The push to build big ocean wind turbine arrays off the U.S. East Coast could bring problems for marine vessel radar, requiring new planning to ensure wind power and other maritime operators can safely operate together, according to a long-anticipated report from the National Academies of Sciences, Engineering and Medicine.
The Biden administration has set a goal to build out 30 gigawatts of offshore wind power capacity by 2030. It is an ambitious endpoint – even wind power advocates say the U.S. plans could be slowed by limited availability of wind turbine installation vessels.
But the National Academies report says change is needed now to ensure vessels can operate safely around the wind turbine generators, or WTGs, envisioned by the federal Bureau of Ocean Energy Management and wind developers.
“Marine vessel radars are not presently optimized to operate in a WTG environment,” the report states up front. “Marine WTGs are very large structures, with towers on the order of several hundred meters and blade lengths exceeding 100 meters. Being heavily composed of steel, the nominal WTG structure has a large radar cross section.”
With the potential for building hundreds of turbines on the East Coast outer continental shelf, “the combination of high radar reflectivity and vast number of WTGs leads to many strong reflected signals entering the radar receiver, further complicated by other factors, such as multipath and range ambiguous returns,” the report says. “In addition, blade motion generates aspect dependent, Doppler-spread interference. These various effects, left unresolved, combine to complicate navigation decision-making.”
Mariners can adapt to operating radars around wind turbines, especially with the latest radar equipment and appropriate training, the report says. But adjusting to the image clutter and scatter from turbines will obfuscate their view of smaller vessels amid the turbine fields – a critical problem for search and rescue operations, the report says.
The National Academies report draws on some studies from wind turbine arrays off the United Kingdom, but the authors stress the proposed size and scale of U.S. offshore projects will involve a whole new scale of bigger turbines with much more profound radar effects.
The wind power industry is looking to taller, more powerful turbines in excess of 800 feet above sea level and 14 MW output. The potential extent of radar shadows cast by those turbines – obscuring radar images of smaller vessels – is concerning to the maritime community, from commercial fishermen to deep-draft vessel operators worried about spotting smaller vessels when they exit wind turbine fields close to traffic separation lanes outside major ports like the New York-New Jersey Harbor complex.
As steel structures the size of moderately high-rise New York City midtown buildings, turbines will “have significant electromagnetic reflectivity and the capacity to interfere with radar systems in their vicinity,” the report states. “Additionally, the rotating blades can return large and numerous Doppler-shifted reflections as the blades move relative to a receiving radar system.”
That has implications for radar systems that support air traffic control, weather forecasting, homeland security, national defense and maritime commerce. In 2014, the U.S. Departments of Defense and Energy, the Federal Aviation Administration, and the National Oceanic and Atmospheric Administration set up an interagency Wind Turbine Radar Interference Mitigation Working Group to begin work on mitigation solutions and strategies. BOEM joined the group in 2018 and in 2020 asked the National Academies to conduct this latest study.
Wind turbines cause radar returns that may appear as interference to vessel radars, “including strong stationary returns from the wind turbine tower, the potential for a strong blade flash return for certain geometries and relative radar-vessel positions, and Doppler-spread clutter generated along the radial extent of the (turbine) blade, which could obfuscate the radar returns of smaller watercraft or stationary objects, such as buoys,” the report states.
“Additionally, multipath reflection from an observer’s own shipboard (radar) platform is a significant challenge for returns from wind turbine generators, leading to ambiguous detections and generating a potentially confusing picture for the operator.”
The report discusses at length the possibilities for radar technology and operator training to address the challenges of navigating around turbines.
“While vessel operators can control the radar detection threshold – via changes to the receiver gain – to mitigate strong returns and manage the number of targets shown on the plan position indicator display, this will frequently lead to the unintended consequence of suppressing detections of small targets in and around wind farms, thereby affecting navigation decision-making and situational awareness,” the report says.
“While the study committee carefully distinguishes performance between magnetron and solid-state classes of marine vessel radars, the corresponding general impact of wind turbine generator-induced degradation will be similar across radar height, radar range, vessel type and size, and other likely parameters,” according to the report.
The study group found no published studies of turbine interference on Doppler-based solid-state marine radars, and their report notes previous studies by British groups “collected wind farm data using magnetron-based radar and did not measure a Doppler signal. Therefore, assertions of the suitability of solid-state radar, or lack thereof, for operation in a wind turbine generator environment are inconclusive from these experiments.”
Among more than two dozen recommendations, the National Academies paper calls for:
• Innovative and collaborative approaches to facilitate data collection, such as the establishment of a marine vessel radar “sensor integration lab” for all classes or types of marine vessel radars and the development of a validated modeling and simulation capability.
• Research, development, and characterization of a reduced radar-cross-section wind turbine generator for marine vessel radar.
• Data collection and analysis using prototype systems, preceding the full deployment of vertical axis wind turbines, if and when they become economically feasible for offshore applications, as a means of characterizing their impacts to marine vessel radars.
• Data collection and analysis on floating wind turbine generators, “which may pose additional challenges for marine vessel radars through their wave-induced movement that will likely provide a less-consistent radar return overall and may also increase clutter and complicate Doppler return interpretation.”
The report lays out detailed recommendations too for changing the design and use of vessel radars, including new training standards, evaluation and standardization of radar mountings to minimize interference, and “deployment of reference buoys adjacent to wind farms to provide mariners a reference target to appropriately adjust marine vessel radar gain and other control settings to assist in the detection of smaller targets operating in the vicinity of wind farms.”