ABSTRACT

This paper presents the results of a study in which a method is developed for estimating maximum turbulent wind drag force acting on a floating offshore structure with low natural frequency. In evaluating the drag force, forces associated with turbulent wind speeds are considered using both the linear and nonlinear reliability analysis approaches. A mathematical wind spectral density formulation is employed. The probability density function applied for the maximum turbulent wind-induced drag force analysis is considered as a random process. The maximum wind force is analytically developed by using an approximate single term expression for the functional relationship between the wind speed and the associated turbulent drag force. At last, the probable maximum and the design maximum values are estimated by applying maximum value statistics and reliability analysis.

INTRODUCTION

Estimation of the magnitude of wind-induced loading (drag force) on offshore structures provides information vital for the design and operation of the structures; in particular for floating structures in a seaway. The wind-induced drag force referred to in the present study is that associated with turbulent winds consisting of various frequencies which span a very wide range. The frequency may vary from nearly zero to l.5 rps or higher. w Although many studies have been carried out evaluate turbulent wind-induced forces on offshore structures, it is common practice to assume that the wind-induced drag force increases linearly with increase in wind fluctuating speed; neglecting the higher order term of the force which is proportional to the square of turbulent wind speed. This is because some natural frequencies of motions of floating offshore structures are very small; on the order of 0.01 to 0.02 Hz for the surging motion of a tension-leg platform, which is the domain where turbulent wind energy over a seaway is extremely high.

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