A six degree of freedom frequency domain model of a Tension Leg Platform (TLP), which includes the effects of non-collinear wind, wave and current, is developed to determine the response and sensitivities to environmental input. The model includes the steady forcing effects due to currents and waves, wind and wave drift. The dynamic effects include Morison type forces due to inertia and drag, spatially correlated wind and slowly varying wave drift effects. Attention is given to the position dependent mooring stiffness incorporating an iterative procedure for the determination of mean static offset position as well as the linearised mooring stiffness at the displaced location. The linearised effects of body motion, current and wave particle velocities are considered for drag determination. This model is applied to predict the response of a TLP to a sample hindcast cyclonic storm off the North West Shelf of Australia. Storm parameters used in the response model include significant wave height, spectral peak period, wave direction, storm current and direction, wind speed and direction. Results show that peak response does not necessarily coincide with the maximum sea state, but rather, different motions have different phasing with the input condition.

INTRODUCTION

In the preliminar3 design stage of offshore structures there is a requirement to understand the relationships between the site specific metocean environment and the global structural response. With this understanding, a structure's main geometric configuration may be optimized to achieve an acceptable design: for example, limiting responses may be maximum overturning moment for a rigid steel jacket or maximum static and dynamic offset for a compliant structure. This process may be relatively simple if the response is governed by a single environmental parameter, or may be quite complex for a structure sensitive to a multi-dimensional metocean input.

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