Global Performance Comparison between Composite and Steel Tendons in Ultra Deepwater TLPs

Abstract

ABSTRACT Tendons that vertical moor TLPs are a major cost driver in deepwater TLPs. Carbon fiber composites have many of the desirable material properties for use as TLP tendon as an alternative to steel. This paper investigates the performance and cost impact on the overall TLP design. The focus is on the comparison of global performance of two generic Gulf-of-Mexico TLPs with steel and composite tendons in 7,000 ft water depth. Fully dynamically coupled TLP-tendon system dynamics of both TLPs has been analyzed in time domain with hydrodynamic input complete to 2nd order. The use of composite tendons leads to improvements on global performance related to airgap, maximum horizontal offset and maximum tendon tension. These improvements have been achieved despite the fact that the heave natural period of the TLP with composite tendons has been increased to 4.72 second from 4.2 second for the TLP with steel tendons. It is demonstrated that use of composite tendons in lieu of steel results in significant reduction in pretension, hull size and hull steel. INTRODUCTION The tension leg platform (TLP) is a proven floating offshore structure for deepwater drilling and production. However, the high cost of steel tendons is a major cost driver, perhaps even limiting its commercial application in water depths beyond 6,000ft. Alternative tendon materials must be investigated for TLPs to remain cost competitive in these water depths. The most promising appears to be carbon fiber composites, which have material properties highly desirable for use as TLP tendons. Although there is lack of experience in the use of these materials as critical components in offshore applications, interest has been growing in their potential use, such as TLP tendons. This paper compares the global performance of Gulf of Mexico TLPs in 7,000 ft water depth configured with steel and composite tendons, respectively. Global analysis is done in time domain by considering the full dynamic coupling of the vessel-tendon system. The hydrodynamic computations are complete to 2nd order to account for the slow drift motions in the horizontal plane as well as the springing motions in the vertical plane. The analysis results have been discussed together with the potential benefits of composite tendons regarding reduction in TLP size, maximum tension on foundation, tendon dynamics and fatigue and platform response. COMPOSITE TENDON Material Carbon fibers possess highly desirable material properties for use as TLP tendons. These include high stiffness/weight ratio, high strength/weight ratio, high resistance to fatigue and corrosion, and high material damping. With proper jacketing, adequate abrasion resistance can be achieved. Table 1 shows the properties of typical carbon fiber composites.