## 9 Discussions  
The joint effort of the NORSEWInD team has resulted in new knowledge and lessons learnt in regard to observation of hub height wind measurements for wind energy using wind profiling lidars on offshore platforms and at the coast. The new knowledge includes three major issues on the device performance:  
* The long term performance consistency for the wind profiling lidars employed is good. The so-called \"NORSEWInD standard\" pre-deployment validation showed excellent results for most of the lidars. Eight lidars were tested. The post-deployment validation tested four lidars and showed only minor deviations from the pre-deployment results. The results are very encouraging. They indicate that the devices have a high absolute accuracy after 6 to 26 months of deployment in the harsh offshore environment. Considering that the need for new bankable wind data for offshore wind farm projects is high wind profiling lidars appear to be a suitable candidate for this task in the future. * The system consistency of the two types of lidars used is encouraging. There are several differences in two types of lidars including the number of observational levels, the difference in volumes of air observed and the sensitivity to cloud and fog, see [8] for further details. Despite the differences both types of lidars passed the \"NORSEWInD standard\" and had similar post-deployment validation results. For winds at hub height both systems appear to perform well. * The system availability for the devices when deployed offshore was lower than is typical on land. Offshore it ranged from 85% to 100% with an average of 95% but is typically higher at sites with better power supply. The data availability offshore ranged from 73% to 97% with an average of 89%. Data availability is typically higher at sites with higher aerosol concentration. System availability may be improved by providing better training to the rig personnel in operating and maintaining the devices. However, on some platforms there were no personnel. Otherwise it is recommended to improve the system reliability by the manufacturers designing future devices which require reduced operational care. Without some type of improvement on the system and data availability there is risk of insufficient long term observations necessary for accurate wind resource assessment when based on unmanned platforms offshore. The new knowledge gained on flow distortion around the offshore platforms using both sub-scale models in a wind tunnel and CFD modeling indicates that the practical use of even rather bulky offshore structures is acceptable for observing free stream winds with lidars at hub height and below. A rough guide is that the flow is not significantly distorted above 2.4 times the deck height. It is clear that neither the wind tunnel experiments, nor CFD modeling is a final proof. It is therefore important to recognize that a critical analysis of the specific wind profiles observed on the platforms should always be performed in order to further verify that the wind information is trustworthy [8]. Although the hub heights and rotor diameters are growing, the lower tip height is not changing. This is fixed by the consenting authority and is usually in the order of 30 m AMSL. At this height it is unlikely a pulsed system (unless inclined and hence well outside potential flow distortion effects) will be able to acquire a signal. A cw system would be able to acquire a signal, however, if a larger host platform is used, then the observation would most likely be higher than the lowest tip height. In short we would not expect to correct for lower tip height values even using flow correction factors for wind lidar observations. The major lessons learnt from the offshore deployment are technical and legal issues. In a research and demonstration project such as NORSEWInD legal issues with the platform owners took a while in several cases. However, it is the technical lessons learnt that will allow improved data collection for the future.