The Evolution of Inert-Gas Systems on SBM FPSOs: The Problem of Venting and a Straightforward Solution

Abstract

Summary 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 FPSO II 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 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 (<2 ms) Employing the downstream vent mast (if two are fitted, one on either side of the vessel) Monitoring the gas concentration by way of the gas-detection system 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.