BV: Rules for LNG cargo containment system with membrane

Classification society Bureau Veritas has published the Rules for the Design and Certification of Membrane Type LNG Cargo Containment System.

This document was issued on March 1st, 2023.

General

1.1 Scope

1.1.1 These Rules apply to cargo containment system fitted on ships granted with the notation liquefied gas carrier or LNG bunkering ship and carrying liquefied natural gas (LNG) as cargo, and for which the cargo containment type is membrane.

1.1.2 Ships for which these Rules are not applicable shall comply with the Rules for the Classification of Steel Ships (NR467).

1.1.3 The purpose of these Rules is to ensure the safe containment of cargo under all design and operating conditions having regard to the nature of the cargo carried.

This will include measures to:

provide strength to withstand defined loads
maintain the cargo in a liquid state
design for or protect the hull structure from low temperature exposure
prevent the ingress of water or air into the cargo containment system.

Application

2.1 General

2.1.1 Unless otherwise specified in [24], the requirements of [3] to [21] shall apply to membrane tanks, including those covered in [22].

Functional requirement

3.1 General

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

3.1.2 Cargo containment systems shall be designed for North Atlantic environmental conditions and relevant long-term sea state scatter diagrams for unrestricted navigation.

Lesser environmental conditions, consistent with the expected usage, may be accepted by the Society for cargo containment systems used exclusively for restricted navigation.

Greater environmental conditions may be required for cargo containment systems operated in conditions more severe than the North Atlantic environment.

3.1.3 Cargo containment systems shall be designed with suitable safety margins:

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 full homogeneous and partial load conditions, partial filling within defined limits and ballast voyage loads; and
being appropriate for uncertainties in loads, structural modelling, fatigue, corrosion, thermal effects, material variability, ageing and construction tolerances.

3.1.4 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 which shall be considered for the design of each cargo containment system are given in [24].

There are three main categories of design conditions:

a) Ultimate 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
thermal loads
sloshing loads
loads corresponding to ship deflections
tank and cargo weight with the corresponding reaction in way of supports
insulation weight
loads in way of towers and other attachments; and
test loads.

b) Fatigue design conditions – the cargo containment system structure and its structural components shall not fail under accumulated cyclic loading.

c) Accident design condition:

The cargo containment system shall meet the following criteria:

Collision: the cargo containment system shall be protectively located in accordance with NR467, Pt D, Ch 9, Sec 2, [4.1.1] and withstand the collision loads specified in [15.2] 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 NR467, Pt D, Ch 9, Sec 8, [4.1] under the fire scenarios envisaged therein.
unfavourable static heel scenario: the cargo containment systems shall sustain, without rupture, a static angle within the range 0° to 30°.

3.1.5 Measures shall be applied to ensure that scantlings required meet the structural strength provisions and be maintained throughout the design life.

Measures may 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, the Society may require a suitable corrosion allowance.

3.1.6 An inspection/survey plan for the cargo containment system shall be approved by the Society.

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 NR467, Pt D, Ch 9, Sec 3, [1.5].

Cargo containment safety principles

4.1 General

4.1.1 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 ship structure to an unsafe level.

4.1.2 However, the size and configuration or arrangement of the secondary barrier may be reduced where an equivalent level of safety is demonstrated in accordance with the requirements of [4.1.3], as applicable.

4.1.3 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 small leak protection system capable of safely handling and disposing of the leakages.

The arrangements shall comply with the following requirements:

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; and
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.