Section 6 Tank types
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 11 Production, Storage and Offloading of Liquefied Gases in Bulk - Chapter 4 Cargo Containment - Section 6 Tank types

Section 6 Tank types

Parent topic: Chapter 4 Cargo Containment

6.1 Type A independent tanks

6.1.1  Design basis
  1. Type A independent tanks are tanks primarily designed using classical ship-structural analysis procedures. Type A independent tanks are to be designed in accordance with Pt 11, Ch 4, 5.2 Construction processes 5.2.3 and Pt 11, Ch 4, 5.2 Construction processes 5.2.4. Where such tanks are primarily constructed of plane surfaces, the design vapour pressure Po shall be less than 0,07 MPa.
  2. If the cargo temperature at atmospheric pressure is below –10°C, a complete secondary barrier is required as defined in Pt 11, Ch 4, 2.3 Secondary barriers in relation to tank types. The secondary barrier shall be designed in accordance with Pt 11, Ch 4, 2.4 Design of secondary barriers.
6.1.2  Structural analysis
  1. A structural analysis shall be performed taking into account the internal pressure as indicated in Pt 11, Ch 4, 3.3 Functional loads 3.3.2, and the interaction loads with the supporting and keying system as well as a reasonable part of the hull of the ship unit.
  2. For parts such as supporting structures not otherwise covered by the requirements of this Part, stresses shall be determined by direct calculations, taking into account the loads referred to in Pt 11, Ch 4, 3.2 Permanent loads to Pt 11, Ch 4, 3.5 Accidental loads as far as applicable, and the deflection of the ship unit in way of supporting structures.
  3. The tanks with supports shall be designed for the accidental loads specified in Pt 11, Ch 4, 3.5 Accidental loads. These loads need not be combined with each other or with environmental loads.

6.1.3  Symbols:

b = width of plating supported, in metres
f = 1,1 – but need not exceed 1,0
fs = 2,7 for nickel steels and carbon manganese steels
= 3,9 for austenitic steels and aluminium alloys
h = vertical distance, from the middle of the effective span of stiffener or transverse to the top of the tank, in metres
l = effective span or girder or web, in metres, see Pt 3, Ch 3, 3.3 Determination of span point of the Rules for Ships
l e = effective length of stiffening member, in metres, see Pt 3, Ch 3, 3.3 Determination of span point of the Rules for Ships

lt, ls, lb, lc are effective spans measured according to Figure 4.6.1 Measurement of spans

ρ = maximum density of the cargo, in kg/m3, at the cargo design temperature
k = higher tensile steel factor, see Pt 3, Ch 2, 1.2 Steel of the Rules for Ships
tp = thickness, in mm, of the attached load bearing plating. Where this varies over the effective width of plating, the mean thickness is to be used
Peq = the internal pressure head, in MPa, as derived from Pt 11, Ch 4, 3.3 Functional loads 3.3.2(a) and measured at a point on the plate one third of the depth of the plate above its lower edge
s = spacing of bulkhead stiffeners, in mm
S = spacing of primary members, in metres

Sw and s1 are as defined in Figure 10.5.1 Bracket toe construction in Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members of the Rules for Ships.

The remaining symbols are as defined in Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads of the Rules for Ships. The lateral and torsional stability of stiffeners should comply with the requirements of Pt 4, Ch 9, 5.6 Stability of longitudinals of the Rules for Ships.

Figure 4.6.1 Measurement of spans

6.1.4  The scantlings of Type A independent tanks are to comply with the following:
  1. Minimum thickness.

    No part of the cargo tank structure is to be less than 7,5 mm in thickness.

  2. Boundary plating.

    The thickness of plating forming the boundaries of cargo tanks is to be not less than 7,5 mm, nor less than:

    mm

    NOTE

    An additional corrosion allowance of 1 mm is to be added to the thickness derived if the cargo is of corrosive nature, see also Pt 11, Ch 4, 2.1 Functional requirements 2.1.6 and Pt 11, Ch 4, 2.1 Functional requirements 2.1.7.

  3. Rolled or built stiffeners.

    The section modulus of rolled or built stiffeners on plating forming tank boundaries is to be not less than:

    cm3

  4. Transverses.

    The scantlings of transverse members are normally to be derived using direct calculation methods. The structural analysis is to take account of the internal pressure defined in Pt 11, Ch 4, 3.3 Functional loads 3.3.2.(d) and also those resulting from structural test loading conditions. Proper account is also to be taken of structural model end constraints, shear and axial forces present and any interaction from the double bottom structure through the cargo tank supports.

    As an initial estimate, the scantlings of the primary transverses may be taken as:

    top transverse

    Z = 720Peq s lt 2 k cm3

    topside transverse

    Z = 520Peq s lt 2 k cm3

    side transverse

    Z = 560Peq s ls 2 k cm3

    bottom transverse

    Z = 560Peq s lb 2 k cm3

    centreline bulkhead transverse

    Z = 400Peq s lc 2 k cm3

    The depth of the bottom transverse web is generally to be not less than lb /4.

