A model study has been made of the aerodynamic loads experienced by a typical TLP superstructure Details are given of
extreme quasi-static aerodynamic loads induced under extreme conditions;
the aerodynamic admittance function relating the spectrum of drag force to the wind gust spectrum.
The economic and safe design of offshore structures requires the availability of reliable codes of practice. Reference is often made to DNV and ABS rules in calculating the overall environmental loads on a structure due to the typical wind and wave climate. These rules base their load and moment calculation procedures on documented field and model tests of specific structures and their elements.
When compliant structures are considered, dynamic effects excited by the periodic nature of waves and the "gusting" structure of wind must also be considered. In the particular case of gust loading, the relationship between the statistical properties of the wind and the induced forces on the structure must first be defined by a suitable aerodynamic "admittance". This frequency dependent function has been shown in many studies to rely on both the structure and its environment 1,2,3,4,5 It cannot generally be defined a priori but requires model or prototype studies to be made for all but the simplest of structures.
In the present investigation, the relationship between the wind characteristics and the resulting forces has been derived for a superstructure geometry typical of a "tension-leg" platform (TLP) in a series of model tests.
The data presented here consists of (i) measurements in a steady uniform flow giving details of the overall drag force, lift force and pitching moment at model leg lengths corresponding to the extremes induced by a design wave, (ii) measurements of that element of the aerodynamic admittance function matrix which relates the unsteady drag force to the velocity fluctuations parallel to the free stream. This will be referred to as the "admittance function".
In both cases variation with the angle of yaw to the wind is considered. The effect of pitching of the structure is considered in the uniform flow while the effect of changing the length scale is examined in turbulent flow.
The resulting set forms a base of unsteady aerodynamic load data which may be used in the modelling of the dynamic behaviour of a TLP.
The experiments were conducted in a closed-return wind tunnel. The working section of the tunnel is 3m long and, in cross-section, is built on a basic frame which is 1.2m square. The working section is fitted with corner fillets so that its cross-section is octagonal. The tunnel produces a uniform flow in the working section with a turbulence level of 0.02% at 15ms1. The model used in the wind tunnel study has a design which includes the main features of proposed tension leg platforms. A plan view of the model in shown in Figure I, and the elevation views are shown in Figure 2.