Steel Gravity Platform Positioning Over Template on the North Sea Maureen Field

Abstract

Summary This paper outlines a design philosophy and a practical solution to the problems of placing a 110,000-ton (110 Mg) steel gravity platform over a predrilled template with 21 live wells in the North Sea. The engineering of the installation operations and the design of all relevant mechanical and instrumentation systems required to solve this unique engineering problem, for which there is no precedent, is described. Introduction A steel gravity, platform is under construction for the development of the Maureen field in Block, 16/29a, located about 160 miles (257 km) northeast of Aberdeen. The platform was designed by Tecnomare S.p.A. on behalf of Phillips Petroleum Co. E and A, which leads a six-member group and acts as operator. The other coventurers are Petrofina, AGIP, Century Power and Light, Ultramar, and British Electric Traction. The Tecnomare Steel Gravity (TSG) concept was dictated by the need forfull topside equipment transportation,installation over a template of predrilled wells, andoil storage capacity to complement the offshore loading system. All these features together with an open structural design are, in fact, peculiar to the TSG platform. Major components of the tripod-type TSG platform are three foundation bases, three storage-stabilizing cylinders, a triangular framed structure connecting the cylinders, and a tower supporting the deck, (Fig. 1). Among the main characteristics of the Maureen TSG platform, this paper deals in detail with the basic philosophy, and practical solutions of the positioning problem. The tight definition of the design premises necessary to cope with tieback operations is reported together with the engineering of the installation operations and the design of the relevant mechanical and instrumentation systems that will permit an easy and safe installation over the template. The importance of the design methods adopted to analyze and simulate the platform behavior during installation is emphasized by means of a comparison between the main theoretical and experimental results obtained. TSG Positioning Two major aspects should be considered when dealing with the positioning of a floating structure over a template of predrilled wells:the safety of the overall operation andthe positioning accuracy needed for the tieback operations. With regard to safety, the main area of concern is the possible template/wellhead vulnerability. During installation, the template and completed wells are vulnerable and susceptible to damage. The damage risk depends mainly on the structure type, being, obviously minimal for a structure having an open structural design. In this case the probability of contact between structure and template/wellhead actually is minimized. This basic requirement is a built-in characteristic for TSG structures, so that the only question was to design the positive locating system so as to eliminate any possible structural contact with the template. Final positioning accuracy is dictated only by the tieback requirements. A minimum horizontal and angular offset normally is allowed to avoid complicated systems for the tieback. Placement inaccuracy then will result in overstresses in tiebacks or eventually in tieback impossibility. In the Maureen case, the required accuracy was 6 in. 15.2 cm); this design premise has been the reference figure throughout the different engineering phases of the positioning. JPT P. 247^