4

Key Findings, Conclusions, and Recommendations

During this study, the committee organized its information gathering around six areas consistent with its statement of task:

1. Navigation safety,
2. Offshore wind turbine generator (WTG) characteristics and deployment,
3. Marine vessel radar (MVR) design and operation,
4. Electromagnetic characteristics of WTGs,
5. The impact of WTGs on MVR performance, and
6. Strategies to mitigate the impact of WTGs on MVR.

With this in mind, in this chapter the committee first summarizes all findings by the six aforementioned areas. Subsequently, the committee provides conclusions and recommendations addressing the impact of WTGs on MVR and mitigating strategies.

This section builds on prior discussion, with the intent of focusing the most important information consistent with the statement of task. 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, a point influencing the committee’s deliberations and the corresponding discussion throughout the report.

FINDINGS

Navigation Safety

Navigation involves directing a ship using all means possible to minimize the potential for collision, allision, or grounding within the Marine Transportation System (MTS). Navigation employs technical instruments as decision aids, of which MVR is a commonly used device. From

earlier discussion, the presence of marine WTG farms affects the marine operating environment in a number of ways, thereby impacting navigation safety.

From the perspective of navigation safety, the U.S. Coast Guard is taking measures to ensure that safe passage to and from U.S. ports does not conflict with an offshore wind farm. The essential guidance from the U.S. Coast Guard involves designating areas of the various waterways as shipping safety fairways, traffic separation schemes, and other routing measures. Similar to the highway road network, safety fairways concentrate vessels into regular traffic patterns. However, the increased traffic density could increase the likelihood of a risk of collision situation occurring, due to reduced available sea space and funneling of vessels into a close proximity, as Finding 1.1 summarizes (see navigation safety findings in Box 4.1).While the fairways and routing measures may increase the likelihood of a risk of collision situation occurring, the designated waters are expected to increase the safety of navigation by providing a corridor free from structures. WTG developers are required to conduct Navigation Safety Risk Assessments when proposing deployment sites, used to assess the proposal but also as input when formulating safety measures.

MVR is a critical tool used to safely pilot vessels throughout the MTS and is generally required for all commercial vessels, as Finding 1.2 enumerates. Mariners employ MVR to navigate WTGs, and the role of MVR increases under adverse weather conditions where visual piloting is restricted. Thus, it stands to reason that factors affecting MVR performance impact navigation safety. Despite this assertion, the committee found that no standard approach to active radar deployment for operation in a WTG environment is available, as the latter part of Finding 1.2 indicates. The U.S. Coast Guard recognizes that addressing the general lack of understanding of how MVR will lose efficacy in a WTG environment, and the corresponding impact on navigation performance, requires in-depth testing and evaluation, a current gap identified in Finding 1.3.

In gathering information on navigation safety, the committee sought to understand the various technical options available to bridge potential performance gaps in the presence of WTGs. A line of reasoning that WTG locations are generally known, at least for monopile deployments, led the committee to understand the availability and use of Automatic Identification Systems (AIS) reporting vessel and WTG locations as a complement to the MVR. As Finding 1.4 indicates, the

committee found that the capability to incorporate AIS into the radar display to enhance context is a standard requirement.

Offshore Wind Turbine Generator Characteristics and Deployment

The characteristics of marine WTG farms determine the impact of corresponding interference on MVR. As discussed earlier in the report, the WTG tower and blade size, orientation, and spacing determine a number of electromagnetic effects, such as the WTG’s effective radar cross section (RCS), Doppler signature due to the rotating blades, and multipath. The axial orientation and anchoring approach result in different properties affecting WTG interference. As Finding 2.1 indicates, the horizontal axis blade configuration is the current standard (Box 4.2). In this configuration, blade Doppler and composite RCS are highly dependent on the orientation of the MVR relative to the nacelle. There are other novel offshore WTG designs that are under investigation such as vertical axis wind turbines (VAWTs) and multi-rotor structures (Deign, 2019). Predictions about if and when these new offshore WTGs will become available to the market are difficult to make. However, any new WTG design will likely have a different radar signature than the typical three-bladed WTG and should be studied. For example, the signature return from a VAWT will be uniform over aspect, with blade Doppler omnipresent. In addition, in the shallower waters of the Atlantic Continental Shelf, the fixed foundation will be standard, indicating a stable WTG location. In the deeper waters of the Pacific, floating foundations with anchoring to the sea floor will predominate. The floating foundations will result in variability of WTG position, and may affect WTG spacing and safe passage to clear the resulting catenary. Finding 2.2 summarizes this latter point.