    Web stiffening is to be in accordance with Pt 4, Ch 9, 10.5 Primary member web plate stiffening of the Rules for Ships with the application of the stiffening requirements as shown in Figure 4.6.1 Measurement of spans.

  5. Tank end webs and girders.

    The section modulus of vertical webs and horizontal girders is to be not less than:

    Z = 850Peq bl 2 k cm3

  6. Internal bulkheads (perforated).

    The thickness of plating is to be not less than 7,5 mm, but may require to be increased at the tank boundaries in regions of high local loading.

    The section modulus of stiffeners, girders and webs is to be in accordance with Pt 4, Ch 9, 8 Non-oiltight bulkheads and Pt 4, Ch 9, 9.8 Primary members supporting non-oiltight bulkheads of the Rules for Ships.

  7. Internal bulkheads (non-perforated).

    Where a bulkhead may be subjected to an internal pressure head, Peq, resulting from loading on one side only, the scantlings of plating, stiffeners and primary members are to be determined from (b), (c) and (d).

    Where no such loading condition is envisaged, the scantlings may be derived as follows:

    The thickness of plating is to be not less than would be required for the tank boundary plating at the corresponding tank depth and stiffener spacing, reduced by 0,5 mm. The section modulus of stiffeners and transverses is to be derived from (c) or (d), respectively, but Peq need not exceed:

    MPa

  8. Tank crown structure.

    Where the minimum thickness of tank boundary plating (7,5 mm) has been adopted, the section moduli of associated stiffeners and transverses are to be derived as above, but P eq is to be not less than that equivalent to the minimum thickness, that is:

    MPa

    The tank crown plating and stiffeners are also to be suitable for a head equivalent to the tank test air pressure where the tank is to be hydro-pneumatically tested.

  9. Connection of stiffeners to primary supporting members.
    In assessing the arrangement at intersections of continuous secondary and primary members, the requirements of Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members are to be complied with using the requirements for ‘other ship types’. The total load, P, in kN, is to be derived using the internal pressure head, Peq, in MPa as given by Pt 11, Ch 4, 3.3 Functional loads 3.3.2.(d) and the following formulae:
    1. In general:

      P = 1000 (S w – 0,5s1)s1 Peq kN

    2. For wash bulkheads:

      P = 1200 (S w – 0,5s1)s1 Peq kN.

6.1.5  On-site operation design condition
  1. For tanks primarily constructed of plane surfaces, the nominal membrane stresses for primary and secondary members (stiffeners, web frames, stringers, girders), when calculated by classical analysis procedures, shall not exceed the lower of R m/2,66 or Re/1,33 for nickel steels, carbon-manganese steels, austenitic steels and aluminium alloys, where Rm and Re are defined in Pt 11, Ch 4, 4.3 Design conditions 4.3.2.(c).

    However, if detailed calculations are carried out for the primary members, the equivalent stress σc, as defined in Pt 11, Ch 4, 4.3 Design conditions 4.3.2.(d), may be increased over that indicated above to a stress acceptable to LR. Calculations shall take into account the effects of bending, shear, axial and torsional deformation as well as the hull/cargo tank interaction forces due to the deflection of the double bottom and cargo tank bottoms.

  2. Tank boundary scantlings shall meet at least the requirements of LR for deep tanks taking into account the internal pressure as indicated in Pt 11, Ch 4, 3.3 Functional loads 3.3.2 and any corrosion allowance required by Pt 11, Ch 4, 2.1 Functional requirements 2.1.6.
  3. The cargo tank structure shall be reviewed against potential buckling.
6.1.6  Accident design condition
  1. The tanks and the tank supports shall be designed for the accidental loads and design conditions specified in Pt 11, Ch 4, 2.1 Functional requirements 2.1.5.(c) and Pt 11, Ch 4, 3.5 Accidental loads, as relevant.
  2. When subjected to the accidental loads specified in Pt 11, Ch 4, 3.5 Accidental loads, the stress shall comply with the acceptance criteria specified in Pt 11, Ch 4, 6.1 Type A independent tanks 6.1.5, modified as appropriate taking into account their lower probability of occurrence see Figure 4.6.2 Hydro-pneumatic tank testing.

6.1.7  Testing

All Type A independent tanks shall be subjected to a hydrostatic or hydro-pneumatic test.

This test shall be performed such that the stresses approximate, as far as practicable, the design stresses, and that the pressure at the top of the tank corresponds at least to the MARVS. When a hydro-pneumatic test is performed, the conditions should simulate, as far as practicable, the design loading of the tank and of its support structure including dynamic components, while avoiding stress levels that could cause permanent deformation.

