Section
7 Cryogenic releases
7.1 Cryogenic liquefied gas spill
control
7.1.1
General
- Cryogenic liquefied gases are liquids that are kept in their
liquid state at very low temperatures.
- 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.
- 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
- 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
- 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.
- 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
- 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.
- A description of the expected method of operation of the
process plant and a diagram showing the process flow are to be
submitted.
- 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.
- A risk assessment, or equivalent method acceptable to LR, is to
be carried out for cryogenic liquefied gas spill.
- Risk assessment is to be carried out by representative
specialists from the Owner, Builder and independent body/third party.
- A summary of the risk assessment is to be documented and
submitted to LR for “for review only”.
- Depending on the likelihood and consequence of failures
identified in the risk analysis, typical prevention and mitigation measures
should be proposed or referred to.
- 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.
- 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
- A detection system shall be provided to give warning of any
cold spot due to the leakage of LNG or natural gas.
- 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.
- The equipment installed for cold spot detection is to be
approved by LR.
7.1.6
Cryogenic spill containment and suppression
- Isolation valves, for inventory control purposes, must be
located as close as practically possible to vessels or equipment.
- Inventory isolation valves that are located within a recognised
fire zone shall be protected against fire and explosion effects.
- 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.
- 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
- Suitable arrangements are to be provided to reduce the chance
of unintentional releases of liquid gas and mitigate the effects of such
releases.
- 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.
- 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.
- 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.
- 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
- 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.(d), 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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