Marine Vessel Radar Design and Operation

As Finding 3.1 indicates, the vast majority of MVRs are magnetron-based (see MVR design and operation findings in Box 4.3). Magnetron transmitters represent older technology, matured and proliferated during World War II. The frequency response of a magnetron lacks sufficient stability to reference return signals to a known phase reference response, thereby limiting the ability of the radar to separate different signal returns in angle and Doppler. Over the past 15 years, solid-state radars have penetrated MVR markets, primarily owing to their increased reliability. Solid-state radars employ stable frequency sources and hence enable coherent signal processing, whereby the radar processor can combine returns from a contiguous set of transmit pulses to filter objects according to Doppler frequency and inferred angle. Thus, solid-state radars provide functionality

substantially exceeding that of their magnetron-based counterparts. Finding 3.1 further indicates that, despite better performance potential, adoption of solid-state radar will be slow, as there are no requirements driving the market. Finding 3.2 relates to the antenna characteristics of MVR; the antenna is a spatial filter, letting signals in from some directions while suppressing signals from others. The current MVR system has fairly narrow azimuth beamwidth but a broad elevation response to accommodate vessel motion. The broad elevation beamwidth allows strong WTG returns to enter the radar over a greater range extent, thereby exacerbating WTG effects.

MVR radars must detect WTGs to avoid allision but then reject the WTG response when looking for weaker targets. This is particularly true due to the thresholding effect discussed in earlier sections of the report: operators will lower the gain function (effectively raising the threshold) to clean up the radar display, at the expense of losing weak target detections. As Finding 3.3 states, MVRs have multiple needs to detect and track weaker targets, such as small boats, birds, and reflectors used to find objects such as fishing nets.

MVRs do follow certain guidance in the form of minimum requirements. These requirements do not address the complexity of the MTS and in present form do not accommodate the anticipated proliferation of WTGs. This latter point, summarized in Finding 3.4, is problematic due to the inertia of proposing, approving, and ultimately deploying new radar requirements. Moreover, the committee found that profit margins for MVR manufacturers are very tight, requiring strict control

over the bill of materials to maintain competitiveness, as described in Finding 3.5. The committee further found that MVR manufacturers do not have financial or regulatory incentive to incorporate ameliorating designs into their product lines to address navigation in a WTG environment.

Electromagnetic Characteristics of Wind Turbine Generators

The electromagnetic characteristics of WTGs determine return signals seen by the MVR. There are two dominant effects: the strong return from the WTG tower, and the strong and Doppler-spread returns from the blades. Magnetron-based radar will see the composite return of tower and blade in a single range-angle cell, whereas the response seen in Doppler-based radar will vary with the processing approach. Concerning the latter, however, any Doppler processing will identify the Doppler return of the moving blades as a function of orientation with respect to the radar. A radar line of sight closer to the edge-on view of the WTG blades leads to more Doppler spread.

At present, the horizontal axis WTG is the primary deployment the community will see over the next 10–15 years. The so-called horizontal axis wind turbine (HAWT) configuration leads to variability in RCS as a function of aspect, primarily due to the orientation of the blades relative to the radar line of sight. Future deployments may use the VAWT configuration. If fielded, the VAWT configuration will lead to a more uniform radar signal response than the HAWT but one where Doppler will be present from virtually all aspects. Finding 4.1 summarizes these considerations (see findings on electromagnetic characteristics of WTGs in Box 4.4).

