A natural gas condensate leak occurred in the aft pump room of the Cidade de São Mateus (CDSM) Floating Production, Storage and Offloading (FPSO) vessel while a cargo tank was being pumped out to the aft slop tank via a temporary line-up and stripping pump. The CDSM was originally a very large crude oil carrier (VLCC) which had been converted to an FPSO in 2008 and was moored in 790 m of water at the Camarupim gas field. Several gas detectors in the pump room alarmed, confirming presence and movement of an explosive atmosphere. However, emergency responders entered the area multiple times to locate, assess and repair the leak. Attempts to clean-up the leaked material with adsorbent mats were unsuccessful, so a fire hose was deployed to perform a “water sweep” while the repair was ongoing. A major explosion occurred, destroying the bulkhead between the engine room and the adjacent pump room, breaching the main deck and wrecking the single access route to the pump room. The hull remained intact, but the vessel developed a severe list to the stern because the pump room flooded (due to fire water and sea water main ruptures and damage to the sea chest valves).

Basic Cause:
Overpressure of an isolation blind on the discharge side of a reciprocating-type stripping pump resulting in loss of primary containment (LOPC) of natural gas condensates and creation of an explosive atmosphere in a confined space.