The Foinaven Umbilical Performance Monitoring System - A New Approach
Authors: Lyons G.J. , Brown D.T. , Cook H.H. , Walls B. , Barnay G. 
Published: October 2008

A novel curvature sensing element forms the basis for monitoring the bending stresses in one of the two subsea umbilicals of the Foinaven floating production 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, and some interesting results presented.

Introduction

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 20 years or so service life. At the time of design of the Foinaven umbilicals (Ref 1) it was considered that no proven service life prediction model esisted tbr umbilical internal elements, and that it was not then possible to determine how the global loadings on the umbilical, particularly tension and curvature, would be distributed 10 the internal elements of the umbilical, ic the armour wires, hoses/tubes and electric conductors. To compensate for this, dynamic testing was camed out to prove the umbilical and bend stiffener. This assisted in enabling service life prediction materials 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. In addition, the system would also provide information to assist in on-line integrity monitoring on Foinaven, and useful feedback for the future design of similar FPSO riser systems. A system was installed 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 (Ref 2). This experience formed the basis for the Foinaven system.

Foinaven FPSO

The Foinaven Petrojal 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 life of the swivel seals. It has a length of 240m, with beam and draft of 34m and 13m respectively. The Foinaven turret includes 15 riser I-tubes.

Subsea

The subsea layout comprises two drilling centres each based upon a manifold and well cluster arrangement, with rigid steel nowlines connecting the riser base locations, flexible risers connecting the riser bases to the FPSO, along with the associated control umbilicals.

Environment

The design parameters for the Foinaven field arc summarized in Table I (Ref 3). In isolation, the wave height, current or water depth arc not unique - the extreme wave heights arc 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.