Development of a Semisubmersible Drilling Unit for 10,OOO-ft Water Depth

Abstract

ABSTRACT Reading & Bates Drilling Co. (R&B) and Ishikawajima-Harima Heavy Industries Coo, Ltd. (IHI) have jointly developed a new semi submersible drilling unit with design goals of greatly increased water depth capability (10,000 ft.), minimum heave response in waves, high variable deck load capacity, and economy of construction. This paper describes the main conceptual features and performance of this new design. INTRODUCTION A new generation semi submersible drilling unit for 10,000 ft. water depth has been developed jointly by R&B and IHI. This new design is targeted primarily for operations in the Gulf of Mexico OCS lease areas with the ability to cope with hurricanes and loop currents. In addition, the rig will also be capable of working efficiently worldwide, in harsh environments such as those found in the northern portion of the North Sea, or west of the Shetlands. The rig is suitable for development, as well as exploration drilling programs_ Hull configuration, dimensions and station keeping performance have been optimized to meet these design goals. Significantly increased variable deck load and very low heave response have been attained with only modest increase in hull size while simplifying the structural configuration, which should result in more economical construction. This was accomplished by adopting laterally slanted columns, longitudinally "dumbbell" shaped pontoons, and the Column Outer Belt (COB) system. Heave motion performance was analyzed numerically and verified by model tank testing. Station keeping is provided by a state-of-the-art triple redundant dynamic positioning and thruster system. Optionally, an externally deployed mooring system may be employed. Either system meets or exceeds all applicable national and international requirements. Considerable effort was made to analyze dynamic positioning requirements, performance, and reliability. Principle power and thruster equipment specifications have been developed. Engineering, cost, and performance study for the external mooring system, designed for survival in a 50-year Gulf of Mexico hurricane, have been carried out. Self contained mooring systems were studied and ruled out as uneconomical. Various concepts of large diameter (21-inch) drilling riser configurations were studied, engineered and analyzed. These included concepts of conventional riser systems, composite steel and FRP, and titanium materials, as well as evaluation of a freestanding riser system. As development of the rig design progressed, the importance of rapid and reliable tubular and riser hand line systems became clear. This led to the selection of symmetrically arranged semi-automated bridge cranes and catwalk pipe handling machines. which allow handling riser. or other tubular from both the forward and aft pipe rack alternately. and permit deployment or recovery of 10.000 ft. of riser in less than 12 hours. DESIGN CRITERIA As stated earlier. the vessel is designed for unrestricted worldwide service as Modu, with particular emphasis for efficient operations in the Gulf of Mexico from 5.000 - 10.000 ft. water depth. Principal environment design criteria for the DP, moored, and transit mode are stated in Table 1. Figures 1a -ld. illustrate graphically station keeping performance in the DP mode for drilling, loop current, stand-by, and severe storm. Table 9 reflects station keeping performance in the moored mode.