Section 2 Cargo containment
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 2 Cargo containment

Section 2 Cargo containment

Parent topic: Chapter 4 Cargo Containment

2.1 Functional requirements

2.1.1  Details of the proposed design of cargo containment systems are to be submitted for consideration, and it is recommended this is done at as early a stage as possible. For a description of LR’s system of approval, refer to the Marine Survey Guidance System. See also Pt 11, Ch 1, 1.4 Alternative arrangements.

2.1.2 The design life of the cargo containment system shall not be less than the design life of the ship unit.

2.1.3  Cargo containment systems shall be designed with site-specific environmental loads for the proposed area of operation. The cargo containment system shall also be designed for all transit conditions as applicable to the operational philosophy of the unit; this includes delivery voyages and sail-away disconnect conditions.

2.1.4 Cargo containment systems shall be designed with suitable safety margins:
  1. to withstand, in the intact condition, the environmental conditions anticipated for the cargo containment system’s design life and the loading conditions appropriate for them, which include loads derived for the following scenarios: on-site operation, inspection/maintenance, transit/disconnect and accidental. The most onerous loading conditions are to be considered.
  2. that are appropriate for uncertainties in loads, structural modelling, fatigue, corrosion, thermal effects, material variability, ageing and construction tolerances.
2.1.5 The cargo containment system structural strength shall be assessed against failure modes, including but not limited to plastic deformation, buckling, and fatigue. The specific design conditions that should be considered for the design of each cargo containment system are given in Pt 11, Ch 4, 6.1 Type A independent tanks to Pt 11, Ch 4, 6.5 Integral tanks. The following three main categories of design conditions are to be assessed:
  1. On-site operation design conditions – The cargo containment system structure and its structural components shall withstand loads liable to occur during its construction, testing and anticipated use in service, without loss of structural integrity. The design shall take into account proper combinations of the following loads:
    • Internal pressure.
    • External pressure.
    • Dynamic loads due to the motion of the ship unit.
    • Thermal loads.
    • Sloshing loads.
    • Loads corresponding to deflections of the ship unit.
    • Tank and cargo weight with the corresponding reaction in way of supports.
    • Insulation weight.
    • Loads in way of towers and other attachments.
    • Test loads.

    The loads are to be calculated at a return period of 100 years.

    The relevant acceptance criteria and allowable stresses are to be in accordance with Pt 11, Ch 4, 6.1 Type A independent tanks 6.1.5, Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.3, Pt 11, Ch 4, 6.3 Type C independent tanks 6.3.3, Pt 11, Ch 4, 6.4 Membrane tanks 6.4.3, Pt 11, Ch 4, 6.5 Integral tanks and Pt 11, Ch 4, 6.4 Membrane tanks 6.4.2 as appropriate.

  2. Fatigue design conditions – The cargo containment system structure and its structural components shall not fail under accumulated cyclic loading.
  3. Accident design conditions – The cargo containment system shall provide the indicated response to each of the following accident conditions (accidental or abnormal events), addressed in this Part:
    • Collision – the cargo containment system shall be protectively located in accordance with Pt 11, Ch 2, 1.4 Location of cargo tanks 1.4.1 and withstand the collision loads specified in Pt 11, Ch 4, 3.5 Accidental loads 3.5.3 without deformation of the supports, or the tank structure in way of the supports, likely to endanger the tank structure.
    • Fire – The cargo containment systems shall sustain without rupture the rise in internal pressure specified in 8.4.1 under the fire scenarios envisaged therein.
    • Flooded compartment causing buoyancy on tank – The anti-flotation arrangements, for independent tanks, shall sustain the upward force, specified in Pt 11, Ch 4, 3.5 Accidental loads 3.5.2 and there should be no endangering plastic deformation to the hull.
    • Loss of heading control – when applicable, the effect of loss of heading control on the cargo containment system shall be considered in the design as specified in Pt 11, Ch 4, 3.5 Accidental loads
    • 10 000 year return period design condition – The cargo containment system integrity and its structural components shall withstand 10 000 year return period loads without loss of containment integrity, and without hydrocarbon release as specified by National Regulations, which could result in a major accident hazard for the unit. The design shall take into account proper combinations of the following loads:
      • Internal pressure.
      • External pressure.
      • Dynamic loads due to the motion of the ship unit.
      • Thermal loads.
      • Sloshing loads.
      • Loads corresponding to deflections of the ship unit.
      • Tank and cargo weight with the corresponding reaction in way of supports.
      • Insulation weight.
      • Loads in way of towers and other attachments.

      The associated safety factor for 10 000 year return period design condition is to be defined by the Owners/designers and is to comply with National Regulations. It shall be advised to LR at the commencement of the project.

The relevant acceptance criteria and allowable stresses are to be in accordance with Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.2, or Pt 11, Ch 4, 6.3 Type C independent tanks 6.3.4, or Pt 11, Ch 4, 6.4 Membrane tanks 6.4.2, or Pt 11, Ch 4, 6.5 Integral tanks 6.5.2 as appropriate.

