Hutton TLP Mooring System

ABSTRACT The tension leg platform (TLP) which is being developed for the Hutton Field in the North Sea is the first application of this new platform concept which could be widely utilized for deep water oil field development. A major novel part 'of the TLP is the mooring system, comprising the tension leg components and the mechanical handling and installation equipment. The paper covers the functional and design requirements for the mooring system and describes the tension leg components. Details of some of the development test programmed are included and a brief out 1ine of the installation procedures and equipment is provided. The mooring system incorporates new hardware for the anchor connector and cross load bearing and a new screwed coupling has been designed and tested. The installation and change out equipment includes a special tensioner/motion compensator which effects translation from the free floating to the TLP mode, as well as a large capacity torque tool and special mechanical handling equipment. The mooring system technology developed for the Hutton TLP would be app licable to future tension leg platforms. INTRODUCTION The TLP is a floating platform fixed by vertical tension legs to piled foundation templates on the sea bed. It is a compliant structure which allows lateral movement (sway, surge and yaw) white the tension legs restrain vertical movements (heave, pitch and roll). Buoyancy is provided by the pontoons and columns of the hull. The excess of buoyancy, greater than the platform weight, maintains the legs in tension in all loading and environmental conditions. The platform supports all the necessary equipment and facilities for drilling and oil production, at a peak rate of 110,000 barrels per day, as well as providing accommodation and life support systems for up to 239 people. The plat form is a permanent installation for the life of the, field and will allow normal operations at all times, except in the most adverse environmental conditions. In this respect it is functionally similar to normal fixed offshore installations. The sixteen tension legs (four at each corner of the platform) provide the "flexible" connection between the hull and the foundations. They withes and very high cyc1ic tension 1oading which could vary from almost 0 to about 2400 tones per leg for extreme design environmental conditions. During their 20 year design life the legs are subject to about 108 load reversals, thus making fatigue an import ant factor in their design. The fact that the tension legs are an essential part of the permanent platform has necessitated careful selection of reliable materials with a high level of quality control in the manufacture of the components as well as rigorous design and analysis. The means of installing the tension legs from the platform at the Hutton Field has been a very important influencing factor in the development of the mooring system.