The following equations calculate the head of water required to model the design pressure, Peq, used in the scantling calculations of the tank structure. If a hydro-pneumatic test is utilised, the head of water hHP is to be taken as:

where

hHP = test head of water, in metres, measured from bottom of cargo tank

Peq = design pressure, in MPa, at location under consideration as derived from Pt 11, Ch 4, 3.3 Functional loads 3.3.2

P = air test pressure, in MPa

RD = ρ/ρfreshwater

ρ = density of test fluid ρfreshwater= 1000 kg/m3 at 4°C

y = the vertical distance, in metres, from bottom of tank to the location under consideration, see Figure 4.6.2 Hydro-pneumatic tank testing

For a given head of water, hHP, the load, in MPa, at the location under consideration would be:

Care is to be taken that the ratio at any point around the tank.

If a hydrostatic test is utilised, the head of water, hHS, is to be taken as:

where

hHS = test head of water, in metres, measured above top of cargo tank of depth h

h = height of tank as defined in Figure 4.6.2 Hydro-pneumatic tank testing)

For a given head of water, hHS, the load, in MPa, at the location under consideration would be:

Care is to be taken that the ratio at any point around the tank.

The test pressure is to be not less than the MARVS.

The design pressure is not to be exceeded at any point, and the test should adequately load all areas of the tank. See also Pt 3, Ch 1, 9.7 Definitions and details of testsin the Rules for Ships. When testing takes place after installation of the tanks on board the ship unit, care is to be taken that the test head does not result in excessive local loading on the hull structure. For this purpose, when the cargo tanks are centrally divided with a non-perforated bulkhead, consideration will be given to testing the port and starboard sides of the tank independently.

Figure 4.6.2 Hydro-pneumatic tank testing

6.2 Type B independent tanks

6.2.1  Design basis
  1. Type B independent tanks are tanks designed using model tests, refined analytical tools and analysis methods to determine stress levels, fatigue life and crack propagation characteristics. Where such tanks are primarily constructed of plane surfaces (prismatic tanks) the design vapour pressure P o shall be less than 0,07 MPa.
  2. If the cargo temperature at atmospheric pressure is below –10°C, a partial secondary barrier with a small leak protection system is required as defined in Pt 11, Ch 4, 2.3 Secondary barriers in relation to tank types. The small leak protection system shall be designed according to Pt 11, Ch 4, 2.5 Partial secondary barriers and primary barrier small leak protection system.

    Figure 4.6.3 Acceleration ellipse

6.2.2  Structural analysis
  1. The effects of all dynamic and static loads shall be used to determine the suitability of the structure with respect to:
    • plastic deformation;
    • buckling;
    • fatigue failure;
    • crack propagation.

    Finite element analysis or similar methods and fracture mechanics analysis or an equivalent approach, shall be carried out.

  2. A three-dimensional analysis shall be carried out to evaluate the stress levels, including interaction with the hull of the ship unit. The model for this analysis shall include the cargo tank with its supporting and keying system, as well as a reasonable part of the hull.
  3. A complete analysis of the particular accelerations and motions of the ship unit in irregular waves, and of the response of the ship unit and its cargo tanks to these forces and motions shall be performed unless the data is available from similar ship units.
    1. Type B independent tanks are to be subjected to a structural analysis by direct calculation procedures at a high confidence level. It is recommended that the assumptions made and the proposed calculation procedures be agreed with LR at an early stage. Where necessary, model or other tests may be required.
    2. Generally, the scantlings of cargo tanks primarily constructed of plane surfaces are not to be less than required by Pt 11, Ch 4, 6.1 Type A independent tanks 6.1.4 for Type A independent tanks. In assessing the cumulative effect of the fatigue load, account is to be taken of the quality control aspects such as misalignment, distortion, fit-up and weld shape. A 97,7 per cent survival probability S–N curve is to be adopted in association with a cumulative damage factor C w value of 0,1 for primary members and 0,5 for secondary members. Alternative proposals will be specially considered.
6.2.3  On-site operation design condition
  1. Plastic deformation

    Allowable stresses for Type B independent tanks are to be in accordance with Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.3 and (ii) as applicable.

    1. For Type B independent tanks, primarily constructed of bodies of revolution, the allowable stresses shall not exceed:
      • σm ≤ f
      • σL ≤ 1,5f
      • σb ≤ 1,5F
      • σL+σb ≤ 1,5F
      • σm+σb ≤ 1,5F
      • σm+σb+σg ≤ 3,0F
      • σL+σb+σg ≤ 3,0F

      where

      • σm = equivalent primary general membrane stress
      • σL = equivalent primary local membrane stress
      • σb = equivalent primary bending stress
      • σg = equivalent secondary stress
      • f = the lesser of (R m /A) or (R e /B)
      • F = the lesser of (R m /C) or (R e /D)

      with R m and R e as defined in Pt 11, Ch 4, 4.3 Design conditions 4.3.2.(c)(i). With regard to the stresses σm, σL and σb see also the definition of stress categories in Pt 11, Ch 4, 7.1 Guidance Notes for Chapter 4 7.1.3. The values A, B, C and D shall have at least the minimum values shown in Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.3.

