New Concepts for Mooring Offshore Structures

ABSTRACT This paper describes the work carried out on behalf of the UK Department of Energy into mooring concepts for the permanent mooring of large floating structures in hostile environments. In particular the mooring of structures for the conversion of wave energy for power generation in the North Atlantic Ocean. The size of the structures envisaged are not particularly large compared with offshore structures, or ships, however unlike offshore structures the wave energy convertors are designed for a high degree of interaction and response to the wave motion and therefore experience high mooring forces. But in one particular installation, a total of 1400 devices are required over a 144 km distance in order to provide a 2 GW output. However, for others it is more likely to be between 150 and 300 devices covering a proportionately smaller sea area. The mooring concepts which have been conceived for wave energy devices are novel in that they lie beyond current mooring practice, nevertheless, they may well be applied to offshore oil and gas production rigs in the future, so that permanent floating structures could be considered in preference to fixed structures, providing more cost-effective options for operating in deeper water and bringing current marginal fields into production. The offshore industry has created a need to improve mooring systems over the past 15 years and projects within the North Sea have been in the forefront of mooring development. It is possible in thenext decade that the development of mooring techniques will receive greater attention as designers evolve new methods to overcome the many restrictions imposed by current financial and technological limitations. The incentive to develop mooring technology already exists in order to moor wave energy or ocean thermal energy systems which could help to answer some of the associated technical problems of drilling and producing oil in water depths in excess of 350 metres and enable the offshore industry greater freedom to exploit new fields beyond the limits of today's technology. INTRODUCTION Moorings have remained substantially unchanged for hundreds of years; it is only in the last decade or two that any significant technological improvements to the traditional ships chain and anchor system have occurred, principally because the offshore hydrocarbon industry has created the need for improved mooring systems with greater efficiency, durability and holding power. The most recent mooring developments include the design of moorings for the Hutton Field Tension Leg Platform in the North Sea and for Ocean Thermal Energy Convertors, (OTEC) which need to be permanently moored in deep water. Moorings for wave energy devices have been the subject of study since 1978 sponsored by the UK Department of Energy as these devices impose special requirements on mooring systems that are unlike those for previous applications. Firstly, the location chosen for wave energy convertors was off the west coast of Scotland in the North Atlantic Ocean generally on the continental shelf in water depths up to 200 metres.