Section 2 Cargo containment

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.

The relevant acceptance criteria and allowable stresses are to be in accordance with Pt 11, Ch 4, 6.1 Type A independent tanks 6.1.6, or 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 addition to the loading conditions defined in Pt 11, Ch 4, 2.1 Functional requirements 2.1.5, a 10 000 year return period design condition is to be considered defined as follows:

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

2.1.9 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).

2.2.1 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 2.2.4 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.

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.

2.5.1 Partial secondary barriers as permitted in Pt 11, Ch 4, 2.2 Cargo containment safety principles 2.2.3 shall be used with a small leak protection system and meet all the requirements in 4.6.2. 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 4.18.2.6, 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.

2.6.1 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 4.12 to 4.15, 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 4.15.1 without plastic deformation likely to endanger the hull structure.

2.6.5 Supports and supporting arrangements shall withstand the loads defined in 4.13.8 and 4.15, 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.

Calculations and analyses are to be performed to show that there would be no gross failure of the supporting arrangements in this event as prescribed above for each tank type.

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).

2.8.1 Thermal insulation shall be provided as required to protect the hull from temperatures below those allowable (see Pt 11, Ch 4, 5 Materials and construction) 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.