    2. For Type B independent tanks, primarily constructed of plane surfaces, the allowable membrane equivalent stresses applied for finite element analysis will be specially considered:
    3. The thickness of the skin plate and the size of the stiffener shall not be less than those required for Type A independent tanks.

      Table 4.6.1 Factors for determining allowable stress for Type B independent tanks

        Nickel steel and carbon manganese steels Austenitic steels Aluminium alloys
      A 3 3,5 4
      B 2 1,6 1,5
      C 3 3 3
      D 1,5 1,5 1,5
  2. Buckling

    Buckling strength analyses of cargo tanks subject to external pressure and other loads causing compressive stresses shall be carried out in accordance with recognised standards. The method should adequately account for the difference in theoretical and actual buckling stress as a result of plate edge misalignment, lack of straightness or flatness, ovality and deviation from true circular form over a specified arc or chord length, as applicable.

6.2.4  Fatigue design condition
  1. Fatigue and crack propagation assessment shall be performed in accordance with the provisions of Pt 11, Ch 4, 4.3 Design conditions 4.3.3.
  2. Fatigue analysis shall consider construction tolerances.
  3. Where deemed necessary by the Administration, model tests may be required to determine stress concentration factors and fatigue life of structural elements.
6.2.5  Accident design condition
  1. The tanks and the tank supports shall be designed for the accidental loads and design conditions specified in Pt 11, Ch 4, 3.3 Functional loads 3.3.9 and Pt 11, Ch 4, 3.5 Accidental loads, as relevant.
  2. When subjected to the accidental loads specified in Pt 11, Ch 4, 3.5 Accidental loads, the stress shall comply with the acceptance criteria specified in Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.3, modified as appropriate, taking into account their lower probability of occurrence.

6.2.6  Testing

Type B independent tanks shall be subjected to a hydrostatic or hydro-pneumatic test as follows:
  • The test shall be performed as required in Pt 11, Ch 4, 6.1 Type A independent tanks 6.1.7 for Type A independent tanks
  • In addition, the maximum primary membrane stress or maximum bending stress in primary members under test conditions shall not exceed 90 per cent of the yield strength of the material (as fabricated) at the test temperature. To ensure that this condition is satisfied, when calculations indicate that this stress exceeds 75 per cent of the yield strength the prototype test shall be monitored by the use of strain gauges or other suitable equipment.

6.2.7  Marking

Any marking of the pressure vessel shall be achieved by a method that does not cause unacceptable local stress raisers.

6.3 Type C independent tanks

6.3.1  Design basis
  1. The design basis for Type C independent tanks is based on pressure vessel criteria modified to include fracture mechanics and crack propagation criteria. The minimum design pressure defined in Pt 11, Ch 4, 6.3 Type C independent tanks 6.3.1.(b) is intended to ensure that the dynamic stress is sufficiently low so that an initial surface flaw will not propagate more than half the thickness of the shell during the lifetime of the tank.
  2. The design vapour pressure shall not be less than:

    P o = 0,2 + AC(ρr)1,5 (MPa)

    where:

    with

    σm = design primary membrane stress
    ΔσA = allowable dynamic membrane stress (double amplitude at probability level Q = 10–8)
    = 55 N/mm2 for ferritic-perlitic, martensitic and austenitic steel
    = 25 N/mm2 for aluminium alloy (5083-O)
    C = a characteristic tank dimension to be taken as the greatest of the following: h, 0,75b or 0,45l

    with

    h = height of tank (dimension in ship unit’s vertical direction) (m)
    b = width of tank (dimension in ship unit’s transverse direction) (m)
    l = length of tank (dimension in ship unit’s longitudinal direction) (m)
    ρr = the relative density of the cargo (ρr = 1 for fresh water) at the cargo design temperature

    When a specified design life of the tank is longer than 108 wave encounters ΔσA shall be modified to give equivalent crack propagation corresponding to the design life.

