North Hoyle Windfarm Report

8 Conclusions and recommendations

8.1 MCA trials

MCA’s programme was intended to assess the effect of the wind farm structures on marine systems in operational scenarios. The trials assessed all practical communications systems used at sea and with links to shore stations, shipborne and shore-based radar, position fixing systems, and the Automatic Identification System (AIS). The tests included basic navigational equipment such as magnetic compasses.

Theeffectsonthemajority ofsystemstestedbytheMCAwerenotfoundtobesignificant enough to affect navigational efficiency or safety, and an on-going collection of data on such systems is expected to prove these conclusions.

Some reported effects, such as those on short range radio devices, will be further investigated as will some scenarios which could not be assessed during the trials period, such as helicopter search and rescue operations within wind farms.

The only significant cause for concern found by the MCA during the trials was the effect of wind farm structures on shipborne and shorebased radar systems. It was determined that the large vertical extent of the wind turbine generators returned radar responses strong enough to produce interfering side lobe, multiple and reflected echoes. While reducing receiver amplification (gain) would enable individual turbines to be clearly identified from the side lobes - and hence limit the potential of collisions with them - its effect would also be to reduce the amplitude of other received signals such that small vessels, buoys, etc., might not be detectable within or close to the wind farm. Bearing discrimination was also reduced by the magnitude of the response and hence the cross range size of displayed echoes. If on passage close to a wind farm boundary or within the wind farm itself, this could in some circumstances affect a vessel’s ability to fully comply with Rules 6, 7 and 19 of the International Regulations for the Prevention of Collisions at Sea and might also affect the performance of its automatic radar plotting aid (ARPA).

With respect to the multiple and reflected echoes produced when wind farm structures lie between the observing radar and a relatively high sided vessel, gain reduction will have similar effects to those described above. If, as in the trial undertaken, a shore or platform based radar is intended to detect and track traffic in port approaches, Vessel Traffic Systems or in the proximity of offshore oil or gas installations, the effects could be significant.

Recommendations from these trials are that:

• This report should be made feely available to all interested parties.
• Information appropriate to the safety of life at sea, such as recommendations with respect to navigating or carrying out activities such as fishing within or close to wind farms, should be promulgated as necessary by the use of Marine Guidance Notes, Marine Information Notes, Merchant Shipping Notices, etc.
• the siting of wind farm boundaries from recognised marine traffic routes should be determined in consultation with MCA HQ and other stakeholders using a recommended risk assessment methodology, prior to the submission of consent applications.
• Similarly the location and relocation of fixed radar surveillance systems should be determined in consultation with relevant organisations.
• Further work to be done, as for example identified in the report with respect to adverse weather conditions, helicopter search and rescue operations, short range radio systems, non type-tested systems, etc., should be carried out as soon as practical.
• The results of such research should be promulgated where significant.
• The collation of data with respect to all offshore renewable energy installations (OREI) should be an ongoing activity.

8.2 QinetiQ trials

Four trials, covering the areas of GPS, VHF communications and radar tracking and radar clutter were performed by QinetiQ.

The QinetiQ GPS trial involved traversing previously defined courses through and around the wind farm. Along each course, the number of satellites visible to two different GPS systems (a Garmin 152 and a Garmin GPSIII) and the position of the ship were recorded. Our results show that on average between 8 and 11 satellites were visible at any one time providing accurate positioning to within 5 metres. The effect of wind turbines on VHF communications was investigated by QinetiQ using a hand-held VHF transceiver that was run in series with an adjustable attenuator. A link margin of 1 dB was achieved in free-space (away from any turbines). This required an attenuation of 16dB to be added to the transceiver.

To explore the shadow region behind the wind turbines, four link margins, 2dB, 3dB, 4dB and 5dB were used. These link margins correspond to a total attenuation of 15dB, 14dB, 13dB and 12dB added to the transceiver. The closest approach to turbine 21 was 500 metres and approximately 5m behind turbine 26. As expected the depth of shadow was greater when closer to a turbine. When behind turbine 21 the shadow was found to be approximately 2dB to 3dB lower than the attenuation needed to give a 1dB link margin in free space. For turbine 26 the shadow was deeper due to the closer proximity of the VHF system. It was found that behind turbine 26 the depth of shadow was approximately 10dB below the link margin in free space. The shadow depths are shallower than predicted theoretically confirming the worst case expectations of the theoretical work.

The QinetiQ radar shadowing trials provided very little evidence that shadowing of targets would present any significant problems. In particular the shadowing observed was, like the VHF trials, less than predicted in the theoretical study. Clutter in the radar display due to the presence of wind turbines was found to be quite considerable. Both ring-around and false plots (side lobe and spurious echoes) were observed. The observed problems could be suppressed successfully by using the gain and range settings of the radar. However, this may have the unwanted side-effect of no longer being able to detect some small targets.

