Integrated Asymmetric Mooring and Hybrid Riser System for Turret Moored Vessels in Deep Water (1997)

ABSTRACT Tanker based Floating Production, Storage and Offloading Systems (FPSO's) are moving into deeper water in the near future. 'The use of turret moored tankers for deep water applications is widely accepted to be a viable and cost effective solution. This paper will discuss the advantages of an unique asymmetric mooring system in combination with a hybrid riser system for a turret moored FPSO in water depths of 1000 m and beyond. A case study for such a mooring-riser system for a typical medium size turret moored tanker in 1000 m water depth in Gulf of Mexico environment will be discussed. 1 INTRODUCTION The use of flexible risers from the floater to the sea floor in deep water requires exceptional flexible pipe design to withstand external hydrostatic pressures and large top tensions and is therefore expected to be very expensive. Additionally, flexibles of the sizes anticipated for export lines are at the current limits of manufacturing capabilities. Moreover, flexible risers can only be installed after the FPSO is hooked-up to its anchor lines and attract therefore a penalty in terms of capital interest due to the FPSO not being able to receive well fluids. An alternative solution is the hybrid riser system (See Figures 1.1 and 1.2). This system basically consist of a large Tethered Riser Buoy (TRB) from which Steel Catenary Risers (SCR's) are suspended to the sea floor and flexible jumper hoses between the riser buoy and the turret of the FPSO. The TRB may be some 150 m below the sea level and is tethered to the seabed. The entire hybrid riser system consist of elements which are proven technology. Large diameter SCR's have already been installed as oil and gas export risers from TLP's in the Gulf of Mexico. The use of flexible risers connected to a tethered buoy in combination with turret moored tankers is also proven technology. It is cost effective to design the riser buoy so that it may accommodate as many risers as possible. Depending on the size of the risers and umbilicals, the riser buoy may be designed to accommodate up to approximately 15 risers and/or umbilicals. In case of that many risers, the horizontal subsea buoy may be long and may therefore require large free space sectors between the mooring lines to avoid riser-mooring interference. To alleviate interference, Bluewater has developed a mooring system with three groups of mooring lines in 120 degrees sectors (refer Figure 1.2). The large anchorline-free sectors in this mooring system allow easy access to the hybrid riser system for installation, maintenance and repair during the field life. In a field lay-out, the asymmetric three-bundle mooring system footprint should now not be expressed as an anchor -point radius but as anchor-point directions. Hence, in congested areas, where the anchor radius is normally kept limited, the entire three bundle mooring system may be rotated to optimise field lay out (see Figure 1.3). The hybrid riser system allows a flexible installation schedule. The Tethered Riser Buoy and Steel Catenary Risers can be installed prior to the arrival of the FPSO in the field.