Summary

This study assesses the impact of wind turbine generators (WTGs) located on the U.S. Outer Continental Shelf (OCS) on marine vessel radar (MVR) and investigates mitigating solutions. The National Academies of Sciences, Engineering, and Medicine (the National Academies) Committee on Wind Turbine Generator Impacts to Marine Vessel Radar drew two major conclusions from its investigations into the issues identified in the statement of task:

These conclusions yield two actionable recommendations centered on (1) filling knowledge gaps through data collection, modeling and analysis, and focused research on an improved understanding of WTG characteristics; and (2) pursuing practicable options to mitigate WTG interference on MVRs through enhanced training, use of radar reflectors on small vessels, use of reference buoys, new radar designs optimized for operation in a WTG environment, and WTGs with reduced radar signatures.

OFFSHORE WIND ENERGY DEVELOPMENTS

Offshore wind energy development is poised to expand across the U.S. OCS, thereby transforming the landscape of this dynamic ocean environment and raising concerns for safe navigation within its boundaries. In an effort to meet growing energy demands in a renewable way, and as part of a government-wide approach to address climate change, the Biden Administration issued Executive Order 14008 in January 2021 prioritizing the expansion of U.S. offshore wind energy. This Executive Order highlighted the need for close coordination and collaboration among federal agencies, states, the private sector, and other key stakeholders to sustainably accelerate offshore wind energy development, and directed the Secretary of the Interior to review siting and permitting processes in offshore waters to determine actions needed to increase renewable energy production. As a result, the U.S. Departments of the Interior (DOI), Energy, and Commerce announced the shared goal to deploy 30 gigawatts (GW) of U.S. offshore wind energy by 2030, while ensuring biodiversity and co-use of the ocean by various stakeholders. To that effect, DOI’s Bureau of Ocean Energy Management (BOEM) announced a plan in March 2021 to advance new lease sales and complete a review of at least 16 Construction and Operations Plans (COPs) by 2025, providing more than 19 GW of energy.

Since that time, the focus of offshore wind energy planning and development has expanded from the U.S. Atlantic OCS to areas in the Gulf of Mexico and the Pacific Ocean, notably the northern and central coasts of California. By September 2021, lessees of 14 offshore wind energy projects submitted COPs for technical and environmental review by BOEM, thus entering into the final phase of the Bureau’s process for authorizing construction of wind energy facilities. Lease areas for these projects cover 1,637,992 acres of the U.S. OCS and are distributed along the Atlantic Coast from Massachusetts to North Carolina.

A myriad of stakeholders operates and intersects across the U.S. OCS, conducting activities vital to the nation’s security and marine economy. The placement of WTGs across the U.S. OCS will result in changes to the Marine Transportation System (MTS)—defined as the waterways, ports, and land-side connections integral to moving people and goods to and from water—and, in some cases, alter the traditional paths followed by mariners operating within this system. While the construction of wind turbines is prohibited in certain areas of MTS waterways to facilitate navigation across the U.S. OCS, the presence of wind farms adjacent to shipping safety fairways and navigational routing measures may provide new risks the mariner must consider in order to navigate safely. Of the many tools a mariner leverages for safe navigation, MVR is widely used and relied upon to avoid collision and allision in the marine environment.

Through public outreach and engagement during the offshore renewable energy leasing process, BOEM received input from commercial vessel maritime and commercial fishing industry stakeholders requesting further insight into the impacts of offshore wind energy installations on MVR. Due to the size, structure, and proposed placement of WTGs offshore, the maritime community expressed concern that WTGs could cast radar shadows, obfuscating smaller vessels exiting wind facilities in the vicinity of deep draft vessels in Traffic Separation Schemes. Commercial fisheries representatives highlighted concerns over the limited amount of research conducted on this topic, in addition to other possible forms of radar interference that may preclude safe navigation within an offshore wind facility. Furthermore, previous studies exploring WTG interference to MVR considered European offshore wind farm structures differing in size and spacing relative to those proposed for planned U.S. facilities.

