Design of the Lena Guyed Tower

ABSTRACT The 1983 Lena platform installation in 1000 feet of water in the Gulf of Mexico culminated twelve years of guyed tower development. During the Lena design, numerous technical challenges were met, resulting in several innovative developments. Technical issues resolved included determining tower dynamic response, developing guying system components, developing a compliant pile foundation, providing permanent tower buoyancy, developing a side-launching method, and assessing the structure's safety and reliability. This paper discusses anal analysis and design with emphasis on features unique to a guyed tower. INTRODUCTION Early guyed tower work involved the development of the concept [l], the development of analytical capabilities [2], and the execution of the joint-industry test tower program [3]. The topics covered in this paper are concept development, design criteria, design methodology, and reliability considerations for the Lena structure. A general Lena project overview is given in Reference 4. A guyed tower is called compliant because it is allowed to move in response to large waves. It responds differently than fixed-base platforms because a guyed tower's fundamental period is typically greater than 25 seconds, compared to fixed-base platforms, which generally have periods less than 5.0 seconds. For fixed-base platforms, the inertial forces are additive to the wave forces because their periods are significantly lower than design-level waves, characterized by periods in the 10–15 second range. For a guyed tower, however, the inertial forces counteract the wave forces. The guylines restrain the tower near the resultant of applied environmental forces, thus significantly lowering the tower's shear and moment forces. This allows the structure to be designed with a narrow cross section, thus saving weight compared to conventional wide-base fixed platforms. The guyline design effort included selecting suitable components for proper system stiffness and resolving concerns of strength, corrosion, wear, and fatigue. Compliancy is accommodated by allowing the tower to tilt about its base. Therefore, a pile foundation was developed which is flexible enough to accommodate the tower tilt allowed by the guying system, yet strong enough to resist base torsion and translation forces. Other compliancy design considerations included accommodating the vertical deck movement relative to the wellheads, attaching flexible pipelines, and minimizing the motion effects on platform operations. Installation requirements had a significant impact on design because of the large overhangs during transportation and the load concentrations during launch. Installation considerations included one- and two-piece tower configurations, and side- and end-launch schemes. MAJOR CONCEPTS Guying System Early studies showed that cable systems radiating directly to anchor points were infeasible for the Lena case.