dnvgl-cg-0135 liquefied gas carriers with independent ......tank type c is currently the only...

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DNV GL AS

CLASS GUIDELINE

DNVGL-CG-0135 Edition February 2016

Liquefied gas carriers with independentcylindrical tanks of type C

FOREWORD

DNV GL class guidelines contain methods, technical requirements, principles and acceptancecriteria related to classed objects as referred to from the rules.

© DNV GL AS February 2016

Any comments may be sent by e-mail to [email protected]

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In this provision "DNV GL" shall mean DNV GL AS, its direct and indirect owners as well as all its affiliates, subsidiaries, directors, officers,employees, agents and any other acting on behalf of DNV GL.

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Class guideline — DNVGL-CG-0135. Edition February 2016 Liquefied gas carriers with independent cylindrical tanks of type C

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CONTENTS

Changes – current.................................................................................................. 3

Section 1 General....................................................................................................51 Introduction.........................................................................................52 Ship application................................................................................... 63 Design basis........................................................................................ 84 Design loads........................................................................................ 9

Section 2 Ultimate strength assessment of tank...................................................141 General.............................................................................................. 142 Scantling due to internal pressure.....................................................153 Scantling due to external pressure....................................................164 Scantling of swash bulkhead............................................................. 165 Reinforcement of openings and attachments.....................................176 Evaluation of saddles and supporting structure.................................177 Stress categories and acceptance criteria..........................................17

Section 3 Cargo hold strength assessment........................................................... 201 General.............................................................................................. 202 Modelling........................................................................................... 203 Design application of loading conditions and load cases....................224 Acceptance criteria for hull structure................................................ 26

Section 4 Local structural strength assessment....................................................271 General.............................................................................................. 272 Locations to be checked.................................................................... 273 Load cases......................................................................................... 274 Acceptance criteria............................................................................ 27

Section 5 Special considerations for other tank designs....................................... 281 Introduction.......................................................................................282 Bi-lobe tanks......................................................................................283 Deck Cargo Tanks..............................................................................28

Section 6 References.............................................................................................301 Reference list.....................................................................................30

Changes – historic................................................................................................31

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SECTION 1 GENERAL

1 Introduction

1.1 ApplicationThis class guideline gives scope, methods and design criteria required for strength analysis of the tanksystem for gas carriers with independent tanks of type C. This class guideline is also applicable for tanksystems 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 normallybe 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 CA 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 ObjectiveThis class guideline provides additional information not covered in RU SHIP Pt.5 Ch.7 Sec.22, Design withcylindrical tanks of type C. Design and assessment procedures are given for hull structures and cargo tanksof cylindrical tanks of type C in accordance with the rules. The objective is to give a general description ofhow to carry out relevant calculations and analyses. In case of discrepancy between the rules and this classguideline, the rule prevails.This class guideline covers the main structural aspects for the ship hull and typical type C tanks consistingof a cylindrical midbody with hemispherical, elliptical or torispherical end caps supported by two saddles. Inaddition some considerations for bi-lobe tanks are included. Associated gas system related aspects are notcovered.

1.4 Scope of documentationStructure-related documents required for approval of independent tank types C are given in RU SHIP Pt.5Ch.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 abbreviationsFor 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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ax = 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/mm2

LEG = liquefied ethylene gas

LNG = liquefied natural gas

LPG = liquefied petroleum gas

MARVS = maximum allowable relieve valve setting

NMA = Norwegian Maritime Authority.

2 Ship application

2.1 GeneralShips with independent tank type C are applicable for carrying a wide range of liquefied gases, all havingparticular properties and design requirements related to general safety. Types of cargoes with their specialrequirements are given in RU SHIP Pt.5 Ch.7 Sec.19. Gas carriers with type C tanks have typically beentransport of cargoes such as LPG, LNG, ethylene, ammonia, including the more recent use for storage of LNGFuel and CO2 transportation.A selection of some typical type C tank applications is listed below:

2.2 LPG carriersTypically cargoes like Propane, Butane, Propylene etc. are transported on LPG carriers. These cargoeshave moderate density and design temperature limited to -48°C. Carbon manganese steel, e.g. VL4-4 orequivalent is normally used as material for the cargo tanks. The ship can be both fully pressurized, semipressurized or fully refrigerated. LPG can also carry a number of other cargoes listed in RU SHIP Pt.5 Ch.7Sec.19 like VCM (high density) etc. Typical sizes range from 4 000 m3 to 22 000 m3.

