During start-up of the platform HP Compressor, pressure was being equalised across the inlet valve SDV-11379 (via a 2” bypass valve). SDV-11379 was in the closed position.
During the pressure equalisation across SDV-11379, a loud bang was heard and smell of gas reported by operators at site. The operators immediately vacated the scene and the plant was shut down and de-pressurised in a controlled manner.
During investigation of the scene, the trunnion from SDV-11379 was found to have parted from the valve body causing a loss of containment. The trunnion, installed in the 6 o’clock position approximately 1 metre from the deck, was projected vertically downwards. The trunnion bounced off some scaffold boards underneath the valve and then struck a 2” cooling medium pipe, before coming to rest circa 1.7 metres from the valve.
The investigation of the valve found that the six of six trunnion fixing cap screws remained screwed into the valve body but all cap screw heads had separated from the cap screw (at the shoulder). Five out of the six cap screw heads were recovered from the deck within a ten metre radius of the valve.
Note that the valve was visually inspected in November 2013 with no faults reported and all capscrews witnessed to be present.
- Installation specifications should take into account possible modes of failure and specify materials to mitigate the risk of occurrence
- An appropriate inspection technique/strategy should be in place which takes into account possible modes of failure during operation
- Undertake a review of asset valve populations to identify those valves potentially at risk from Chloride induced SCC considering valve; operating temperature; design and materials of construction; location and potential for chlorides environment.
- Replace trunnion cap screws of valves potentially at risk with cap screws not susceptible to Chloride induced stress corrosion cracking
- Review company material / valve specifications and revise as required to recognise failure mode and specify materials accordingly
- Develop and implement a Long Term Strategy for valve cap screw inspection and replacement
- Review and revise the asset ALE strategy based upon learning from this incident
Actuated, 16” 300# trunnion mounted ball valve in 25Cr Super Duplex Stainless Steel. Trunnion fixed externally to ball valve body with capscrews. Plant Tag Number SDV-11379