The committee further found that WTG RCS will vary substantially with geometry because of near-field phasing effects, as given in Finding 4.1. It is unlikely the radar will operate in the far field of the WTG, so the result of being in the near field is an increase in destructive interference with changing angle, thereby resulting in a broad range of anticipated WTG RCS values, which affects system performance assessment and mitigating strategies. Furthermore, as Finding 4.2 indicates, WTG height presents unique issues at or beyond the normal radar horizon presented by a surface vessel. A point of concern centers on range ambiguous returns from WTGs at far range that mask targets at near range or add to an already confusing operator picture: these ambiguous returns, while coming from a farther range beyond the expected horizon, will appear to the operator to emanate from near ranges. Similarly, at the horizon, the radar may only see part of the target, such as the mast, which is lower RCS than the whole ship, while the majority of a tall WTG in the vicinity will present itself fully to the radar, thereby degrading target detectability through masking effects.

The proposed density of the WTGs as seen in the submitted Construction and Operations Plans raises some concerns about shadowing, as stated in Finding 4.3. The committee found that while shadowing will be present, it is a higher-order effect, and other factors play a more serious role in degrading MVR performance. Nevertheless, the issue was raised by members of the MVR development community.

As given in Finding 4.4, at a nominal blade tip speed of roughly 100 meters per second (m/s), the Doppler signature of the WTG can cover a broad range of frequencies. Close to the hub, the blade Doppler is virtually zero, whereas the response increases in Doppler to a maximum at the blade tip. The resulting Doppler at X-band is on the order of 6 kilohertz (kHz), thereby requiring a pulse repetition frequency (PRF) of 12 kHz to avoid signal aliasing. The higher PRF constrains the unambiguous range of the radar. At lower PRF, the maximum Doppler frequency of the blade will alias, filling up the Doppler spectrum at the corresponding individual WTG’s range and angle location.

Finally, the committee found that modeling and simulation of WTG interaction with radar use approximate models that may miss important, higher-order effects impacting the mariner’s use of the MVR as a decision aid. Finding 4.5 identifies this matter and further notes a lack of detailed validation and documentation of WTG models.

Wind Turbine Generator Impacts on Marine Vessel Radar

From the committee’s information-gathering sessions and collective experience, it is evident that WTGs decrease the effectiveness of MVR, and the sizes of anticipated marine WTG farms will exacerbate this situation. This decreased efficacy applies to both traditional, magnetron-based MVRs and as-fielded, solid-state MVRs, with some discriminating factors, as identified in Finding 5.1 (see findings on WTG impacts to MVR in Box 4.5). A combination of factors manifests as issues degrading effectiveness of MVR, as described in Finding 5.2, leading to lost contact with smaller objects, such as recreational watercraft and buoys, and presenting a confusing navigation picture. The large RCS of a WTG leads to a strong signal return, with own vessel multipath adding angle ambiguous returns. WTG blades also exhibit high RCS and will result in a strong Doppler return over a continuum of frequencies generated by both reflections along the extent of the blade and the varying geometry between the MVR and the multiplicity of WTGs in the MVR field of view. As identified in Finding 5.3 and Finding 5.4, a natural operator response to strong WTG reflections is to reduce the radar gain, effectively raising the detection threshold, with the consequence of losing detections of lower RCS targets. A combination of lost contacts, as well as shadowing effects, may complicate search and rescue (SAR) in proximity to the WTG farm. As Finding 5.4 indicates, there is currently no available bespoke WTG mode to facilitate improved use of MVR when operating in a WTG environment.

Comprehensive data collections will improve the stakeholder community’s understanding of WTG effects on MVR. As identified in Finding 5.5, there is a lack of experimental data to inform the regulatory, investment, engineering, training, and operational constituents. This is especially valid over a range of parameters necessary to make the best-informed decisions. A good example of this latter point is given in Finding 5.6, where the committee notes that the well-known Kentish Flats experiments (Marico Marine, 2007) were limited in scope to only include magnetron-based radar: assertions on the impact of WTGs on Doppler-based radar from the Kentish Flats collections necessarily yield an incomplete perspective, since no Doppler radars were used to collect data at the time.

Consistent with several of the findings in this section, the lack of MVR tailoring to the WTG environment raises two findings related to radar data processing. Finding 5.7 asserts that post-processing methods to suppress range-Doppler sidelobes in Doppler radar, without customization for nuances presented by the WTG operating environment, will potentially lead to a loss of small object detections. Similarly, as Finding 5.8 indicates, the Automatic Radar Plotting Aid (ARPA) ingests measurements and outputs tracks and key predictions (i.e., CPA and TCPA); there was no

evidence that ARPA design accommodates WTG effects, such as logic-based exclusion of WTG corrupted measurements or approaches to maintain custody of smaller RCS objects resulting from episodic or lost contacts due to WTG presence.