2.1.6 Measures shall be applied to ensure that scantlings required meet the structural strength provisions and will be maintained throughout the design life. Measures include, but are not limited to, material selection, coatings, corrosion additions, cathodic protection and inerting.

Corrosion allowance need not be required in addition to the thickness resulting from the structural analysis. However, where there is no environmental control, such as inerting around the cargo tank, or where the cargo is of a corrosive nature, LR may require a suitable corrosion allowance.

2.1.7  In areas where excessive corrosion might be expected, a corrosion addition may be required if means of protection are not installed.

2.1.8 An inspection/survey plan for the cargo containment system shall be developed and approved at the time of build. The inspection/survey plan shall identify areas that need inspection during surveys throughout the cargo containment system’s life and in particular all necessary in-service survey and maintenance that was assumed when selecting cargo containment system design parameters. Cargo containment systems shall be designed, constructed and equipped to provide adequate means of access to areas that need inspection as specified in the inspection/survey plan. Cargo containment systems, including all associated internal equipment shall be designed and built to ensure safety during operations, inspection and maintenance (see Pt 11, Ch 3, 1.5 Access to spaces in the cargo area).

2.2 Cargo containment safety principles

2.2.1 The containment systems shall be provided with a full secondary liquid-tight barrier capable of safely containing all potential leakages through the primary barrier and, in conjunction with the thermal insulation system, of preventing lowering of the temperature of the structure of the ship unit to an unsafe level.

2.2.2 However, the size and configuration or arrangement of the secondary barrier can be reduced where an equivalent level of safety can be demonstrated in accordance with the requirements of Pt 11, Ch 4, 2.2 Cargo containment safety principles 2.2.3 to Pt 11, Ch 4, 2.2 Cargo containment safety principles 2.2.5 as applicable.

2.2.3 Cargo containment systems for which the probability for structural failures to develop into a critical state has been determined to be extremely low, but where the possibility of leakages through the primary barrier cannot be excluded, shall be equipped with a partial secondary barrier and small leak protection system capable of safely handling and disposing of the leakages.

The arrangements shall comply with the following requirements:
  1. Failure developments that can be reliably detected before reaching a critical state (e.g. by gas detection or inspection) shall have a sufficiently long development time for remedial actions to be taken.
  2. Failure developments that cannot be safely detected before reaching a critical state shall have a predicted development time that is much longer than the expected lifetime of the tank.

2.2.4 No secondary barrier is required for cargo containment systems, e.g. Type C independent tanks, where the probability for structural failures and leakages through the primary barrier is extremely low and can be neglected.

2.2.5 No secondary barrier is required where the cargo temperature at atmospheric pressure is at or above –10°C.

2.3 Secondary barriers in relation to tank types

2.3.1 Secondary barriers in relation to the tank types defined in Pt 11, Ch 4, 6.1 Type A independent tanks to Pt 11, Ch 4, 6.6 Semi-membrane tanks shall be provided in accordance with Table 4.2.1 Secondary barriers in relation to tank.

Table 4.2.1 Secondary barriers in relation to tank

Cargo temperature at atmospheric pressure –10°C and above Below –10°C down to –55°C Below –55°C
Basic tank type No secondary barrier required Hull may act as secondary barrier Separate secondary barrier where required
Integral   Tank type not normally allowed, see Note 1
Membrane   Complete secondary barrier
Semi-membrane   Complete secondary barrier see Note 2
Independent    
Type A   Complete secondary barrier
Type B   Partial secondary barrier
Type C   No secondary barrier required
NOTES
1. A complete secondary barrier should normally be required if cargoes with a temperature at atmospheric pressure below –10°C are permitted in accordance with Pt 11, Ch 4, 6.5 Integral tanks 6.5.1
2. 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.

2.4 Design of secondary barriers

2.4.1 Where the cargo temperature at atmospheric pressure is not below –55°C, the hull structure may act as a secondary barrier based on the following:
  1. the hull material shall be suitable for the cargo temperature at atmospheric pressure as required by Pt 11, Ch 4, 5.2 Construction processes 5.2.1 (d); and
  2. the design shall be such that this temperature will not result in unacceptable hull stresses.
2.4.2 The design of the secondary barrier shall be such that:
  1. it is capable of containing any envisaged leakage of liquid cargo for the RD, as specified in Pt 11, Ch 4, 4.3 Design conditions 4.3.3 (f) , unless different project-specific requirements apply, taking into account the load spectrum referred to in Pt 11, Ch 4, 4.3 Design conditions. Project-specific requirements are to be submitted for consideration.
  2. physical, mechanical, or operational events within the cargo tank that could cause failure of the primary barrier shall not impair the due function of the secondary barrier, or vice versa.
  3. failure of a support or an attachment to the hull structure will not lead to loss of liquid tightness of both the primary and secondary barriers.
  4. it is capable of being periodically checked for its effectiveness by means acceptable to LR of a visual inspection or a pressure/vacuum test or other suitable means carried out according to a documented procedure agreed with LR.
  5. The methods required in Pt 11, Ch 4, 2.4 Design of secondary barriers 2.4.2.(d) shall be approved by LR and shall include, where applicable to the test procedure:
    1. Details on the size of defect acceptable and the location within the secondary barrier, before its liquid tight effectiveness is compromised.
    2. Accuracy and range of values of the proposed method for detecting defects in Pt 11, Ch 4, 2.4 Design of secondary barriers.
    3. Scaling factors to be used if full scale model testing is not undertaken.
    4. Effects of thermal and mechanical cyclic loading on the effectiveness of the proposed test.
  6. The secondary barrier shall fulfil its functional requirements at a static angle of heel of 30°.
  7. Proposals for the periodical examination of the secondary barrier are to be submitted for consideration.

