The industry has deployed over 20 disconnectable mooring systems that allow ship-shaped floating production units to leave their station to avoid extreme events; such as tropical storms or impact from icebergs. This paper reviews the evolution of these systems; together with the design considerations leading to their selection for various projects. By examining the key features of each type of system; the paper provides insight into the drivers for their development; and identifies their benefits and limitations. A summary of projects employing disconnectable mooring systems is provided; including the systems' principal characteristics; and the timeline of their deployment.
Permanently mooring a ship shaped floating production unit (FPU) on station; from commissioning through to demobilization; was a key enabling step in the development of floating production. The first Floating Production Storage and Offloading (FPSO); Shell Castellan; was installed in 1977 using a Single Anchor Leg System (SALS) (Eijkhout; 1978; McLeod and Smulders; 1982); and since then a wide variety of mooring system designs have been developed.
The moorings of the early permanent systems were connected to a floating body; which was in turn connected to the hull of the FPU. These moorings systems were either tubular steel structures fixed to a foundation via an articulation; or chain mooring legs used in conjunction with flexible risers. Multiple mechanical parts were required to connect the floating body and FPU; both as articulations to allow movement; and as swivels to transfer well fluids between the fixed and rotation parts of the system.
Figure 1 shows an example of such an arrangement. The articulations decouple the pitch of the vessel from the floating body supporting the mooring legs; and allow the vessel to weathervane around the mooring system to adopt the position of least resistance to the prevailing wind; waves and current.
