Abstract

In October 1983, EMH was awarded by BRITOIL the contract for the redesign of the THISTLE SALM. This paper describes the work covered by the contract and underlines the modifications to the design of the structure.

The contract was divided into two distinct phases.

The first phase, which started in November 1983, concerned the engineering of the project: a full hydrodynamic and fatigue analysis of the structure was carried out, reinstallation procedures defined, and the articulations and connection pieces redesigned.

The second phase, which stated in March 1984, concerned the fabrication of the articulations and supervision by EMH.

Each step of the re-design is clearly defined and a detailed description of the articulations is illustrated by pictures of the most important components during fabrication.

The experience gained and the improvement to the system as a result of the modifications of the articulations is the subject of this paper.

Background

In 1977, the Single Anchor Leg Mooring was installed in 162 meters (531 feet) water depth in the THISTLE FIELD, north-east of the Shetland Islands in the North Sea. This SALM moored tankers for loading purposes until 1978 - as scheduled - when a pipeline to shore was commissioned. The SALM performed well during this time and fulfilled its design role. After this date, the SALM was placed on stand-by. During the first year of stand-by, water leaked into the buoy which led to the disconnection of the buoy from the riser. The structure was removed for repair and reinstalled in mid-1979. The SALM was still on stand-by on site in January 1983 when once more the buoy section broke free from the riser.

After a full damage investigation, BRITOIL decided to carry out an extensive analysis of the structure.

As a result it was highlighted that several areas required redesigning. BRITOIL decided to make the necessary repairs and to store the different parts of the SALM ready for deployment in the event of a major pipeline parts of the SALM ready for deployment in the event of a major pipeline breakdown. The contract for the redesign was awarded to EMH in October 1983.

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