Clasification Society Rulefinder 2016 - Version 9.25
Clasification Society Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, January 2016 - Part 11 PRODUCTION, STORAGE AND OFFLOADING OF LIQUEFIED GASES IN BULK - Chapter 5 Process Pressure Vessels and Liquids, Vapour and Pressure Piping Systems and Offshore Arrangements - Section 7 Cryogenic releases

Section 7 Cryogenic releases

Parent topic: Chapter 5 Process Pressure Vessels and Liquids, Vapour and Pressure Piping Systems and Offshore Arrangements

7.1  Cryogenic liquefied gas spill control

7.1.1  General
  1. Cryogenic liquefied gases are liquids that are kept in their liquid state at very low temperatures.
  2. Cryogenic liquefied gas release can cause or contribute to the failure of safety critical structures and equipment due to the embrittlement of steel or control systems in contact with the release.
  3. This Section considers the brittle fracture of critical structures and equipment as well as the failure of control systems due to cooling to a critical temperature following a leak of a cryogenic liquefied gas.
7.1.2  Scope
  1. The requirements of this Section are additional to those of this Chapter and are applicable to offshore units which are intended for the processing and carriage of cryogenic liquefied gas(es) in bulk.
7.1.3  Application
  1. The following Rules are applicable to the cryogenic process equipment, associated cryogenic piping systems and pipework serving essential safety systems in way of or within the vicinity of the cryogenic process area on board an offshore unit.
  2. Requirements for fire safety are not included in these Rules; instead they are subject to the satisfactory requirements of the National Administration.
7.1.4  Documents and plans
  1. Plans, together with particulars as detailed in this Section, are to be submitted for approval. Any subsequent modifications are subject to approval before being put into operation.
  2. A description of the expected method of operation of the process plant and a diagram showing the process flow are to be submitted.
  3. Particulars of the proposals for isolating the offshore unit from the shore/subsea installation and/or vessels, where applicable, are to be submitted, including:
    • Feedstock supply and product discharge, with details of the arrangements showing the location of shut-off valves and of the control and indicating stations.
    • The process plant parameters and analysis of transient conditions under which emergency shut-down will be initiated and the time estimated to obtain a safe environment.
    • The proposed emergency procedures for controlled shut-down of the process plant, i.e. depressurising, inerting, etc. and the arrangements for the continued operation of the essential services necessary to allow for such controlled shut-down under the emergency conditions.
  4. A risk assessment, or equivalent method acceptable to LR, is to be carried out for cryogenic liquefied gas spill.
  5. Risk assessment is to be carried out by representative specialists from the Owner, Builder and independent body/third party.
  6. A summary of the risk assessment is to be documented and submitted to LR for “for review only”.
  7. Depending on the likelihood and consequence of failures identified in the risk analysis, typical prevention and mitigation measures should be proposed or referred to.
  8. Each component of the cryogenic process equipment such as, and not limited to tanks, pumps, compressors, pipelines, valves and vessels must be considered as a potential source of cryogenic release. Special consideration shall be made for components which are not generally considered to be a source of cryogen release such as all welded pipelines, pressure vessels and associated welded instrumentation.
  9. During the risk assessment each identified accident/casualty scenario shall where necessary be graded with respects to severity of consequence and likelihood of occurrence. This grading shall provide a risk ranking that can be related to an appropriate risk matrix. The risk matrix shall distinguish risk into a series of groupings;
    • unacceptable or intolerable;
    • tolerable if ALARP; and
    • acceptable, tolerable or negligible.

The risk matrix may be adapted from the IMO Guidelines on Formal Safety Assessment (FSA) – MSC/Circ.1023 MEPC/Cic.392 and the target individual risk levels for crew members given by the FSA for LNG-Carriers - MSC 72/16.

