The design of a new generation steel jacket for the Tern field, located in the northern North Sea in a water depth of 167m, has been completed with the incorporation of a number of innovative features. The Tern jacket has achieved a total estimated weight saving of 20% when compared to an equivalent conventional jacket utilising similar design criteria. This saving comprises an estimated 6000 tonnes for the installed jacket with a further reduction of 1000 tonnes for temporary steelwork. The corresponding cost benefit is estimated to be £20 million, or approximately 25% of the total fabricated cost for the jacket.
The Tern platform will be installed during 1988 in a water depth of 167m, relative to lowest astronomical tide (LAT), in block 210/25 in the UK sector of .the northern North Sea, as shown in Fig.l. The Tern field is being developed by Shell UK Exploration and Production, the operator for the joint venture in the North Sea between Shell UK Exploration and Production, and Esso Exploration and Production UK Ltd (Shell/Esso).
The field was discovered in 1975 and has estimated recoverable oil reserves of 175 million barrels. The platform comprises a piled steel jacket and module support frame (MSF) which supports modularised topside facilities including drilling, process and accommodation packages. The Tern jacket and MSF have been designed to support a total topside weight of 24000 tonnes which includes contingencies for present operating requirements and future operational developments. The platform provides 30 drilling slots for 0.660m outside diameter conductors, with 10 j-tubes for possible future subsea developments for outlying areas of the field.
As a result of the low reservoir energy, the Tern platform incorporates water injection facilities. Gas lift facilities have also been included to ensure adequate production rates, with gas imported, as necessary, by pipeline from the nearby Shell/Esso North Cormorant platform (see Fig. 1). The oil produced at Tern will be exported by pipeline to the North Cormorant platform for onward transportation by pipeline to the oil terminal at Sullom Voe in the Shetland Islands. The nearby Shell/Esso Eider platform, which will also be installed during 1988, will receive treated water for reservoir injection from the Tern platform through a pipeline.
The detailed design of the Tern platform has been completed with fabrication to commence in early 1986 and first oil scheduled for early 1989. The jacket has been designed to resist a storm wave condition corresponding to the maximum wave height having an average recurrence period of 100 years for eight compass directions as shown in Fig. 2, with the greatest value being 30.5m. During offshore installation, the jacket will be barge launched and, subsequent to upending, will be positioned such that the major axis has an orientation of approximately 22 degrees west of true north. Thirty-two 2.l34m outside diameter piles, arranged in clusters of eight at the four corner legs, will be driven to a design penetration of 46.5m.
