In April 2023, the New Energy and Industrial Technology Development Organization (NEDO) will develop the first "Offshore Wind Observation Guidebook” has been published.
This guidebook provides a detailed summary of the necessary information about wind condition surveys, which are essential for business planning and wind turbine design for offshore wind power plants. To help you make the most of these surveys, Relatec will explain the key points from the perspective of a wind condition consultant.
The "Offshore Wind Observation Guidebook" incorporates the latest knowledge from both Japan and abroad. ReraTech members are involved in the aforementioned NEDO project as implementing parties and are also cooperating in the creation of the guidebook.
Learn about the new unified offshore wind observation methodology
With the aim of achieving carbon neutrality by 2050, plans for offshore wind power generation are currently moving forward at a rapid pace in Japan.
When planning a wind power generation project, it is necessary to conduct accurate wind condition surveys, but until now, the lack of standardized methods for observing offshore wind conditions in Japan has been pointed out as an issue.
The basic method of observing wind conditions for wind power generation is to directly observe wind conditions using an observation tower called a wind mast.
Figure 1. Image of observation using a wind condition mast
Figure 2. Examples of offshore wind masts (from left: Germany's FINO, Choshi, Kitakyushu, and Mutsu Ogawara)
(Source: FINO website, NEDO website)
To use such a wind mast for offshore wind observation, construction is very costly, and the installation process is also time-consuming and difficult, as it requires coordination with local authorities and approval procedures.
A method of observation that is expected to solve these problems is one that utilizes remote sensing technology, using a laser-based instrument called a Doppler LIDAR.
We have talked in more detail about Doppler LIDAR in a previous article, but there are various types of LIDAR, as shown in Figure 3, for example.
Figure 3. Wind observation method
Doppler lidar (SL, FLS) developed for oceanic applications is still new, and in order to introduce it in Japan, it was necessary to verify its accuracy in the country's natural environment and develop a unified observation method using it.
Therefore, in 2019, NEDO's "Fixed-bottom Offshore Wind Farm Development Support Project (Establishment of Offshore Wind Condition Survey Methods)" carried out accuracy verification using multiple models, which enabled the characteristics and challenges of the observation methods to be understood. The results of this work have been compiled into the "Offshore Wind Condition Observation Guidebook."
5 points to note in the guidebook - ①②③
We will introduce the key points of this guidebook from the perspective of Relatec, a wind condition consulting company.
① Lidar is clearly defined as an observation device that can be used to measure offshore wind conditions.
Wind observation is based on a wind mast, one of the reasons being that the cup anemometers mounted on the wind mast are the standard for measuring turbulence (wind disturbances) that affect wind turbine design.
In the accuracy verification project mentioned above, sufficient accuracy equivalent to that of a wind condition mast was obtained, and the following items have been specified as "usable observation equipment" (see Table 1).
For lidar measurements of turbulence intensity, "DSL" is recommended, and for lidar measurements of wind speed, wind direction, etc., "DSL, SSL, FLS, VL" are recommended.
Table 1. Wind condition evaluation items required for power generation prediction and wind turbine design (adapted from Guidebook Table 2.1)
| Evaluation item | Usable observation equipment*1 |
| Average wind speed | MM, DSL, SSL, FLS, VL |
| Wind speed occurrence frequency distribution | MM, DSL, SSL, FLS, VL |
| Frequency distribution by wind direction | MM, DSL, SSL, FLS, VL |
| Wind shear exponent | MM, DSL, SSL, FLS, VL |
| Turbulence Intensity | MM/DSL |
*1: Equipment that is currently deemed technically usable.
*2: Abbreviations are as follows:
MM (Wind Mast), DSL (Dual Scanning Lidar), SSL (Single Scanning Lidar), FLS (Floating Lidar System), VL (Vertical Lidar)
According to Table 1, if you are using a lidar to observe turbulence intensity, you should choose DSL. However, in offshore areas where the laser cannot reach, DSL cannot be used and it is expected that FLS will be used, but this guidebook does not recommend measuring turbulence intensity with FLS.
As noted in *1 in the table, the findings in this guidebook represent the current findings, and further verification is required in order to put turbulence intensity measurements using FLS into practical use.
