3
SECTION 3 CARGO HOLD STRENGTH ASSESSMENT
1 General
1.1 Hull and saddle supports
The following describes acceptable methods for the strength analysis, with focus on finite element models
of the cargo hold area. The FE analysis shall be carried out for the vessel with length 150 m and above to
confirm that the stress levels are acceptable when the structures are loaded in accordance with the described
design conditions.
The analyses shall be carried out in accordance with this section and the rules RU SHIP Pt.5 Ch.7 Sec.22 [3]
to verify the hull and saddle support structures.
1.2 Cargo tank
In case that FE analysis for cargo tank is required by the Society, the cargo tank can be modelled separately
and evaluated without incorporation with the hull. The inclusion of saddle support in the model shall be
decided case by case considering the boundary condition to be applied. The details of analysis such as mesh
size and load cases need to be agreed with the Society. In general, the mesh size for stress concentration
area shall be fine enough to consider plate bending stress properly. Stresses induced by thermal loads shall
be considered in the calculation for both stationary thermal load and transient thermal load of cool down.
For cargo tank and supporting structures the acceptance criteria given in RU SHIP Pt.5 Ch.7 Sec.22 [2.8]
shall be applied.
When the tank is supported by other than two saddles and interaction forces are induced by the double
bottom deflection, the cargo tank shall be integrated in the FE cargo hold model for hull structure and
calculated.
2 Modelling
2.1 General
Modelling of hull and tank structure shall follow RU SHIP Pt.3 Ch.7 of the rules and DNVGL CG 0127 unless
otherwise given in this section. This cover the following:
—
—
—
—
Geometric modelling of hull and tank structure in general
Element types and mesh size
Boundary conditions
Load application.
2.2 Model extent
For the vessels having three or more cargo holds longitudinally, extent of the model shall be over three cargo
tank lengths (1+1+1), where the middle tank/hold of the model is used to assess the yielding and buckling
strength. For the vessels having 2 cargo holds longitudinally, see [2.4].
2.3 Consideration of cargo tank
Cargo tank shall be considered in the cargo hold analysis in a way that the reaction forces from cargo tank
are properly applied in the cargo hold model. To achieve this cargo tank is recommended to be included in
the cargo hold model. In this case the connection between cargo tank and the saddle supports need to be
modelled with special consideration. One way of doing it is to idealize the wooden material with truss (spring)
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2.4 Ships with 2 cargo tanks in longitudinal direction
The cargo hold FE analysis of ships having 2 cargo tanks in longitudinal direction may be carried out
according to standard procedures given in DNVGL CG 0127 Sec.3, with the following:
— Normally, the model should represent entire cargo hold area, extending from engine room bulkhead to
collision bulkhead, as shown in Figure 1.
— Hull girder bending moments can be adjusted to the required target values according to the procedure
for midship cargo hold region as given in DNVGL CG 0127 Sec.3 [6.3.8]. The target location should be
considered at different positions within midship area in order to maximize stresses from hull girder loads.
— Hull girder shear force adjustment procedure as given in DNVGL CG 0127 Sec.3 [6.3.5] is not applicable
for 2 cargo tanks configuration. This adjustment may be disregarded in case of a ship with high margin
of hull girder shear strength. Otherwise, the adjustment of hull girder shear forces needs to be specially
considered.
— Normally, hull girder torsional moment may be disregarded in the analysis. The torsional moments due to
applied loads shall be adjusted to zero at middle of the FE model.
— Yield and buckling strength assessment shall be carried out within an evaluation area of the FE model.
The evaluation area in 2 cargo tanks model needs to be defined based on the analysis results. The areas
where abnormal stresses caused by boundary conditions shall be outside the evaluation area. Midship
area is normally suitable for the results evaluations with a standard boundary conditions.
— The standard boundary conditions as given in DNVGL CG 0127 Sec.3 [3] may apply in general. These
boundary constraints may introduce abnormal stresses towards the model ends due to unbalanced loads.
For structures with a low stress contribution from hull girder loads, e.g. transverse web frames in forward
or aft of cargo hold area ends, a separate analysis with a new boundary conditions may be carried out. In
such a case only local loads shall be applied to the model. The model should be locally supported in areas
of considered structures in such a way that the effects from unbalanced loads are eliminated, for instance
by supporting the model at transverse bulkhead locations.
Figure 1 Example of 2 cargo tank model of LPG carrier with C type independent tank (shows only
port side of the full breadth model)
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Section 3
elements considering only axial property. The truss (spring) elements in tension shall be removed during the
analysis. Some iteration may be necessary to be done for this.
Section 3
3 Design application of loading conditions and load cases
3.1 General
Hull girder and local loads according to RU SHIP Pt.3 Ch.4 shall be applied to the model.
3.2 Loading conditions
The loading conditions described in RU SHIP Pt.5 Ch.7 Sec.22 [3.2.2] shall be considered.
3.3 Design load cases
Load cases that can be considered in the cargo hold analysis are shown in Table 1 below.
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.
Ships with 2 cargo tanks in longitudinal direction design load cases given in Table 1 may apply with a
consideration given in [2.4].
