12.7 Marine Radar

  1. This section summarises the results of trials and studies undertaken in relation to Radar effects from offshore wind farms in the UK. It is important to note that since the time of the trials and studies discussed, wind turbine technology has advanced significantly, most notably in terms of the size of wind turbines available to be installed and utilised. The use of these larger wind turbines allows for a greater spacing between wind turbines than was achievable at the time of the studies being undertaken, which is beneficial in terms of Radar interference effects (and surface navigation in general) as detailed below.

12.7.1 Trials

  1. During the early years of offshore renewables within the UK, maritime regulators undertook a number of trials (both shore-based and vessel-based) into the effects of wind turbines on the use and effectiveness of marine Radar.
  2. In 2004 trials undertaken at the North Hoyle Offshore Wind Farm (MCA and QinetiQ, 2004) identified areas of concern regarding the potential impact on marine- and shore-based Radar systems due to the large vertical extents of the wind turbines (based on the technology at that time). This resulted in Radar responses strong enough to produce interfering side lobes and reflected echoes (often referred to as false targets or ghosts).
  3. Side lobe patterns are produced by small amounts of energy from the transmitted pulses that are radiated outside of the narrow main beam. The effects of side lobes are most noticeable within targets at short range (below 1.5 nm) and with large objects. Side lobe echoes form either an arc on the Radar screen similar to range rings, or a series of echoes forming a broken arc, as illustrated in Figure 12.1.

Diagram of a diagram of a circular object Description automatically generated

Figure 12.1: Illustration of Side Lobes on Radar Screen

 

  1. Multiple reflected echoes are returned from a real target by reflection from some object in the Radar beam. Indirect echoes or ‘ghost’ images have the appearance of true echoes but are usually intermittent or poorly defined; such echoes appear at a false bearing and false range, as illustrated in Figure 12.2.

A diagram of a pie chart Description automatically generated

Figure 12.2: Illustration of Multiple Reflected Echoes on Radar Screen

 

  1. Based on the results of the North Hoyle Offshore Wind Farm trials, the MCA produced a Shipping Route Template designed to give guidance to mariners on the distances which should be established between shipping routes and offshore wind farms. However, as experience of effects associated with use of marine Radar in proximity to offshore wind farms grew, the MCA refined their guidance, offering more flexibility within the most recent Shipping Route Template contained within MGN 654 (MCA, 2021).
  2. A second set of trials conducted at Kentish Flats Offshore Wind Farm in 2006 on behalf of the British Wind Energy Association (BWEA) – now called RenewableUK (BWEA, 2007) – also found that Radar antennas which are sited unfavourably with respect to components of the vessel’s structure can exacerbate effects such as side lobes and reflected echoes. Careful adjustment of Radar controls suppressed these spurious Radar returns but mariners were warned that there is a consequent risk of losing targets with a small Radar cross section, which may include buoys or small craft, particularly yachts or Glass Reinforced Plastic (GRP) constructed craft; therefore, due care should be taken in making such adjustments.
  3. Theoretical modelling of the effects of the development of the proposed Atlantic Array Offshore Wind Farm, which was to be located off the south coast of Wales, on marine Radar systems was undertaken by the Atlantic Array project (Atlantic Array, 2012) and considered a wider spacing of wind turbines than that considered within the early trials[6]. The main outcomes of the modelling were the following:
  • Multiple and indirect echoes were detected under all modelled parameters.
  • The main effects noticed were stretching of targets in azimuth (horizontal) and appearance of ghost targets.
  • There was a significant amount of clear space amongst the returns to ensure recognition of vessels moving amongst the wind turbines and safe navigation.
  • Even in the worst case with Radar operator settings artificially set to be poor, there is significant clear space around each wind turbine that does not contain any multipath or side lobe ambiguities to ensure safe navigation and allow differentiation between false and real (both static and moving) targets.
  • Overall it was concluded that the amount of shadowing observed was very little (noting that the model considered lattice-type foundations which are sufficiently sparse to allow Radar energy to pass through).
  • The lower the density of wind turbines, the easier it is to interpret the Radar returns and fewer multipath ambiguities are present.
  • In dense, target rich environments S-Band Radar scanners suffer more severely from multipath effects in comparison to X-Band Radar scanners.
  • It is important for passing vessels to keep a reasonable separation distance between the wind turbines in order to minimise the effect of multipath and other ambiguities.
  • The Atlantic Array study undertaken in 2012 noted that the potential for Radar interference was mainly a problem during periods of reduced visibility when mariners may not be able to visually confirm the presence of other vessels in proximity (those without AIS installed which are usually fishing and recreational craft). It is noted that this situation would arise with or without wind turbines in place.
  • There is potential for the performance of a vessel’s ARPA to be affected when tracking targets in or near the array. Although greater vigilance is required, during the Kentish Flats Offshore Wind Farm trials it was shown that false targets were quickly identified as such by the mariners and then by the equipment itself.
  1. In summary, experience in UK waters has shown that mariners have become increasingly aware of any Radar effects as more offshore wind farms become operational. Based on this experience, the mariner can interpret the effects correctly, noting that effects are the same as those experienced by mariners in other environments such as in close proximity to other vessels or structures. Effects can be effectively mitigated by “careful adjustment of Radar controls” (MCA, 2022).
  2. The MCA has also produced guidance to mariners operating in proximity to OREIs in the UK which highlights Radar issues amongst others to be taken into account when planning and undertaking voyages in proximity to OREIs (MCA, 2022). The interference buffers presented in Table 12.1 are based on MGN 654 (MCA, 2021), MGN 371 (MCA, 2008a), MGN 543 (MCA, 2016), MGN 372 Amendment 1 (MCA, 2022) and MGN 372 (MCA, 2008b).

 

Table 12.1: Distances at Which Impacts on Marine Radar Occur

Distance at Which Effect Occurs (nm)

Identified Effects

0.5

  • Intolerable impacts can be experienced.
  • X-Band Radar interference is intolerable under 0.25 nm.
  • Vessels may generate multiple echoes on shore-based Radars under 0.45 nm.

1.5

  • Under MGN 654, impacts on Radar are considered to be tolerable with mitigation between 0.5 and 3.5 nm.
  • S-band Radar interference starts at 1.5 nm.
  • Echoes develop at approximately 1.5 nm, with progressive deterioration in the Radar display as the range closes. Where a main vessel route passes within this range considerable interference may be expected along a line of wind turbines.
  • The wind turbines produce strong Radar echoes giving early warning of their presence.
  • Target size of the wind turbines echo increases close to the wind turbines with a consequent degradation on both X and S-Band Radars.

 

  1. As noted in Table 12.1, the onset range from the wind turbines of false returns is approximately 1.5 nm, with progressive deterioration in the Radar display as the range closes. If interfering echoes develop, the requirements of the Convention on International Regulations for Preventing Collisions at Sea (COLREGs) ‘Rule 6 Safe Speed’ are particularly applicable and must be observed with due regard to the prevailing circumstances (IMO, 1972/77). In restricted visibility, ‘Rule 19 Conduct of Vessels in Restricted Visibility’ applies and compliance with ‘Rule 6’ becomes especially relevant. In such conditions mariners are required, under ‘Rule 5 Look-out’ to take into account information from other sources which may include sound signals and VHF information, for example from a VTS or AIS (MCA, 2016)