8.3 Summary

Most of the effects of offshore wind farm structures on the operational use of marine radar, communicationsandnavigation systemsdonot significantlycompromisemarine navigation or safety. Where there are questions about specific systems they will continue to be monitored and assessed when possible.

There are however some concerns about the use of both shipborne and shorebased radar in the proximity of wind farms. Wind farm structures generally have high vertical extents and therefore will return very strong responses when observing radars are close. The magnitude of such responses will vary according to transmitted radar power and proximity to the structures but may affect both the visual detection of targets and the effective operation of automatic radar plotting aids (ARPA).

These effects can be mitigated by vessels keeping well clear of wind farms in open water or, where navigation is restricted, keeping the wind farm boundaries at suitable distances from established traffic routes, port approaches, routing schemes, etc.

With respect to shorebased or offshore platform based systems, the careful siting of radar scanners in relation to traffic routes and wind farm configurations should enable any degrading effects to be minimised.

The overall results are summarised as:

i Global Positioning System (GPS)

No problems with basic GPS reception or positional accuracy were reported during the trials.

ii Magnetic compasses

The wind farm generators and their cabling, interturbine and onshore, did not cause any compass deviation during the MCA trials. As with any ferrous metal structure, however, caution should be exercised when using magnetic compasses close to turbine towers.

iii Loran C

Although a position could not be obtained using Loran C in the wind farm area, the available signals were received without apparent degradation.

iv Helicopter radar and communications systems

These trials were not carried out due to helicopter call-outs to emergencies on the trial days. The emergency services are keen that they should be undertaken when convenient with the co-operation of HM Coastguard Holyhead MRSC.

v VHF and other communications

The wind farm structures had no noticeable effects on any voice communications system, vessel to vessel or vessel to shore station. These included shipborne, shorebased and hand held VHF transceivers and mobile telephones. Digital selective calling (DSC)was also satisfactorily tested. The VHF Direction Finding equipment carried in the lifeboats did not function correctly when very close to turbines and the BHP telemetry link was similarly reported to suffer interruptions.

vi The Automatic Identification System (AIS) carried aboard MV "Norbay" and monitored by HM Coastguard MRSC Liverpool was fully operational.

vii Small Vessel radar performance.

1. The wind turbine generators (WTG) produced blind and shadow areas in which other turbines and vessels could not be detected unless the observing vessel was moving.

2. Detection of targets within the wind farm was also reduced by the cross and down-range responses from the WTGs which limited range and bearing discrimination.

3. The large displayed echoes of WTGs were due to the vertical extent of the turbine structures.

4. These returned strong responses from sectors of the main beam outside the half power (-3dB) points and the side lobes outside 10° from the main beam.

5. Although such spurious echo effects can be limited to some extent by reducing receiver amplification (gain) this will also reduce the amplification of other targets, perhaps below their display threshold levels.

6. Sea and rain clutter will present further difficulties to target detection within and close to wind farms. Weather conditions at the time of the trials were such that these effects could not be examined.

viii Shore based radar performance

1. Short range performance (less than 6 nm)
   When a small shore based radar was sited such that the height of its antenna was about six metres above sea level, its performance with respect to small vessels was similar to that of the vessel-mounted systems in terms of range and bearing discrimination and target detection within the wind farm.
   When moved to a height of 200 metres above sea level there was an improvement in range discrimination.
   When the higher powered and narrower beam width BHP Billiton radar was used, at the same height, the visual detection of targets within, and beyond, the wind farm was again improved.

2. Larger vessel detection
   A larger vessel was easily detected within and beyond the wind farm. However, while it was broadside on to the direction of the shore radar, reflections from the turbines produced strong multiple echoes. At an oblique aspect to the radar, multiple echoes did not occur, but some reflected echoes were observed.

3. Long range radar (more than 12 nm)
   When the wind farm was observed at long range by the Mersey docks and Harbour Board radar the vessel was easily detected and tracked

ix Radar and ARPA carried on larger vessels

As with small vessel radars, range and bearing discrimination were affected by the response from the WTGs. Definition was less on S band radar than on X band. Numerous spurious echoes from side lobes and reflections were reported by MV "Norbay" starting at a range of about 1.5 nm. The ship’s ARPA had difficulty tracking a target vessel within the wind farm due to target swop to the stronger response. This substantiated a similar report with respect to the BHP Billiton radar’s own tracking system

x Non type-tested radar, communications and navigational equipment

The effects on such systems will be similar to those tested during the trials but will vary individually with respect to transmitted power, antenna performance, radar beam width, etc. The Royal Yachting Association is assisting MCA by providing ongoing information through the experiences of its membership.