Table 1 Design load cases for cargo hold analysis in midship area
No.
Application
Loading pattern
Draught
% of perm.
SWBM
% of perm.
SWSF
Dynamic load case/
Comments
≤ 100%
Mw = 0
Tank Load (S)
Static conditions
LC 1
Hull/Support
TSC
100%
(hog.)
Sea Press. (S)
100%
7)
Max SFLC
LC 2
Hull/Support
TSC
1)
100%
(hog.)
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100%
8)
Max SFLC
Mw = 0
Tank Load (S)
Sea Press. (S)
Mw = 0
Tank Load (S)
Sea Press. (S)
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Application
Loading pattern
Draught
% of perm.
SWBM
% of perm.
SWSF
100%
7)
Max SFLC
LC 3
LC 4
Hull/Support
Support
TA
2,3)
TSC
100%
(sag.)
100%
(hog.)
100%
8)
Max SFLC
≤ 100%
Dynamic load case/
Comments
Mw = 0
Tank Load (S)
Sea Press. (S)
Mw = 0
Tank Load (S)
Sea Press. (S)
Mw = 0
Inclination of 30° with tank
pressure corresponding to g
and a transverse component
equal to g·sin30° = 0.5·g (S).
Inclined static sea pressure
(S)
LC 5
Support
TA
2,3)
100%
(sag.)
≤ 100%
Mw = 0
Inclination of 30° with tank
pressure corresponding to g
and a transverse component
equal to g·sin30° = 0.5·g (S).
Inclined static sea pressure
(S)
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Section 3
No.
Application
Loading pattern
Draught
% of perm.
SWBM
% of perm.
SWSF
Dynamic load case/
Comments
Seagoing conditions
LC 6
Hull/Support
TSC
100%
(hog.)
≤ 100%
HSM-2, FSM-2, HSA-2,
BSR-1P, BSR-2P,
BSP-1P, BSP-2P, OST-1P,
OST-2P, OSA-1P, OSA-2P
9)
100%
Max
4,5,6)
HSM-2, FSM-2
SFLC
LC 7
Hull/Support
TSC
1)
100%
(hog.)
≤ 100%
HSA-2, BSR-1P, BSR-2P,
BSP-1P, BSP-2P, OST-1P,
OST-2P, OSA-1P, OSA-2P
9)
100%
Max
4,5,6)
HSM-1, FSM-1
SFLC
LC 8
Hull/Support
TA
2)
100%
(sag.)
≤ 100%
HSA-2, BSR-1P, BSR-2P,
BSP-1P, BSP-2P, OST-1P,
OST-2P, OSA-1P, OSA-2P
9)
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Section 3
No.
Application
Loading pattern
Draught
% of perm.
SWBM
% of perm.
SWSF
Dynamic load case/
Comments
Accidental conditions
Collision crash stop (forward)
condition
Mw = 0
LC 9
Support
TSC
100%
(hog.)
≤ 100%
Tank Load (S+D)
Sea Press. (S)
Acceleration ax = 0.5g
forward combined with gravity
g
LC 10
LC 11
Support
Support
10)
TSC
TSC
100%
(hog.)
100%
(hog.)
Collision crash stop (aftward)
condition
Tank Load (S+D)
≤ 100%
Sea Press. (S)
Acceleration ax = 0.25g
aftward combined with gravity
g
N/A
Flooded condition
- one tank empty
Mw = 0
Tank Load (S)
Sea Press. (S)
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Section 3
No.
Application
Loading pattern
Draught
% of perm.
SWBM
Dynamic load case/
Comments
% of perm.
SWSF
Damaged condition - tank full
Mw = 0
LC 12
Transverse
BHD
TDAM
N/A
N/A
Heeled damage waterline to
be applied to the transverse
bulkhead. The vertical
distance shall not be less than
actual damage draught at C.L.
Inclined static sea pressure
(S).
Notes:
1)
Maximum draft with one cargo tank empty may be used instead of scantling draft TSC, if this is stated as an
operational information in the loading manual
2)
Actual minimum draught at any hold loaded condition from Trim and Stability (T&S) booklet
3)
Draught not to be taken greater than minimum of 2 + 0.02L and the minimum ballast draught
4)
For the mid-hold where xb-aft < 0.5L and xb-fwd > 0.5L, the shear force shall be adjusted to target value at aft
bulkhead of the midhold
5)
For the mid-hold where xb-aft < 0.5L and xb-fwd > 0.5L, the shear force shall be adjusted to target value at forward
bulkhead of the mid-hold. Otherwise this load combination may be omitted.
6)
This load combination shall be considered only for the mid-hold where xb-aft > 0.5L or xb-fwd < 0.5L
7)
The shear force shall be adjusted to target value at aft bulkhead of the mid-hold
8)
The shear force shall be adjusted to target value at forward bulkhead of the mid-hold
9)
The beam sea and oblique sea dynamic load cases calculated for P and S shall 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.
10) Anti-floatation support and hull structures in way of anti-floatation supports.
4 Acceptance criteria for hull structure
4.1 Yielding
Acceptance criteria for yielding are given in the rules RU SHIP Pt.5 Ch.7 Sec.22 [3.3.1]. For the saddle
support structure 10% less criteria than hull structure are applied.
