Ukooa Fire and Explosion Guidance

UKOOA FIRE AND EXPLOSION GUIDANCE Part 1: Avoidance and mitigation of explosions

There is always the possibility that the application of deluge may generate extra ignition sources if acting on unsealed electrical equipment and in particular lighting units. If lighting seals are routinely replaced at the same time as expired light bulbs then the impact of this will be reduced or eliminated.

4.7.3

Rate of Deluge Coverage

The minimum deluge rate for general process area coverage as recommended in ISO 13702 [34] is 10 l/min/m2. Full-scale tests undertaken as part of the Phase 2 JIP [9] investigated coverage in the range 13 to 21 l/min/m2. This showed an increasing benefit of deluge in reducing overpressures as area coverage rate increases, however the increase in benefit between these values is slight. The usual general area coverage rate of 10 l/min/m2 (even with 15% added for hydraulic imbalance) falls outside the rate tested. If explosion mitigation is considered critical a deluge flow rate of at least 13-15 l/min/m2 is recommended for general area coverage.

4.7.4

Firewater Extinguishing of Jet Fires

Initiation of deluge on fire detection is general practice where firewater systems are installed. For gas fires the aim is not to put out the fire but to cool equipment and the structure to prevent escalation. Recent research [65] however has shown that deluge can successfully extinguish some jet fires. This would then result in gas cloud accumulation where previously the gas had been burning. The potential for explosion is then high, especially as hot surfaces will have been created by the initial fire which can then act as ignition points. Any subsequent explosion should however be attenuated by the firewater deluge generally will continue to run. Unless special circumstances exist, the fear of extinguishing a jet fire should not result in any decision not to activate deluge since the initial jet fire is likely to hold the highest risk potential. Special circumstances may occur where an installation is particularly susceptible to blast (even at the deluge attenuated level) and where fire scenarios are short lived due to inventory depletion with low escalation risk.

4.7.5

Deluge Delay

There will inevitably be a delay in the supply of deluge to the module, this is caused by;

•

delay in detecting the gas cloud

•

delay in starting the firewater pump driver

•

delay pressurising the system (if no jockey pump is used)

•

delay in flowing water to the nozzles.

The total delay may be in the region of 60 to 90 seconds. If ignition of the flammable cloud occurs within this time there will be no explosion mitigation by deluge. For fires this delay can still be significant, but a delayed supply will still provide considerable benefit especially for cooling where direct jet impingement has not occurred. Until deluge coverage occurs the module is susceptible to full overpressures. Time delay can be minimised by:

•

rapid detection of the gas cloud. Acoustic leak detection is likely to give quicker detection, especially as it does not need to be in the hazardous cloud to detect the release;

Issue 1, October 2003

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PUBLISHED BY UK OFFSHORE OPERATORS ASSOCIATION London Office:

2nd Floor, 232-242 Vauxhall Bridge Road, London, SW1V 1AU. Tel: 020 7802 2400 Fax: 020 7802 2401 Aberdeen Office:

9, Albyn Terrace, Aberdeen, AB10 1YP Tel: 01224 626652 Fax: 01224 626503 626503 Email: [email protected] Website: www.oilandgas.org.uk

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PUBLISHED BY UK OFFSHORE OPERATORS ASSOCIATION London Office:

2nd Floor, 232-242 Vauxhall Bridge Road, London, SW1V 1AU. Tel: 020 7802 2400 Fax: 020 7802 2401 Aberdeen Office:

9, Albyn Terrace, Aberdeen, AB10 1YP Tel: 01224 626652 Fax: 01224 626503 626503 Email: [email protected] Website: www.oilandgas.org.uk

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Foreword This document has been prepared under a joint industry project sponsored by UKOOA and the UK HSE. The project was managed by fireandblast.com limited, limited, and the production of the initial text and of the back up documentation was undertaken by a consortium headed by MSL Engineering Ltd. The other members of the consortium were Aker Kværner, Century Dynamics, Genesis Oil and Gas, IC I C Consultants, Morgan Safety Solutions and WS Atkins inc. This document is part of a series being produced by UKOOA and HSE on fires and explosions, the full series being: Part 0 Hazard management management (formerly FEHM) Part 1 Avoidance and mitigation of explosions Part 2 Avoidance and mitigation of fires Part 3 Detailed design and assessment guidance This Part 1 document is taken from MSL Engineering Reports C26800R006 Rev 2 and C26800R007 Rev 2. Part 0:- Fire and explosion hazard hazard management management Describes Hazard Management principles and practices with particular emphasis on the management of fire and explosion hazards

