Waves, buoys, offshore wind energy: Wind measurement in the German Bight

Fraunhofer IWES Wind Lidar Buoys floated in the German Bight, east of the Bard Offshore 1 wind farm, at times last year. These yellow buoys ensure that comprehensive wind measurement data and meteorological parameters can be precisely recorded and evaluated. In this way measured wind conditions, in particular wind speed and direction, contribute to an efficient and cost-effective design of future offshore wind farms.

Why are we measuring the wind in the German Bight?

In 2019, the German Federal Maritime and Hydrographic Agency (BSH) issued an invitation to tender for a meteorological measuring campaign of the N-7.2 area in the scope of preliminary investigations. We completed this assignment with IWES experts in wind measurement and characterization.

During the 12-month wind measurement campaign in the scope of the Site Development Plan (FEP), we measured the wind conditions in the German Bight. In addition, further meteorological parameters such as the temperature, air pressure, and humidity were also recorded. The results of our measurements are additionally verified by an independent third party and then made available to the potential operators of the future wind farms via the BSH. They can then utilize the measurement data to design the future offshore wind farm, compile a yield assessment, and thus ensure the economic viability of the wind farm project. The Light Detection and Ranging wind measurement buoy, or Lidar buoy for short, is a floating lidar system developed at Fraunhofer IWES in Bremerhaven was deployed in the N-7.2 area to determine the wind there using state-of-the-art science and technology. With an overall height of almost 10 meters, the buoy is by no means small and weighs around 6 metric tons.

What is so special about the Fraunhofer IWES Wind Lidar Buoy?

The IWES Wind Lidar Buoy design is based on a conventional navigation light buoy which is in successful use for over 40 years. For this navigation light buoy to be used for wind measurements, a few adaptations were required:

The buoy is equipped with a lidar device for wind measurement, which is completely encapsulated by a specially manufactured aluminum housing. This hermetically protects the highly sensitive equipment from all of the sea’s environmental influences, especially the pounding of the waves. Alongside the main lidar measuring device, there is also further sensor technology installed for recording of the buoy’s movements and its precise GPS position as well as additional sensors for the measurement of other environmental parameters. The robust energy supply system for maintaining operation even in rough seas is also just as important.

Why do we use a buoy for the measurements and how does it even make it to the North Sea?

The advantage in the use of the buoy is that there is no need to install and dismantle an offshore measuring mast. That saves a considerable amount of money, as the costs of a measuring campaign with a wind lidar buoy are at least 10–15 times lower than the costs of a measuring mast. Additionally, the approval and installation process for a wind measurement buoy, which is also more mobile with regard to its deployment, is considerably shorter.

It is important to ensure high system and data availability of the buoy over the entire measuring period so as to be able to make as much convincing measurement data as possible available to the future operator of the wind farm. A robust buoy design and energy supply concept are therefore the most important aspects for being able to survive the rough sea with the equipment just 3 meters above the waterline.

Well organized logistics are also important, not only with regard to the initial deployment of the buoy in the area to be measured and its retrieval once the measuring campaign is complete, but also as far as rapid responses in the case of possible failures are concerned. The period of data loss should be as short as possible, as it can have tremendous consequences for the measurements.

A combination of the following factors is fundamental to ensure high system availability:

• Good weather conditions – a low wave height is required due to the size and weight of the buoy during crane operations;
• Vessel availability, incl. necessary operational documentation;
• Spare parts availability;
• Personnel availability;
• Logistics availability – low loaders and truck-mounted cranes, means of travel for the operations team;
• and customs clearances.

Performance of these offshore operations requires thorough and complete preparations. When the factors above are combined correctly, it is possible to reach the buoy and perform repairs within just a few days in the case of a buoy failure or fault in one of the installed components.

Following successful deployment, the location of the buoy is marked on the nautical charts so that the shipping traffic in the area also receives information regarding its exact position, see Figure 4.

And what do we do with the massive data set?

Prior to the actual measuring campaign, the buoy is verified at an IEC-compliant measuring mast in the North Sea. The verification measurement – in our case at FINO3 – is to ensure the buoy is functioning as intended and measuring the wind correctly. The buoy is then transported to the area of sea to be measured.

The system availability over the entire measuring period at N-7.2 was 91.86%. The few periods in which measurements were not taken due to faults were restored with modeled time series based on WRF data (Weather, Research and Forecasting) corrected for the position of the N-7.2 area using an MCP (measure-correlate-predict) method.

The data collected were already evaluated, analyzed, and plausibilized during the measuring campaign. We do so using software we developed ourselves, which corrects the measurement data, as buoy movements caused by wind and waves affect the measured values.

All measurement data were then subsequently evaluated and summarized again in a final report. This report presents wind directions and speeds ostensibly in different plots, e.g., wind roses and Weibull distributions of wind speed. In addition, the measurement uncertainty is quantified and information provided regarding the determined turbulence intensity.

What comes next after the measurements?

Our measurement of the N-7.2 area was the first commercial 12-month measuring campaign with an FLS (Floating Lidar System) in the German Bight. We assume that the preliminary area surveys in the offshore expansion zones will be carried out here primarily using floating lidar measurements in the future. This is necessary due to the excessive distance of the areas to be surveyed from the mainland and other neighboring offshore structures. With the use of these floating lidar measurements, Germany is catching up with countries such as the United Kingdom, where lidar buoys are already increasingly used to measure wind potential.

We are currently conducting measuring campaigns with two lidar buoys in the N-9 area comprising three subareas (N-9.1 to N-9.3) as part of another assignment from the BSH. But the IWES is also active beyond the German Bight: since 2017, two Fraunhofer IWES Wind Lidar Buoys have been measuring in Scottish waters to investigate the wind conditions there. In addition to four other lidar buoys (currently under construction), the IWES is also operating two further lidar buoys in research projects. Alongside the continuous further development of the lidar buoy technology in the scope of funded projects, we are also co-developing a technical specification for wind measurement with floating lidar systems (e.g., IEC 61400-50-4).

With our scientific expertise and many years of valuable experience in the field of wind measurement, the IWES is making an important contribution to the efficient use of areas for the wind energy sector.

Sources:

• OpenSeaMap: Home page / Visited on 02/11/2022 at 1:49 p.m.
• Garrad Hassan & Partners Ltd, DNV KEMA, M. MacDonald, ECN, Frazer-Nash Consultancy, DNV GL, Multiversum Consulting, and Fraunhofer IWES: “Accelerator Roadmap for the Commercial Acceptance of Floating LiDAR Technology,” Carbon Trust Offshore Wind, 2018, Version 2.0.
• J. Gottschall and M. Dörenkämper: “Understanding and mitigating the impact of data gaps on offshore wind resource estimates” Wind Energy Science, 6, p. 505–520, 2021.