Dynamic Analysis Models of the Tension Leg Platform

Abstract

Abstract Tension leg platf9rms are designed with natural periods of oscillation which are well separated from the dominant wave excitation frequencies so that displacements remain acceptably small even in severe weather conditions. However, the results presented in this paper show that the tensioned tethers can resonate at periods close to those of wave excitation if the platform is deployed in deep water where its economic advantages are held to be greatest. Analysis which ignores the tether dynamics gives good estimates of the wave induced motion amplitudes of the platform but can be seriously in error when predicting the dynamic tether stresses. An additional uncertainty is associated with interactions between the varying longitudinal tether tensions and lateral platform dynamics which can lead to a Mathieu type of instability. A simple energy balance approach is presented in this paper to quantify the effects of linear and square law damping on the severity of this instability. It is demonstrated that the effect of nonlinear square law damping is to impose an upper bound on the otherwise unstable oscillations. An algebraic expression for this upper bound is derived by consideration of an extreme 'worst case' situation. Introduction The use of a tensioned leg platform(TLP) as an alternative to offshore oil production from fixed or catenary moored structures is receiving serious attention within the offshore industry. Research has advanced to the stage where plans have been announced to produce oil from a north sea field using a tension leg design. A TLP possesses a combination of desirable characteristics as an offshore work platform. The absence of a fixed rigid structure from sea surfaceto sea bed, coupled with excellent station keeping characteristics, satisfy the two important demands of economyand practicality that are made on any offshore production system. A TLP exhibits wave induced heave roll and pitch motions which are much smaller than those of a conventional floating structure. Larger off station surge and sway motions can, however, result due to the low restoring stiffnesses in these directions. This feature serves as a positive advantage since horizontal wave loads are then not fully reacted by the surface platform. Despite these obvious advantages, the compliant nature of a TLP does raise uncertainties regarding the detailed platform dynamics particularly for deep water applications. Field and Flogeland suggest a number of problem areas which have not yet been adequately resolved. Two of these are particularly important for the TLP and are concerned with the influence of the detailed dynamics of a realistic tether system on platform motions and tether stresses and the limiting effects of fluid damping on the size and occurrence of subharmonic resonance or instabilities.