Feasibility of a Fixed Platform for Use in 1,300 ft of Water
Authors: Loftin T.D. 
Published: December 2007

A deep-water fixed platform for drilling and production operations in the Bass Strait off the coast of Australia seems technically feasible. This conclusion is based on analysis and preliminary, design of a fixed-bottom steel platform for 1,300 ft of water. This paper emphasizes the analytic results that play a significant part in the platform design.

Introduction

The technical feasibility of a fixed platform for 1,300 ft of water was studied as part of a preliminary assessment of exploration prospects in deep water in the Gippsland Basin area of the Bass Strait off the coast of Australia.

The 1,300-ft water depth is considerably deeper than the depth in which the largest fixed-bottom platform has been installed to date. Fixed platforms have been installed in the North Sea in depths to 475 ft and in the Gulf of Mexico in depths to 375 ft. Construction and installation have been completed on a fixed platform for 850 ft of water off the coast of California, and fabrication has begun on a 1,000-ft-depth fixed platform in the Gulf of Mexico. Obviously, the design of a 1,300-ft fixed platform is not simply an extension of present technology, but will require many innovative techniques.

Many preliminary studies have been conducted for deep-water fixed platforms, but no detailed designs have been carried out for platforms in water as deep as 1,300 ft. A parameter study for deep-water fixed platforms has been conducted, and designers are finding that deepwater platform designs are dominated by factors that are less important in shallow water. For example, fabrication procedures must be extended to accommodate the larger procedures must be extended to accommodate the larger structures, and dynamic response becomes a key design factor. Exxon Production Research Co. and outside contractors have conducted studies for fixed platforms in 1,000 and 1,200 ft of water. The initial configuration formulated for the 1,300-ft platform was based on several features of these preliminary designs.

Structural Configuration

A perspective of the 1,300-ft fixed platform is shown in Fig. 1, principal dimensions are presented in Fig. 2, and typical member sizes are shown in Fig. 3 for the preliminary design. A detailed design has not been developed.

The platform would be 1,355 ft tall and would have 12 legs. The design base width is 346 x 396 ft, the top of the jacket is 75 x 210 ft, and the deck is 100 x 210 ft. The base dimensions were derived from results of a deep-water fixed-platform parameter study. A large base width increases stiffness and reduces dynamic response. The deck size is based on other deck designs with similar oil and gas production rates.

The large-diameter legs below elevation - 860 ft serve two purposes. They greatly increase the stiffness of the structure and can provide integral buoyancy for flotation. Buoyancy requirements dictated that the legs on the east face (the face on which the platform floats) of the platform below elevation -860 ft be 30 ft in diameter. platform below elevation -860 ft be 30 ft in diameter. The 16-ft-diameter legs from elevation -390 to -860 ft on the east face also provide integral buoyancy. The vertical x-bracing in the lower three bays of the structure increases the platform stiffness.

The platform is designed for 30 conductors that are 20 in. in diameter. The three rows of conductors are spaced 7.5 ft apart and the conductors are spaced 7.0 ft apart. The platform would be supported by 16 conventional skirt piles and 40 skirt piles in groups of four per perimeter leg. The piling would extend only 150 ft above the perimeter leg. The piling would extend only 150 ft above the mud line and would be grouted to the structure.

JPT

P. 613