WTGs are large structures predominantly constructed of steel. As a result, they generally have significant electromagnetic reflectivity and the capacity to interfere with radar systems in their vicinity. Additionally, the rotating blades can return large and numerous Doppler-shifted reflections as the blades move relative to a receiving radar system. The installation of WTGs towering hundreds

of meters above the sea surface across the U.S. OCS therefore poses potential conflicts with a number of radar missions supporting air traffic control, weather forecasting, homeland security, national defense, maritime commerce, and other activities relying on this technology for surveillance, navigation, and situational awareness. In 2014, the U.S. Departments of Defense and Energy, the Federal Aviation Administration, and the National Oceanic and Atmospheric Administration established the interagency Wind Turbine Radar Interference Mitigation Working Group (WTRIM), observed by the U.S. Department of Homeland Security and BOEM personnel, to identify and develop mitigation solutions and strategies with an emphasis on WTG interference to aircraft and weather surveillance radar systems. BOEM joined the WTRIM as an active member in 2018. In 2020, BOEM called for the National Academies to conduct a study to determine and characterize the impacts of WTGs on MVR used by vessel operators in and adjacent to offshore wind facilities. A second objective of this study is the identification of plausible techniques to mitigate WTG impacts in order to preserve radar effectiveness for navigational awareness and safety on vessels both in and adjacent to offshore wind facilities.

In 2021, members of the National Academies committee undertaking this study met virtually to gather information from public sessions with federal employees, industry representatives, researchers, and other stakeholders, and conducted a review of literature in order to develop this report. To clarify the scale, scope, and nature of WTG impacts on MVR, the committee organized its information-gathering efforts around six areas consistent with the study’s statement of task (Box S.1): (1) navigation safety, (2) offshore WTG characteristics and deployment, (3) MVR design and operation, (4) electromagnetic characteristics of WTGs, (5) the impact of WTGs on MVR performance, and (6) mitigation strategies. Through this process, the committee also identified tailored mitigation methods to address WTG effects on MVR, including those related to operational procedures, wind turbine design and deployment, radar design, signal and data processing, and other combined approaches, and considered the feasibility of adopting these methods for relevant stakeholders.

WIND TURBINE GENERATOR IMPACTS TO MARINE VESSEL RADAR

Since the world’s first offshore wind farm was installed in Vindeby, Denmark, in 1991, offshore wind farms have become operational in several European and Asian nations. Offshore wind energy generation commenced on the U.S. OCS relatively recently, including the installation of the 30-megawatt (MW), five-turbine Block Island Wind Farm in Rhode Island state waters in 2016 and two 6-MW turbines in federal waters off of Virginia Beach in 2020. Concurrently, WTGs have grown in size and capacity. Globally, from 2010 to 2019, the capacity-weighted average offshore turbine size has grown from 3.5 MW to more than 6 MW, and rotor (blade) diameters have grown from 100 meters (m) to more than 150 m. Taller WTG towers with longer blades and higher capacities are planned for deployment in the coming decade. Upcoming COPs include WTGs with hub heights and rotor diameters approaching 175 m and 250 m, respectively, spaced apart by roughly 1 nautical mile, with most developers submitting WTG capacities of at least 14 MW. Similar to onshore WTGs, WTG towers—the largest part of the structure—are made of steel, as already noted, and are consequently highly conductive, resulting in large, aspect-independent radar reflections. In contrast, returns from the turbine blades are aspect-dependent and can appear even larger than reflections from the tower for certain geometries. It is important to note that blade composition, construction, and orientation all affect the magnitude of the blade contribution to the overall WTG signal visible on a vessel operator’s display.

Innovation in WTG engineering has resulted in the design of new configurations to maximize efficiency. The three-bladed horizontal axis wind turbines (HAWTs) will be the standard marine deployment in the near term (10–15 years). Vertical axis wind turbines (VAWTs) may be deployed in the medium to long term (10–20 years) to take advantage of their lower center of gravity for deep water applications and potential for energy-generating efficiency. Both deployments require a stationary tower in their super-structure. Doppler returns from VAWTs will show less dependence on the angle between the WTG and the MVR, such that the Doppler return from VAWTs will generally be more extensive than that of HAWTs. Additionally, monopile (fixed-bottom) WTG foundations will be the standard for the shallow OCS of the U.S. East Coast, whereas floating WTG foundations will be the predominant deployment for the deeper waters of the West Coast.

MVR is a critical instrument for navigation, collision avoidance, and other specialized purposes such as small target detection and tracking, especially in restricted visibility. The Safety of Life at Sea international conventions set forth by the International Maritime Organization require that MVRs (which operate within one of two frequency bands centered on 9.4 gigahertz [GHz] in the X-band and 3 GHz in the S-band, depending on vessel size) be installed on a multitude of commercial vessels for navigation safety. The past decade has seen a shift in MVR design from magnetron-based transmission to solid-state transmission, resulting in the production of radar systems with faster response times, lower transmit power, longer lifespans, and greater frequency stability. Solid-state radars can also accommodate the incorporation of more sophisticated processing techniques, such as Doppler processing used to measure the velocity of moving targets with respect to the radar, and Doppler beam sharpening to improve the resolution of the features of distributed, stationary targets.

