Norway's Den norske stats oljeselskap AS will decide this summer whether to build a production ship that would be a modification of its recent submerged turret loading (STL) shuttle tanker design.
STL involves a tanker positioned above a buoy moored to the seabed and held 30 m below sea level. The tanker pulls the buoy into a turret in its hull and loads oil through a hose passing through the buoy (see illustration, OGJ, Oct. 25, 1993, p. 34).
Statoil plans to modify the STL concept to submerged turret production (STP). An STP ship would be positioned above a buoy, which would be linked by risers to subsea wellheads.
Oil would be produced directly through the buoy and into the tanker for processing.
Use of similarly dimensioned turrets for loading and production would allow the tanker to be converted from STL to STP, mainly by installation of process modules on the tanker deck.
Tore Lea, field systems manager in Statoil's marine technical department, explains that the standard sizing of the turret would limit the number of risers an STP tanker could handle to four to six.
"This would limit production to 60,000 120,000 b/d depending on the reservoir, so the STP concept is not suitable for large fields," Lea said.
Storage capacity would be 550,000 bbl of oil.
"However," Lea said, "two STP ships could be used at once for initial plateau production to bring early profits from field developments. One STP ship could be moved on as production from the field declines."
Lea said the incremental cost between shuttle tanker and production ship versions is so small that Statoil can build a vessel on speculation and use it as a shuttle tanker until a suitable production opportunity appears.
Statoil has asked for construction bids. Lea expects to be able to confirm cost estimates for construction, on which hinge the decision to build, during May.
"If we decide to build an STP ship this summer, it could be completed by early autumn of 1996," Lea said.
The vessel could find ready use as one of Statoil's shuttle tanker fleet.
Lea said, "At that time, Norway would require extra shuttle tanker capacity to deal with new oil from Norne, peak production from Draugen and Heidrun, and new production from Statfjord satellite fields."
A full scale production turret has not been built, but a prototype is to be finished this summer. A critical element is sealing, although a suitable technique has been developed for a subsea multiphase pump used in Draugen field.
The STL concept has been proved in Shell's Fulmar field in the U.K., where an STL tanker is being used as a floating storage unit (OGJ, Oct. 25, 1993, p. 34).
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
