Control methods

Fires that have not been prevented should be detected and then controlled to reduce the size, duration, and escalation potential of the fire. The following control methods are commonly in use offshore. All platforms are different, but many of these control methods will be relevant to most installations. Note that extinguishants and manual firefighting are considered below as control methods. Deluge systems and passive fire protection methods are classed as mitigation methods because they generally protect against the impact of an existing fire rather than working to control the fire itself. The mitigation methods are addressed in Section 3.2.8.

Some typical fire related operational and design considerations are provided for each control method in Table 3.3 below.

Table 3.3 - Summary of methods of controlling fire

Control method

Control mechanism Fire related design considerations

Process

Emergency Shut Down Valves (ESDVs)

Automatic - Reduces inventory available to leak or fire by isolating process into separate, smaller, segments.

Ease of testing and maintenance. Regular test of process ESDVs often neglected.

Specify and justify test interval and acceptable leak rate as part of design. Record in performance standard documentation

In fire situations several ESDVs plus adjacent pipework may be engulfed at one time, releasing several inventories to prolong fire.

ESDVs are frequently used at module boundaries to prevent inventories from one module feeding a fire in another, these divisions then match the designated fire areas and their associated firewater coverage. Where no such boundary isolations are in place, it becomes possible for hydrocarbon which is stored in one module to be released into another module and fuel escalation of further fires..

Note that riser ESDVs are a requirement in the UK Continental Shelf under the Pipeline Safety Regulations.

Topsides valves to fail close

Locate away from process fire areas wherever possible.

Protect valve and exposed riser sections against foreseeable fire scenarios

Always consider benefits of subsea pipeline isolation, even a simple NRV may provide

Control method

Control mechanism Fire related design considerations

Sub-sea Isolation

Valves (SSIVs) Automatic - Isolates platform from pipeline inventories at a defined distance.

Topsides valves to fail close

Locate away from supply vessel routes, incoming jack-ups and other potential sources of dropped objects or dragging anchors. Locate the valve such that uncontrolled events just the far side of the SSIV will not pose a radiation problem for the installation, distances are often of the order of about 250-350m.

Well head and downhole

isolation valves

Automatic - Isolates platform

from reservoir inventories Surface and downhole valves to fail close on confirmed fire or gas release event.

General Platform

Alarm (GPA) Automatic - Removes people

to place of relative safety Any prolonged fire necessitates evacuation as a precaution

OIM and deputies must understand escalation mechanisms and timeframes for all emergency scenarios in order to be able to make competent decisions.

Blowdown and blowdown valves (BDVs)

Automatic or manual - Removes gases to flare or cold vent

See also Section 7.7 for a general review of Process Responses

See also Sections 5.5.1.9 to 5.5.1.12 and 7.7.2 to 7.7.5 for details on how the approach described in ISO 23251:2006 [3.3] impacts blow down rates

and subsequent consequences.

BDVs to be fail-open, unless this endangers helicopter operations and pre-warning not feasible.

Automatic facility recommended. Any manual arrangements need clear and detailed instructions for operation to offshore staff.

Appropriate blowdown time to be developed from escalation scenarios

Process drain facility

Automatic or Manual - Removes main liquid inventories from vicinity of fire to a safer location (e.g. cellar deck surge tanks)

Usually manual facility

Consider vulnerability of dump line route Consider time required for draining

Manual fire

fighting Manual fire intervention with hydrants, fire hoses, foam monitors, extinguishers etc

Appropriate for very small fires - Immediate intervention on discovery of small fire can prevent fire taking hold. All personnel trained for small fire intervention.

Fire fighting, equipment cooling and helideck fire control only possible where trained fire teams available. Effectiveness depends on understanding of installation-specific fire and escalation scenarios and plus realistic offshore exercises.

Note that even with training, fire fighting teams that remain to fight a fire will be at greater risk.

Control method

Control mechanism Fire related design considerations

Remote manual

fire fighting Initiation of fixed or oscillating fire monitors, with or without foam.

Often used on helideck or open upper or weather decks. May be affected by strong winds.

Inerting agents Prevents fire from starting/taking holds by rendering the atmosphere inert – Inergen ^©, CO₂ etc.

Useful in enclosed, remote spaces difficult to access in fire situations (e.g. pump rooms in semi sub or ship hulls)

Static discharge may ignite atmosphere, causing explosion – check potential with vendor.

Inerted atmosphere may not be breathable so warnings and pre-discharge alarms required.

Extinguishants Stops fire burning by preventing oxygen reaching fuel, removing heat, or otherwise interfering with combustion process – water-mists, foams, some types of deluge etc

Useful in enclosed spaces such as machinery enclosures

Deluge and fuel/water wash-off needs weight-control consideration, particularly on floating installation.

Foam application Reduces evaporation of vapours.

Creates film/foam to prevent oxygen reaching liquid fuel thus reducing, or extinguishing pool fire.

Suitable for contained liquid fires. Less effective on running pool fires, not effective on jet fires

Dirivent systems Disperses very small leaks to prevent flammable cloud build-up.

Only effective for fugitive (very tiny) leak scenarios System shuts down on in major release scenarios as may spread leak and mix release to flammable concentrations.

Other HVAC systems

Provides air exchange within an enclosed area to prevent or slow flammable cloud build-up.

System needs special attention to be able to provide adequate air flow rates and be safe, i.e. to not introduce any ignition sources and also not move the fuel/air mixture to other areas hitherto safe within the context of the originating accident.

Bunds Control spread of liquid releases

While bunds can contain a liquid release/fire, they can also concentrate a fire around the equipment in the bund and should be used in conjunction with foam. Design must ensure deluge does not cause bund overflow by being sized for maximum foreseeable liquid volume release.

Drains Remove liquid and deluge

releases to drain system. Small releases are usually within drain system capacity. The drain capacity needs to be capable of removing maximum foreseeable liquid volume release although the effects of burning liquids in the drain system must be checked.

Sea-fire possibilities and consequences need checking

In emergency scenarios environmental issues become secondary to preservation of life.