Feasibility of a Fixed Platform for 1300 Feet of Water

ABSTRACT A deepwater fixed platform for use in 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 1300 feet of water. Wave forces, wave-induced dynamic response, cyclic load considerations (fatigue), hydrostatic collapse, and axial buckling were included in the analysis. Fabrication and installation procedures were investigated, but this paper emphasizes the analytic results, particularly the dynamic response, which plays a significant part in the design of the platform. This feasibility study found that much conventional, shallow-water fabrication could be used, but some innovative techniques unique to deep-water platform construction must be incorporated. The proposed installation of the 1300ft structure departs from conventional practices. The proposed jacket could be installed as a single piece by using integral and auxiliary buoyancy for flotation, and this requires the use of a graving dock for construction. The l300-ft steel jacket would weigh about 60,000 tons and would require 56 piles to support it. For a possible Bass Strait application, the 60-inch sleeve piles would be driven to a depth of 250 feet and the 48-inch insert piles would be drilled and grouted to a depth of 450 feet below the mudline. The results indicate that the platform would be costly and difficult to fabricate, and time requirements would be extensive. INTRODUCTION The technical feasibility of a fixed platform for 1300 feet of water was studied as a part of a preliminary assessment of potential exploration prospects in deepwater in the Gippsland basin area of the Bass Strait. The l300-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 water depths to 475 ft, and in the Gulf of Mexico in water depths to 375 ft. Construction is now under way for a fixed platform for 850 ft of water off the coast of California, and a design has been initiated for a 100D-ft fixed platform in the Gulf of Mexico. Obviously, the design of a l300-ft fixed platform is not simply an extension of present-day technology, but will require many innovative techniques. Many preliminary studies have been conducted for deepwater fixed platforms, but no detailed designs have been carried out for platforms in water as deep as 1300 ft. EPR has conducted a parameter study for deepwater fixed platforms [1], 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 structures, and dynamic response becomes a key design factor. Exxon [2] and outside contractors have conducted studies for fixed platforms in 1000 and 1200 feet of water. The initial configuration formulated for the l300-ft platform was based on several features of these preliminary designs.