The Disconnectable Mooring System Of The FPSO Vessel At Huizhou, South China Sea, Lives Up To Its Promises

Abstract

ABSTRACT The system chosen to develop the HZ-21 and HZ26 oilfield offshore the People's Republic of China was a tanker based FPSO incorporating an innovative disconnectable mooring design to allow the tanker to leave the area in the event of an approaching typhoon. This paper gives an outline of the design and construction, and describes the installation and first 18 months operational experience. INTRODUCTION The HZ-21 and HZ-26 fields, jointly developed by the ACT OPERATORS GROUP (ACT-OG) comprising Agip, Chevron, Texaco and the Chinese NanhaiEast Oil Corporation, lies in the South China Sea approximately 100 miles S.E. of Hong Kong in 116 meters of water. The area is notorious for typhoon conditions where a combination of wind, wave and storm driven currents result in far more severe weather conditions than encountered, for instance, in the North Sea. However, the seasonality, low frequency andpredictability of typhoons made the choice of a disconnectable rather than permanent mooring system more attractive. In addition to the relative costs, down time due to disconnection would be minimal since the production facility in the field required evacuation and hence production would be halted in any case. The system involved locating an internal turret inthe forepeak structure of the FPSO supported by a three race roller bearing allowing the tanker to weathervane around the mooring (refer to Figure 1). Connection of the turret to the submerged mooring buoy carrying the eight catenary anchor legs is effected by means of a mechanical connector once the mooring buoy has beenwinched up to the tanker from its neutral buoyancy position 35 m below mean sea level (refer to Figure 2). The design concept is described in greater detailin OTC 6251, Innovative Disconnectable Mooring System for Floating Production System of HZ-21-1 Oil Field at Huizhou, South China Sea, while SPE 22377, First 12 Months' Experience of Disconnectable Mooring System for HZ-21/HZ-26 FPSO, gives a review of the first year of operation of the system. MOORING SYSTEM The Buoyant Turret Mooring (BTM) was designed to allow reconnection in 3.5 m significant sea states. As with any disconnectable mooring system there are three major elements:the mooring buoy which carries the risers and the anchor legs,the turret around which the tanker weathervanes,the swivel which provides the connectionbetween the earthbound risers and the piping on the rotating tanker. Mooring Buoy The mooring buoy carries the eight catenary anchor legs, comprising of wire rope and chainterminated with a high holding power anchor as well as the risers from the production platform. The buoy is of novel design since the buoyancy is controlled to allow the buoy to sink, when disconnected, to a depth of 35 m where the net buoyancy is equal to the suspended weight of the chains and risers.