As a floating production, storage, and offloading vessel (FPSO) continually
loads stabilized crude oil to the cargo tanks, the inert-gas (IG) blanket
within the tanks is compressed. At a certain pressure, below that which could
cause damage to the vessel structure, this mixture of now IG and volatile
organic compounds (VOCs), emanated from the loaded crude, must be vented safely
to atmosphere by some means.
From the very first FPSO that Single Buoy Moorings (SBM) operated, the
FPSOII in 1980, errant gas (IG) has resulted in a number of
emergency shutdowns (ESDs) while venting the cargo tanks during calm weather.
With no wind to disperse the heavier-than-air gas, the result is the mixture
falling onto the vessel main deck or the process modules, triggering the FPSO
gas-detection system and subsequently causing an ESD. The cost can quickly
become considerable because of loss or delay of production in this manner.
More importantly, while venting the cargo tanks, personnel are relocated to
a safe area to ensure their safety, and any hot-work being undertaken is
postponed. Crane and helicopter operations also are suspended. There is, of
course, a cost associated with the loss of personnel productivity during these
To prevent reoccurrence of IG venting-related incidents, a number of
operational measures are used.
As a high concentration is attained [usually approximately 40% of the
lower-explosive limit (LEL)], venting is stopped until the gas has been
dispersed. Typically, the alarm is activated at 20% LEL, and the executive
action (in this case, an ESD) is set at 60% LEL.
With FPSOs becoming larger, and production rates correspondingly so
(sometimes more than 300 000 B/D), the financial penalties resulting from an
unplanned shutdown are even more significant. Although using operational means
can be a successful way of mitigating both the safety and production risks
associated with IG venting, a system that would obviate the need for such
procedures has been sought by SBM since their first operating FPSO, more than
25 years ago.
This paper describes the evolution of the IG arrangements on board SBM’s
units, past and present systems that have been used with varying degrees of
success and the recent (July 2005) retrofit of an IG eductor to 2 SBM FPSOs
operating west of Africa. The details of the patented IG eductor, from
conception, design, safety studies, construction, installation, and operational
success, are also presented.
© 2007. Society of Petroleum Engineers
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- Original manuscript received:
16 May 2006
- Revised manuscript received:
1 November 2006
- Manuscript approved:
19 January 2007
- Version of record:
20 June 2007