## 6 Flow Distortion due to Offshore Platform and Terrain
Comparing this data with the data acquired in the empty tunnel the effect of the presence of the rig on the undisturbed flow field was determined. Figure 8 shows the results of four traverses above a rig with the flow approaching the rig from different azimuthal angles. The X on the plan form view of the rig shows the location above which the probe was traversed in the positive Z direction. Probe heights were normalised by the height of the rig deck and the speed was normalised by the free stream velocity of the wind tunnel [22]. The data from the wind tunnel tests served two purposes: to assess the height above the platform that a point measurement device, such as a cup and vane anemometer, might be affected by flow distortion, and to verify CFD simulations which were required to assess the effect of flow distortion on the measurements made by lidars. By rotating the platform 360\({}^{\circ}\) in the wind tunnel and measuring the velocity profiles, the boundary where the flow velocity magnitude was within a certain percentage of the free stream velocity could be determined, see Figure 9. The result of this analysis for a number of platforms is shown in Table 6. The CFD model results compared well with the wind tunnel experiment [9, 23] and were used to determine the effect of flow distortion on the measurements made by the lidars both onshore and offshore. The effect of flow distortion on the cup and vane type anemometer, being essentially a point measurement, is easily understood and measured. However, remote sensing devices, such as lidars and sodars, determine the wind vector from a spatially averaged set of measurements. Some attempts have been made to measure the effect of flow distortion on lidars in complex terrain as might be found when measuring in hilly or mountainous terrain [2, 5, 19, 24, 25, 26, 27]. In the WAsP Engineering software [28], a program for wind site assessment, a script is available that accounts for the error due to the flow distortion created by orography when scanning con  
Figure 9: Height above rig required for 99% free stream velocity magnitude as a function of the azimuth angle from wind tunnel and Computational Fluid Dynamic model (CFD) results. lidars [25]. The authors of [8] investigated the influence of the landscape to the wind profile observed by the lidar on the island of Utsira and found significant influence to the wind profile at all levels and with clear azimuthal dependence. However, the effect of the flow distortion on lidars in close proximity to large structures, such as buildings and oil rigs, had not been investigated to date. To understand the difficulty of estimating the effect of flow distortion on the measurements made by a lidar it is necessary to understand the fundamental difference between the point measurement of a cup anemometer and the spatially averaged velocity measurement of a lidar. The measurement technique employed by lidar systems relies on spatially averaged line of sight velocity measurements of the flow field. To measure a 3D velocity vector three or more line of sight velocity vectors are required. Depending on the instrument and the technique employed the number of line of sight vectors can be as low as 4 (WindCube) or as high as 150 (ZephIR). In order to assess the likely impact of an inhomogeneous flow field on such measurement techniques it was necessary to simulate more than a single point in the flow and assess any interference that might exist at each measurement point. Only when this interference at every measurement location had been found the effect on the final velocity vector could be determined. To assess the effect of a platform's flow distortion on the lidars, the flow field over each platform was simulated using CFD and so the measurements performed by a scanning lidar. In this way the extent to which the platform affected the measurements made by a lidar mounted on that platform could be determined. The CFD data also provided information on the distortion observed by a point measurement device such as a cup anemometer.