Section 5 Materials and construction
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 5 Materials and construction

Section 5 Materials and construction

Parent topic: Chapter 4 Cargo Containment

5.1 Materials

5.1.1  The specification and plans of the cargo containment system including the insulation are to be submitted for approval. The materials used are to be approved by LR, see Pt 11, Ch 6 Materials of Construction and Quality Control . For the plans to be submitted, see Pt 11, Ch 1, 1.7 Information and plans.

5.1.2  Materials forming the structure of the ship unit
  1. To determine the grade of plate and sections used in the hull structure, a temperature calculation shall be performed for all tank types when the cargo temperature is cooler than –10°C. The following assumptions should be made in this calculation:
    1. The primary barrier of all tanks shall be assumed to be at the cargo temperature.
    2. In addition to item (i), where a complete or partial secondary barrier is required it shall be assumed to be at the cargo temperature at atmospheric pressure for any one tank only.
    3. The ambient temperatures for air and sea-water are to be based on those levels for which these temperatures have a probability of exceedance of 99,6 per cent. The ambient temperatures are to be rounded down to the nearest degree Celsius.
    4. Still air and sea water conditions shall be assumed, i.e. no adjustment for forced convection.
    5. Degradation of the thermal insulation properties over the life of the ship unit due to factors such as thermal and mechanical ageing, compaction, ship motions and tank vibrations as defined in Pt 11, Ch 4, 5.1 Materials 5.1.4(g) and Pt 11, Ch 4, 5.1 Materials 5.1.4(h) shall be assumed.
    6. The cooling effect of the rising boil-off vapour from the leaked cargo should be taken into account where applicable.
    7. No credit shall be given for any means of heating, except as described in Pt 11, Ch 4, 5.1 Materials 5.1.4(e) and provided the heating arrangements are in compliance with Pt 11, Ch 4, 5.1 Materials 5.1.4.
    8. For members connecting inner and outer hulls, the mean temperature may be taken for determining the steel grade.
    9. Consideration should be given to scenarios in the lifecycle of the unit, other than on-site operation, where the cargo containment system may be used, tested or operated e.g. exposure to hotter or colder environmental temperatures.
    10. The data set used for the ambient temperatures calculation should be sufficiently large to minimise uncertainties. Where limited data is available sufficient margins shall be included in the calculation to account for statistical uncertainties. It is the responsibility of the Owner to determine and propose the final ambient temperatures most suitable for a particular unit's operational requirements.
    11. These site specific ambient temperatures are supplementary to any other statutory requirements which may be applicable to the unit. As such, this Section is not intended to duplicate, alter, amend or supersede any requirements that may be imposed by the National Authority or Administrations.

    When heat transmission studies are carried out, the heat balance method is acceptable to LR.

