New FPSO mooring technology aimed at deeper developments

For operators of floating production, storage, and offloading vessels (FPSO's), disconnectable mooring systems typically do not come cheap. SBM Atlantia aims to change that with its new MoorSpar system, a disconnectable mooring system that allows lower-cost, higher-efficiency steel catenary risers (SCR's) to be used in the development of deep and ultradeepwater fields.

According to Philippe Lavagna, engineering project manager for the MoorSpar system, development of the new technology arose from a perceived technology gap for floating production systems in deepwater areas exposed to hurricanes, typhoons, or other extreme weather events. "Typically, up to now deep and ultradeepwater FPSO's had no choice but to use expensive riser towers or hybrid riser systems, as the vessel's heave motions prohibit the use of SCR's."

SCR's are preferred in ultradeepwater applications because they are seen as a cost-effective alternative to hybrid riser systems. Further, they become the only real option when flexible risers and jumpers are not feasible due to high temperature and pressure limitations and extreme water depths, which cause hydrostatic pressures to exceed the collapse tolerance of the risers.

"In traditional connection scenarios where the riser would be attached directly to the FPSO, SCR's are sensitive to fatigue arising from floater heave motions," Lavagna continued. "For this reason, SCR's have been limited to use on spars, semisubmersibles, and tension leg platform-type floaters, where the heave motions are small enough (acceptable) for SCR's. However, space limitations on these floaters allow significantly lower production capacity and basically no storage compared with FPSO's."

SBM Atlantia incorporated this low heave response advantage into its design. "That's why we call it the MoorSpar system" Lavagna said, "because we essentially moor an FPSO to a 'SCR friendly', spar-type floater. For people familiar with spars, at first look, our MoorSpar buoy would resemble a truss spar turned upside down."

The MoorSpar unit consists of a truss-like structure set atop a long, submerged, slender buoy which is moored to the seafloor by a combination of vertical tethers and lateral polyester lines. The FPSO and buoy are connected through an articulated yoke system terminated by female (on the yoke) and male (on the buoy) conical sections which match in dimension.

Situated at the top of the MoorSpar unit, the male conical section is the structural link between a main roller bearing and a gimbal table (a heavy duty uni-joint at the tip of the yoke). This arrangement reportedly accommodates the vessel's roll and pitch motions, and also allows the FPSO to weathervane.

SCR's connect to the MoorSpar unit at riser porches located along the keel of the buoy. The risers are then linked to internal piping, which is routed up through the central column and then across hard piping before swiveling to the FPSO. The high pressure rating can be reduced upstream of the swivel stack arrangement, within the piping manifold located at the MoorSpar buoy top above the extreme wave crests.

"Connecting the FPSO to the buoy with an articulated yoke allows us to filter out the FPSO heave motions. The truss limits the wave loads and the wave-induced motions of the MoorSpar buoy, and consequently the motions of the SCR's porches are minimized," Lavagna explained. "Having a 'wave transparent' design like this is especially important during a severe weather event, and basically eliminates the FPSO-induced fatigue issues in the SCR's. With this design advantage, the MoorSpar system can be used in water depths up to 10,000 ft."

In a severe weather event when the FPSO must be moved, the yoke system is easily disconnected and goes with the vessel. The buoy stays at the field location. Once the weather event passes, the FPSO can return to the field location and reconnect to the MoorSpar within a 4-6 hr period.

The design of the MoorSpar system was confirmed in a series of wave basin model tests at the MARIN facility in the Netherlands in 2007, which also verified the feasibility of connection and disconnection.

To learn more about the MoorSpar system, visit www.sbmoffshore.com or contact Philippe Lavagna by email or by phone at +281.899.4356.

Ted Moon is the Technology Editor of JPT Online. He brings information on emerging technologies, R&D successes, new field applications, updates from SPE papers about recent innovations, and more. If you have a question or suggestion for future article topics, email Ted at teched@spe.org.