Mitigating Solutions for Wind Turbine Generator Effects on Marine Vessel Radar

The committee found that plausible options exist to improve MVR operating in the presence of marine WTGs. However, as Finding 6.1 indicates, there is no evidence of investments to support the development of mitigating methods for decreases in MVR effectiveness, in contrast to the attention given to military and Federal Aviation Administration radars operating in the vicinity of land-based WTGs (see findings on mitigating solutions in Box 4.6).

Solid-state radar holds the potential to improve MVR performance in the presence of WTG interference. As Finding 6.2 identifies, the current generation of solid-state Doppler radars employs many strategies similar to the magnetron-based radars that they replace, leading to questions about their effective utilization. Specifically, the committee found that currently fielded, solid-state MVR can be used more effectively in a WTG environment, in addition to the currently touted benefits of better reliability and rapid identification of approaching and receding targets based on Doppler frequency. This observation leads to Finding 6.3, where the committee explicitly notes that solid-state radar holds the potential for improved performance due to its ability to separate and filter signals based on Doppler frequency (and, hence, angle for stationary objects, where angle and Doppler are dependent measurements), as well as provide measurements available to improve data processing and logic in a WTG environment. In general, it appears that software upgrades can enhance solid-state MVR performance, as filtering and data processing are software functions.

Modifications to the WTGs themselves to reduce radar signature are possible, as stated in Finding 6.4, where the use of radar absorbing materials and tower shaping will decrease the WTG radar signature.

CONCLUSIONS

From the findings presented in the prior section, the committee draws two primary conclusions.

WTGs cause interference to MVR, including strong stationary returns from the wind turbine tower; the potential for a strong blade flash return for certain geometries; and Doppler spread clutter generated along the radial extent of the WTG blade, which may obfuscate smaller watercraft or stationary objects, such as buoys. Additionally, multipath reflection from own vessel platform is a significant challenge for returns from WTGs, leading to ambiguous detections and a potentially confusing operator picture. As presently deployed, WTGs reduce the effectiveness of both magnetron-based and Doppler-based radar; however, similarities and differences exist between both radar classes as to the actual mechanisms leading to WTG-induced effectiveness loss. MVR strives to detect both moving and stationary objects to aid safe navigation. While vessel operators can control the radar detection threshold—via the receive gain function—to manage the number of targets shown on the plan position indicator (PPI) display and mitigate strong returns, this approach 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.

WTG interference decreases the effectiveness of MVR, and the sizes of anticipated marine WTG farms across the U.S. Outer Continental Shelf 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 ARPA performance. Navies, coast guards, and rescue vessels searching for smaller boats as their primary targets in the conduct of ordinary operations will experience a loss of contact with lower RCS vessels due to the various forms of identified WTG interference. Specifically, WTG interference will complicate MTS operations and is therefore particularly consequential when conducting maritime surface SAR operations in and adjacent to an offshore wind farm.

MVRs are not optimized for operations 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. Solid-state radar technology makes it possible to apply 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. It therefore offers greater potential in overcoming WTG interference than magnetron-based radar; however, assertions of the suitability of Doppler-based solid-state radar, or lack thereof, for operation in a WTG environment are inconclusive from previous experiments. Furthermore, adoption of solid-state radar is expected to be slow due to cost of replacement, long life cycle of existing MVRs, and lack of regulations requiring enhancements in radar capability.

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 other radar applications used elsewhere to deal with strong clutter returns from large RCS objects can, however, be considered as a low-cost or alternative means of mitigating WTG interference. These methods can include enhancing the RCS of small vessels or other objects that are difficult to detect, reducing topside scattering from the own vessel structure to reduce false returns, and improving operator training. These techniques apply to both magnetron-based and solid-state MVRs.

Finally, the environmental complexity that an offshore WTG farm presents to the MVR, its PPI display, and other output products highlights the need to carefully evaluate training and performance of MVR operators.

RECOMMENDATIONS

In light of the key conclusions, the committee offers the following recommendations:

REFERENCES

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