Hutton TLP Installation

ABSTRACT Installation of the Hutton TLP required precise simultaneous control of several complex systems. This paper describes the equipment and operations involved in the deployment and connection of the sixteen tension legs, and the documentation used for control. The equipment trials and personnel training programmes undertaken to ensure the first-time success of this unique operation are presented. The paper concludes with an appraisal of the installation process and the significance of the principles employed to the development of deep water TLPs. INTRODUCTION Installation of the Hutton Tension Leg Platform, a new concept in oilfield development(l), was carried out in under four days. The process of installation involved translation of the free floating vessel into a vertically restrained platform. This operation was unprecedented in its reliance on multiple systems, novel equipment and specially trained personnel. It entailed the assembly and deployment of sixteen tension legs, their connection to four pre-installed foundations, and the application of 13,000 tonnes of tension. These unique operations required the deve1opment of detail ed procedures to control the installation systems and activities. The use of equipment unproven in service and personnel inexperienced in the special operations necessitated intensive trials and training programmes to ensure a safe and efficient installation. The installation of the TLP, 90 miles north-east of the Shetland islands and in a water depth of 148 metres, was completed on 15th July and first oil produced on 6th August, 1984. PRINCIPLES OF INSTALLATION The TLP is a compliant structure where the vertical motions of heave, pitch and roll are suppressed by leg tension. The tension is induced by an excess of buoyancy over platform weight. This concept can be represented by the equation of static equilibrium:(Equation available in full paper) The static 'pre-tension' must be sufficient to maintain the legs in tension for all operating conditions and extreme environmental events. Approximate values are:(Equation available in full paper) This condition is achieved by restraining the platform approximately 10 metres deeper in the water than its free floating draft. Significant changes in operating loads, riser tensions and platform centre of gravity can be accommodated by the transfer of ballast. Installation requirements were a major factor influencing the design of the mooring system, thus engineering for installation proceeded in parallel with mooring system design. An early decision in the design process was the method of 'stabbing' the legs into the foundation mooring sleeves. Unaided stabbing was considered the most reliable, efficient and easily engineered method, provided the platform position could be maintained within acceptable tolerances. The design environmenta1 conditions were established as a wind speed of 10 m/s and a sea state typically of 2m significant wave height and 7s period. Analyses and model tests demonstrated acceptable leg motions and established a vessel positioning tolerance requirement of 1.5m during stabbing.