A New Deep-Water Platform: The Guyed Tower (1978)

A new structure, the guyed tower, is proposed as a deep-water drilling and production platform. The concept is discussed and the dynamic production platform. The concept is discussed and the dynamic characteristics of the structure are presented. A large-scale model of the tower has been installed in the Gulf of Mexico for testing. Introduction A new, compliant offshore structure, the guyed tower, has been proposed as a deep-water production and drilling platform. The concept has two major advantages over other proposed schemes:the structure can be manufactured and installed using available equipment and technology, andthis production platform is anticipated to be less expensive to build and maintain than present alternatives proposed for water depths from 600 present alternatives proposed for water depths from 600 to 2,000 ft. This paper describes the basic guyed-tower concept and illustrates the design procedures used to assure that the compliant structure will safely withstand severe environmental forces and yet be slender, lightweight, and economical. Prototype Guyed Tower Prototype Guyed Tower The guyed tower is a trussed structure that rests on the ocean floor, extends upward to a deck supported above the waves, and is held upright by multiple guy lines (Fig. 1). The base of the tower is supported on a truss-reinforced shell foundation called a spud can. During installation, the spud can is forced into the ocean bottom until the desired load-carrying capability is attained. The amount of design penetration is, of course, dependent on the load to be carried and the soil parameters of the site. The main truss of the tower would have four equally spaced legs connected primarily with cross-bracing. However, other geometric configurations could be used. For a structure supporting 24 wells in 1,500 ft of water, the legs would be spaced 100 ft apart and range form 5 to 8 ft in diameter. Ideally, the deck would be designed to support all the equipment needed to drill and produce a large number of wells (20 to 40). The deck for the 24-well, 1,500-ft structure would have two levels 150 ft on a side and would support a 7,500-ton deck payload, which is adequate for many areas of the world. To carry larger payloads, the support capacity of the tower truss, spud can, and guying system would have to be increased proportionally. proportionally. The Guying System The means for supporting the tower horizontally is structurally appealing to the designer. Because the guy lines support the top of the tower, the structure can function basically as a pinned-pinned beam. Thus, the spud can is not required to resist any overturning moment. Conventional platforms, on the other hand, essentially are cantilevered platforms, on the other hand, essentially are cantilevered from the ocean floor. Further, by positioning the guy lines about 50 ft below the mean water level, the restoring force of the guying system is essentially colinear with the center of pressure of the applied design wave load. Thus, the guy lines resist most of the wave load and the horizontal reaction at the spud can is minimal. To have some redundancy, a large number of guy lines (10 to 20) would be arranged symmetrically around the tower (Fig. 1). Each line would be secured at the deck of the platform by two wedge-type cable grips placed in series to form a hydraulic jacking unit. JPT P. 537