During the summer of 200 1, Phillips Petroleum successfully completed the re-float and removal of the Maureen steel gravity platform from its offshore location in the North Sea, following the end of production in the field in October 1999. This paper discusses the geotechnical aspects of the removal process, and the engineering processes which were used to ensure that the removal of the platform could be performed successfully.
The Maureen platform was the world's largest steel gravity platform, weighing about 1100 MN (1 10,000 tomes) and incorporating 650,000 barrels of oil storage within three tanks, as shown in Figure 1. The platform was located m 95 metres of water, in the central North Sea, as shown in the location plan given m Figure 2. Following the end of production in October 1999, there was a requirement for the Maureen facilities to be removed from location, to satisfy International conventions.1 This paper concerns the geotechnical engineering necessary for the planning and execution of the re-float operations, which led to the successful removal of the Maureen platform and associated facilities. The engineering studies were performed by Fugro Limited, under the overall guidance of Phillips Petroleum. The main contractor appointed by Phillips Petroleum for the offshore removal operations was Aker-Maritime a.s.
The original platform design, which was undertaken by Tecnomare, had premised that at the end of field life the platform would be hydraulically jacked from the fully penetrated condition (in the seabed) by the injection of water through the system of drainage spears In 1992, Phillips Petroleum, m conjunction with Tecnomare, undertook feasibility studies on the skirt abstraction and re-float of the platform.2
Whilst these studies were one of the starting points for assessing the optimum method of refloating the Maureen platform, the platform posed some further particular geotechnical problems. These various issues were discussed prior to the platform removal3 This current paper reviews the practical and theoretical work performed to define the extent of these problems, and the engineering solutions developed for the platform removal. The actual performance of the platform during the removal, and how this compared with predictions, is also discussed.
The Maureen platform was supported on three steel bases, each 47 metres in diameter. The platform was held in place on the seabed both by its own weight, and by the lateral resistance provided by the underbase steel skirts. Figure 3 shows a cross-section through the centre of one of the bases, indicating the dimensions of the base and the extent of skirts penetrating into the seabed It may be seen from Figure 3 that there are both radial and circumferential skirts, which therefore create compartments beneath the bases. The deepest skirts penetrate 3.4 metres into the seabed, and these are structurally supported by stiffeners, the sue and geometry of which are also shown in Figure 3.