  3. Alternative means of calculating the design vapour pressure referred to in (b) will be accepted.
  4. The Administration may allocate a tank complying with the criteria of Type C, minimum design pressure as in (b), to a Type A or Type B, dependent on the configuration of the tank and the arrangement of its supports and attachments.
  5. Before construction of the pressure vessels is commenced, the following particulars, where applicable, and plans are to be submitted for approval:
    • Nature of cargoes, together with maximum vapour pressures and minimum liquid temperature for which the pressure vessels are to be approved, and proposed hydraulic test pressure.
    • Particulars of materials proposed for the construction of the vessels.
    • Particulars of refrigeration equipment.
    • General arrangement plan showing location of pressure vessels in the ship unit.
    • Plans of pressure vessels showing attachments, openings, dimensions, details of welded joints and particulars of proposed stress relief heat treatment.
    • Plans of seatings, securing arrangements and deck sealing arrangements.
    • Plans showing arrangement of mountings, level gauges and number, type and size of safety valves.
6.3.2  Shell thickness
  1. The shell thickness shall be as follows:
    1. For pressure vessels, the thickness calculated according to (e) shall be considered as a minimum thickness after forming, without any negative tolerance.
    2. For pressure vessels, the minimum thickness of shell and heads including corrosion allowance, after forming, shall not be less than 5 mm for carbon-manganese steels and nickel steels, 3 mm for austenitic steels or 7 mm for aluminium alloys.
    3. The welded joint efficiency factor to be used in the calculation according to (e) shall be 0,95 when the inspection and the non-destructive testing referred to in Pt 11, Ch 6 Materials of Construction and Quality Control are carried out. This value may be increased up to 1,0 when account is taken of other considerations, such as the material used, type of joints, welding procedure and type of loading. For process pressure vessels LR may accept partial non-destructive examinations, but not less than those of Pt 11, Ch 6 Materials of Construction and Quality Control , depending on such factors as the material used, the cargo design temperature, the nil-ductility transition temperature of the material as fabricated and the type of joint and welding procedure, but in this case an efficiency factor of not more than 0,85 should be adopted. For special materials the above-mentioned factors shall be reduced, depending on the specified mechanical properties of the welded joint.
  2. The design liquid pressure defined in Pt 11, Ch 4, 3.3 Functional loads 3.3.2 shall be taken into account in the internal pressure calculations.
  3. The thickness of pressure parts subject to internal pressure is to be in accordance with Pt 5, Ch 11 Other Pressure Vessels of the Rules and Regulations for the Classification of Ships, July 2022 except that:
    1. the welded joint efficiency factor, J, is to be as defined in (a)(iii) above;
    2. the allowable stress is to be in accordance with Pt 11, Ch 4, 6.3 Type C independent tanks 6.3.3;
    3. the constant thickness increment (0,75 mm) included in the formulae in Pt 5, Ch 11, 2 Cylindrical shells and drums subject to internal pressure of the Rules and Regulations for the Classification of Ships, July 2022 may require to be increased in accordance with Pt 11, Ch 4, 2.1 Functional requirements 2.1.6.
  4. The design external pressure Pe , used for verifying the buckling of the pressure vessels, shall not be less than that given by:
    Pe = P1 + P2 + P3 + P4 (MPa)

    where

    P1 = setting value of vacuum relief valves. For vessels not fitted with vacuum relief valves P1 shall be specially considered, but shall not in general be taken as less than 0,025 MPa
    P2 = the set pressure of the pressure relief valves (PRVs) for completely closed spaces containing pressure vessels or parts of pressure vessels; elsewhere P2 = 0
    P3 = compressive actions in or on the shell due to the weight and contraction of thermal insulation, weight of shell including corrosion allowance and other miscellaneous external pressure loads to which the pressure vessel may be subjected. These include, but are not limited to, weight of domes, weight of towers and piping, effect of product in the partially filled condition, accelerations and hull deflection. In addition, the local effect of external or internal pressures or both shall be taken into account
    P4 = external pressure due to head of water for pressure vessels or part of pressure vessels on exposed decks; elsewhere P4 = 0.
  5. Scantlings based on internal pressure shall be calculated as follows:

    The thickness and form of pressure-containing parts of pressure vessels, under internal pressure, as defined in Pt 11, Ch 4, 3.3 Functional loads 3.3.2, including flanges, should be determined. These calculations shall in all cases be based on accepted pressure vessel design theory. Openings in pressure-containing parts of pressure vessels shall be reinforced in accordance with recognised Standards.

  6. Stress analysis in respect of static and dynamic loads shall be performed as follows:
    1. Pressure vessel scantlings shall be determined in accordance with (a) to (e) and Pt 11, Ch 4, 6.3 Type C independent tanks 6.3.3.
    2. Calculations of the loads and stresses in way of the supports and the shell attachment of the support shall be made. Loads referred to in Pt 11, Ch 4, 3.2 Permanent loads to Pt 11, Ch 4, 3.5 Accidental loads shall be used, as applicable. Stresses in way of the supporting structures shall be to a recognised standard acceptable to LR. In special cases a fatigue analysis may be required by LR.
    3. If required by LR, secondary stresses and thermal stresses shall be specially considered.
6.3.3  On-site operation design condition
  1. Plastic deformation
    For Type C independent tanks, the allowable stresses shall not exceed:
    • σm ≤ f
    • σL ≤ 1,5f
    • σb ≤ 1,5f
    • σL+σb ≤ 1,5f
    • σm+σb ≤ 1,5f
    • σm+σb+σg ≤ 3,0f
    • σL+σb+σg ≤ 3,0f

    where

    σm = equivalent primary general membrane stress
    σL = equivalent primary local membrane stress
    σb = equivalent primary bending stress
    σg = equivalent secondary stress
    f = the lesser of (Rm/A ) or (Re/B )
    with R m and R e as defined in Pt 11, Ch 4, 4.3 Design conditions 4.3.2.(c). With regard to the stresses σm, σL, σb and σg see also the definition of stress categories in Pt 11, Ch 4, 7.1 Guidance Notes for Chapter 4 7.1.3. The values A and B shall have at least the minimum values shown in Table 4.6.2 Factors for determining allowable.