  2. The shell and deck plating of the ship unit and all stiffeners attached thereto shall be in accordance with the requirements of Pt 10 Ship Units and this Part. If the calculated temperature of the material in the design condition is below –5°C due to the influence of the cargo temperature and ambient sea and air temperatures, the material shall be in accordance with Table 6.1.5 Plates and sections for hull structures required by . The ambient sea and air temperatures are to be determined as defined in (a)(iii).
  3. The materials of all other hull structures for which the calculated temperature in the design condition is below 0°C, due to the influence of cargo temperature and ambient sea and air temperatures, and that do not form the secondary barrier, shall also be in accordance with Table 6.1.5 Plates and sections for hull structures required by . This includes hull structure supporting the cargo tanks, inner bottom plating, longitudinal bulkhead plating, transverse bulkhead plating, floors, webs, stringers and all attached stiffening members. The ambient sea and air temperatures are to be determined as defined in (a)(iii).
  4. The hull material forming the secondary barrier shall be in accordance with Table 6.1.2 Plates, sections and forgings (see Note 1) for cargo tanks, secondary barriers and process pressure vessels for cargo design temperatures below 0°C and down to –55°C, maximum thickness 25 mm (see Note 2). Where the secondary barrier is formed by the deck or side shell plating, the material grade required by Table 6.1.2 Plates, sections and forgings (see Note 1) for cargo tanks, secondary barriers and process pressure vessels for cargo design temperatures below 0°C and down to –55°C, maximum thickness 25 mm (see Note 2) shall be carried into the adjacent deck or side shell plating, where applicable, to a suitable extent.
  5. Means of heating structural materials may be used to ensure that the material temperature does not fall below the minimum allowed for the grade of material specified in Table 6.1.5 Plates and sections for hull structures required by . In the calculations required in (a), credit for such heating may be taken in accordance with the following:
    1. for any transverse hull structure;
    2. for longitudinal hull structure referred to in (b) and (c) where colder ambient temperatures are specified, provided the material remains suitable for the ambient temperature conditions of +5°C for air and 0°C for sea-water with no credit taken in the calculations for heating; and
    3. as an alternative to (ii), for longitudinal bulkhead between cargo tanks, credit may be taken for heating provided the material remains suitable for a minimum design temperature of –30°C, or a temperature 30°C lower than that determined by Pt 11, Ch 4, 5.1 Materials 5.1.2 with the heating considered, whichever is less. In this case, the longitudinal strength of the ship unit shall comply with SOLAS Regulation Regulation 3-1 - Structural, mechanical and electrical requirements for ships for both when those bulkhead(s) are considered effective and not.
  6. The means of heating referred to in Pt 11, Ch 4, 5.1 Materials 5.1.2.(e) shall comply with the following requirements:
    1. the heating system shall be arranged so that, in the case of a single failure of a mechanical or electrical component in any part of the system, standby heating can be maintained equal to not less than 100 per cent of the theoretical heat requirement;
    2. the heating system shall be considered as an essential auxiliary. Where the requirements specified by Pt 11, Ch 4, 5.1 Materials 5.1.2.(f).(i) are met by duplication of the system components, i.e. heaters, glycol circulation pumps, electrical control panel, auxiliary boilers etc., all electrical components of at least one of the systems are to be supplied from the emergency switchboard;
    3. where duplication of the primary source of heat, e.g. oil-fired boiler, is not feasible, alternative proposals such as an electric heater capable of providing 100 per cent of the theoretical heat requirement is to be provided, which is to be supplied by an independent circuit, arranged separately on the emergency switchboard. Other solutions may be considered acceptable towards satisfying the requirements specified in Pt 11, Ch 4, 5.1 Materials 5.1.2.(f).(i) provided that a suitable risk assessment is conducted to the satisfaction of the Administration. The requirement in paragraph Pt 11, Ch 4, 5.1 Materials 5.1.2.(f).(ii) continues to apply to all other electrical components in the system; and
    4. the design and construction of the heating system shall be included in the approval of the containment system by LR.

    Details of the proposed heating system are to be submitted.

