• APPENDIX
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Ship Collisions, Radars and Radio Signals, and Other Impacts

 

SHIP COLLISIONS

Ship collisions with the turbines are one of the potential risks associated with offshore wind energy development. Colliding with a wind turbine foundation could damage or possibly destroy a ship. The potential danger to the environment is the spillage of oil or chemicals from the ship into the water (Koeller et al, 2006).

Evaluation of several collision scenarios between three different types of turbine foundations (monopile, jacket and tripod) and different ship types (single and double hull tankers, bulk carriers and container ships) has been carried out in several locations of the North Sea and Baltic Sea off Germany (Koeller et al, 2006). The results have demonstrated two main results: the first is that monopile and jacket foundations are safer than tripod structures, and the second is related to the risk of collision which can be reduced, but not totally avoided.

There are several safety approaches applicable to avoid or minimise this potential risk:

• Redundant navigation and control systems such as radar and ships optimised to survive collisions;
• Prohibition on navigation into the wind farm area for certain kind of unsafe ships;
• Introduction of traffic management systems;
• Wind farm monitoring;
• Availability of tug boats for emergencies; and
• Crew training.

(Koeller et al, 2006)

 

 

RADARS AND RADIO SIGNALS

The wind turbines may impact on aviation activity, both civil and military, due to interference with radars that manage aircraft operations. Radar is a system for detecting the presence or position or movement of objects by transmitting radio waves, which are reflected back to a receiver. The radio wave transmitted by radar can be interrupted by an object (also called target), then part of the energy is reflected back (called echo or return) to a radio receiver located near the transmitter.

Wind turbines are vertical structures that can potentially interfere with certain electromagnetic transmissions. Mobile structures such as rotating blades may generate more interference on the radars than stationary structures. The effects depend on type of radar, specific characteristics of wind turbines and the distribution of wind turbines. Air traffic management is susceptible to being negatively affected by wind turbine installations. The systems managed by radars are air traffic control, military air defence and meteorological radars (DTI, 2002).

Table 2.5 summarises the functions and the mitigation measures according to the different types of radars and wind turbines effects in the UK.

Table 2.5. Effects and Mitigation Measures by Radar Types

 

Systems	Air traffic control		Meteorological control	Air defence
Mission	Control of arrival,  departure and transit in vicinity of airport and transit over the country		Weather forecasting; very important to aviation safety	Detect and identify aircraft approaching, leaving or flying over the territory of a country
Types	Primary radar	Secondary surveillance radar	Weather radar Wind profile radar	Ground based radars	Airborne radars
Wind turbines’ effects	False radar responses or returns	Masking genuine aircraft returns; reflection from wind turbines could cause misidentification or mislocation of aircraft	Reflection Reflection	Highly complex and not completely understood	Highly complex and not completely understood
Mitigation measures at the beginning of project planning	Ensuring location in area with low aircraft traffic; ensuring location not in line of sight of any aircraft radar	Avoiding close vicinity to radars; minimum safe distance between wind farms and these types of radars not defined	Avoiding wind farm installation at 10 km or less of radar facility	Minister of Defence of UK does not permit any wind farm located at less than 74 km from an air defence radar, unless developers can demonstrate no interferences with the defence radar	Moving the location of wind farm or adjusting the configuration of turbines to avoid interference; providing alternative site for the affected radar; contribute to investment in additional or improved radar system

Source: Based on DTI (2002)

The impacts associated with wind turbines are masking, returns/clutter and scattering.

 

MASKING

Radar systems work at high radio frequencies and therefore depend on a clear 'line of sight' to the target object for successful detection. When any structure or geographical feature is located between the radar and the target, it will cause a shadowing or masking effect. The interference varies according to turbine dimensions, type of radar and the aspect of the turbine relative to the radar. The masking of an aircraft can occur by reflecting or deflecting the returns when the aircraft is flying in the 'shadow' of wind turbines and thus is not detected. Also the masking can occur when returns from the towers and blades of the wind turbines are so large that returns from real aircraft are lost in the 'clutter' (radar returns from targets considered irrelevant to the purpose of the radar.)

 

RETURNS/CLUTTER

Radar returns may be received from any radar-reflective surface. In certain geographical areas, or under particular meteorological conditions, radar performance may be adversely affected by unwanted returns, which may mask those of interest. Such unwanted returns are known as radar clutter. Clutter is displayed to a controller as 'interference' and is of concern primarily to  air surveillance and control systems - ASACS and aerodrome radar operators, because it occurs more often at lower altitudes.

The combination of blades from different turbines at a wind farm can give an appearance of a moving object, which could be considered as an unidentified aircraft requiring controllers to take action to avoid a crash with another aircraft.

 

SCATTERING, REFRACTION AND/OR FALSE RETURNS

Scattering occurs when the rotating wind turbine blades reflect or refract radar waves in the atmosphere. The source radar system or another system can absorb the waves and provide false information to that system. This effect is not well known, but it has been reported in Copenhagen airport as a result of the Middelgrunden offshore wind farm.

The possible effects are:

• Multiple, false radar returns are displayed to the radar operator such as blade reflections are displayed to the radar operator as false radar contacts;
  
• Radar returns from genuine aircraft are recorded but in an incorrect location; and
  
• Garbling or loss of information.

Marine radars and communication and navigation systems may suffer interference from nearby wind farms. Howard & Braun (2004) stated that most of the effects of Hoyle offshore wind farm do not significantly compromise marine navigation or safety. Mitigation measures in open water include the definition of vessel routes distant from wind farms, while in restricted areas the boundaries of wind farms must be kept at appropriate distances from navigation routes or port approaches (Howard and Braun, 2004).

 

 

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