Abstract
Development concepts for an off shore oilfield is dependant of reserves and infrastructure in the area. Often minor reserves close to existing infrastructure can be developed as tie-back to a host platform where the processing of the well fluid takes place.
The type of development can roughly be divided into two categories: Wet and dry wells or sub-sea wells and Normally Unmanned Installations (NUI). NUI's are often favoured in shallow water (less than 70 meters water depth) due to the cost associated to subsea development as well as lower operating costs for a NUI compared to a subsea development.
A NUI is a compromise between flexibility and purpose. The more equipment you install the less chance is there of "normally unmanned operation". Power is always required. For subsea solutions this is typically provided with an umbilical that also provides hydraulics, I/O, chemicals etc. Local storage of chemicals and power generation is often chosen for the NUI due to lower capital cost. However, especially power generation can cause a lot of grief and lead to high operating costs due to bunker, service and down time.
Use of FC as an alternative to diesel generators has been investigated as a concept for power generation for a NUI. The NUI is assumed to be located in shallow water and North Sea conditions. The proposed solution is benchmarked against an existing installation in the North Sea. The basic assumption is that the FC can serve as the single source of power, that the Lift Gas (LG) is required at the NUI and that this LG can be used as Fuel Gas (FG) only requiring pressure and temperature control for the LG.
The major obstacle to using LG as fuel for FC for a NUI is the low temperatures occurring when letting down the high pressure LG to FC operating conditions and disposing of potential condensate that might appear when the LG pressure is let down. The required equipment will add complexity to the NUI going against the philosophy of the NUI design and potentially increase the risk of extended shut down and start-up problems. The equipment and volumes of condensate is strongly dependant on the LG composition and may also compromise requirements for limiting the hydrocarbon inventory that often governs the design of a NUI. Furthermore, operation of the LG treatment may be made difficult due to the boundary conditions given by the production export system and the need for disposing of condensate and impose less than optimal conditions for the LG treatment. There is a potential for saving on operating cost for diesel and bunkering operations that may justify a significant price difference in capital cost between a diesel generator solution and a FC. The reduced activity level will also mean a safer operation of the facilities when considering transfer to the NUI.
A potential alternative to FC could be a gas engine generator but this set-up would essentially face the same type of problems as would the FC with regards to using LG as fuel. However, the gas engine is a well know technology that does face the technology barrier that the FC does.