5.1.3  Materials of primary and secondary barriers
  1. Metallic materials used in the construction of primary and secondary barriers not forming the hull, shall be suitable for the design loads that they may be subjected to, and be in accordance with Table 6.1.1 Plates, pipes (seamless and welded, see Notes 1 and 2), sections and forgings for cargo tanks and process pressure vessels for cargo design temperatures not lower than 0°C, Table 6.1.2 Plates, sections and forgings (see Note 1) for cargo tanks, secondary barriers and process pressure vessels for cargo design temperatures below 0°C and down to –55°C, maximum thickness 25 mm (see Note 2) or Table 6.1.3 Plates, sections and forgings (see Note 1) for cargo tanks, secondary barriers and process pressure vessels for cargo design temperatures below –55°C and down to –165°C (see Note 2), maximum thickness 25 mm (see Notes 3 and 4).
  2. Materials, either non-metallic or metallic but not covered by Table 6.1.1 Plates, pipes (seamless and welded, see Notes 1 and 2), sections and forgings for cargo tanks and process pressure vessels for cargo design temperatures not lower than 0°C, Table 6.1.2 Plates, sections and forgings (see Note 1) for cargo tanks, secondary barriers and process pressure vessels for cargo design temperatures below 0°C and down to –55°C, maximum thickness 25 mm (see Note 2) and Table 6.1.3 Plates, sections and forgings (see Note 1) for cargo tanks, secondary barriers and process pressure vessels for cargo design temperatures below –55°C and down to –165°C (see Note 2), maximum thickness 25 mm (see Notes 3 and 4), used in the primary and secondary barriers may be approved by LR considering the design loads that they may be subjected to, their properties and their intended use.
  3. Where non-metallic materials, including composites, are used for or incorporated in the primary or secondary barriers, they shall be tested for the following properties, as applicable, to ensure that they are adequate for the intended service:
    • compatibility with the cargoes;
    • solubility in cargo;
    • absorption of cargo;
    • ageing;
    • density;
    • mechanical properties;
    • thermal expansion and contraction;
    • abrasion;
    • cohesion;
    • resistance to vibrations;
    • resistance to fire and flame spread;
    • resistance to fatigue failure and crack propagation;
    • influence of water;
    • resistance to cargo pressure.
  4. The above properties, where applicable, shall be tested for the range between the expected maximum temperature in service and 5°C below the minimum cargo design temperature, but not lower than –196°C.
  5. Where non-metallic materials, including composites, are used for the primary and secondary barriers, the joining processes shall also be tested as described above.
    1. Guidance on the use of non-metallic materials in the construction of primary and secondary barriers is provided in Appendix 1, 1,5.
  6. Consideration may be given to the use of materials in the primary and secondary barrier, which are not resistant to fire and flame spread, provided they are protected by a suitable system such as a permanent inert gas environment, or are provided with a fire retardant barrier.
5.1.4  Thermal insulation and other materials used in cargo containment systems
  1. Load-bearing thermal insulation and other materials used in cargo containment systems shall be suitable for the design loads.
  2. Thermal insulation and other materials used in cargo containment systems shall have the following properties, as applicable, to ensure that they are adequate for the intended service:
    • compatibility with the cargoes;
    • solubility in the cargo;
    • absorption of the cargo;
    • shrinkage;
    • ageing;
    • closed cell content;
    • density;
    • mechanical properties, to the extent that they are subjected to cargo and other loading effects, thermal expansion and contraction;
    • abrasion;
    • cohesion;
    • thermal conductivity;
    • resistance to vibrations;
    • resistance to fire and flame spread;
    • resistance to fatigue failure and crack propagation.
  3. In addition to the requirements given in (b), fatigue and crack propagation properties for insulation in membrane systems are also to be submitted. Insulation materials are to be approved by LR. Where applicable, these requirements also apply to any adhesive, sealers, vapour barriers, coatings or similar products used in the insulation system, any material used to give strength to the insulation system, components used to hold the insulation in place and any non-metallic membrane materials. Such products are to be compatible with the insulation.
  4. The above properties, where applicable, shall be tested for the range between the expected maximum temperature in service and 5°C below the minimum cargo design temperature, but not lower than –196°C.
  5. Due to location or environmental conditions, thermal insulation materials shall have suitable properties of resistance to fire and flame spread and shall be adequately protected against penetration of water vapour and mechanical damage. Where the thermal insulation is located on or above the exposed deck, and in way of tank cover penetrations, it shall have suitable fire resistance properties in accordance with a recognised Standard acceptable to LR or be covered with a material having low flame spread characteristics and forming an efficient approved vapour seal.
  6. Thermal insulation that does not meet recognised Standards acceptable to LR for fire resistance may be used in hold spaces that are not kept permanently inerted, provided its surfaces are covered with material with low flame spread characteristics and that forms an efficient approved vapour seal.
  7. Testing for thermal conductivity of thermal insulation shall be carried out on suitably aged samples.
  8. Where powder or granulated thermal insulation is used, measures shall be taken to reduce compaction in service, for example due to vibrations, and to maintain the required thermal conductivity and also prevent any undue increase of pressure on the cargo containment system.
  9. Particular attention is to be paid to the cleaning of the steelwork prior to the application of the insulation. Where insulation is to be foamed or sprayed in situ, the minimum steelwork temperature at the time of application is to be indicated in the specification in addition to environmental conditions.

5.2 Construction processes

5.2.1  A construction, testing and inspection (CTI) plan for the installation of the containment system is to be submitted for approval. This plan is to list the following sequentially for each stage of installation, testing and inspection:
(a) The method to be used.
(b) The acceptance criteria.
(c) The form of record to be made.
(d) The involvement of the shipyard, containment system designer where relevant, and LR Surveyor.
The testing and inspection should be commensurate with assumptions made in the design of the containment system, see Pt 11, Ch 4, 4.3 Design conditions 4.3.3.(f). Further detailed documents, which may be cross-referenced by the CTI plan, are to be submitted for approval as applicable.

5.2.2  A detailed quality assurance/quality control (QA/QC) programme shall be applied to ensure the continued conformity of materials in the containment system during installation and service. The quality assurance/quality control programme shall include the procedure for fabrication, storage, handling and preventive actions to guard against exposure of a material to harmful effects. The proposed procedure is to be submitted to LR for consideration. All materials in the containment system are also to be considered and included in the procedure. See also Appendix 1, 1.5.

5.2.3  Weld joint design
  1. All welded joints of the shells of independent tanks shall be of the in-plane butt weld full penetration type. For dome-to-shell connections only, tee welds of the full penetration type may be used depending on the results of the tests carried out at the approval of the welding procedure. Except for small penetrations on domes, nozzle welds are also to be designed with full penetration.

    Except for the dome-to-shell connections, T-butt welds will not be accepted in the shell.

    This requirement is applicable to independent tanks of type A or type B, primarily constructed of plane surfaces. This includes the tank corners which are constructed using bent plating which is aligned with the tank surfaces and connected with in-plane welds.