2.3 LNG carriers (feeders)Lately it has been an increased demand for small LNG Carriers for coastal service where the independenttank type C has shown to be competitive and flexible for the operators. The LNG tanks need to be designedfor the low temperature of LNG of -163°C. Tank material is typically 9% Nickel steel or austenitic steel, withtypical ship sizes ranging from 1100 – 3000 m3. An example of a typical coastal LNG carrier is shown inFigure 1.

2.4 Ethylene carriers (LEG)Ethylene carriers are normally LPG Carriers applying tank material suited for the low temperature of ethyleneof -104°C including ethane of -88°C. Tank material is normally 5% Nickel steel. Typical sizes are the same asLPG Carriers.

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2.5 Ammonia carriersAmmonia is carried on both LPG and LEG vessels. Particular attention should be paid to the selection ofmaterial and heat treatment to prevent possible stress corrosive cracking from the ammonia. Typical sizesare 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 withregard to location of tanks, ship arrangement, damage stability and special requirements as given in RU SHIPPt.5 Ch.7 Sec.19 must be met. Due to the special requirements mentioned above, independent tank types Care 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 carriersCO2 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, thepressure has to be above 5.18 bars.Most of the particular design requirements for CO2 carriers are system related. Taking into account thespecific design pressures, temperatures and the density of CO2 (heavier than water), no special structuraltank 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 corrosionshould however be made.Existing ships have typically sizes below 1500 m3, however larger ships may be expected in the future.

2.8 Vessels designed to carry combination of cargoes e.g. CO2 and LPGShips intended to carry both normal LPG cargoes and CO2 must comply with all the requirements for bothLPG carriage and CO2. Special considerations with regard to cleaning procedures must be considered toprevent e.g. contamination of LPG.

Figure 1 Coastal LNG carrier with independent cylindrical tank type C

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3 Design basis

3.1 Hull structureHull 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 tankThe independent type C cargo tanks are covered by the requirements given in RU SHIP Pt.5 Ch.7 Sec.22 ofthe rules.Table 2 gives an overview of the different load components and conditions to be used as basis for thestrength assessment of the tank structure. Allowable stress and buckling acceptance criteria to be used forthe different structural members are specified.

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Table 2 Overview of strength assessment for cargo tanks

4 Design loads

4.1 GeneralThe design loads which shall be considered for strength evaluation of independent cylindrical tanks are ingeneral 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 Other loads

— Cargo weight (static pressure)— Tank system self weight

— tank shell— insulation— domes and piping

— Internal and external overpressure

— Still water interaction forces*)

— Vertical, transverse and longitudinalaccelerations acting on the system(dynamic pressure)

— Sloshing loads— Dynamic interaction forces from

wave loads*)

— Stationary temperature distribution— Transient temperature distribution

of initial cool down— Vibration

*) Interaction forces may normally not be relevant, see RU SHIP Pt.5 Ch.7 Sec.22 [1.2.7]

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In addition to the design loads above, the following load scenarios shall be considered:

1) 30° static inclination of tank2) Collision force acting on the tank corresponding to half of the mass of the tank and cargo in the forward

direction1)

3) Collision force acting on the tank corresponding to a quarter of the mass of the tank and cargo in the aftdirection1), 2)

4) Flooding of cargo hold5) Tank test condition.

The different load components described in RU SHIP Pt.5 Ch.7 Sec.22 [1.2] and in the following shall becombined as described in the rules RU SHIP Pt.5 Ch.7 Sec.22 [2.2].

