Explosion design loads were in the past derived from the worst credible event assuming a gas cloud of maximal extent with stoichiometric composition ignited at the worst time in the worst position. Usually, the ultimate peak overpressure derived in this way is far too large to be accommodated by the structure.
NORSOK Z-013 (2010) defines dimensioning accidental load as the most severe accidental load that the function or system shall be able to withstand during a required period of time in order to meet the defined risk acceptance criteria. The dimensioning accidental load (DAL) is typically established as the load that occurs with an annual probability of 10-4.
Design accidental load is a chosen load that is to be used as the basis for design. The applied/chosen design accidental load may sometimes be the same as the dimensioning accidental load (DAL), but it may also be more conservative based on other input and considerations such as ALARP. Hence, the design accidental load may be more severe than the DAL. The design accidental load should as a minimum be capable of resisting the dimensioning accidental load (DAL).
In the modelling of escalation events and in determining the frequencies of impairment of Temporary Refuge, explosion resistance and failure probability of the following types of components and systems need to be evaluated:
- Module support frame (MSF) and topside modules
- Deck plating, module cladding, etc.
- Escape ways, tunnels
- Fire/blast walls
- Process piping and process vessels
- Drilling derrick and flare boom etc.
Detailed explosion risk assessment and its implementation into the design is carried out by Nowatec for clients’ development projects. Typical scope of work and deliverables include:
- Fire & Explosion Risk Assessment (FERA)
- Assessment of explosion overpressures and drag forces for defined explosion scenarios using CFD tools (FLACS)
- Development of probabilistic models of explosion loads to be used in design
- Structural integrity analysis against explosions, global and local structures
- Structural integrity analysis of explosions including explosion effects on critical equipment and pipe supports.