Dynamic Effects of Wind on Tension Leg Platforms

Abstract

Introduction The Tension Leg Platform is relatively new concept in offshore platforms. It is anticipated that the TLP will have great applicability to deep water situations. The TLP, due to the flexibility of its legs, is more susceptible to the dynamic effects of wind than is the conventional fixed-bottom platform. This is especially true in the low frequency range of wind spectra to which this type of platform is exposed; The presence of wind, waves and currents causes the platform to have a displacement offset in the detraction of these effects. Waves cause an oscillation about thus displacement offset due to the time-dependent nature of waves. Another type of oscillation is generated by the fluctuations present in the wind. These fluctuations are manifested as the gustiness of the wind. This study will neglect the effects of currents and waves which have frequencies typically an order of magnitude higher than those in the wind and which also are responsible for the low frequency drift of a TLP. The focus will be on the dynamic response of a TLP to the gustiness in a wind field in excess of the displacement caused by the steady part of the wind velocity. The mean velocity field of the wind is of the boundary-layer type where the fluid motion is that of a turbulent flow. This type of flow can be characterized by a logarithmic or a power-law expression. The gustiness of the wind field is expressed as a fluctuation about the mean wind field at a given elevation. The description of the fluctuation is expressed by means of the digital simulation of a random process such as the turbulent fluctuations present in a wind field. The emphasis of this study will be on the interaction of a gusty wind field with a TLP to determine the total fluctuating load on the structure. It is necessary to assume that the steady-flow drag description applies to the structure. This means that the drag coefficient does not vary with instantaneous velocity, but is determined by the assumption that the Reynolds number is always high enough for the drag coefficient to be constant. Representation Of The Velocity Field The mean description of the turbulent wind field is assumed to be of either of two forms [1]. One is the logarithmic law,(Mathematical Equation Available In Full Paper) where(Mathematical Symbol Available In Full Paper) is the reference velocity at lore, is the vertical coordinate and Z. is the roughness length which is approximately 0.005m for a rough area surface. The second form is the power-law expression given by(Mathematical Equation Available In Full Paper) where ? is the power-law exponent. For the analysis herein, ? is taken to be 0.16 [1]. The gustiness of the wind is due to the level of turbulence in the wind. The turbulence intensity is expressed as the rums value of the wind velocity relative to the mean value. It is influenced by the stability of the air layer and the roughness of the surface.