4.2 Cargo weight and tank system self weightFor design purposes the specific density of the intended cargo shall be used. A minimum density is generallyto be taken as ρ = 500 kg/m3.The static weight of the cargo tank, including piping, insulation etc. should be included in the assessment. Ifthe weight of the additional piping etc. is not known, this may conservatively be taken as a small percentageof 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 describedin RU SHIP Pt.5 Ch.7 Sec.4 [6.1.1].

4.3 Calculation of maximum cargo tank pressureThe maximum simultaneous combination of the acceleration components is however not considered to actover a larger area of the tank. Hence, for design purposes of cylindrical shell and tank ends, it is normallyconsidered sufficient to calculate the accelerations aβ at transverse and longitudinal direction at the angle ofβmax X and βmax Y according to the rules RU SHIP Pt.5 Ch.7 Sec.4 Figure 1. The design pressure Peq in MPa atany location of the tank can be determined from the following formula:

over the range 0 < β ≤ βmax

where

ρ = specific density of the cargo, in kg/m3

d = horizontal distance to pressure point, in m= x longitudinal distance, in m, from the end of the cylindrical part, Figure 2= y transverse distance, in m, from the tank centre, Figure 2

z = vertical distance, in m, from tank centre to pressure calculation point (positive downwards)R = tank radius, in mβx, βy = 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.βmax = maximum angle as defined in RU SHIP Pt.5 Ch.7 Sec.4 Figure 1

1 Stricter requirements issued by other authorities may apply (e.g. Norwegian Maritime Authority (NMA)requirements for fuel tanks, ref. Sec.6 /4/)

2 Normally covered by item 2) for symmetrical tanks.

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aβ = resulting acceleration vector in fraction of gravity (g) for inclination angle βx and βy as shown inRU SHIP Pt.5 Ch.7 Sec.4 Figure 3.

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 calculatedfrom the ellipsoid (aβ) should be used.

The governing design pressure in the transversal or longitudinal direction is used as input to the minimumscantling 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.5Ch.7 Sec.4 [6.1.2]. These formulas are considered to give conservative design accelerations for traditionaltype C gas carriers. As an alternative, the design accelerations may be determined by direct calculationsand used in connection with the acceleration ellipse, but this has not been the general practice for past andcurrent designs. This is mainly because the dynamic contribution on the scantling is normally significantlyless compared to the static contribution, and an optimization with regard to the dynamic pressure has notbeen considered necessary.

If direct calculations are carried out (i.e. a wave load analysis) to determine the accelerations, the followingassumptions 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— Loads shall be taken at probability of exceedance equal to 10-8. This corresponds to the most probable

largest load the ship will experience during 108 wave encounters in the North Atlantic and is normallyinterpreted 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 inthe hydrodynamic analysis shall reflect the vessels loading manual with full load and part loading conditionsas relevant.

4.4 Sloshing loadsEvaluation of the sloshing loads on the tank shell, supports and the internal structure (except for swashbulkheads) may be disregarded provided that the free surface liquid length ℓslh < 0.13 L and/or bslh < 0.56B, where the parameters L and B are the ship length and breadth, respectively.

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Based on the current practice and past experience, cargo tanks with lengths exceeding 0.1 L have beenarranged with swash bulkheads to reduce the free liquid surface length.

The rationale for introducing a restriction on the free liquid surface length is to avoid exposure to largeamplitude 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 amplitudeoperator (RAO) for a vessel with length 110 m, and the right hand side diagram the same RAO for a vesselwith length 220 m. The hatched regions represent the sloshing resonance period range for a cylindrical tankwith length 35 m for two tank filling levels. For the small vessel this tank length corresponds to 0.32 L. Forthe 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 withperiods coinciding with the sloshing resonance periods of the tank is small, and is hence not likely to exciteviolent and potentially harmful liquid motions in the tank. Increasing tank/ship length ratios will increase theamplitude 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 thatsloshing loads for tanks with free liquid surface lengths up to 0.35 L are moderate in terms of the strength ofthe tank shell and the tank supports. Violent sloshing is, however, expected to occur, and particular attentionshould be given to the lateral impact pressure on the ring stiffeners and the routing of tank internals (pipingetc.).

