Prevention, Control and Mitigation of Explosions
A wide variety of measures may be employed to prevent, control and mitigate the effects of explosions. Whilst the emphasis should always be on explosion prevention (e.g. through prevention of leaks or elimination of ignition sources), the possibility of accumulation and subsequent ignition of a flammable hydrocarbon-air mixture cannot always be eliminated. Therefore control and mitigation measures may additionally be required.
Current standards for the selection and specification of measures to prevent, control and mitigate the effects of explosions comprise:
- UKOOA Fire and Explosion Hazard Management Guidelines (UKOOA, 1995)
- Interim Guidance Notes (SCI, 1992) and associated Technical Notes.
- UKOOA/HSE Fire and Explosion Guidance Part 0: Fire and Explosion Hazard Management, Oct 2003.
- UKOOA/HSE Fire and Explosion Guidance Part 1: Avoidance and Mitigation of Explosions, Oct 2003.
- ISO/FDIS 13702, Petroleum and Natural Gas Industries - Control and Mitigation of Fires and Explosions on Offshore Production Installations - Requirements and Guidelines (ISO, 1998).
- NORSOK Standard S-001, Rev 3, Technical Safety (NORSOK, 2000).
- API RP2A (21st Ed) Section 18.
- Engineering Handbook on the Design of Offshore Facilities to Resist Gas Explosion Hazard (Czujko, 2001).
- BP Corporation 'Guidance for the Protection of Offshore Structures against Fires and Explosions' (BP, 2001) (See also Walker et al, 2001).
The Interim Guidance Notes have been updated (above), whilst the API RP2A standard has been replaced with a new standard Design and Assessment of Offshore Structures for Fire and Blast'. The recent BP guidance (see above) forms the basis for the new API standard.'
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Since the Piper Alpha disaster installations in the UK sector of the North Sea have undergone significant modification to prevent, control and mitigate the effects of explosions. From Safety Cases reviewed recently, significant modifications continue to be implemented as the understanding of explosion hazards improves. For new installations a wide variety of measures are being employed with the emphasis on measures to prevent explosions or reduce explosion overpressures through good ventilation and installation layout.
Explosion development barriers
- Limit State Approach to Blast Resistant Design. This approach is widely used in the field of structural design. It forms the basis of the new BP guidance (BP, 2001) on fire and explosion engineering and has been utilised in the update of the Interim Guidance Notes. Its main advantage is that it avoids the use of an arbitrary design explosion load, but instead introduces a probabilistic element. Various limit states are defined with associated levels of performance (e.g. 10-3 per year event - elastic response; 10-5 per year event - plastic deformation, but not collapse). It can also take explicit account of the uncertainty associated with the loading event, although this aspect is not yet developed for gas explosions. Of concern to OSD is how the limit state approach fits in with the ultimate criteria that explosion risks should be ALARP. The limit state approach may place undue emphasis on the prediction of the probability of a given severity of explosion (highly uncertain), whereas under the ALARP principle OSD may look for low cost strengthening measures regardless of the predicted explosion frequency.
- Scope of current guidance on explosion prevention, control and mitigation systems. In the development the new BP guidance for fire and explosion engineering, Walker et al (2001) identified various shortfalls in the Interim Guidance Notes. These included:
- explosion drag load estimation (important for blast resistant design of pipework, supports, primary structure etc);
- simple methods for the calculation of blast wave loading on adjacent structures;
- structural details for blast resistance; and
- floating structure issues.
- Technical Note 2 (Explosion Mitigation Systems), published in 1994, has been superseded by the UKOOA/HSE Fire and Explosion Guidance for Fire and Explosion Hazard Management and Avoidance and Mitigation of Explosions.