7.1.5  Detection of cryogenic spill
  1. A detection system shall be provided to give warning of any cold spot due to the leakage of LNG or natural gas.
  2. Detecting of cold spots may include but are not limited to:
    • Gas detection.
    • Metal temperature monitoring.
    • Thermal imaging.
    • Visual inspection.
    • Video monitoring.
    • Monitoring of process parameters.
  3. The equipment installed for cold spot detection is to be approved by LR.
7.1.6  Cryogenic spill containment and suppression
  1. Isolation valves, for inventory control purposes, must be located as close as practically possible to vessels or equipment.
  2. Inventory isolation valves that are located within a recognised fire zone shall be protected against fire and explosion effects.
  3. For inventory control under fire conditions consideration shall be given to automatic operation of valves where:
    • manual operation of valves may involve danger to operators;
    • require a rapid response;
    • need unusual strength or dexterity.
  4. Where inventory isolation valves are automatically operated, they will need to be an emergency shut-down valves which are actuated by a process trip/alarm and/or actuated by gas or fire alarm.
7.1.7  Limiting liquid gas spills and releases
  1. Suitable arrangements are to be provided to reduce the chance of unintentional releases of liquid gas and mitigate the effects of such releases.
  2. Spray shields shall be fitted in way of all demountable joints, such as the terminal manifolds where leakage may occur at valves and pipe joints. Propriety shields or clamps, surrounding each demountable flange, fabricated from a material suitable for the pipework’s contents, may be proposed.
  3. Where open drive pumps are installed, splash guards and drip trays around and below the pump shaft seal shall be provided. Guards and drip trays shall be constructed of a suitable material as per the requirements of Pt 11, Ch 6, 1.4 Requirements for metallic materials.
  4. Where the inventory of liquid gas necessitates, the use of impoundments shall be proposed. In process areas where there are numerous valves, fittings, pumps and flanges a common impoundment, covering the area of possible liquid release, may be required.
  5. The capacity of the drip trays/impoundment shall be based on an assessment of the largest credible containable spill. For guidance, if means are provided to automatically detect liquid releases, this capacity may be the contents of the pipework between isolating valves. For discharge facilities this capacity may be outboard section of one transfer arm, or one cargo hose, plus the volume of liquid between one of the unit’s manifold valves and the highest point in the crossover.

7.2 Blowdown/depressurisation

7.2.1  Blowdown is defined as
  1. The depressurisation of a system, part of a system and its equipment to allow the safe disposal of both vapour and liquid discharged from blowdown valves. Depressurisation is used to mitigate the consequences of a pipeline or vessel leak by reducing the leakage rate and/or inventory within the pipe or vessel prior to a potential failure.

7.2.2 A depressurisation and blowdown system shall be provided for depressurising the high pressure liquid and gas pumps, vessels and pipework. The recommendations and guidelines given in standards such as ISO 23251 due to it being applicable to liquefied natural gas (LNG) and oil and gas production facilities shall be used for establishing a basis of design.

7.2.3  Where a liquid depressurisation system is provided, adequate provision shall be made in the design and installation for the effects of back pressure in the system and vapour flash-off when the pressures of liquids in the blowdown system are reduced.

7.2.4  Manual and automatic activation of the depressurisation system shall be provided.

7.2.5  Manual activation of the depressurisation system shall be possible from the process control station, local to the vessel or system being protected. Activation from other locations, as determined by the type, number, location and position of the process systems and equipment, shall also be possible. The designer of the system should recognise that a manual control may not be accessible during a fire.

7.2.6  Automatic activation shall be part of the emergency shutdown arrangements.

7.2.7  The maximum potential system release inventory due to depressurisation should be calculated for both individual systems and the maximum common-mode event. Consideration can be given to project specific philosophies such as staged blowdown. The disposal system is to be sized to deal with the maximum common-mode event inventory and resultant flash gas. To prevent exceeding the flare system capacity, the use of a liquid blowdown collection drum, knockout drum or liquid return to the storage tanks where possible, shall be proposed.

7.2.8  Substances which will react with each other are to be provided with separate systems.

7.3 Protection of steelwork against brittle fracture

7.3.1  Requirements are to be provided to minimize the risks associated with the uncontrolled release of low temperature liquids. In locations where a release of low temperature liquid could occur, suitable mitigation methods are to be provided. The techniques selected need to consider; the inventory volume, maximum liquid pressure, minimum liquid temperature and the direction of possible leakage.

7.3.2  Where drip trays and impoundment are used the material shall be selected to withstand exposure at the saturation temperature of the released liquid. The boundaries of the drip tray and impoundments are to be such to remain effective at the angles of inclination stated in Pt 5, Ch 1, 2.1 Inclination of unit 2.1.1.

7.3.3  Drip trays and impoundments containing low temperature liquid are not to adversely affect supporting or adjacent steelwork. The fitting of thermal breaks to drip tray supports and insulation between impoundments and supporting steelwork structure is to be considered.

7.3.4  Where it is established that the liquid release may be substantial, the ability to drain drip trays and impoundments to be drained to an appropriate location or collection vessel is to be provided.

7.3.5  Unless the material has been selected accordingly, a water distribution system shall be fitted in way of the hull under the discharge connections to provide a low-pressure water curtain for additional protection of the hull steel and the side structure. This system is in addition to the requirements of Pt 11, Ch 11, 1.3 Water-spray system 1.3.1, and shall be operated when discharging is in progress.

7.3.6  Personnel access ways, escape routes and refuge areas are to be protected against the possibility of uncontrolled release of low temperature liquids.

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