Based on the above, the following steps should be taken to ensure that tanks are designed to sustainsloshing loads:

1) In case of tank length 0.13L < ℓslh ≤ 0.16La) Tank may be designed without internal swash bulkheadsb) Sloshing evaluation is not required

2) In case of tank length 0.16L < ℓslh ≤ 0.35La) Designed with swash bulkheads (free liquid length ℓslh ≤ 0.16L)

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— Sloshing evaluation not required, except dimensioning of swash bulkhead. Sloshing loadsspecified in the rules RU SHIP Pt.3 Ch.10 Sec.4 [2] should be used for the assessment of swashbulkheads. The sloshing pressure values are assumed to be a return period equivalent to 104

wave encounters.

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 > 0.35La) 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 carriedout.

Sloshing evaluations may be based on model testing, CFD analyses or similar. Simple assessments of thetank natural periods can be used as basis for further evaluations. The following formula may be used forestimating the liquid sloshing natural period in seconds in the longitudinal tank direction:

where

Ccyl =

ℓ = cargo tank length, in mg = 9.81 m/s2

h = filling height, in mR = tank radius, in m.

4.5 Vibration analysisSeparate vibration analysis may in special cases be required for type C tank designs, RU SHIP Pt.5 Ch.7Sec.4 [3.3.5].

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SECTION 2 ULTIMATE STRENGTH ASSESSMENT OF TANK

1 General

1.1 IntroductionIn the following, scantling requirements for the tank structure are described in detail. The requirementsinclude allowable stress and buckling assessment of the tank shell, internal structure and support, anddimensioning control of opening and attachments, as described in RU SHIP Pt.5 Ch.7 Sec.22 [2] and RUSHIP Pt.4 Ch.7 Sec.4, as applicable. Some of the formulations given are re-produced from the rules forconvenience, otherwise specific references to the rules are given.

1.2 Division of tankA 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 Scantling due to internal pressure

2.1 Shell plate thickness calculation based on allowable stress1) 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 Tank Part Formula

1 Cylindrical tank body — Cylindrical Shells RU SHIP Pt.4 Ch.7 Sec.4 [3.2.1]

2 Dished end/ spherical shells — Hemispherical ends— Spherical shells

RU SHIP Pt.4 Ch.7 Sec.4 [3.3.1]

3 Dished end /transition— Hemispherical ends— Elliptical ends— Torispherical ends

RU SHIP Pt.4 Ch.7 Sec.4 [4.1.1]

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 alloys2)

, (mm)

where

Di = Inside diameter in mm of shell or inside diameter of cylindrical shirt for dished ends.

2) It should be noted that the thickness formula for the dished ends is taking into account the increasedstress level at the conjunction with the cylindrical tank body, and no extra considerations in the transitionzone 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 thethickness requirement for a sphere.

3) If the end cap has a complete hemispherical shape, if may be beneficial to divide the end cap into twoparts. The part adjacent to the cylindrical shell body (transition zone) may be dimensioned according toformula 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 supports1) 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 PD5500Annex G.3, ref. Sec.6 /5/, may be followed. Alternative equivalent procedures specified in otherrecognized standards may be used.In connection with the evaluation of the global response of the tank structure, the following aspects needto 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 tankThe calculated longitudinal stress according to the rules RU SHIP Pt.5 Ch.7 Sec.22 [2.3.2] consists ofa combination of the global axial forces created in the tank due to the design pressure (hydrostatic

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and overpressure) and the global bending of the tank between the supports. The longitudinal stressformulation consists of the following three terms:

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 theacceleration ellipse (yz-plane), i.e. 0 ≤ βy ≤ βmax and/or the 30° static heel condition. In addition, thetank 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 internaloverpressure should be used. For buckling assessment the internal overpressure should generally bedisregarded.The second term includes the average sectional axial stress due to static and dynamic cargo loading. Theaxial 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, andmay 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 calculatedaccording to BS5500 Annex G3.3.2.5, using the relevant K-factors.

