Lessons Learned from the Lena Guyed Tower

Abstract

The design, fabrication, and installation of the Lena guyed tower required the development of new offshore technology in several areas. Lessons learned from this experience include an understanding of the consequences of major design decisions and insight into the interactive behavior of the structural components. The guyed tower is now a proven deepwater production platform concept for the Gulf of Mexico and production platform concept for the Gulf of Mexico and is also judged to be applicable for a wide range of water depths and deck loads in the North Sea and other areas of the world. Introduction A major milestone was reached in the summer of 1983 when the Lena guyed tower was installed in 1000 ft (305m) of water in the Gulf of Mexico. The installation was achieved after 12 years of developnent work and represents the efforts of many individuals who contributed innovative solutions to challenging technical problems. The solutions not only have advanced the state of guyed tower technology but also have contributed to offshore platform technology in general. Many lessons were learned as a result of the Lena experience, as would be expected with a first-of-a- kind structural concept. The insight gained will allow future designers to better evaluate alternatives and thus promote further optimization of the concept. The design principles and construction techniques successfully applied to the Lena tower can be adapted to other water depths and environments, thus enhancing the applicability of the guyed tower as a production platform. The brief description of the Lena guyed tower presented below is followed by a discussion of the consequences of several major design decisions. The paper concludes with a discussion of the impact of the lessons learned from Lena and in particular focuses on how this knowledge would be particular focuses on how this knowledge would be applied to North Sea guyed towers. THE LENA TOWER Structural Configuration Above the water level, the Lena guyed tower appears similar to a conventional platform. The most obvious differences below the water line are the shape of the jacket (tower) and the system of guylines. The jacket has a constant cross-sectional dimension of 120 ft (37m) square (Fig. 1). The tower is supported vertically by eight main piles, which are located in a circular array near the center of the jacket. Twisting restraint is enhanced by six short piles driven through guides placed around the perimeter of the base. Twelve buoyancy tanks located centrally in the upper part of the jacket support about 75 percent of the deck load. The tower is supported laterally by 20 guylines symmetrically located around the jacket. Driven piles anchor the guylines to the seafloor. A 200-ton clump weight is attached to each guyline and partially rests on bottom. During severe environmental events the articulated clumps are designed to reduce mud suction forces by allowing the clump segments to "peel" off the bottom. The anchor line (between the pile anchor and the clump) is longer than the depth of the water to allow surface make-up to the clump after installation of the pile anchor. The guylines enter the jacket well below the water surface and are redirected vertically to the deck by bending shoes located near the center of the jacket. Fairleads located around the exterior of the jacket serve to route the guyline through the structure and accommodate any structure-guyline misalignment. The guyline tensions are transmitted to the jacket by wedge-type cable grips located on the +15-ft (4.6m) elevation. The end of the guyline terminates in a field-poured socket located at the +53-ft (16m) elevation. The principal supporting members (the piles, buoyancy tanks, and guylines) are located in the upper region of the jacket. p. 161