    The applicability of the expression ‘For dome-to-shell connections only’ is clarified as follows:

    • Welded corners (i.e. corners made of weld metal) shall not be used in the main tank shell construction, i.e. corners between the shell side (sloped plane surfaces parallel to hopper or top side inclusive, if any) and bottom or top of the tank, and between the tank end transverse bulkheads and the bottom, top or shell sides (sloped plane surfaces inclusive, if any) of the tank. Instead, tank corners which are constructed using bent plating aligned with the tank surfaces and connected with in-plane welds, are to be used.
    • Tee welds can be accepted for other localised constructions of the shell such as suction well, sump, dome, etc. where tee welds of full penetration type shall also be used.
  2. Welding joint details for Type C independent tanks, and for the liquid-tight primary barriers of Type B independent tanks primarily constructed of curved surfaces, shall be as follows:
    1. All longitudinal and circumferential joints shall be of butt welded, full penetration, double vee or single vee type. Full penetration butt welds shall be obtained by double welding or by the use of backing rings. If used, backing rings shall be removed except from very small process pressure vessels. Other edge preparations may be permitted, depending on the results of the tests carried out at the approval of the welding procedure.
    2. The bevel preparation of the joints between the tank body and domes and between domes and relevant fittings shall be designed according to a standard acceptable to LR. All welds connecting nozzles, domes or other penetrations of the vessel and all welds connecting flanges to the vessel or nozzles shall be full penetration welds.

    See also Pt 5, Ch 10, 14 Construction of the Rules for Ships.

    The applicability of the expression “Other edge preparations” is clarified as follows:
    • Cruciform full penetration welded joints in a bi-lobe tank with centreline bulkhead can be accepted for the tank structure construction at tank centreline welds with bevel preparation subject to the approval of LR, based on the results of the tests carried out at the approval of the welding procedure (see Figure 4.5.1 Cruciform full penetration weld).
  3. Where applicable, all the construction processes and testing, except that specified in Pt 11, Ch 4, 5.2 Construction processes 5.2.5 shall be done in accordance with the applicable provisions of Pt 11, Ch 6 Materials of Construction and Quality Control .


Figure 4.5.1 Cruciform full penetration weld

5.2.4  Design for gluing and other joining processes

The design of the joint to be glued (or joined by some other process except welding) shall take account of the strength characteristics of the joining process.

5.2.5  Testing during construction
  1. All cargo tanks and process pressure vessels shall be subjected to hydrostatic or hydro-pneumatic pressure testing in accordance with Pt 11, Ch 4, 6.1 Type A independent tanks to Pt 11, Ch 4, 6.6 Semi-membrane tanks, as applicable for the tank type.
  2. All tanks shall be subject to a tightness test which may be performed in combination with the pressure test referred to in (a).
  3. Requirements with respect to inspection of secondary barriers shall be decided by LR in each case, taking into account the accessibility of the barrier. See also Pt 11, Ch 4, 2.4 Design of secondary barriers 2.4.2.
  4. The Administration may require that, for ship units fitted with novel Type B independent tanks or tanks designed according to Pt 11, Ch 4, 8 Cargo containment systems of novel configuration, at least one prototype tank and its supporting structures shall be instrumented with strain gauges or other suitable equipment to confirm stress levels. Similar instrumentation may be required for Type C independent tanks, depending on their configuration and on the arrangement of their supports and attachments.
  5. The overall performance of the cargo containment system shall be verified for compliance with the design parameters during entry into service in accordance with the survey procedure. Records of the performance of the components and equipment, essential to verify the design parameters, shall be maintained and be available to the Administration.
  6. The overall performance of the cargo containment system is to be verified for compliance with the design parameters during initial acceptance cargo trials. The initial trials are to be witnessed by LR’s Surveyors, and are to demonstrate that the system is capable of being inerted, cooled, loaded and discharged in a satisfactory manner, and that all safety devices function correctly.

    The temperature at which these tests are carried out is to be at or near the minimum cargo temperature. Where a refrigeration plant is fitted, its operation is to be demonstrated to the Surveyors. Records of the plant performance taken during entry into service at minimum temperature are to be submitted. Logs of plant performance are to be maintained for examination by the Surveyors when requested.

  7. Heating arrangements, if fitted in accordance with Pt 11, Ch 4, 5.1 Materials 5.1.2.(e) and Pt 11, Ch 4, 5.1 Materials 5.1.2.(f), shall be tested for required heat output and heat distribution.
  8. The cargo containment system shall be inspected for cold spots during or immediately following entry into service. Inspection of the integrity of thermal insulation surfaces that can not be visually checked shall be carried out in accordance with recognised Standards.
  9. Repair Procedures shall define imperfection and defects and their allowable limits, identification of failure type and subsequent repair processes.

    Repairs shall be of a quality standard as defined in Pt 11, Ch 4, 5.2 Construction processes.

    Records of the performance of the repaired components and equipment, essential to verify the design parameters, shall be maintained and be available.

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