The Foinaven riser and umbilical system comprises of 10 flexible pipes and 2 umbilical and provides production, test, gas injection, water injection and control for two drilling centre.
The size and duty of the risers is as follows:-
2 × 10" Production
2 × 8" Production/Test
1 × 10" Water Injection
1 × 8" Gas Injection
I × Dynamic Umbilical
2 × 10" Production
2 × 8" Production/Test
I × Dynamic Umbilical
The design pressure for the system is 3689 psi (254 bar g). The configuration of the risers is a "pliant wave" (figure I) which is anchored to a gravity base structure by tethers and has buoyancy modules distributed along the lower end. The system is designed such that it can be released from the FPSO in extreme emergency conditions.
The Foinaven field is the deepest ever application of a "pliant wave" riser configuration and has to withstand very high currents over the full water column of up to 2 m/s (3.9 knots), When combined with the 100 year design wave of 18 m (sign) this provided very harsh conditions for the design of the pipe (figure 2), the critical areas being the vessel interface, where a bend stiffener is required, and at the riser touchdown point where extreme near and far vessel positions resulted in the need for a hold back anchor to prevent pipe movement and subsequent over bending of the pipe. The large waves in the Atlantic also impose severe fatigue loading on the risers, the most critical risers being the gas and water injection risers, due to the high operating pressure within the system.
Next to the environmental conditions the next major influence on the system design was the vessel offset. This is closely linked to the design of the mooring system for the vessel which was far more compliant than originally anticipated in the riser design. When procuring a floating production system the risers influence both the sub sea layout as well as the mooring system design. Care must therefore be taken in addressing this interface. If the interface is defined at the riser touchdown the cost benefit of manufacturing flexible flow line jumpers (especially the flow line to manifold jumpers) at the same time as the riser maybe lost.
Also for the umbilical, vessel interface was one of the major challenges. The region that experiences the highest loads both with respect to tension and bending is found at the I-tube exist. Bend stiffener design as well as cross-section design is critical.
At touch down two clump weights were needed, One main clump weight taking up virtually all the tension, leading to that the touch down area saw no tension at high bending. the resulting catenary below the tether clamp, however, induced such a high bottom tension in the extreme current and far vessel offset cases that the interface joint between dynamic and static umbilical was anchored to a second, smaller slump weight. Its role is to prevent axial movement of the interface joint thereby preventing potential unwanted configuration changes.
