Abstract

After 16 years of field production, the Phillips Maureen Alpha Platform was refloated and towed to deep water moorings at Stord in Norway in the summer of 2001, as part of the Maureen Field Decommissioning process.

The Platform, weighing about 110,000 tonnes, consists of the Tecnomare Steel Gravity (TSG) substructure, supporting an integrated deck. To date, this platform is the largest and heaviest offshore production platform to be removed from an offshore location as part of decommissioning operations. The substructure consists of a triangular lattice steel frame interconnected with three cylindrical storage tanks (having a total storage capacity of 650,000 barrels).

Aker Maritime ASA was the main contractor for detailed engineering, refloat execution and tow to inshore mooring of the TSG.

The main objective in planning and making preparations for the operations were to achieve a successful and controlled extraction of the platform skirts from the seabed followed by a controlled and stable ascent of the platform to the towing draft.

This Paper addresses the development of the optimal refloat method, through extensive in-field and model testing, geotechnical engineering, and float-up stability analyses. It further describes the design and testing of the temporary refloat equipment, including the three subsea pump skids required for deballasting of the storage tanks and for underbase water injection. Results from the actual platform skirt extraction process are also provided and discussed.

Introduction

The Maureen Oil Field was developed with the installation of the world's largest steel gravity platform and an export pipeline to an offshore articulated loading column, as shown by the schematic arrangement in Figure 1. Field production at the Maureen location ceased in 1999, and the producing wells were then cemented in and abandoned. In order to satisfy recent international conventions (1), the Maureen facilities were removed from location in the summer of 2001, as part of the field decommissioning process. This process is summarised in the "Maureen Decommissioning Programme", (2), which was submitted to the U.K. Department of Trade and Industry(DTI) in May 2000. The largest element for removal was the steel gravity platform, which consists of a steel lattice frame structure, supporting an integrated steel deck, and integral with three large diameter oil storage tanks. The storage tanks were each supported at seabed level by 47 metre diameter bases. The underside of each base consists of a series of circumferential and radial steel skirts, which penetrated the seabed to a maximum depth of 3.4 metres, as shown in the cross-sectional view given in Figure 2. The concept for an integrated storage and production steel platform was outlined previously (3), as was the concept for platform removal (4).

The platform was removed from the seabed by deballasting the 3 oil storage tanks and other elements of the structure to provide the platform with near neutral buoyancy, prior to providing underbase hydraulic pressure sufficient to jack the platform (and skirts) from the seabed.

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