In April, 1977 the deepest Single Anchor Leg Mooring in the World was installed at the Thistle Field, Block 211/18 in the North Sea, for the British National Oil, Corporation. As the SALM location was the most northerly, most exposed and in the deepest water to date, it presented the severest test of Single Point Mooring technology so far. The system has been in operation now for one year, capable of loading into dedicated tankers of up to the 120,000 dwt class.
It was appreciated (at an early stage) that the operation and maintenance of this export system would be crucial to the economics of the initial field development: so, the SALM chosen contains many additional monitoring and maintenance features. For example the ease of maintenance is improved by increased accessibility provided in the novel design of the buoy, and instrumentation has been incorporated to measure the dynamic operating conditions to enable the SALM to be used in conditions which would normally not be considered practicable.
The mooring of the loading export tankers is closely monitored by a fully integrated tanker/ SALM/platform instrumentation system. The system monitors mooring loads, buoy and riser angles and tanker status, the latter being radio telemetred to the platform. Crude may be automatically diverted to platform storage following a radio signal from the tanker during emergencies. These aspects, plus ongoing developments, will be discussed together with some of the first year's experiences in operating and maintaining the SALM.
The Thistle oil field, is located 210 km north-east of the Shetland Islands and lies mainly in block 211/18 with an easterly extension into block 211/19 (see Figure 1). The Thistle field is the most northerly development so far undertaken in the UK Continental Shelf. The oil produced from the field has been transported by direct loading into tankers through a SALM until the Shetlands (Sullom-Voe) pipeline was commissioned. During this phase, three 100,000 dwt tankers were chartered for transportation of crude oil from the field to various European terminals.
The Thistle Jacket is a conventional steel piled construction and incorporates crude oil storage tanks having a maximum capacity of only 11,000 m3 (70,000 bbls). Because of the possible delay in completing pipelines and the Sullom-Voe Terminal it was decided to get the field on stream as early as possible by utilising a SPM tanker export system. Consequently the overall export efficiency is primarily dependent on the environmental constraints which dictate the SALM operation limits and also the number of export tankers required.
The 154 km long 910 mm (36 inch) diameter export pipeline to Su110m-Voe which services 5 major fields in the Brent System including Thistle, was completed and transferred first oil in December 1978. It is obviously important to Thistle producing at peak 180,000 bbls/day, to reduce shutdowns to a minimum, therefore it was considered prudent to maintain the SALM in good condition as a standby alternative export outlet should the main trunk1ine become unserviceable.
