North Hoyle Windfarm Report

6 MCA Radar trials

6.1 Overview

The wind turbine generators (WTG) are very large structures in the vertical plane and significantly so in the horizontal plane. Although the towers are cylindrical, their diameter of 5 metres and height above the water - around 70 metres - is such that they have a comparatively large reflecting surface area. This is compounded by the reflecting surfaces of the platforms, ladders and other structural features of the towers, an average total of about 80 square metres of signal returning surface at any time and from any direction. The three bladed rotors have a total reflecting area of around 200 square metres when their plane is at right angles to the direction of the radar scanner, and around half that when in line with it. The nacelle and boss have reflecting areas of up to 16 square metres. Thus in the vertical plane the North Hoyle WTGs can have a radar signal returning area of around 300 square metres. The sections of turbine which are other than at right angles to the shipborne radar, i.e. non-returning, may produce reflected and other spurious echoes. The scale of the structures is better illustrated in Figure 6-1.

Figure 6-1: North Hoyle Vestas wind turbines

This is a critically important factor when shipborne or VTS radars are close to the WTGs. Here the vertical beam width, for most ships’ radars this being between 25 and 30 degrees, has a greater effect than the horizontal beam width, usually between 1 and 2 degrees.

When close to turbines, the response from individual transmitted pulses may therefore be significantly greater than if, for example, at the same range from a large ship which would be unlikely to have an equivalent vertical extent.

This has some advantages in, for example, detecting wind farm structures by radar, but can have disadvantages with respect to the use of radar in SAR, automatic radar plotting aids (ARPA), collision avoidance or vessel traffic services (VTS). It will also have implications for the siting of radar beacons (RACONS).

Figure 6-2: Typical radar scanner horizontal and vertical beamwidths

As the radar station increases in distance from the wind farm, this effect reduces in significance. For example, as will be seen in subsection 6.16.1, at the range of the Mersey Docks and Harbour Board’s Seaforth radar from the wind farm, 14 nautical miles (nm), the vertical extent of the WTGs has little effect and larger vessels such as the "Norbay" (17,464 Gross Tons ) could be detected and tracked. Smaller vessels, such as the lifeboats and service craft could not be detected at this range.

Technical details of all the radar systems used by the MCA during the trials can be found in Appendix B.

This report is not intended to explain marine radar systems or their operation. A number of publications are available that deal with this and other marine navigation subjects. An example is suggested in reference [4].