Explosions are the result of a delayed ignition of a flammable vapour or gas release. Any damage from explosions to plant, supporting structure or protection systems which leads to major escalation and/or inhibits personnel escape should be regarded as unacceptable. Procedures for consideration of explosions are straightforward in principle even if challenging to implement in practice. As far as practicable,
All process plants should be designed and operated to minimise releases that might lead to the build-up of an explosive mixture
Ventilation should be arranged to minimise the accumulation of a vapour/gas cloud
The design should minimise potential ignition sources
• Layout should be designed to minimise the magnitude of any overpressure that would develop from an explosion
• Personnel and critical equipment should be protected from anticipated overpressures
• Potential damage to process plant and structure should be minimised thus reducing the risk of escalation
• Where potential overpressures are unacceptable, consideration should be given to explosion mitigation techniques.
Explosion Loads
It is necessary that the interaction between the following factors be considered in order to assess the loads resulting from explosions:
• The leak source and type of gas/vapour
• The dispersion of the fuel relative to the release rate
• The source, type and location of the ignition point
• The geometry of the surroundings
• The size of the enclosure
• The interaction of the burning gases with any obstructions. Turbulence increases the rate of flame propagation and severity of explosion.
• The restriction to expansion of the burning gases by obstructions
• The type of vents and their locations
• The existence of an explosion mitigation system.
Detailed guidance on explosion loading can be found in the following key references:
1) Interim Guidance Notes for the design and protection of topside structures against explosion and fire, SCI, 1992
2) Fire and Explosion Guidance, Oil and Gas UK, 2007
3) Gas Explosion Handbook, CMR-Gexcon
4) Design of offshore facilities to resist gas explosion hazard - Engineering Handbook, Czujko, J. (ed.), 2001
5) Guidelines for evaluating the characteristics of vapor cloud explosions, flash fires and BLEVEs, Center for Chemical Process Safety (CCPS), 1994
Explosion Response
In order to decide on the level of assessment that is required for various parts of the structure and at different stages of the design process, the philosophy for the explosion design cases and corresponding performance standards need to be established.
For explosions, the prime objective is often energy absorption and not static strength, particularly for the extreme low probability events although criteria for limiting deformations need to be applied to some parts of the structure which may be influenced by plant, equipment and piping.
Two methodologies are available for the computation of the response of structures to explosion loads: single degree of freedom (SDOF) analysis and multi degree of freedom (MDOF) analysis.
SDOF analysis
The SDOF method can be applied as a simple hand method to generate quasi-static load cases and combinations that can be applied in conventional static finite element analysis. Its ease of application makes it a practical design tool and reliable results can be obtained in many situations. It is limited by the need to characterise the structure in a form to which the method can be applied. Traditionally, much of the guidance developed is based on the Biggs method which has several shortcomings:
• It does not incorporate the effects of support flexibility, since it assumes either pinned or fixed conditions.
• It does not account for different moment capacities at the two supports.
• It ignores the catenary effect, which has a significant influence on the large displacement member response in the presence of axial restraint at the supports.
• It ignores the influence of material rate-sensitivity.
• It ignores the influence of strain-hardening, through assuming elastic perfectly-plastic material and cross-sectional responses.
• It does not account for the beam-column effect in load-bearing members that sustain significant compressive axial forces.
Recently, an approach that overcomes several of the shortcomings of the Biggs method has been developed by Izzuddin and described in FABIG Technical Note 7 and FABIG Technical Note 10. To facilitate the adoption of the method and its uptake among engineers, the new SDOF approach has been implemented in a software tool (SATEL).
Detailed guidance on explosion response and SDOF analysis can be found in the following key references:
1) Interim Guidance Notes for the design and protection of topside structures against explosion and fire, SCI, 1992
2) FABIG Technical Note 4, Explosion resistant design of offshore structures
3) FABIG Technical Note 7, Simplified methods for analysis of response to dynamic loading
4) FABIG Technical Note 10, Simplified methods for analysis of response to dynamic loading: Material rate sensitivity
5) Introduction to Structural Dynamics, Biggs, J.M., 1964
6) Fire and Explosion Guidance, Oil and Gas UK
MDOF analysis
MDOF analysis is a full analysis of the structure run in the time domain. Theoretically, the most complex structural forms can be handled. There are two types of computer programs that can handle MDOF analysis: conventional linear finite element analysis and non-linear finite element analysis (NLFEA).
The accuracy of NLFEA depends partly on the capabilities of the selected program and partly on the interaction between the analyst and the program. All NLFEA work must be supplemented by a system of checks. Further details on NLFEA can be found in textbooks on the Finite Element method and software user and theory manuals.