Exposed Location Single Buoy Mooring

ABSTRACT Since 1961 Single Point Moorings (SPM's) have been used successfully in nearly all parts of the world. Nevertheless the feasibility of applying a conventional SPM in the North Sea, where sea conditions are very bad, was considered doubtful in 1972. At that time the practical experience with loading hoses at installed SPM's was such that no long hose life was expected at a North Sea SPM location. Consequently several oil companies developed much more complicated S}'M systems such as ALP, ELSBM, SPAR etc. These mooring systems include facilities to keep floating hoses out of the water and to restrict submarine hose motion. Moreover, helicopter landing facilities are available to improve accessibility. The design of and operational experience with a typical example of a more advanced SPM the ELSBM, are described in this paper. A performance comparison between a single ELSBM and two conventional SPM's has been made. INTRODUCTION Use of a SPM as an offshore loading/unloading facility for crude carriers is generally accepted now. At present Over 150_SPM's are being used successfully in all parts of the world. Most of the SPM's installed are of the Catenary Anchor Leg Mooring (CALM) type (1). In addition some Single Anchor Leg Mooring (SALM)buoys (2) are being used successfully. Characteristic properties of the CALM and SALM type SPM's are:unmanned facilityaccessible by boat onlyloading hoses and moorings are free floating in the waterbuoy is livelydesign is relatively simplelow costs of installed unit. In the early seventies Shell U.K. Expro discovered their Auk Field in the central part of the North Sea. As the Auk Field production was estimated to be 40,000 barrels per day only, neither a pipeline to the shore nor a storage facility at the Auk Field location appeared economically justified. At that time the application of a conventional SPM in the Central North Sea was not recommended, for the following reasons:Unsatisfactory experience had been gained with the use of large-diameter hoses at SPM's installed at more exposed locations.The natural period for heave and pitch is in the wave period range; consequently the buoy motion in rough seas is serious.Problems were expected with entanglement of loading hoses and moorings.Accessibility of the buoy for repairs and maintenance is worse. DESIGN REQUIREMENTS OF AUK FIELD ELSBM As a conventional SPM was not considered suitable for Auk Field it was decided to design an Exposed Location Single Buoy Mooring (ELSBM). For this buoy the following requirements would apply:No floating hosesNo floating mooringsOptimum behaviour of submarine hosesMinimum buoy motionAccessible by helicopterDedicated tanker would be acceptableBuoy would be unmanned. Manning of an SBM provides some very important beneficial effects such as (3):No need for remote-controlled operation Tanker mooring easierDeveloping failures are discovered sooner and more easily