- Activation of water deluge on gas detection. The advantages and disadvantages of activation of water deluge on gas detection has been the subject of considerable debate and study, e.g. see IGN Technical Note 2 (SCI, 1994), OTH 94 463 [PDF 1.9mb], OTO 95 026 [PDF 1mb], FABIG Technical Meeting 8 (SCI, 1996), OTO 2000 042 and FABIG Technical Meeting 20 (SCI, 2000). The effectiveness of deluge for explosion mitigation is dependant on module venting configuration and is not appropriate in all situations. This mitigation system has been adopted on some existing and newer installations (e.g. Shearwater and Elgin/Franklin). The main reason for non implementation in many cases appears to be the potential for water to enter electrical equipment and create an ignition hazard (as is believed to have occurred in two explosion incidents), requirements for uniform coverage and modifications to spray nozzles. A recent study commissioned by HSE (OTO 2000 042) concluded that 'activation of deluge on gas detection can make a significant improvement in the level of safety '...... providing the implementation is appropriate'. The consensus appears to be activation of deluge on gas detection is not the universal answer. However, there is a lack of guidance as to the circumstances under which adoption of this mitigation measure would be beneficial and, if so, the design considerations which apply (deluge activation time, protection of electrical equipment, nozzle type, application rate etc). Issues relating to the benefits (or otherwise) of deluge on gas detection include:
- the dependence of any risk reduction on the effectiveness of gas detection;
- reliability initiation of deluge systems;
- creation of turbulence, hence increased rate of pressure rise (initially);
- low benefit in confined situations (low flame speeds);
- issue of where gas is displaced to as a result of activation of deluge (e.g. to more hazardous location);
- increased probability of ignition;
- capability of deluge to provide explosion mitigation in addition to primary function, i.e. fire protection; and
- enhanced corrosion of equipment.
Emerging technologies in the prevention, control and mitigation of explosions. These technologies comprise: barrier technologies (Tam, 2000), water mist explosion suppression systems (Tam et al, 2000). The barrier technologies can be sub-divided as follows:
Inventory control barriers:
- Passive barriers: hard (e.g. blast walls), soft (membrane gas barriers)
- Active barriers: suppressive, non-suppressive
(e.g. to control pressure piling effects or turbulence generation).
Some of these types of barrier have been implemented, e.g. 'weak wall' gas barriers and membrane gas barriers, whilst others are only at the concept stage. Some of the outstanding technical issues include the choice of materials for the barrier, the impact of the barrier during and after collapse, the impact on natural ventilation and consideration of equipment layout conducive to the implementation of barrier methods.
Water mist suppression systems utilising superheated water have been tested at medium scale under partial funding from HSE. This has demonstrated that such devices can suppress a developed explosion in a partially-confined module. However the proponents of this method recommend that a greater understanding of the interaction of mists and flames is obtained and that further testing is undertaken with different geometries and scales. The question of effectiveness of initiation the system on gas cloud ignition is also outstanding.
The efficacy of blast-induced atomisation from water containers has been demonstrated at small scale. Further work is required to develop and then test a device at large scale. A key advantage of this system is that it is passive (no activation required). The outstanding concerns are that are that it does not mitigate the effects explosions within a module (i.e. acts at the end of a duct or module boundary) and that it represents an additional mitigation system with an associated maintenance requirement.
- Industry adherence to current guidance for the selection of prevention, control and mitigation measures. It is not always apparent that the duty-holder had undertaken an assessment such as referred to in the UKOOA Guidelines (Fire and Explosion Hazard Management process), Interim Guidance Notes (flowchart for explosion resistance methodology) or ISO 13702 (Fire and Explosion Strategy). Through adherence to such guidance the dutyholder's consideration of the hierarchy of available measures for explosion prevention, control and mitigation will be clearer.
- Prevention, control and mitigation of explosions on floating installations. A recent FABIG Technical Meeting (SCI, 2001) has highlighted a number of issues for floating installations which represent a challenge to existing design practices. These include:
- diffraction and drag loading (both may be important on open decks),
- explosion loading on the deck potentially leading to a cargo fire or loss of vessel integrity (FPSO's)
- potentially severe turret explosion hazard with potential escalation to risers, deck, cargo tanks and/or TR impairment (FPSO's)
- engine room explosions due to use of dual fuel engines (HP gas)
Strategy development issues
Scope of current guidance on explosion prevention, control and mitigation
- The relationship between explosion risk assessment, the 'limit state' approach to blast resistant and the ALARP principle.
Activation of water deluge on detection of gas
- Consider the need for further guidance to better define the circumstances in which the activation of deluge on detection of gas is likely to be beneficial.
- Consider the need for further studies to support effective implementation of this measure (e.g. concerning deluge activation time, protection of electrical equipment, nozzle type, application rate etc).
- Maintain an up-to-date knowledge and appraisal of emerging technologies for the control and mitigation of explosions, some of which have both positive and negative impacts (e.g. gas barriers - prevent movement of gas, but may also hinder ventilation) whilst others are at the early stage of development (e.g. water mist suppression, use of passive water containers) with outstanding issues on cost and practicality.
Industry adherence to existing guidance on the explosion prevention, control and mitigation
- Promote greater adherence by industry to existing guidance on the selection of explosion prevention, control and mitigation measures.