Response of Tension Leg Platforms to Wind, Waves, and Currents: A Frequency Domain Approach
Authors: Kareem A. , Li You  Sun 
Published: October 2008

Abstract
This paper reports a computationally efficient frequency domain analysis procedure for coupled six-degree-of-freedom motion analysis of TLPs exposed to the combined action of wind, waves, and currents. This analysis includes complex interactions between waves and oscillatory platform motions, and more importantly, the second-order forces in random seas and the coupling of the TLP degrees of freedom which generally preclude a straightforward application of the numerical methods applied to conventional platforms. The computational procedure has been made possible by the development of a stochastic decomposition approach in combination with a tri-variety Hermite polynomial expansion of the viscous forces and a new approach to incorporate feedback forces of potential and viscous nature resulting from the displaced position of the platform. The procedure presented here is illustrated by way of an example utilizing a typical TLP subjected to the simultaneous action of wind, waves, and currents. The results from this study provide excellent agreement with the time domain approach. The sensitivity of the platform response to the environmental loading conditions and the mechanical and hydrodynamic characteristics of the platform is discussed.

INTRODUCTION
The development of innovative structural systems such as a tension leg platform (TLP) in deep water has placed a growing importance on their safety and structural integrity. Their reliable design can be ensured through enhanced prediction capability with respect to the effects of environmental loads. The dynamic behavior of TLPs is significantly different from conventional platforms. Not only is me applied loading complex in nature due to the interaction between wave and oscillatory platform motions, but more importantly, second-order forces in random seas and the coupling of the TLP degrees of freedom precludes straightforward application of me numerical methods generally applied to conventional platforms. Clearly, an enhanced response prediction capability is needed.
The tension leg platforms, by virtue of their compliantbehavior in me horizontal plane, Le., surge, sway and yaw motions, are more sensitive to low frequency loads (e.g., wind fluctuations and slowly varying wave drift loads) than conventional offshore structures. A typical TLP is exposed to a combination of environmental loads. These loads may be described in terms of aerodynamic and. hydraulic load effects. Aerodynamic loads are obtained by Utilizing space-time structure of the Wind field and the appropriate aerodynamic characteristics of the structural shapes. The fluctuations in the wave surface profile introduce hydrodynamic loads on structures at typical wave frequencies and second-order nonlinear exciting forces which have steady and oscillatory components outside the range of typical wave frequencies. These nonlinear forces are often referred to as the slowly varying drift and high-frequency springing forces. The TLP motions in the vertical plane, Le., heave, pitch and roll are susceptible to the springing excitation in deep water applications.
The current frequency domain analysis capabilities in the literature do not contain adequate descriptions of the nonlinear hydrodynamic (viscous and potential) load effects, the feedback effects introduced by the platform displacement, and the dynamic effects of wind. The time domain techniques can be quite tedious and time-consuming, rendering them inappropriate for concept selection.. This paper presents one of the most advanced frequency domain-based computational dynamics procedures to predict the response of a complex system such as a TLP with respect to the Simultaneous action of wind, waves, and currents.