    Table 4.6.2 Factors for determining allowable

      Nickel steels and carbon-manganese steels Austenitic steels Aluminium alloys
    A 3 3,5 4
    B 1,5 1,5 1,5
  2. Buckling criteria shall be as follows:

    The thickness and form of pressure vessels subject to external pressure and other loads causing compressive stresses shall be based on calculations using accepted pressure vessel buckling theory and shall adequately account for the difference in theoretical and actual buckling stress as a result of plate edge misalignment, ovality and deviation from true circular form over a specified arc or chord length.

6.3.4  Fatigue design condition

For large Type C independent tanks where the cargo at atmospheric pressure is below –55°C, LR may require additional verification to check their compliance with Pt 11, Ch 4, 6.3 Type C independent tanks 6.3.1, regarding static and dynamic stress.

6.3.5  Accident design condition
  1. The tanks and the tank supporting structures shall be designed for the accidental loads and design conditions specified in Pt 11, Ch 4, 2.1 Functional requirements 2.1.5.(c)(i) and Pt 11, Ch 4, 3.5 Accidental loads, as relevant.
  2. When subjected to the accidental loads specified in Pt 11, Ch 4, 3.5 Accidental loads, the stress shall comply with the acceptance criteria specified in Pt 11, Ch 4, 6.3 Type C independent tanks 6.3.3, modified as appropriate taking into account their lower probability of occurrence.
6.3.6  Testing
  1. Each pressure vessel shall be subjected to a hydrostatic test at a pressure measured at the top of the tanks, of not less than 1,5 P o. In no case during the pressure test shall the calculated primary membrane stress at any point exceed 90 per cent of the yield stress of the material. To ensure that this condition is satisfied where calculations indicate that this stress will exceed 0,75 times the yield strength, the prototype test shall be monitored by the use of strain gauges or other suitable equipment in pressure vessels other than simple cylindrical and spherical pressure vessels.
  2. The temperature of the water used for the test shall be at least 30°C above the nil-ductility transition temperature of the material, as fabricated.
  3. The pressure shall be held for 2 hours per 25 mm of thickness, but in no case less than 2 hours.
  4. Where necessary for cargo pressure vessels, a hydro-pneumatic test may be carried out under the conditions prescribed in (a) to (c) .
  5. When a hydro-pneumatic test is performed, the conditions are to simulate, so far as practicable, the actual loading of the tank and its supports.
  6. Special consideration may be given to the testing of tanks in which higher allowable stresses are used, depending on service temperature. However, the requirements of (a) shall be fully complied with.
  7. After completion and assembly, each pressure vessel and its related fittings shall be subjected to an adequate tightness test, which may be performed in combination with the pressure testing referred to in Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.6.
  8. Pneumatic testing of pressure vessels other than cargo tanks shall only be considered on an individual case basis. Such testing shall only be permitted for those vessels designed or supported such that they cannot be safely filled with water, or for those vessels that cannot be dried and are to be used in a service where traces of the testing medium cannot be tolerated.

6.3.7  Marking

The required marking of the pressure vessel shall be achieved by a method that does not cause unacceptable local stress raisers.

6.4 Membrane tanks

6.4.1  Design basis
  1. The design basis for membrane containment systems is that thermal and other expansion or contraction is compensated for without undue risk of losing the tightness of the membrane.
  2. A systematic approach, based on analysis and testing, shall be used to demonstrate that the system will provide its intended function in consideration of the identified in service events as specified in Pt 11, Ch 4, 6.4 Membrane tanks 6.4.2.
  3. If the cargo temperature at atmospheric pressure is below –10°C a complete secondary barrier is required as defined in Pt 11, Ch 4, 2.3 Secondary barriers in relation to tank types. The secondary barrier shall be designed according to Pt 11, Ch 4, 2.4 Design of secondary barriers.
  4. The design vapour pressure P o shall not normally exceed 0,025 MPa. If the hull scantlings are increased accordingly and consideration is given, where appropriate, to the strength of the supporting thermal insulation, P o may be increased to a higher value but less than 0,07 MPa.
  5. The definition of membrane tanks does not exclude designs such as those in which non-metallic membranes are used or where membranes are included or incorporated into the thermal insulation.
  6. The thickness of the membranes is normally not to exceed 10 mm.
  7. The circulation of inert gas throughout the primary insulation space and the secondary insulation space, in accordance with Pt 11, Ch 9, 1.2 Atmosphere control within the hold spaces (cargo containment systems other than Type C independent tanks) 1.2.1, shall be sufficient to allow for effective means of gas detection.
6.4.2  Design considerations
  1. Potential incidents that could lead to loss of fluid tightness over the life of the membranes shall be evaluated. These include, but are not limited to:
    1. Ultimate design events
      • Tensile failure of membranes.
      • Compressive collapse of thermal insulation.
      • Thermal ageing.
      • Loss of attachment between thermal insulation and hull structure.
      • Loss of attachment of membranes to thermal insulation system.
      • Structural integrity of internal structures and their supports.
      • Failure of the supporting hull structure.
    2. Fatigue design events
      • Fatigue of membranes including joints and attachments to hull structure.
      • Fatigue cracking of thermal insulation.
      • Fatigue of internal structures and their supports.
      • Fatigue cracking of inner hull leading to ballast water ingress.
    3. Accident design events
      • Accidental mechanical damage (such as dropped objects inside the tank while in service).
      • Accidental over-pressurisation of thermal insulation spaces.
      • Accidental vacuum in the tank.
      • Water ingress through the inner hull structure.