4) Circumferential stresses at supportThe 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 pressureBuckling requirements due to external pressure are given in RU SHIP Pt.5 Ch.7 Sec.22 [2.4.1] for thecylindrical shells, [2.4.2] for spherical shells and hemispherical ends, [2.4.3] for torispherical or ellipsoidalends and [2.4.4] for stiffening rings. The various members exposed to external pressure should be checkedagainst 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 bulkheadSwash bulkhead shall be designed in accordance with RU SHIP Pt.5 Ch.7 Sec.22 [2.6] with consideration ofthe sloshing load given in Sec.1 [4.4].

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5 Reinforcement of openings and attachments

5.1 GeneralThe main requirements with comments are included in the below. Please refer to the rules for applicabilityand limitations of the below requirements.

5.2 Reinforcement platesThe strength requirements for the reinforcements around openings and penetrations are in the rulesformulated 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 attachmentsScantling control of attachments such as domes and sumps should in principle follow the same calculationprocedures as for the main shell of the tank, taking into account the actual dimensions. Effects of openings orpenetrations should be included.

5.4 Requirements for manhole coversThe requirements for thickness of manhole covers are given in Pt.4 Ch.7 Sec.4 [5]. Standard dimensioningaccording to e.g. ASME standards is accepted. The same applies to flanges and standard fittings.

6 Evaluation of saddles and supporting structure

6.1 GeneralThe connection of the tank to the saddle, the saddle structure and its connection to the hull, and thesupporting 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 intoaccount the relevant requirements in RU SHIP Pt.5 Ch.7 Sec.22 [2.5]. Fatigue assessment of the saddle andrelevant parts of the supporting structure should be considered if large dynamic stresses are present.

6.2 Material selectionMaterial selection for saddles and supporting structures shall be made based on the resulting temperature ofstationary temperature analysis. The temperature for saddle and supporting structures shall be calculated inaccordance with RU SHIP Pt.5 Ch.7 Sec.6 [8].

7 Stress categories and acceptance criteria

7.1 Stress categoriesThe calculated stresses are in general divided into different stress categories depending on their type ofnature and their criticality for the safety of the system. For independent tank types C consisting mainly of a

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cylindrical body and dished ends, the main stress component will be membrane stresses in the shell, and thestress can easily be calculated based on classical formulas. In special areas or designs, where other stresscomponents or categories are dominating, the stress evaluation may be carried out using other relevantcriteria as specified below.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 averagevalue of the stress across the thickness of the section under consideration

Primary membranestress

σm membrane stress which is so distributed in the structure that no redistribution of loadsoccurs as the result of yielding

Primary localmembrane stress

σL membrane stress produced by pressure or other mechanical loading and associated witha primary and/or a discontinuity effect produces excessive distortion in the transfer ofloads to other portions of the structure

Bending stress σb the variable stress across the thickness of the section under consideration, after thesubtraction of the membrane stress

Secondary stress σg a normal stress or shear stress developed by the constraint of adjacent parts or by self-constraint of a structure. The basic characteristic of a secondary stress is that it is self-limiting.

7.2 Acceptance criteriaAcceptance criteria for each requirement are according to the rules RU SHIP Pt.5 Ch.7 Sec.22 [2.8] withconsideration of the following practical application:

1) Acceptance criteria for buckling strength assessmentThe increased safety factor for buckling of cylindrical shells is included to reflect that deviations froma perfect circular cylindrical form will reduce the critical load compared to the formulation. It is foundthat practical out-of-roundness deviations reduce the capacity of up to 25%, which is compensated byincreasing the safety factor from 3 to 4.In addition there are separate requirements for the moment of inertia of stiffening rings, RU SHIP Pt.5Ch.7 Sec.22 [2.4.4].

