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Provision of active fire protection on offshore installations

Offshore Information Sheet No. 5/2009

(Issued April 2009)

Contents

Introduction

This information sheet provides guidance for dutyholders on the provision of active fire protection. On an offshore installation, particularly in hydrocarbon processing areas, the most effective fire protection provided for the occupants is, generally, water deluge. Recent new, limited facility designs, have been put forward that do not incorporate active fire protection systems. The lack of such systems is not considered good practice.

Background

Risks to persons from fires and explosions on installations are mitigated by a series of protection barriers. These barriers are classed as passive and active and use a diverse range of technologies. Good design recognises that the failure or the absence of one barrier should not have a measurable detrimental effect on the risk profile of an installation. Although active fire protection systems are not specifically recommended, it should be noted that this mitigation system can provide a wider level of protection for a wider range of fire scenarios than other, more specific, protection systems.

There have been several cases recently concerning the lack of the active fire and explosion protection barrier of water deluge. This sheet is aimed at providing a coherent approach to evaluating the case for active fire protection.

Standards and guidance

The most comprehensive and up to date source of information and data for fire attack on offshore installations is OTO 2000 0511.

This report summarises the new knowledge gained since 1991 and identifies gaps in the currently available information, particularly with respect to establishing sound modelling procedures and guidelines for industry. It was funded by HSE and Shell International Exploration and Production BV.

The most specific standard, ISO 137022, requires that fire and explosion hazards are identified and associated risks evaluated and managed. The standard emphasises the Fire and Explosion Strategy (FES) and the Evacuation, Escape, and Rescue Strategy (EERS). In the UK the issues are also covered by the Offshore Installations (Prevention of Fire and Explosion, and Emergency Response) Regulations 1995 (PFEER).

ISO 104183

API RP 14C4 covers industry practice for the provision of basic surface safety systems, including active fire protection systems.4

API 5215 covers pressure relieving and depressurising systems but is being revised and will be replaced by ISO23251.

Hazard identification and management

An installation's FES and EERS will require risk reduction measures; the most significant measures being passive and active fire protection systems, and emergency depressurisation systems. The role of emergency depressurisation systems (EDS) is not included in this note, but is described in API 521 and API RP 14C.

Passive fire protection (PFP) is generally used to protect main structural elements that support walkways, open escape routes etc. and hydrocarbon containing process vessels. It is also used to provide long (up to 2 hours) fire resistance for fire and blast walls. This includes providing protection for covered escape routes such as tunnels on FPSO's. It is highly effective against jet fires and pool fires. PFP is a passive system and in many ways superior to water deluge in terms of its lower maintenance burden; it is in position permanently and does not require a detection and initiation system.  However, passive protective materials are susceptible to physical damage and water ingress that can give rise to an unrevealed failure mechanism. Failure can be rapid on fire exposure such that it fails and compromises the performance of the component it is protecting.

Water deluge is an active system whose effectiveness needs to be periodically checked and this requires frequent testing that is time consuming and expensive. It requires linked gas, heat and flame detection and initiation systems that have their own reliability and availability problems, as well as large pumps and associated large and small-bore pipework. Salt water is used and this causes nozzle blockage and internal corrosion.

Water deluge however has the advantage of being able to provide general area protection to personnel and open escape routes as well as vulnerable plant items such as safety critical elements (SCE's), which may also have personnel protection functions. Deluge systems do tend to provide a wider level of protection for a range of fire scenarios than other, more specific, protection systems. On installations with minimum facilities and limited space, a general area protection system will tend to give a higher level of personnel protection than other more specialised systems. This is particularly relevant to open or  unshielded  escape routes.

Water deluge is ineffective against jet fires but is highly effective against pool fires when used with aqueous film forming foam (AFFF) and liquid containing bunds.

Risk balance and sensitivity analysis

On installations with a small POB, a water deluge system and its attendant detection and initiation systems could significantly increase the maintenance burden and POB/occupancy. However, the requirement for appropriate measures to be provided is based on the PFEER Regulation 5 assessment and is not subject to judgments on overall risk profiles. Provision of fire protection measures should therefore be viewed as a fundamental requirement of the design specification unless the dutyholder can provide a robust counter argument.

When dealing with normally unattended installations (NUIs), it is important that a sensitivity analysis is carried out, and is included in the risk balance arguments to ensure that the assumptions within the arguments are valid and are weighted in a reasonable, sensible manner. The likely increase in POB and or life extension should form part of this analysis as historically the UKCS has witnessed increases in both for several installations.

It may be argued that installations with a small POB or a NUI where occupancy is limited will probably have a low risk profile when there is a high integrity temporary refuge (TR) and evacuation or escape is rapid. Rapid escape is clearly associated with a low POB, although this is not guaranteed. A high integrity TR component, discounting benefits from deluge, in the demonstration of compliance for PFEER regulation 5 assessments, will often be advanced but deterioration over time of TR integrity is well known and undermines such arguments. 

In the low POB scenario the sensitivity of the arguments to the ease and speed of evacuation in relation to the POB size need to be scrutinised in detail, ie if a small increase in POB/occupancy increases the evacuation times disproportionately then the arguments for not having a water deluge become weaker since deluge will 'buy' time for EER purposes. Risk arguments put forward in defending the lack of active mitigation systems highlight a low inventory (for example ~350kg gas) which can be blown down within minutes while the few workers on board are protected in the TR prior to evacuation. The TEMPSC being accessed on the 'sheltered' side of the TR and the boarding area is protected, during escape or evacuation.

The argument is based on the short fire duration (of 350Kg gas), so that a deluge would have little risk benefit, and resources are concentrated on protecting the workforce in the TR; for example, a fire and blast wall on the "process face" of the TR. In such an incident scenario, the strategy is to muster in the TR, close in the reservoir and blow down the inventory. HSE has accepted these arguments but the continued integrity of the passive measures and effective control measures are vital and will require a higher level of regulatory scrutiny.

Risk assessments

Overall the benefits for and against water deluge will depend upon the risk profile of the installation; specifically the inventory size, blowdown effectiveness, and will be sensitive to the level of personnel occupancy. Active water deluge systems provide protection to personnel both directly by heat flux attenuation and by protecting the structure and the TR. Additionally the integrity of active measures can be determined more easily than the integrity of passive measures by simple inspection and test. It is recommended therefore that all installations other than NUIs, even those with low POBs, should be provided with water deluge active protection.

It is recommended that the issue of provision for water deluge on NUI's is considered on a case by case basis taking account of an installation's hazard profile, and its primary mitigation systems. A high performance TR, including protected escape routes and a rapid blowdown system have been put forward by dutyholders as the best option for personnel protection for NUI's without deluge systems. However, to deal with a wide variation in fire scenarios the fundamental principle should be to provide active fire protection as the first option.

Relevant legal requirements

References

  1. OTO 2000 051 Review of the Response of Pressurised Process Vessels and Equipment to Fire Attack [689KB]
  2. ISO 13702 Control and mitigation of fires and explosions on offshore production installations
  3. ISO 10418 Petroleum and natural gas industries - Offshore production installations - Analysis, design, installation and testing of basic surface process safety systems
  4. API RP 14C Recommended practice for analysis, design, installation and testing of basic surface safety systems for offshore production platforms
  5. API 521 Pressure-relieving and depressuring systems

This information sheet contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do

Updated 2011-09-29