North Hoyle Windfarm Report
6 MCA Radar trials The results are presented in Figure 6-32 to Figure 6-37. In Figure 6-32 the raw radar display as "Norbay" begins to pass at a distance of 800 metres across the northern boundary of the wind farm is shown. Whilst in Figure 6-33 the filtered recording of "Norbay" passing turbine 30 is presented. Note that in both the raw and processed radar displays, strong multiple echoes of turbines are visible. As the "Norbay" passes turbine 29 multiple echoes are still visible as is shown in Figure 6-34 and in Figure 6-35 as the vessel passes turbine 28. In Figure 6-36 the raw radar display, as the "Norbay" roundsNWcorner of the wind farm, shows heavy multiple and reflected echoes. In Figure 6-37 the filtered display, with high persistence is shown. As the "Norbay" leaves the wind farm it resumes its passage with a hull aspect of about 150 degrees. No multiple echoes are see at this aspect, but some small reflected echoes are visible. 6.16.1 The Mersey Docks and Harbour Board long range radar This radar, at a range of 14 nautical miles (26 km) from the wind farm, successfully tracked the "Norbay" during her passage around and through the turbine array, with the Norcontrol VOC500 tracking and recording equipment. However, no smaller vessels could be detected or tracked at this range. 6.16.2 Reflected and multiple echoes in general Since the WTGs are strongly reflecting when vessels and / or shore based radars are close by they can produce significantly interfering reflected and multiple echoes. Reflected echoes occur when signals are reflected at an angle from one structure to another and returned to the radar via the same route. The latter target will then be indicated on the display in the direction of the initial reflecting surface, and at a range equivalent to the total distance from radar to initial reflector plus the distance from it to the second surface.The target may additionally be indicated at its correct range and bearing. This effect occurred within the wind farm when signals were reflected between WTGs. Multiple echoes occur similarly when two strongly reflecting surfaces reflect signals backwards and forwards between them, such that echoes of the latter target occur a number of times behind the initial reflecting target, the distance between each such spurious echo being that of the two targets. This was found to occur with the BHP Billiton radar sited at Gwaenysgor, whose purpose is to monitor traffic in and around the Douglas and Hamilton oil and gas fields. These fields lie 14 nm from the radar site, the North Hoyle wind farm lying in the same direction but only 5.2 nm from the radar site. The Gwaenysgor radar scanner is 200 metres above sea level. When the P & O ferry "Norbay" was proceeding along the northern boundary of the wind farm and at a distance of around 800 metres from it (as indicated by the radar ranges) very strong multiple echoes were found to occur on its far side (see subsection 6.16) At this time the "Norbay" was almost broadside on to the scanner direction, such that its reflected echoes to the WTGs would be maximum. Both of these effects may have implications for port approaches, Vessel Traffic Services, search and rescue, and for collision avoidance. As with side lobe echoes, the effects can be reduced by turning down the receiver gain, but again with the penalty of reducing the displayed response of other vessels or buoyage. For radars used in Vessel Traffic Services, for monitoring infringements, or in port approaches the effects of multiple and reflected echoes may be significant, particularly where a number of vessels may be required to pass or anchor close to a wind farm boundary. However, they may be reduced by the careful siting of shore radars relative to shipping routes and wind farms, or if necessary, by using radars at different sites to resolve ambiguities. Previous laboratory studies have indicated that there is high potential for such reflected signals to trigger Racons when a turbine is within 1000 metres of them. No Racons were at this distance from the North Hoyle turbines and therefore this could not be substantiated. However, if Racons were to be considered for use in marking wind farms, this effect should be determined. Trinity House Lighthouse Service, which maintains a number of Racons, have agreed to investigate this. 6.16.3 Report from the Master, MV "Norbay" mv.NORBAY
Figure 6-32: Raw radar data as the "Norbay" passes turbine 30 at a range of 800 metres Figure 6-33: Filtered radar data as the "Norbay" passes turbine 30 at a range of 800 metres Figure 6-34: Filtered radar data as the "Norbay" passes turbine 29 Figure 6-35: Raw radar data as the "Norbay" passes turbine 28 Figure 6-36: Raw radar data as the "Norbay" rounds NW corner of the wind farm Figure 6-37: Filtered radar data as the "Norbay" leaves the wind farm
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