2) Acceptance criteria for evaluation of swash bulkheadThe load and strength formulation for sloshing is based on a return period equivalent to 104 waveencounters, i.e. probability level 10-4. 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, Csand Ct.If the bulkhead is designed such that the prescriptive formulations in the rules do not apply, equivalentformulations may be derived. The plate formulation may be derived allowing development of plastichinges (mechanisms). Supporting members (stiffeners, girders etc.) should be designed based on anelastic approach. Allowable stresses equal to 0.9∙C∙ReH should be used.If the swash bulkhead is part of the strength of the tank, the ULS condition should be checked againstthe tank design loads defined at 10-8 probability level in Sec.1 [4] (exclusive the frequent loads at a 10-4

probability level for sloshing) and in RU SHIP Pt.5 Ch.7 Sec.22 [1.2]. In this case acceptance criteriaas specified for the tank in RU SHIP Pt.5 Ch.7 Sec.22 [2.8] apply. Considerations with regard to thepossibility of developing cracks in the shell should be made if applicable.Supporting springs etc. should be designed using acceptance criteria similar to those above, taking intoaccount the assumed probability level of the loads.

3) Acceptance criteria for accidental load cases

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The collision load case is normally not dimensioning for the tank structure itself, but should beconsidered in the evaluation of the supports.For the flooding case, it has normally only been required to evaluate the strength of the anti-flotationsupports. The tank strength itself (e.g. buckling of empty/heeled tank) has normally not beenconsidered.

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SECTION 3 CARGO HOLD STRENGTH ASSESSMENT

1 General

1.1 Hull and saddle supportsThe following describes acceptable methods for the strength analysis, with focus on finite element modelsof the cargo hold area. The FE analysis shall be carried out for the vessel with length 150 m and above toconfirm that the stress levels are acceptable when the structures are loaded in accordance with the describeddesign 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 tankIn case that FE analysis for cargo tank is required by the Society, the cargo tank can be modelled separatelyand evaluated without incorporation with the hull. The inclusion of saddle support in the model shall bedecided case by case considering the boundary condition to be applied. The details of analysis such as meshsize and load cases need to be agreed with the Society. In general, the mesh size for stress concentrationarea shall be fine enough to consider plate bending stress properly. Stresses induced by thermal loads shallbe 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 doublebottom deflection, the cargo tank shall be integrated in the FE cargo hold model for hull structure andcalculated.

2 Modelling

2.1 GeneralModelling of hull and tank structure shall follow RU SHIP Pt.3 Ch.7 of the rules and DNVGL CG 0127 unlessotherwise 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 extentFor the vessels having three or more cargo holds longitudinally, extent of the model shall be over three cargotank lengths (1+1+1), where the middle tank/hold of the model is used to assess the yielding and bucklingstrength. For the vessels having 2 cargo holds longitudinally, see [2.4].

2.3 Consideration of cargo tankCargo tank shall be considered in the cargo hold analysis in a way that the reaction forces from cargo tankare properly applied in the cargo hold model. To achieve this cargo tank is recommended to be included inthe cargo hold model. In this case the connection between cargo tank and the saddle supports need to bemodelled with special consideration. One way of doing it is to idealize the wooden material with truss (spring)

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elements considering only axial property. The truss (spring) elements in tension shall be removed during theanalysis. Some iteration may be necessary to be done for this.

2.4 Ships with 2 cargo tanks in longitudinal directionThe cargo hold FE analysis of ships having 2 cargo tanks in longitudinal direction may be carried outaccording 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 tocollision bulkhead, as shown in Figure 1.

— Hull girder bending moments can be adjusted to the required target values according to the procedurefor midship cargo hold region as given in DNVGL CG 0127 Sec.3 [6.3.8]. The target location should beconsidered 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 applicablefor 2 cargo tanks configuration. This adjustment may be disregarded in case of a ship with high marginof hull girder shear strength. Otherwise, the adjustment of hull girder shear forces needs to be speciallyconsidered.

— Normally, hull girder torsional moment may be disregarded in the analysis. The torsional moments due toapplied 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 areaswhere abnormal stresses caused by boundary conditions shall be outside the evaluation area. Midshiparea 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. Theseboundary 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 forwardor aft of cargo hold area ends, a separate analysis with a new boundary conditions may be carried out. Insuch a case only local loads shall be applied to the model. The model should be locally supported in areasof considered structures in such a way that the effects from unbalanced loads are eliminated, for instanceby supporting the model at transverse bulkhead locations.