      Designs where a single internal event could cause simultaneous or cascading failure of both membranes are unacceptable.

  2. The necessary physical properties (mechanical, thermal, chemical, etc.) of the materials used in the construction of the cargo containment system shall be established during the design development in accordance with Pt 11, Ch 4, 6.4 Membrane tanks 6.4.1.(b).
  3. Loads, load combinations

    Particular consideration shall be paid to the possible loss of tank integrity due to either an overpressure in the interbarrier space, a possible vacuum in the cargo tank, the sloshing effects, to hull vibration effects, or any combination of these events.

  4. Structural analyses
    1. Structural analyses and/or testing for the purpose of determining the strength and fatigue assessments of the cargo containment and associated structures, e.g. structures as defined in Pt 11, Ch 4, 2.7 Associated structure and equipment shall be performed. The structural analysis shall provide the data required to assess each failure mode that has been identified as critical for the cargo containment system.
    2. Structural analyses of the hull shall take into account the internal pressure as indicated in Pt 11, Ch 4, 3.3 Functional loads 3.3.2. Special attention shall be paid to deflections of the hull and their compatibility with the membrane and associated thermal insulation.
    3. The analyses referred to in Pt 11, Ch 4, 6.4 Membrane tanks 6.4.2.(d).(i) and Pt 11, Ch 4, 6.4 Membrane tanks 6.4.2.(d).(ii) shall be based on the particular motions, accelerations and response of ship units and cargo containment systems.
    4. The hull structure supporting the membrane tank is to be incorporated into the structural finite element model of the ship unit. The scantlings of the inner hull are to be not less than required by Pt 10 Ship Units.
    5. Strength analysis is also to be carried out for structures inside the tank, i.e. pump towers, and its attachments. This should take account of hydrodynamic loads due to the sloshing motion of the cargo, inertia loading due to the accelerations of the vessel, and thermal effects due to loading and unloading of the tanks in accordance with the operational philosophy. The assessment is to consider stress levels, including shear stresses in the welds, buckling, fatigue (including fatigue due to thermal effects), and vibration.
6.4.3  On-site operation design condition
  1. The structural resistance of every critical component, sub-system, or assembly, shall be established, in accordance with Pt 11, Ch 4, 6.4 Membrane tanks 6.4.1.(b), for in-service conditions.
  2. The choice of strength acceptance criteria for the failure modes of the cargo containment system, its attachments to the hull structure and internal tank structures, shall reflect the consequences associated with the considered mode of failure.
  3. The inner hull scantlings shall meet the requirements for deep tanks, taking into account the internal pressure as indicated in Pt 11, Ch 4, 3.3 Functional loads 3.3.2 and the specified appropriate requirements for sloshing load as defined in Pt 11, Ch 4, 3.4 Environmental loads 3.4.4.
6.4.4  Fatigue design condition
  1. Fatigue analysis shall be carried out for structures inside the tank, i.e. pump towers, and for parts of membrane and pump tower attachments, where failure development cannot be reliably detected by continuous monitoring.
  2. The fatigue calculations shall be carried out in accordance with Pt 11, Ch 4, 4.3 Design conditions 4.3.3, with relevant requirements depending on:
    • The significance of the structural components with respect to structural integrity.
    • Availability for inspection.
  3. For structural elements for which it can be demonstrated by tests and/or analyses that a crack will not develop to cause simultaneous or cascading failure of both membranes, C w shall be less than or equal to 0,5.
  4. Structural elements subject to periodic inspection, and where an unattended fatigue crack can develop to cause simultaneous or cascading failure of both membranes, shall satisfy the fatigue and fracture mechanics requirements stated in Pt 11, Ch 4, 4.3 Design conditions 4.3.3.(h).
  5. Structural elements not accessible for in-service inspection, and where a fatigue crack can develop without warning to cause simultaneous or cascading failure of both membranes, shall satisfy the fatigue and fracture mechanics requirements stated in Pt 11, Ch 4, 4.3 Design conditions 4.3.3.(i).
  6. Selected details of the containment system are to be investigated by fatigue analysis, which should take into account interactions with the vessel-supporting structure of the ship unit, including local, transverse and longitudinal hull girder effects, also pressure loading from the cargo and from ballast acting on the supporting structure. The number of cycles of full and partial loading and unloading are to be consistent with the operational philosophy of the unit. For investigation of the fatigue damage sustained by the secondary barrier following failure of the primary barrier, a simplified load distribution over the RD, as specified in Pt 11, Ch 4, 1.1 Definitions 1.1.9, may be used, unless different project-specific requirements apply, as described in Pt 11, Ch 4, 2.4 Design of secondary barriers 2.4.2. Project-specific requirements are to be submitted for consideration.
  7. The fatigue damage factor of both the containment system and internal structures such as pump towers is generally to be no greater than 0,5, but is to be no greater than 0,1 for any structural detail which is not accessible for survey during the service life of the vessel and whose failure would cause the simultaneous breach of both the primary and secondary barrier, such as the attachment weld of the pump tower base support.
6.4.5  Accident design condition
  1. The containment system and the supporting hull structure shall be designed for the accidental loads specified in Pt 11, Ch 4, 3.5 Accidental loads. These loads need not be combined with each other or with environmental loads.
  2. Additional relevant accident scenarios shall be determined based on a risk analysis. Particular attention shall be paid to securing devices inside of tanks.
6.4.6  Design development testing
  1. The design development testing required in Pt 11, Ch 4, 6.4 Membrane tanks 6.4.1.(b) should include a series of analytical and physical models of both the primary and secondary barriers, including corners and joints, tested to verify that they will withstand the expected combined strains due to static, dynamic and thermal loads. This will culminate in the construction of a prototype scaled model of the complete cargo containment system.

