A Large-Scale Model Test of Turret Mooring System for Floating Production Storage Offloading (FPSO)

ABSTRACT A new generation turret mooring system for floating production storage offloading (FPSO) system has been developed by Mitsui Engineering & Shipbuilding Co., Ltd. (MES) Jointly with Technology Research Center of Japan National Oil Corporation (JNOC). This paper refers to the concept design of FPSO, detailed designs of turret structure and bearing system, and large scale (one-fourth) model test. The turret mooring system is characterized by the application of self-lubricated plain bearings below water level. Under the simulated vessel motion and mooring load, the large scale model tests were conducted to verify and evaluate the effects of the fabrication tolerances on the bearing pressure and its distribution. Results of the theoretical computer analysis and model testing are also compared. INTRODUCTION The importance of offshore oil development for marginal fields in deep water or hostile environment is more and more increasing. Many companies are seriously considering, as an economical and reliable scheme, the use of a turret moored FPSO for such fields. A turret mooring .system has high mobility, weathervaning capability and lower installation cost, and is sui table for extended flow test, early production and marginal field production. Existing conventional turret mooring system, used on drilling vessels and also production test ship supports, all mooring loads by means of rollers/rails at main deck level. This inevitably makes the turret structure very heavy and sophisticated, and very high fabrication /assembly techniques are required. A three-year joint development program was accordingly initiated in 1985 to develop more compact, reliable and economical turret mooring system, which comprises :Phase I - Conceptual design of FPSO, and basic design of turret mooring systemPhase II - Detail design and fabrication of 1/4 scale turret mooring equipment, and performance test of plain bearing materialsPhase III - Evaluation test of 1/4 scale turret mooring system, and design improvement of the turret system Major objectives of this program were to design a new generation turret mooring system and, conducting large scale model test, to evaluate how the fabrication tolerance and rigidity of turret/hull structure will affect the maximum bearing pressure and its distribution for feedback into the turret design. FEATURES OF THE TURRET SYSTEM Features of the system compared with conventional turret system are :Use of underwater lateral load bearingsUse of self-lubricated slide bearingsEase of fabrication and assemblyLower cost of total mooring systemAbility to replace turret bearings without drydocking TURRET LOCATION There are two considerations regarding the location of the FPSO turret. One is to let the vessel naturally weather-vane with changes in environmental forces such as wind, waves and currents. This results in the minimum total load on the mooring system. The turret, in this case, should be located further forward of the vessel to attain higher weathervaning capability.