As an FPSO continually loads stabilised 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 (VOC's), emanated from the loaded crude, has to be vented safely to atmosphere, by some means.
From the very first FPSO that Single Buoy Moorings operated, the FPSO II in 1980, errant inert gas (IG) has resulted in a number of emergency shut downs (ESD's) whilst 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 due to loss or delay of production in this manner can quickly become considerable.
More importantly, whilst venting the cargo tanks, personnel are relocated to a safe area to ensure their safety and any hotwork being undertaken is postponed.
Crane and helicopter operations are also suspended. There is, of course, a cost associated with the loss of personnel productivity during these events.
To prevent reoccurrence of IG venting related incidents, a number of operational measures are used. These usually consist of:
delaying venting during low wind periods (<2ms−1)
employing the downstream vent mast (if two are fitted, one either side of the vessel)
monitoring the lower explosive limit (LEL) via the gas detection system. As a high LEL is attained (usuallyaround 40%) 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.
Many of the problems associated with IG venting can be attributed to the design of the IG facilities being a copy of those employed onboard crude oil and product tankers.
