Floating offshore wind: challenges for marine wildlife

The paper titled Potential impacts of floating wind turbine technology for marine species and habitats by Sara M. Maxwell, Francine Kershaw, Cameron C. Locke, Melinda G. Conners, Cyndi Dawson, Sandy Aylesworth, Rebecca Loomis, Andrew F. Johnson, published in the Journal of Environmental Management provides an excellent summary of floating offshore wind turbines and the wildlife impacts that may result from its development.

Overview

Offshore wind is projected to grow dramatically in the United States in the next decade from a push to decarbonize energy sources, a more consistent wind regime compared to terrestrial wind, and the increasing cost-competitiveness with traditional fossil fuels. Nearly all installed offshore wind facilities are attached to the ocean surface using fixed foundations, where pile driving is used to insert towers into the seafloor. This approach works in areas where water depths are ≤ 60 m, but beyond this depth fixed-foundation installations are not practical. In much of the Pacific Ocean, water depths are too deep to permit fixed-foundation turbines. Instead, floating offshore wind technology will be necessary for offshore wind energy growth in the deepwater regions of the Pacific Ocean from 60 to 1000 m.

This paper reviews the different floating offshore wind technologies and provides an overview of the potential wildlife impacts from floating wind facilities. The main impacts include the following:

• Collisions with turbines and vessels associated with the project
• Entanglements with mooring lines
• Increased energy use from displacement and avoidance of turbines
• Noise disturbance from site assessment and construction
• Impacts of electromagnetic fields from cables

Benefits from floating wind turbines compared to fixed-foundation turbines include:

• Minimal to no pile driving is required for construction (more minor noise impacts)
• Reduced construction times at-sea because much of the construction can be accomplished on land
• Turbines can be moved if necessary, and thus there is more flexibility if social, environmental, or economic reasons dictate

Summary

Types of Floating Offshore Wind Infrastructure

The primary way floating offshore wind turbines (FOWTs) differ from traditional fixed-foundation turbines is from the platform and anchoring systems used to secure the turbine. FOWT uses platforms anchored to the seabed using mooring cables and anchors. Power cables that transport the energy to an offshore substation are suspended in the water column. The substation connects to a static power cable to the onshore landing site and the electrical grid.

Types of Potential Impacts

Entanglement

Primary entanglement refers to animals entangled in the mooring lines and cables themselves. Secondary entanglements occur when derelict objects become entangled on lines and cables, which consequently causes animals to become entangled. Entanglements are influenced by factors including:

• Size and shape of mooring lines
• Depth of mooring lines
• Animal behavior
• Animal detection of mooring lines
• Derelict trash
• Proximity to fishing grounds

Primary Entanglement

Marine mammals (e.g., whales) are the taxon with the highest risk of primary entanglement, though the absolute risk is likely low given the tautness and size of cables and the lack of entanglements reported from oil platforms. Marine mammals are likely to detect large mooring lines through echolocation, vibrations, or hearing rope noise and thus avoid them. Baleen whales are thought to be at the highest risk of primary entanglement due to foraging while their mouths open.

Secondary Entanglement

Secondary entanglements occur when animals become entangled in derelict trash on mooring lines. Species with large appendages, such as humpback whales and sea turtles, are likely to be at the highest risk for secondary entanglement, along with diving seabirds and elasmobranchs (e.g., sharks, rays, etc.). Secondary entanglements also threaten the endangered Northern Atlantic Right Whale, as up to 83% of North Atlantic Right Whales have shown evidence of entanglement.

Collision

Collisions occur when an animal impacts a foreign object, causing injury or death. Little is known about the collision risks of FOWT, given the few number of sites and the difficulty of observing interactions with turbines far offshore. Because bird presence typically decreases as the distance from shore increases and FOWT are typically installed further offshore than fixed-foundation turbines, it is possible that collision risks could be less than collisions occurring at fixed-foundation turbines. This finding, if true, would be more because of where the turbines are installed rather than because of the turbine design per se.

However, FOWT is not stationary like fixed-foundation turbines. Thus, collision risk could increase if turbine monopoles move dynamically due to wave action. There is considerable uncertainty with these statements, and there are no empirical data comparing collision rates from fixed-foundation and floating offshore wind turbines.

Collisions with vessels are also possible, and wind facilities typically increase vessel traffic during construction and operations and maintenance phases. There could be fewer vessel collisions with FOWT compared to fixed-foundation turbines because less construction is required, and most can be performed on land.

Displacement

Displacement occurs when a foreign object results in an animal avoiding an area that was used before the presence of the foreign object. Depending on the species of interest, the impacts of FOWT could be either positive or negative depending on behavioral and habitat preferences. Turbines could displace species from feeding and breeding areas; however, they could also create foraging habitats for marine mammals and seabirds, as platforms could provide a space for prey to attach. Turbines also create a perching habitat for birds, especially in the offshore environment where natural perches are rare. Loons, gannets, and fulmars are known to be displaced by offshore wind, while cormorants and gulls frequently use turbines for perching.

Effects on fish are likely to be highly species-dependent. Some fish species are sensitive to electromagnetic fields, which are used for orientation. The flow of electricity through a conductor produces an electric and magnetic field; however, some studies have shown mixed or non-significant effects on fish. Noise emitted from FOWT is mainly in the lower frequencies of <1 kHz; however, how/if noise levels differ between FOWT and fixed-foundation turbines is unknown.

Habitat disturbance and degradation

Fishes and benthic communities may experience more direct habitat degradation effects than other animal taxa. FOWT may cause sedimentation from anchor use, and effects could be augmented by wave action dragging the anchors along the seabed which could degrade benthic habitat.

Mitigation

Several mitigation options are available to reduce the impacts of FOWT on marine wildlife. Many forms of mitigation involve initial monitoring followed by corrective action if needed. Because there is so little known about FOWT effects on wildlife, monitoring for impacts is highly recommended to understand if/when mitigation is needed. The following measures would be useful for monitoring marine wildlife’s response to FOWT:

• Monitor tension of mooring lines to detect entanglements
• Use autonomous vehicles to monitor for entanglements
• Study mooring line structure and materials to understand the influence on entanglement
• Evaluate the use of pingers to deter animals and reduce entanglements
• Develop a reporting approach for cataloging entanglement incidents to facilitate further study
• Conduct construction when animals of concern are least likely to occur
• Understand how noise levels from different activities vary across distances from the source
• Use bubble nets to reduce noise, especially during construction
• Use accelerometers, microphones, and optical sensors to detect collisions
• Limit the number of vessels and reduce vessel speeds
• Use acoustic monitoring and aerial surveys to understand species presence and when concerns should be the highest
• Place cables and anchors in areas of less ecological importance
• Use reef balls and rocks as anchor locations and reduce seafloor scouring from mooring lines

Commentary

I found this paper to be a good summary of the types of floating offshore wind technology and potential species impacts from development. As there are currently so few installed floating wind facilities, many of the concerns about FOWT are based on expert opinion and experience with other anthropogenic structures. Though many of the concerns seem to be well-reasoned, it is unclear if and to what extent FOWT will impact marine wildlife, especially compared to fixed-foundation structures. Further studies are needed to quantify the extent of impacts to wildlife from floating offshore wind.

Section numbering seems to have been misnumbered on the mitigation section on page 9. The mitigation section should have started a new top-level section 4 but instead remained part of section 3.