The world's first tension leg platform has now been producing oil from the Hutton Field since August 1984. During its first 3 years, the separation facilities have undergone a testing period of platform motion which has exposed a number of design deficiencies. The most significant has been the inability of the separators to achieve acceptable oil and produced water quality. In order to maximise recoverable reserves, it is planned to increase seawater injection which will consequently increase the total produced fluid rate by 50 per cent above the original design specification. The challenge to Conoco was to improve the separation performance for the existing and predicted future conditions without radically modifying the existing facilities, and incurring massive expenditure and loss of production uptime.
The primary cause of the poor separation performance was the internal design of the separators which were inefficient in damping motion and created a detrimental effect on separation. As a result of extensive model testing and offshore trials, solutions to this problem have been identified. This work has led to vastly improved operational efficiency and control of oil and produced water quality, the results of which will be invaluable for future designs of process facilities on floating production systems.
Operated by Conoco on behalf of a consortium of 8 companies, the Hutton Field is located 90 miles north east of the Shetland Islands in the UK Sector of the North Sea. Hutton was discovered in 1973 and has an estimated recoverable reserve of 190 million barrels of oil. In 198O, it was decided to develop the field using a Tension Leg Platform (TLP). This is the first commercial application of a revolutionary design of offshore production platform. Intended for oil and gas production in water depths beyond the economic limits of traditional fixed structures, the TLP is a floating platform which is fixed to the seabed by steel tethers. The tethers are constantly kept in tension so that vertical movement is suppressed, while limited horizontal movements (sway and yaw) can occur.
The crude oil separation train and produced water handling systems, installed on the TLP, were similar to those found on many other North Sea platforms. They did have modifications incorporated which were recommended by the equipment suppliers to counteract the predicted platform motions. Operational experience has now shown the profound and detrimental effect the TLP motion has had on the process systems and features incorporated into the equipment during the design phase to nullify the effects of wave motion have been detrimental to separation even in calm conditions. The system as a whole has proven to be very sensitive to small changes in operating conditions and this has been compounded by the nature of the reservoir fluids which have proven difficult to separate. Severe winter storm conditions in fact almost immobilised the control over the separation and clean-up systems.