    Testing conditions considered in the analytical and physical model shall represent the most extreme service conditions the cargo containment system will be likely to encounter over its life.

    Proposed acceptance criteria for periodic testing of secondary barriers required in Pt 11, Ch 4, 2.4 Design of secondary barriers 2.4.2 is to be based on the results of testing carried out on the prototype scaled model.

  2. The fatigue performance of the membrane materials and representative welded or bonded joints in the membranes shall be determined by tests.

    The ultimate strength and fatigue performance of arrangements for securing the thermal insulation system to the hull structure shall be determined by analyses or tests.

6.4.7  Testing

In ship units fitted with membrane cargo containment systems, all tanks and other spaces that may normally contain liquid and are adjacent to the hull structure supporting the membrane, shall be hydrostatically tested.

All hold structures supporting the membrane shall be tested for tightness before installation of the cargo containment system.

Pipe tunnels and other compartments that do not normally contain liquid need not be hydrostatically tested.

6.5 Integral tanks

6.5.1  Design basis

Integral tanks that form a structural part of the hull and are affected by the loads that stress the adjacent hull structure shall comply with the following:

  1. The design vapour pressure P o as defined in Pt 11, Ch 4, 1.1 Definitions 1.1.2 shall not normally exceed 0,025 MPa. If the hull scantlings are increased accordingly, P o may be increased to a higher value, but less than 0,07 MPa.
  2. Integral tanks may be used for products provided the boiling point of the cargo is not below –10°C. A lower temperature may be accepted by LR subject to special consideration, but in such cases a complete secondary barrier shall be provided.
6.5.2  Structural analysis
  1. On-site operation design condition

    Integral tanks are to be designed and constructed in accordance with the requirements for cargo tanks in Pt 10 Ship Units, using the actual cargo density and additional vapour pressure.

6.5.3  Accident design condition
  1. The tanks and the tank supports shall be designed for the accidental loads specified in Pt 11, Ch 4, 2.1 Functional requirements 2.1.5.(c) and Pt 11, Ch 4, 3.5 Accidental loads, as relevant.

6.5.4  Testing

All integral tanks shall be hydrostatically or hydro-pneumatically tested. The test shall be performed so that the stresses approximate, as far as practicable, to the design stresses and that the pressure at the top of the tank corresponds at least to the MARVS.

6.6 Semi-membrane tanks

6.6.1  Design basis
  1. Semi-membrane tanks are non-self-supporting tanks when in the loaded condition and consist of a layer, parts of which are supported through thermal insulation by the adjacent hull structure; the rounded parts of this layer connecting the above-mentioned supported parts are designed also to accommodate the thermal and other expansion or contraction.
  2. The design vapour pressure P o shall not normally exceed 0,025 MPa. If the hull scantlings are increased accordingly, and consideration is given, where appropriate, to the strength of the supporting thermal insulation, P o may be increased to a higher value but less than 0,07 MPa.
  3. For semi-membrane tanks the relevant requirements in this Section for independent tanks or for membrane tanks shall be applied as appropriate.
  4. A structural analysis and other analyses and calculations should be performed in accordance with the requirements for membrane tanks or independent tanks as appropriate, taking into account the internal pressure as indicated in Pt 11, Ch 4, 3.3 Functional loads 3.3.2.
  5. In the case of semi-membrane tanks that comply in all respects with the requirements applicable to Type B independent tanks, except for the manner of support, the Administration may, after special consideration, accept a partial secondary barrier.
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