The Exact Positioning of Oseberg 'A' Platform

ABSTRACT This paper describes the planning and execution of the exact positioning of the 800,000 tonne Oseberg A production platform. Oseberg A is the tenth Condeep concrete platform to have been designed, constructed and installed in the North Sea by Norwegian Contractors, and of all the Condeep structures installed to date, none have been positioned with the same degree of accuracy as this one. As have all Condeep platforms, Oseberg A was constructed to near 100% completion inshore, in sheltered waters, on the west coast of Norway. And then finally, in June 1988, the floating structure was towed to the Oseberg field in the North Sea where the platform was positioned and installed on site in 109 metres of water. Whereas previous positioning of such giant structures had been carried out through the utilization of free running tugs in combination with dowels, a new method of positioning had to be developed for the Oseberg A allowing for much more precision. Compared with the generous installation tolerances of the past, that had always been met with ample margins, the absolute requirements in regard to the exact positioning of Oseberg A on the field represented quite a marine engineering and operation challenge. This paper describes how that challenge was met, and how the platform was successfully positioned less than 0.1 meter (4 inches) from theoretical centre. INTRODUCTION A major milestone in the development of the Oseberg field in the North Sea was reached in June 1988 with the tow-out and installation of the Oseberg A platform. The platform was installed in a water depth of 109 meters. Oseberg A is the tenth Condeep platform designed, constructed and installed by Norwegian Contractors. On previous occasions, free running tugs have been used to position platforms. To obtain sufficient control during the final stage of positioning, dowels i.e. large piles underneath the platform, are brought into contact with the seabed to provide the resistance necessary to control the motion of the platform. The combination of tugs and dowels provide both station keeping ability and the means to accurately position and orientate the platform. The method has been proven in practice to meet generous installation tolerances with ample safety margins. The installation of the 800,000 tonne Oseberg A platform close to the Oseberg B jacket and preinstalled subsea piplines called for very careful positioning and presented Norwegian Contractors with a new challenge. It was essential to ensure that no damage was done to the jacket or the prelaid pipelines. At the same time a high degree of accuracy of position and orientation was required as the two platforms were to be connected by a prefabricated bridge. The method of using free running tugs and dowels was therefore improved upon. For better control during this latest positioning, four of the nine tugs were anchored. For safe maneuvering of the platform towards target winches were used onboard the anchored tugs.