|Publisher||Offshore Technology Conference||Language||English|
|Content Type||Conference Paper|
|Title||LOADOUT TO INSTALLATION OF THE MUCHISON JACKET|
|Authors||Raymond G. Guy and William R.H. Clifford, CJB-Earl and Wright Ltd.|
Offshore Technology Conference, 5-8 May 1980, Houston, Texas
|Copyright||1980. Offshore Technology Conference|
At 25500 tonnes, the heaviest ever barge launched steel jacket for Conoco North Sea was successfully installed in the Murchison field last August after transfer from the fabrication yard in Scotland. The technology involved in manoeuvring a large barge launched jacket from the fabrication yard to the installation site has only been put to the test previously for jackets smaller than Murchison. Outlined here are some of the features, as seen by the consulting engineers responsible for the design, in taking the jacket through loadout from the fabrication yard, transportation to the site, launch and upending. The predicted behaviour from analytical methods and model tests are compared with the behaviour actually observed during installation.
It was the task of the consulting engineer for the largest barge launched jacket to ensure its satisfactory performance during the period of loadout, transportation, launch and upending (fig. 1). In service analysis and design has been established on large self floating platforms such as Thistle (31000 Tonne) and Ninian Southern (21000 Tonne) whereas previous barge launched jackets have been considered only for shallower water depths, for example Piper (14000 Tonne), Heather (16000 Tonne) and Tartan (19000 Tonne). Hence the proposal for a barge launched jacket in 156 metres of water with a prospective launch weight of 25000 Tonne was a bold step for Conoco to take, the decision being based on the anticipated fabrication costs. The Murchison jacket was to support 25000 Tonnes of topside loading and 27 conductors and was to resist an extreme storm wave height of 29 metres. Due to the pioneering nature of the Murchison jacket it was considered prudent to perform an extensive series of model tests to investigate its behaviour for the stages during its transportation launch and upending. For the loadout stage model tests were not considered practical, but reference could be made to the loadout of previous smaller jackets. Fabrication techniques had previously been developed, although fabrication of the Murchison jacket involved the heaviest crane lift to date.
Contingencies were considered at all stages as far as possible. At the design stage the jacket was modelled with the most highly stressed members removed to examine its reserve. Loadout was monitored to ensure that the jacket could be returned to the slipways until it was ascertained that the barge was sustaining the whole jacket weight. For the transportation stage pre-designated harbours would have been made for in the event of adverse weather conditions. The upending system contained redundant sub-systems to ensure satisfactory control should there be any valve malfunction.
During the loadout operation, the Conoco Murchison jacket was moved approximately 250 metres from the slipway on which it was built, onto a grounded barge at the quayside. It was loaded out head first over the bow of the barge and negotiated a 20 metre span between the ends of the slipway and the barge skid rails (fig. 1). The 20m span was necessary because of the poor ground conditions adjacent to the quay.
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