An Investigation of Dynamic Wind Loads on Offshore Platforms

Abstract

ABSTRACT A wind tunnel investigation of dynamic wind loads on a 1:400 scale model of the Exxon Company, U.S.A. Lena guyed tower offshore platform was conducted. The objectives of this investigation were to (1) investigate the dependence of wind loads on flow structure and turbulence intensity, and (2) obtain data necessary to permit an assessment of theoretical and semi-empirical methods of estimating the effects of dynamic wind loads on offshore platforms. There are two fundamental reasons for conducting this investigation. First, relevant experimental data on dynamic wind effects on offshore platforms is limited, and there is a need to improve the experimental data base upon which dynamic wind design procedures can be evaluated. Second, dynamic wind loads can contribute significantly to the design requirements of compliant offshore structures (e.g., the Lena guyed tower) because of the relatively long-period response of these structures. This paper presents the major results of the experimental investigation and shows the results of a measured vs. calculated comparison of dynamic wind loads. The comparison shows that dynamic wind loads calculated using free-stream wind engineering theories are conservative relative to the model test data. INTRODUCTION Compliant offshore structures such as guyed-tower or tension-leg platforms are susceptible to wind loads which, under design conditions, may be responsible for over 25% of the overall response to wind, wave and current. Constant loads due to steady winds and currents are not counteracted by inertia loads induced by platform movement (compliancy) and thus, unlike wave loads, must be fully resisted by the platform restraints. In addition, the relatively long natural period or low natural frequency places such structures in the energy containing region of the wind turbulence spectrum and, despite the comparatively high hydrodynamic damping, the dynamic response to wind can be significant. The development of compliant offshore structures has necessitated the development of wind design procedures that explicitly account for the dynamic interaction of the structure and the Wind. Unfortunately, the relevant experimental data base upon which the accuracy of such procedures can be evaluated is limited. A previous investigation of wind loads on offshore platforms by the authors [1] was concerned with mean loads under predominately steady, fully-coherent wind conditions. In the case of dynamic wind loads, however, the structure of wind turbulence, particularly the spatial characteristics of wind gusts relative to the spatial dimensions of the platform, should be considered. The study presented herein is concerned with a detailed examination of both the mean and dynamic wind loads acting on a 1:400 scale model of the Exxon Company, U.S.A. Lena guyed tower offshore platform when placed in correctly scaled turbulent shear flow.