A novel curvature sensing element forms the basis for monitoring the bending stresses in one of the two subsea umbilical of the Foinaven floating facility Additionally, load cells are located at the top of the umbilical to measure tension From a combination of the these the total stresses in the individual components of the umbilical are derived, and rainflow counting used to assess the fatigue life expended The system in operation is examined.
In any offshore field development the umbilical is a critical component for the production of hydrocarbons It provides electrical and hydraulic power and signals to the subsea components in the system from the surface vessel. It will be subject to dynamic and static loads throughout its anticipated 15 years or so service life.
At the time of design of the Foinaven umbilicals (Walls, 1995) it was considered that no proven service life prediction model existed for umbilical internal elements, and that it was not then possible to detemine how the global loadings on the umbilical, particularly tension and curvature, would be distributed to the internal elements of the umbilical, ie the armour wires, hoses/tubes and electric conductors To compensate for this dynamic testing was carred out to prove the umbilical and bend stiffener This assisted in enabling service life prediction models to be validated
The question remaining was, would the dynamic response of the umbilicals be as predicted, and so would the service life predictions (backed up by dynamic testing) be appropriate? Provision of a monitoring system would help provide this information BPP Technical Services Ltd installed a system on the Emerald FPF in 1991 which continued in operation until 1996 This monitored the tension and bending moments at the top of a 8 inch water injection flexible riser (Lyons, 1996) This experience formed the basis for the Foinaven system.
The Foinaven Petrojarl IV FPSO is located in 465m of water in the newly developed Atlantic Frontier region West of Shetland The FPSO arrived on station in November 1996 It is catenary moored with 10 wire rope mooring lines, and has thruster assistance to aid station-keeping Turret position is adjusted incrementally rather than continuously to increase the seal life of the swivel connections It has a length of 240m, with beam and draft of 34m and 13m respectively The Foinaven turret includes 15 riser I-tubes
The subsea layout comprises two drilling centres each based upon a manifold and well cluster arrangement, with rigid steel flowlines connecting the riser base locations, flexible risers connecting the riser bases to the FPSO, along with the associated control umbilicals
The design parameters for the Foinaven field are summarised in Table 1 (Grant 1995) In isolation, the wave height, current or water depth are not unique - the extreme wave heights are slightly higher than Gulf of Mexico hurricanes or Far East typhoons, the current is comparable to a Gulf of Mexico eddy current, and the water depth is considerably less than those for Campos Basin floating facilities But the combination of the design conditions for West Shetlands, and the persistence of them, presents the challenges to the designer to provide a system which can survive the extreme loads, and the fatigue loading.