Figure 1 Example of 2 cargo tank model of LPG carrier with C type independent tank (shows onlyport side of the full breadth model)

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3 Design application of loading conditions and load cases

3.1 GeneralHull girder and local loads according to RU SHIP Pt.3 Ch.4 shall be applied to the model.

3.2 Loading conditionsThe loading conditions described in RU SHIP Pt.5 Ch.7 Sec.22 [3.2.2] shall be considered.

3.3 Design load casesLoad 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 alwaysfilled, 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 aconsideration 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

Static conditions

LC 1 Hull/Support

TSC100%(hog.) ≤ 100%

Mw = 0Tank Load (S)

Sea Press. (S)

100%Max SFLC7)

Mw = 0Tank Load (S)

Sea Press. (S)

LC 2 Hull/Support

TSC1) 100%

(hog.)100%

Max SFLC8)

Mw = 0Tank Load (S)

Sea Press. (S)

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% of perm.SWSF


100%Max SFLC7)

Mw = 0Tank Load (S)

Sea Press. (S)

LC 3 Hull/Support

TA 2,3) 100%(sag.)

100%Max SFLC8)

Mw = 0Tank Load (S)

Sea Press. (S)

LC 4 Support

TSC100%(hog.) ≤ 100%

Mw = 0Inclination of 30° with tankpressure corresponding to gand a transverse componentequal to g·sin30° = 0.5·g (S).

Inclined static sea pressure(S)

LC 5 Support

TA 2,3) 100%(sag.) ≤ 100%

Mw = 0Inclination of 30° with tankpressure corresponding to gand a transverse componentequal to g·sin30° = 0.5·g (S).

Inclined static sea pressure(S)

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% of perm.SWSF


Seagoing conditions

LC 6 Hull/Support

TSC100%(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

SFLC4,5,6)HSM-2, FSM-2

LC 7 Hull/Support

TSC1) 100%

(hog.)

≤ 100%

HSA-2, BSR-1P, BSR-2P,BSP-1P, BSP-2P, OST-1P,OST-2P, OSA-1P, OSA-2P

9)

100%Max

SFLC4,5,6)HSM-1, FSM-1

LC 8 Hull/Support

TA2) 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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% of perm.SWSF


Accidental conditions

LC 9 Support

TSC100%(hog.) ≤ 100%

Collision crash stop (forward)conditionMw = 0

Tank Load (S+D)

Sea Press. (S)

Acceleration ax = 0.5gforward combined with gravityg

LC 10 Support

TSC100%(hog.) ≤ 100%

Collision crash stop (aftward)conditionTank Load (S+D)

Sea Press. (S)

Acceleration ax = 0.25gaftward combined with gravityg

LC 11 Support 10)

TSC100%(hog.) N/A

Flooded condition- one tank empty

Mw = 0

Tank Load (S)

Sea Press. (S)

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% of perm.SWSF


LC 12 TransverseBHD

TDAM N/A N/A

Damaged condition - tank fullMw = 0

Heeled damage waterline tobe applied to the transversebulkhead. The verticaldistance shall not be less thanactual 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 anoperational information in the loading manual

2) Actual minimum draught at any hold loaded condition from Trim and Stability (T&S) booklet3) Draught not to be taken greater than minimum of 2 + 0.02L and the minimum ballast draught4) 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 midhold5) 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 < 0.5L7) The shear force shall be adjusted to target value at aft bulkhead of the mid-hold8) The shear force shall be adjusted to target value at forward bulkhead of the mid-hold9) 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 andoblique sea dynamic load cases calculated for P may be applied only to the model (i.e. S may be omitted) providedthe 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 YieldingAcceptance criteria for yielding are given in the rules RU SHIP Pt.5 Ch.7 Sec.22 [3.3.1]. For the saddlesupport structure 10% less criteria than hull structure are applied.

4.2 BucklingAcceptance criteria for buckling are given in the rules RU SHIP Pt.5 Ch.7 Sec.22 [3.3.2].

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SECTION 4 LOCAL STRUCTURAL STRENGTH ASSESSMENT

1 GeneralLocal structural analyses with fine mesh finite element shall to be carried out in accordance with the rules RUSHIP Pt.5 Ch.7 Sec.22 [4].