Part 0

Part 1:- Avoidance and mitigation mitigation of explosions

Part 1

Part 2

Part 2:- Avoidance and and mitigation mitigation of fires Describe design considerations for the prevention, control and mitigation of fires

Describes design considerations for the prevention, control and mitigation of explosions

Part 3

Part 3:- Design practices for fire and explosion engineering engineering Contains advice on the engineering implementation of the measures outlined in principle in Parts 1 & 2

Basis Documents for Parts 1, 2 & 3 Contains base position papers as guidance was developed. Available on www.fireandblast.com for those wishing to understand the logic and data gathered for the positions taken in the guidance

The treatment described in this part of the guidance draws on the experience gained during the period since the Interim Guidance Notes [1] were prepared. This has allowed simplifications to be made and a more clearly defined approach to be adopted in some circumstances, without compromising compromising safety.


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This Guidance does not have the force of a Standard and contains information on good practice which may or may not be on a firm scientific basis and may require clear justification. Where this is the case the uncertainties are highlighted and the limitations of the methods are identified. There is a recognized need to provide such guidance to avoid decisions being made out of context during the explosion assessment process. The term ‘assessment’ is taken to include the assessment of a design in progress and the assessment of an existing installation. This part of the Guidance identifies methodologies for explosion assessment, the circumstances in which the methods may be applied and their limitations. Part 3 of the Guidance will deal with the detailed implementation of these and other methodologies. methodologies. Alternative methods to those presented may be used so long as they are justified by a risk assessment and provided their use leads to reducing risks to As Low As Reasonably Practicable (ALARP).

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Contents 1

Introduction........................................................................................................................... 7 1.1 History ......................................................................................................................... 7 1.2 Objectives.................................................................................................................... 7 1.3 Fire and Explosion Hazard Management .................................................................... 8 1.4 Overview of Guidance ............................................................................................... 10

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Explosion hazard philosophy.............................................................................................. 12 2.1 General...................................................................................................................... 12 2.2 The explosion hazard ................................................................................................ 14 2.3 Goals, aims and Principles........................................................................................ 17 2.4 Legislation, standards and guidance......................................................................... 18 2.5 Inherently safer design .............................................................................................. 20

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Explosion hazard management .......................................................................................... 28 3.1 General...................................................................................................................... 28 3.2 Safety Management Systems.................................................................................... 29 3.3 Risk reduction............................................................................................................ 33 3.4 Risk screening........................................................................................................... 36 3.5 Hazard identification and scenarios........................................................................... 38 3.6 Detection, control and mitigation ............................................................................... 41 3.7 Control systems and Safety Critical Equipment......................................................... 47 3.8 Mitigation and consequence minimization................................................................. 51 3.9 Acceptance criteria .................................................................................................... 58 3.10 Implementation and monitoring ................................................................................. 60 3.11 Analysis methods ...................................................................................................... 61 3.12 Existing installations .................................................................................................. 65

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INTERACTION WITH FIRE HAZARD MANAGEMENT ..................................................... 69 4.1 Overview fire and explosion hazard management................................................... .. 69 4.2 Common areas .......................................................................................................... 70 4.3 Considerations by design phase......................................... ....................................... 70 4.4 Special issues relating to installation type ................................................................. 73 4.5 Potential areas of interaction and conflict.................................................................. 75 4.6 Explosion damage to passive fire protection (PFP) Systems .................................... 76 4.7 Use and effectiveness of deluge ............................................................................... 76 4.8 Operational issues..................................................................................................... 79

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Derivation of explosion loads.............................................................................................. 80 5.1 Introduction to explosion load determination............................................................. 80 5.2 Tasks for the determination of explosion loads ......................................................... 82 5.3 Determination of explosion frequency ....................................................................... 84 5.4 Dispersion.................................... .............................................................................. 85 5.5 Ignition ....................................................................................................................... 89 5.6 Explosion overpressure determination ...................................................................... 89 5.7 Development and application of Nominal Explosion Loads....................................... 98 5.8 Impulse and duration related to peak overpressure ................................................ 101 5.9 Design explosion loads............................................................................................ 102 5.10 Generating Exceedance curves .............................................................................. 105 5.11 Loads on piping and equipment .............................................................................. 111 5.12 Reporting Template for ALARP demonstration ....................................................... 116


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Ukooa Fire and Explosion Guidance

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