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Precis File
SHIP NAME: Mobil Petrel KEY: NUM. ENTRIES: 2
source MAIB
type L
volume
material
dead 0
link http://www.maib.gov.uk/publications/investigation_reports/1990_to_1998/mobil_petrel.cfm


source CTX
type C
volume
material
dead 0
link

This casualties was a series of errors on a part-loaded major oil company VLCC. She was discharging at Fawley after part-discharging in Rotterdam, where she has a small shell leak high in 1P. This had no effect on the Fawley problems. At Fawley she was simultaneously discharging and ballasting. On the 2BC cargo tank IG branch valve the handle had broken off. Photos in the MAIB report show very bad corrosion in the IG system and this valve was a total mess. The crew had rigged an iron bar to open and close, but it was difficult if not impossible to tell if the valve was open or closed. Apparently, it was closed, for in the process of ballasting, there was a big bang and the aft bulkhead of 2BC blew out. 3C the next tank aft still had cargo in it.

This ship had a pipe tunnel running the length of the ship, plus access trunks to the pipe tunnel. (This feature is a Mobil idiocyncracy which in CTX's opinion causes far more dangerous problem than it solves.) The oil and water from 3C and 2BC poured into the vertical access trunk and down to the pipe tunnel. The pipe tunnel flooded. The door between the pipe tunnel and the pump room had not been properly secured, so then the pump room flooded, eventually to a depth of 10 m. The glands in the supposedly gas tight bulkhead between the pump room and the engine room leaked badly, so the engine room flooded, eventually to a depth of 4 m. There were strong hydrocarbon odors in both the CCR and the engine room. The latter was shut down and evacuated. It is a miracle this ship did not blow up.

MAIB found the quality of maintenance on the ship extremely poor. The pictures show very bad corrosion on deck. The discharge at both Rotterdam and Fawley was delayed by a series of repairs to IG fans, IG piping. The really strange thing was the leaking ER/PR glands. Normally even a lousy crew maintains the pump room to engine room seals for they know that, if they don't, they will probably die. The ship was fully approved by ABS. In fact, 10 months earlier (1989-01-04) during the annual survey, ABS had explicitly stated that the seals had been inspected and passed.

Both Mobil and MAIB estimated the over-pressure at the point that 2BC blow out. Mobil came up with 12 to 23 psig, MAIB said 10 to 28 psig. Both calculations indicate that the structure took at least 0.7 bar over-pressure, and possibly as much as 2 bar. Two pumps were being used to ballast the tank and the pump flow rates were known. These calculations were based on estimating the pressure loss in the individual tank P/V valve and vent piping.