2.5 Partial secondary barriers and primary barrier small leak protection system

2.5.1 Partial secondary barriers shall be used with a small leak protection system and meet all the requirements in Pt 11, Ch 4, 2.4 Design of secondary barriers 2.4.2. The small leak protection system shall include means to detect a leak in the primary barrier, provision such as a spray shield to deflect any liquid cargo down into the partial secondary barrier, and means to dispose of the liquid, which may be by natural evaporation.

2.5.2 The capacity of the partial secondary barrier shall be determined, based on the cargo leakage corresponding to the extent of failure resulting from the load spectrum referred to in Pt 11, Ch 4, 4.3 Design conditions 4.3.3 (f), after the initial detection of a primary leak. Due account may be taken of liquid evaporation, rate of leakage, pumping capacity and other relevant factors.

2.5.3 The required liquid leakage detection may be by means of liquid sensors, or by an effective use of pressure, temperature or gas detection systems, or any combination thereof.

2.6 Supporting arrangements

2.6.1 The cargo tanks shall be supported by the hull in a manner that prevents bodily movement of the tank under the static and dynamic loads defined in Pt 11, Ch 4, 3.2 Permanent loads to Pt 11, Ch 4, 3.5 Accidental loads, where applicable, while allowing contraction and expansion of the tank under temperature variations and hull deflections without undue stressing of the tank and the hull.

2.6.2  Tank supporting arrangements are generally to be located in way of the primary support structure of the tank and the hull of the ship unit. Steel seatings are to be arranged, where possible, on both the inner bottom and underside of the cargo tank so as to ensure an effective distribution of the transmitted load and reactions into the cargo tanks and double bottom structure.

2.6.3  The strength of supporting arrangements is to be verified by direct calculation.

2.6.4 Anti-flotation arrangements shall be provided for independent tanks and be capable of withstanding the loads defined in Pt 11, Ch 4, 3.5 Accidental loads 3.5.2 without plastic deformation likely to endanger the hull structure.

2.6.5 Supports and supporting arrangements shall withstand the loads defined in Pt 11, Ch 4, 3.3 Functional loads and Pt 11, Ch 4, 3.5 Accidental loads , but these loads need not be combined with each other or with wave-induced loads.

2.6.6  An adequate clearance is to be provided between the anti-flotation chocks and the hull of the ship unit in all operational conditions.

2.7 Associated structure and equipment

2.7.1 Cargo containment systems are to be designed for the loads imposed by associated structure and equipment. This includes pump towers, cargo domes, cargo pumps and piping, stripping pumps and piping, inert gas piping, access hatches, ladders, piping penetrations, liquid level gauges, independent level alarm gauges, spray nozzles, and instrumentation systems (such as pressure, temperature and strain gauges).

2.8 Thermal insulation

2.8.1 Thermal insulation shall be provided as required to protect the hull from temperatures below those allowable (see Pt 11, Ch 4, 5.1 Materials 5.1.2) and to limit the heat flux into the tank to the levels that can be maintained by the pressure and temperature control system applied in Pt 11, Ch 7 Cargo Pressure/Temperature Control .

2.8.2 In determining the insulation performance, due regard should be paid to the amount of the acceptable boil-off in association with the liquefaction or reliquefaction plant on board, gas consumers if present or other temperature control system.

Copyright 2022 Clasifications Register Group Limited, International Maritime Organization, International Labour Organization or Maritime and Coastguard Agency. All rights reserved. Clasifications Register Group Limited, its affiliates and subsidiaries and their respective officers, employees or agents are, individually and collectively, referred to in this clause as 'Clasifications Register'. Clasifications Register assumes no responsibility and shall not be liable to any person for any loss, damage or expense caused by reliance on the information or advice in this document or howsoever provided, unless that person has signed a contract with the relevant Clasifications Register entity for the provision of this information or advice and in that case any responsibility or liability is exclusively on the terms and conditions set out in that contract.