WTGs cause radar returns that may appear as interference to MVR, 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 WTG blade, which could obfuscate the radar returns of smaller watercraft or stationary objects, such as buoys. Additionally, multipath reflection from an observer’s own shipboard MVR (also known as “own vessel”) platform is a significant challenge for returns from WTGs, leading to ambiguous detections and generating a potentially confusing picture for the operator. As presently deployed, WTGs reduce the effectiveness of both magnetron-based and Doppler-based MVR;

however, similarities and differences exist between both radar classes as to the actual mechanisms leading to WTG-induced degradation. MVR strives to detect both moving and stationary objects to aid safe navigation. 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. While the study committee carefully distinguishes performance between magnetron and solid-state classes of MVR, the corresponding general impact of WTG-induced degradation will be similar across radar height, radar range, vessel type and size, and other likely parameters.

It is noteworthy that there are no published studies of WTG interference on Doppler-based solid-state radar used for marine navigation. Previous studies of WTG interference on MVR, such as the 2007 British Wind Energy Association study of the U.K. Kentish Flats Wind Farm, 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 WTG environment are inconclusive from these experiments.

WTG interference decreases the effectiveness of MVR mounted on all vessel classes, and the sizes of anticipated marine WTG farms across the U.S. OCS will exacerbate this situation. WTG interaction with MVRs at the scale of the proposed U.S. deployment will lead to unforeseen complications due to heightened effects of propagation, multipath, shadowing, and degraded Automatic Radar Plotting Aid performance. Maritime search and rescue (SAR) assets rely on MVR to search for smaller boats as their primary targets in the conduct of ordinary SAR operations. A loss of contact with smaller vessels due to the various forms of MVR interference could complicate MTS operations, and is therefore particularly consequential when conducting maritime surface SAR operations in and adjacent to an offshore wind farm.

MITIGATION ACTIONS

MVRs are not optimized to operate in the complex environments of a fully populated, continental shelf wind farm. There is no simple MVR modification resulting in a robust WTG operating mode. Additionally, in contrast to investments by developers and operators of air traffic control and military radar systems, compelling WTG mitigation techniques for MVR have not been substantially investigated, implemented, matured, or deployed. Approaches external to the MVR radar design successfully employed for radar applications used elsewhere to deal with strong clutter returns from objects with a large radar cross section (RCS), however, could be considered as a low-cost or alternative means of mitigating WTG interference. These methods could include enhancing the RCS (defined as a measure of the strength of the backscattered signal from a target to the radar with units of square meters) of small vessels or other objects that are difficult to detect, reducing topside scattering from the own vessel structure to reduce false (angle and range ambiguous) returns, and improving operator training. These techniques apply to both magnetron-based and solid-state MVRs.

The environmental complexity that an offshore WTG farm presents to the MVR, its plan position indicator display, and other output products necessitates careful evaluation of training methods and tools to properly assess real-world performance of MVR operators, incorporating realistic scenarios and verified, physics-based and effects-based models.

Solid-state radar technology allows for the application of coherent signal processing methods to filter out both static and dynamic WTG clutter returns to improve detection of moving targets and stationary objects, such as buoys. Thus, solid-state radar offers greater potential in overcoming WTG interference than magnetron-based radar. The MTS stakeholder community could incentivize innovation in MVR products by manufacturers to promote radar designs with increased immunity to WTG interference. For example, development of new, Doppler-based, solid-state MVRs with WTG resilience is possible. However, the majority of MVRs in operation today are still magnetron-based systems, and widespread adoption of solid-state radars will, at present, likely be a gradual process due to the cost of replacement, the long life cycles of existing MVRs, and a lack of regulations that require the functionality provided by solid-state radars.

Additionally, modifications to the WTGs themselves could potentially reduce the WTG radar signature. Previous modeling and simulation efforts have shown, for example, that incorporation of radar absorbing materials and tower shaping can reduce the RCS of WTGs. Preliminary research and development of a reduced-RCS WTG shows promise. However, with the exception of the 2018 QinetiQ Stealth Wind Farm Case Study, to which the committee did not have full access, those efforts have not been fully proven and are not available in the near term.

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