2 Locations to be checked

2.1 GeneralThe areas to be considered are according to the rules RU SHIP Pt.5 Ch.7 Sec.22 [4.2]. The location is ingeneral to be decided by screening procedure described in DNVGL CG 0127 Sec.4 [3].

2.2 Vertical stiffeners on transverse bulkheads to inner bottomFor vertical stiffeners on transverse bulkheads connection to inner bottom it is recommended to doverification by fine mesh analysis due to damaged flooding condition, LC12 in Sec.3 Table 1. If horizontalstringers are arranged on transverse bulkhead and the vertical stiffeners are supported by the stringers thenthe fine mesh analysis for this location may not be carried out.

2.3 Saddle supportIn general higher stresses are found in way of saddle support and it may be required to carry out fine meshanalysis. Model shall be extended at least two web frame spaces on each side longitudinally. And full breadthand depth of cargo hold model is recommendable.

2.4 Anti floatation KeyFor anti-floatation support and adjoining hull structures it is recommended to do verification by fine meshanalysis due to flooded condition when cargo tank is empty, LC11 in Sec.3 Table 1. The reaction force on antifloatation support estimated from LC11 in Sec.3 Table 1 shall be applied on the surface of the support.

3 Load casesFine 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 holdanalysis, shall be applied to the model when separate sub-modelling is used.

4 Acceptance criteriaAcceptance criteria for stress results from local structural analysis are given in the rules RU SHIP Pt.5 Ch.7Sec.22 [4.3].

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SECTION 5 SPECIAL CONSIDERATIONS FOR OTHER TANK DESIGNS

1 IntroductionThe procedure described in Sec.3 covers only a normal pressure tank consisting of a single cylindrical tankbody with end caps horizontally supported by saddles. For other tank designs, such as multi-lobe tanks anddeck cargo tanks, some considerations are made below.

2 Bi-lobe tanks

2.1 GeneralFor the multi-lobe tanks the introduction of the longitudinal bulkhead affects the stress flow in the tank andspecial attention should be given to the evaluation of the bulkhead structure and the Y-connection at theintersection between the longitudinal bulkhead and the tank shell. As the longitudinal bulkhead is consideredbeing a strength member of the tank, the tank acceptance criteria in Sec.2 [7] apply. The bulkheads may belongitudinally or transversely stiffened, and special consideration should be given to how the stiffeners areterminated or connected to other strength members.

2.2 Strength assessmentFor the Y-connection and the stiffening rings, the stress response due to geometry change should bespecially considered. A finite element analysis of the details in question may be required, taking into accountall relevant load components and using the relevant acceptance criteria as specified in Sec.2 [7]. Theacceptance 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 oneside only.The remaining part of the tank shell may be calculated in a similar way as given above for cylindrical tanks.

2.3 FatigueFor the multi-lobe tanks tank joint especially in way of supports and the connections between the cylindersand their longitudinal bulkhead (Y-joint) need to be evaluated.

3 Deck Cargo Tanks

3.1 GeneralIndependent cylindrical C-type tanks may be arranged above deck for exchange of cargoes and cooling downof cargo tanks, Figure 1.

3.2 MaterialsDoubling plates directly in contacted with the deck cargo tanks should be of same material grades as thedeck tank.

3.3 Natural periodIf liquid sloshing in deck cargo tanks is anticipated, possible resonance with the ship motion periods shouldbe 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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motion may be estimated according to the formula in Sec.1 [4.4] and roll motion is in general not necessaryto consider.

Figure 1 Supporting structures of deck cargo tank

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SECTION 6 REFERENCES

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 ofFederal Register), Part 154, § 154.170/172/176

/3/ USCG: Alternate Pressure Relief Valve Settings on Vessels Carrying Liquefied Gases in Bulk inIndependent 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), CCC1/WP.3, Annex 2

/5/ PD 5500 “Specification for Unfired Fusion Welded Pressure Vessels”

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CHANGES – HISTORICThere are currently no historical changes for this document.

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