3.1 FIONA (31 August 1988)
The forward cargo tank of the FIONA exploded while
a surveyor measured cargo temperature prior to unloading, resulting in one death.
The National Transportation Safety Board (NTSB) concluded that
a steam leak in the tank caused static charge to be generated, that the
charge accumulated on an ungrounded temperature probe and discharged as
the probe was withdrawn from the tank, and that the resulting sparks ignited
explosive vapors from the residue of the tank's previous cargo.
NTSB's recommendations addressed the foregoing items
and other contributory factors:
FIONA's cargo tanks should have been inerted,
and the ISGOTT should state more clearly that the inert gas system (IGS)
should be used with all cargoes unless tanks are gas free.
The main source of the explosive vapors
was contamination of the cargo (No. 6 fuel oil) by previous condensate
cargo, while release of light hydrocarbons by the No. 6 fuel oil may have
been contributory. Masters of vessels carrying Grade E cargoes should certify
that explosive vapors are not present prior to sampling or measuring cargoes
with a combustible gas indicator device.
The static charge was generated by a steam
leak in the cargo heating pipes and accumulated on an ungrounded temperature
probe. Better maintenance might have prevented the casualty.
The probe lacked a precautionary nameplate stating the, need for grounding
the instrument during use. Underwriters Laboratory UL) should adopt the
Canadian Standards Association requirement for such a nameplate. The internal
grounding wiring on these probes should also be checked periodically.