Deepwater Platform Design

ABSTRACT Results of preliminary platform design studies enable engineers"to identify and appraise major variables affecting deepwater platform capabilities. Transient dynamic analyses are used in carrying out the preliminary designs. Design results are presented in terms of required fabrication capabilities (platform base dimensions) and relative steel weights as these factors are affected by depth, environmental conditions, and design deck loads. Pile foundation requirements are determined, and integral buoyancy for transport is discussed. The preliminary design results corroborate the dominant effects of dynamics on platform design loads for depths substantially greater than 500 feet. Fatigue may dominate over the most severe storm as an environmental design condition for deepwater platforms. Fabrication capabilities for base widths in the range of 170 ft are adequate for platform construction for depths of at least 900 ft with moderate deck loads. A fabrication capability for 350-ft base widths would permit platform depths of at least 1200 feet. Platform weights and fabrication requirements for a large deck load capacity show a strong incentive for accurate estimates of the capacity actually required. Developing adequate foundation capacity is one of the most difficult aspects of deepwater platform design. Additional weight and fabrication requirements for integral buoyancy as a transport means provide an incentive for developing alternatives. Overall results show a strong incentive for optimum platform design for a specific environment. INTRODUCTION To provide a choice among alternative ways to develop deepwater reservoirs, design engin7ers and fabricators will extend pile-founded, fixed-platform designs to the greatest water depths at which they are competitive. In accomplishing this, the industry will reduce new design procedures to practice, find that deepwater platform designs are dominated by factors unimportant in shallow water, and extend fabrication procedures to accommodate the inherently larger structures. As platform designs are extended to greater water depths, dynamic response becomes a key factor., Dynamic response analyses have been discussed in several papers l - 3, and in this paper the subject is restricted to a summary of the transient analyses used to develop base-case platform designs. Based on the present study, comments are given on the significance of dynamics in deepwater platform design. Preliminary design results presented in this paper are given in terms of relative steel weights and of required fabrication capabilities (base dimensions) for fixed platforms as these factors are affected by depth, environmental wave conditions, and functional requirements (deck load and deck area). Foundation design is shown to be a major aspect of deepwater platform engineering. Structures with integral buoyancy for transport are also discussed. Fabrication, installation, and operating requirements strongly affect total steel require ments for a platform. Because these factors were not considered in the preliminary designs, only relative weights of steel for the various cases are given as results. The weights include conductors, piling, deck, and jacket. With the assumption that add-ons are about proportional to total platform weight, these relative steel weights can be interpreted in terms of weights for existing designs to provide guidelines as to total platform steel requirements. From the authors' viewpoint, the relative weights are most meaningful in terms of practices for the Gulf of Mexico.