However, I feel that it is a big step forward that LIDAR has been explicitly stated as a method for observing offshore wind conditions, and that the guidebook also recommends DSL for observing turbulence intensity.
②Indicate the installation standards and setting methods for scanning lidar (SL)
As shown in Figure 4, DSL is the observation method of choice for including turbulence intensity, and it is expected to become the main observation method for coastal areas where lasers can reach from land.
Until now, there were no clear standards for setting up steam locomotives, and observers had to set them up by trial and error. However, this guidebook now provides a set of standards for the first time.
For example, regarding the "dual scanning lidar (DSL)" method, which uses two SLs for measurement, page 2 of the guidebook clearly states that "The angle between the two lasers should be between 16° and 2°, with 30° being ideal."
Until now, this has been based on experience, and it was common to see antennas being operated at around 30°, but now that a range of included angles has been provided that can be used as a reference, installation has become easier.
Figure 4. Example of SL installation standards (left) and observation overview (right)
In addition, Appendix A describes the "hard target adjustments" that are required at the start of observations, and it is also important to note that it specifically describes the setting methods that should be taken into consideration when conducting observations.
However, it cannot be said that all of the numerical values given in the guidebooks are appropriate, and in some cases it may be necessary to consult with an expert before making a decision.
For example, on page 18 it says that "The elevation angle for DSL observations should be within 5 degrees," but when DSL is deployed on land, it is often difficult to keep the top of an offshore wind turbine within 5 degrees. Furthermore, considering that wind turbines will continue to get larger in the future, this is not a very realistic number.
On the other hand, it has been confirmed that the larger the elevation angle, the lower the observation accuracy, which is why the "5°" was specified. I think that in the future, it will be necessary to consider better standards from the perspectives of both accuracy and operation.
3) Clarification of installation procedures for Floating Lidar Systems (FLS)
Another important point is that the safety and approval procedures for FLS, which has relatively few installations in Japan, are also explained (pp.30-31, Appendix D).
Based on experience gained in Japan, the document summarizes the installation work flow, points to note to ensure safety, and examples of permit procedures (Table 2).
For example, if a buoy drifts at sea, it may cause damage to nearby facilities (ships, fishing nets, etc.), so FLS observation requires even greater safety. Therefore, it is considered to be a part that must be read when introducing FLS in Japan.
Table 2. Examples of permit procedures for installing an FLS (excerpt from Guidebook Table 7.1)
| Counterparty | Item |
| Coast Guard (station) | ・Application for permission to install navigation aids, etc. ・Construction permit application, etc. |
| Prefectures | Application for permission to use sea area |
| Port Authority | Use of port facilities such as wharves and yards |
In addition to the turbulence intensity error of the vertical lidar (VL), the FLS is also affected by wind wave motion, resulting in a larger measurement error than the cup anemometer on the wind condition mast.
In the future, research into quantitatively understanding this measurement error and its correction technology will be important, focusing on FLS observations in actual ocean areas and on determining what kind of corrections should be made, including measurements of turbulence intensity.
Kobe University, where Relatec is headquartered, is already working on this issue, and we would like to continue to enthusiastically pursue research in cooperation with Kobe University.
Figure 5. Research example related to FLS oscillation correction (left) and observation overview (right) (Left: Asakura et al., 2022)
In the first part, we explained points ① to ③ of the guidebook.
In the second part, we will discuss "Point 4: Clearly stating the method for verifying accuracy in advance" and "Point 5: Valid data rate and how to fill in periods of missing data," as well as points to keep in mind when using this guidebook.
References
- NEDO, Offshore Wind Observation Guidebook, 2023, https://www.nedo.go.jp/library/fuukyou_kansoku_guidebook.html
- FINO1, https://www.fino1.de/en/
- NEDO, https://www.nedo.go.jp/fuusha/
- S. Asakura, T. Osawa, Y. Aso, A. Land Motion Experiment for Performance Evaluation of Floating Lidar (Part 44), Proceedings of the 2022th Wind Energy Utilization Symposium, XNUMX.
As wind condition consultants, Rera Tech Inc. will conduct optimal wind condition surveys that combine "observation" and "estimation" for wind power generation. Please feel free to contact us if you have any inquiries regarding wind conditions.