4.2 Buckling
Acceptance criteria for buckling are given in the rules RU SHIP Pt.5 Ch.7 Sec.22 [3.3.2].
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Section 3
No.
1 General
Local structural analyses with fine mesh finite element shall to be carried out in accordance with the rules RU
SHIP Pt.5 Ch.7 Sec.22 [4].
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.22 [4.2]. The location is in
general to be decided by screening procedure described in DNVGL CG 0127 Sec.4 [3].
2.2 Vertical stiffeners on transverse bulkheads to inner bottom
For vertical stiffeners on transverse bulkheads connection to inner bottom it is recommended to do
verification by fine mesh analysis due to damaged flooding condition, LC12 in Sec.3 Table 1. If horizontal
stringers are arranged on transverse bulkhead and the vertical stiffeners are supported by the stringers then
the fine mesh analysis for this location may not be carried out.
2.3 Saddle support
In general higher stresses are found in way of saddle support and it may be required to carry out fine mesh
analysis. Model shall be extended at least two web frame spaces on each side longitudinally. And full breadth
and depth of cargo hold model is recommendable.
2.4 Anti floatation Key
For anti-floatation support and adjoining hull structures it is recommended to do verification by fine mesh
analysis due to flooded condition when cargo tank is empty, LC11 in Sec.3 Table 1. The reaction force on anti
floatation support estimated from LC11 in Sec.3 Table 1 shall be applied on the surface of the support.
3 Load cases
Fine mesh analysis shall be carried out for the load cases specified in Sec.3 [3.3].
All local loads, including any vertical loads applied for hull girder shear force correction in the cargo hold
analysis, shall be applied to the model when separate sub-modelling is used.
4 Acceptance criteria
Acceptance criteria for stress results from local structural analysis are given in the rules RU SHIP Pt.5 Ch.7
Sec.22 [4.3].
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Section 4
SECTION 4 LOCAL STRUCTURAL STRENGTH ASSESSMENT
1 Introduction
The procedure described in Sec.3 covers only a normal pressure tank consisting of a single cylindrical tank
body with end caps horizontally supported by saddles. For other tank designs, such as multi-lobe tanks and
deck cargo tanks, some considerations are made below.
2 Bi-lobe tanks
2.1 General
For the multi-lobe tanks the introduction of the longitudinal bulkhead affects the stress flow in the tank and
special attention should be given to the evaluation of the bulkhead structure and the Y-connection at the
intersection between the longitudinal bulkhead and the tank shell. As the longitudinal bulkhead is considered
being a strength member of the tank, the tank acceptance criteria in Sec.2 [7] apply. The bulkheads may be
longitudinally or transversely stiffened, and special consideration should be given to how the stiffeners are
terminated or connected to other strength members.
2.2 Strength assessment
For the Y-connection and the stiffening rings, the stress response due to geometry change should be
specially considered. A finite element analysis of the details in question may be required, taking into account
all relevant load components and using the relevant acceptance criteria as specified in Sec.2 [7]. The
acceptance criteria should reflect the refinement and type of analysis used in the evaluation.
The bulkhead has normally been evaluated based on a load scenario where liquid pressure is applied on one
side only.
The remaining part of the tank shell may be calculated in a similar way as given above for cylindrical tanks.
2.3 Fatigue
For the multi-lobe tanks tank joint especially in way of supports and the connections between the cylinders
and their longitudinal bulkhead (Y-joint) need to be evaluated.
3 Deck Cargo Tanks
3.1 General
Independent cylindrical C-type tanks may be arranged above deck for exchange of cargoes and cooling down
of cargo tanks, Figure 1.
3.2 Materials
Doubling plates directly in contacted with the deck cargo tanks should be of same material grades as the
deck tank.
3.3 Natural period
If liquid sloshing in deck cargo tanks is anticipated, possible resonance with the ship motion periods should
be checked. A swash bulkhead may be necessary if the natural period of a cargo tank and the ship is within
+/-2 seconds, if partial filling is anticipated during operation. Natural period of deck cargo tank against pitch
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Section 5
SECTION 5 SPECIAL CONSIDERATIONS FOR OTHER TANK DESIGNS
Figure 1 Supporting structures of deck cargo tank
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Section 5
motion may be estimated according to the formula in Sec.1 [4.4] and roll motion is in general not necessary
to consider.
1 Reference list
/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, § 154.170/172/176
/3/
USCG: Alternate Pressure Relief Valve Settings on Vessels Carrying Liquefied Gases in Bulk in
Independent Type B and Type C-Tanks, 16710, CG-ENG Policy Letter, No. 04-12, August 8, 2012
/4/
IMO: Draft International Code of Safety for Ships using Gases or Lower Flashpoint Fuels (IGF), CCC
1/WP.3, Annex 2
/5/
PD 5500 “Specification for Unfired Fusion Welded Pressure Vessels”
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Section 6
SECTION 6 REFERENCES
Changes – historic
CHANGES – HISTORIC
There are currently no historical changes for this document.
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Liquefied gas carriers with independent cylindrical tanks of type C
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