Through its audits and from questions from various stakeholders, the Petroleum Safety Authority Norway has discovered a need to map how the industry defines the "Worst Credible Process Fire" (WCPF) that a facility is designed for.
A WCPF can be interpreted as the worst fire from a single process segment when emergency shutdown valves, pressure relief valves and relevant safety systems work as intended. The effect of firewater and process shutdown valves must not be credited in the definition of WCPF. When designing for such a fire, account must be taken however of potential internal escalation within the area.
Room for interpretation
A detailed recipe for how to define "unfavourable combinations of heat load/action, fire size and fire duration" is not provided in the present regulations and standards. In other words, there is certain room for interpretation in terms of how conservative the underlying assumptions should be, when defining which fires the facility should withstand.
How WCPF is defined imposes key constraints on the design of a facility in respect of the requirements for preventing the spread of fire and for maintaining structural integrity, including the need for passive fire protection of equipment and the protection of load-bearing structures, including firewall requirements. It has therefore been of interest to assess whether the industry has a uniform practice for defining WCPF, whether current practice complies with the intentions of the regulations, and whether the regulations, standards and practice can be improved.
Robust and comprehensive work
The report that Sintef has prepared is based on relevant documentation from six operators representing seven recent projects and facilities. There was also dialogue with the commissioning agency and selected technical personnel from the companies during the execution of the project.
The impression that emerges from the work on the report is that robust and comprehensive activities are being undertaken to model, simulate and analyse various fire scenarios. It appears from risk and fire analyses that many, detailed calculations are being made and a range of fire scenarios are being analysed.
New definition in the pipeline
The report points to some variation in practice in terms of taking account of PSD valves in the definition and calculation of the worst design fires. It is also pointed out that escalation is defined and practised differently.
One provisional conclusion is therefore that, in order to achieve more uniform industry practice in respect of defining WCPF, a more detailed description is needed, as well as more refinements concerning the assumptions to apply. This will also save the industry unnecessary and time-consuming discussions. At present, WCPF is not sufficiently clearly and consistently defined in the regulations and standards, which is also the reason why the update of NORSOK S-001 currently in progress is working on a new definition of WCPF.