## 6 Flow Distortion due to Offshore Platform and Terrain  
Nine of the lidars were deployed on offshore platforms. These platforms included large gas and oil drilling rigs with tall derrick structures (Beatrice, Siri, Taqa, ORP), smaller unmanned production  
Figure 6: Lidar observations offshore from four lidars. (**a**) The carrier-to-noise ratio (CNR) as a function of observations height above installation. (**b**) The average data availability as a function of observations height above installation. platforms (Jacky, Schooner, Babbage), wind farm transformer stations (Horns Rev 2 in the North Sea, Denmark) and a platform mounted meteorological mast (Fino3 in the North Sea, Germany). One lidar was deployed on the coast of the island of Utsira, see [8] for details. Common to all installations was the risk of flow distortion around the structures or influence from the surrounding landscape on the observed wind profile. The aim of the NORSEWInD project was to accurately observe free stream winds at hub height; thus it was desirable to minimize the flow distortion on the lidar wind profile observations by selecting the observational heights with care. In certain cases it could, however, become necessary to correct the wind profile observations. To investigate the flow distortion around the platforms and to validate the Computational Fluid Dynamic (CFD) simulations, measurements in a low speed wind tunnel were made with a calibrated DANTEC Streamline constant temperature (CTA), triple wire anemometer mounted on a three dimensional traversing rig as shown in the diagram of Figure 7.  
By traversing the hot wire probe vertically above the location of the simulated lidar the velocity profile in a vertical line above the rig could be determined. This velocity profile was then compared with the results of the CFD simulation of the rig. Initially, to create a base line against which the effect of the rig on the flow field could be assessed, the flow in the wind tunnel was traversed without the rig model present in the tunnel. The measured vectors were then non-dimensionalised by a reference wind speed measured by a single hot wire probe upstream and to the right of the proposed model location,  
Figure 7: Diagram of constant temperature (CTA) probe traverse system showing the wind tunnel coordinate system and a plan view of wind tunnel layout. with due care taken to ensure the reference speed was outside any likely flow disturbance that might be caused by the presence of the rig model. This provided the non-dimensional, undisturbed, free stream velocity at the measurement locations above the rig for neutral conditions. Simulations were undertaken at model scale and full scale to identify any issues regarding Reynolds number effects in the subscale wind tunnel tests and none were found. Length scale for oil production platforms was typically between 0.5 m and 1 m and 1.5 m for models of the island. Tunnel free stream speed was 15 m\(\cdot\)s\({}^{-1}\) in all cases. Platform models were typically 100 th scale and 1,250 th scale of the platforms and island, respectively. The CFD simulations were carried out for turbulent flow and the turbulence intensity in the 1.5 m low speed wind tunnel is approximately 1%. The k-omega turbulence model was selected because the model is a mature and established algorithm intended for general use with external flows [21]. To confirm the validity of the CFD simulation and to evaluate the most appropriate turbulence model the data collected by the hot wire traverses above the rig were compared to the CFD data at the same locations using a range of turbulence models including the k-\(\omega\) and the standard k-\(\varepsilon\) model. Figure 8: Non-dimensional velocity magnitude profiles measured above the platform with the flow approaching from four different azimuth angles. The rig was then placed in the tunnel and the velocity profiles above the rig measured.