Lidar Technology for Offshore Turbines

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Lidar technology has proven performance in onshore applications and now a growing track record for offshore wind measurement. The rapid increase of

offshore wind farm projects has given rise to the need for accurate, easy to use equipment to measure the wind on locations several kilometers from the coast. If installed offshore, the traditional mast equipped with anemometers requires a large and heavy platform to sustain the weight and the size of the structure. Lidar technology allows for the development of innovative and cheaper solutions, while achieving similar measurement accuracy to cup anemometers. In terms of logistics, installations of met towers offshore are dependent of the availability of large vessels, involve several days of work by highly trained engineers and specific calm sea conditions. Meanwhile, the installation of a Lidar on any type of structure takes less than a day. Taking into account all these considerations, the use of Lidars allows for the saving of thousands of dollars in construction costs, installation and maintenance. This poster focuses on the different types of Lidars available to realize such measurements during the different phases of any offshore project, from site assessment to power curve measurements.

The first measurement campaigns offshore were realized using existing oil & gas platforms or platforms already equipped with masts, such as FINO 1, the first German off-shore research platform. DEWI has performed an analysis of the Lidar data on this platform and has proved the excellent availability of Lidar data over one year of measurement as well as the good reliability of the equipment. Lidars are also widely used in the framework of the NORSEWInD project, with Lidars

operating for more than four years. For wind measurement from a fixed platform

for site assessment, ground based Lidar is becoming the equipment of choice.

Although scanning Lidars are not widely used yet in the wind energy market, there

are proofs of interest, especially offshore. While the wind resource tends to vary

less spatially in offshore wind fields, there is interest in using such devices for site

assessment in comparison to ground based devices. There is also interest in using scanning Lidars after the construction of the wind farm thanks to their capability to measure the wind away from the device and on large surfaces (several km2). Scanning Lidars can be used for wake analysis, as the phenomena is not perfectly understood and the existing models are showing their limitations.

Another field of application is the realization of power curve by placing the device on the available platform at the base of the wind turbine and by scanning at the rotor level at the distance suggested by the standards. Even though the price to erect a platform with a Lidar is cheaper than with a mast, it is usually not the least expensive solution. The adaptation of the Lidar technology to a buoy solution is an even more cost-effective option.

The usage of Lidar on a buoy requires specific adaptations. For ground based

Lidar, the device is located a few tens of meters above the sea level whereas

buoy-mounted Lidars are at the sea level where they suffer from the sea state

and also the assault of waves. Specific adaptations must be developed to

make sure that the high tech device can operate properly for long periods

without being compromised. The second challenge is the measurement. Buoys

are is in constant motion and this movement needs to be taken into account to

reach the same level of wind measurement accuracy as on fixed platforms or

onshore. Below, you can find the solution developed by 3E in partnership with

Leosphere and OWA.

Turbine mounted Lidars offer a cost efficient and very accurate mean to measure

power curves and to improve wind farm economic performance.

DTU Wind, DONG Energy, Siemens Wind and Avent Lidar Technology have

developed nacelle Lidar power curve measurement procedures compliant to the

IEC 61400-12-1 standard. Agreement to within 1% of Class One cup anemometry was achieved, thereby establishing nacelle Lidar as a valid alternative for power curve verification. Thanks to its flexible installation, permanent upwind alignment and low maintenance requirements, nacelle Lidar allows for verification of any turbine. Monitoring inflow wind conditions to every turbine (turbulence, wakes, wind speed losses) allows for the optimization of wind sector management, with improved

power output and reduced loads.