Wind Drag Coefficients For Bluff Offshore Ocean Platforms
Authors: Aiston S. Thomas , Nath John H. 
Published: October 2008

The purpose of this paper is to determine experimentally an overall wind drag coefficient for the design of bluff offshore ocean platforms subjected to wind forces. The results of this investigation show that an overall drag coefficient of 0.9 may be used for design purposes in relatively thick boundary layer conditions which is somewhat lower than published drag coefficients for standard land based buildings.

INTRODUCTION

Bluff offshore ocean platforms, those multi-legged giant structures that support men and equipment above the ocean's surface, present many design problems. The forces that the sea and air exert on the structure are many and varied and, yet, all must be determined by the designer. One force, which may greatly influence an ocean platform, is the force exerted by the wind, often referred to as the wind drag force. Little is known about the wind force on elevated ocean platforms. Ordinarily, the total force exerted on an object immersed in a flowing fluid would be determined by integrating the pressure distribution over the entire area of the object. However, it is difficult to measure or theoretically determine the pressure distribution on an irregular, bluff object. Thus, the total force obtained by integration may be considerably in error. In this work the total force was measured directly without reference to pressure distribution. An overall wind drag coefficient applicable for design purposes was determined.

An offshore ocean platform is usually supported by a number of cylindrical legs. A radio - or drilling - tower may be erected on it. The platform may present various plan views and will usually have bluff or jagged edges. This investigation was limited to a bluff, square platform supported by four cylindrical legs. There was found nowhere in the literature a successful determination of an overall wind drag coefficient for as complicated a structure as a platform elevated on legs supporting a tower. Drag coefficients have been determined for conventional buildings, but the immediate question is how will the drag coefficient for an elevated platform differ from that of a conventional building of the same general proportions?

The drag coefficient of a conventional building is a result of the positive pressure distribution on the front face, plus the negative pressure distribution at the rear of the building. Any means of decreasing the intensity of the pressure on the front or rear faces of the building will decrease the overall drag coefficient. If a building were elevated above the ground surface, two phenomena could be expected to occur. First, the pressure distribution on the front face could be expected to change shape and possibly reduce in magnitude. Secondly, the added flow or jet under the building could be expected to reduce the intensity of the negative pressure at the rear. Thus, an elevated building or platform may exhibit a drag coefficient which is lower than that for a conventional building of comparable shape.

In 1961, testing to determine drag coefficients for a great number of structures, including conventional buildings, was carried out at the Institute of Aerodynamics of zurich, Switzerland; the results having been incorporated into the Swiss Building Code.