Challenge

A brownfield modification to an onshore gas plant meant that depressurizing the whole plant in accordance with API 521 would be impracticable. It would require a new stainless steel flare header due to cold temperatures. A staggered depressurisation system was proposed and the customer wished to verify if  the design concept would reduce the risk and, if not, what action would be required.

DNV solution

Using the Vessfire model, DNV assessed the fire consequences to determine the potential for escalation between different areas, which is dependent on the speed of blowdown. This included constructing models to understand the response of process equipment under flame impingement, predict the time to and conditions at failure, to assess the consequences of the escalation failures and determine their significance. DNV also identified potential risk reduction measures and performed cost benefit analysis to determine if they were reasonably practicable.

Outcome and benefits

The assessment concluded that escalation failure of large bore pipework was a significant risk. This resulted in a change to the design to further segment a large inventory and leave the inventory in associated heavy walled vessels shut-in to allow faster depressurisation of the pipework. The risk of escalation was reduced to ALARP without a new flare header and the requirement for extensive and costly passive fire protection (PFP) on pipework was avoided.