Because offshore wind energy is created over the ocean but consumed on land, efficient transmission of the energy produced offshore to consumers on land will play a critical role in achieving offshore wind deployment goals. Expanding the electric transmission systems and upgrading the onshore grid are necessary components to provide access to offshore wind energy sources in the Mid-Atlantic. The energy generated by offshore wind farms will require underwater transmission cables, called export cables, to traverse through federal and state waters to onshore landing sites, where onshore transmission systems deliver the power to customers.
Undersea cables are not a new concept - in fact, cables have been installed in the ocean for over 150 years. Cables are typically buried in order to reduce risks to the cables themselves, as well as interactions with ocean users and resources. As previously explored in Ocean Stories, there are telecommunication cables located within U.S. waters throughout the Mid-Atlantic region that are essential for society to function as it does - for example, the trans-Atlantic cables that keep us connected to the internet.
Although not unique or unfamiliar, siting offshore wind transmission cables requires careful planning, design studies, and cost considerations to minimize environmental impacts and maximize economic value. Each export cable is a custom design for the project, often taking more than a year lead-time to manufacture.
There are generally two technology options considered for export cables: high-voltage alternating current (HVAC) and high-voltage direct current (HVDC). The decision to select HVAC vs. HVDC is highly dependent on a number of factors (e.g., cost, distance, transmission capacity, onshore and offshore space constraints, heat dissipation, electromagnetic fields [EMF]). For example, HVDC cables are able to transmit more energy over greater distances due to reduced electrical losses and higher transfer capability (e.g.,1,200 megawatts [MW] compared to 400 MW per HVAC cable), whereas HVAC cables are usually more economical for offshore wind projects located within 60 nautical miles from shore (as outlined in this New Jersey Board of Public Utilities report). Subsea interconnection cables are also envisioned as a critical component to making our onshore grids more resilient and reliable and delivering renewable energy more efficiently from multiple offshore wind projects to high demand areas.
Basics of Cable Siting
As offshore wind development in the Mid-Atlantic progresses, the need for more cost-effective and efficient transmission options becomes increasingly important. Transmission cable siting is a complex process informed by desktop reviews, routing surveys, and modeling to optimize route options. Often, several cable routing options are identified and then carefully refined through an iterative process to first avoid then minimize risks. Below are a few examples of considerations for siting offshore wind energy transmission cables (“export cables”).
Existing infrastructure. When planning an export cable route, existing infrastructure must be considered. Ironically, our wireless world relies on the vast network of existing telecommunication wires (cables) to transmit large amounts of data rapidly from one point to another. Cables must maintain a certain lateral separation distance from one another due to thermal interactions and to safely install and ensure repairs can be completed without damaging nearby infrastructure. Cable crossings can occur, although this typically requires installing the export cable at a shallower depth and entering into agreements with the asset owner.
Offshore industry and recreation. Many wind energy areas (WEAs) and existing wind farms are located where commercial and/or recreational fishing occurs. Fish are constantly on the move, and depending on the time of year, installation can be in conflict with where fish species are located or where regulated fishing grounds are open for harvest. Additionally, seafloor disturbance, electromagnetic fields, and use of cable protection can influence fish use and fishing practices as well.
Wildlife and Habitat. The seafloor is not void of features and life. The sediment of the seafloor can indicate where and which species will thrive in a given area. Shoals, canyons, slopes and artificial reefs provide habitat for marine life of all kinds. There are dozens of deep-sea canyons that cut into the continental slope in the Mid-Atlantic. While some are well known like the Norfolk Canyon, Baltimore Canyons and Hudson Canyon, there are many that are still relatively unknown. Canyons provide rich biomes comprised of complex benthic habitats and hydrologic conditions, making these suitable locations for cold water corals and sponges to grow and thrive. Larger marine life including large whale species are attracted to the areas’ rich food sources.
Maritime commerce and navigation. The ocean is a busy place, and the Mid-Atlantic is home to the East Coast’s busiest commercial ports. Accommodating safe access to these ports requires anchorage areas, regularly maintained navigation channels, and aids to navigation which all must be considered in cable route designs.
Sand borrow areas. Sand for beach nourishment comes from offshore underwater locations. Cables are sited to avoid known borrow areas where possible so that sand harvesting for beach nourishment projects can continue. Without enough sand, our valuable beaches and the coastal infrastructure would be at risk for erosion.
Seafloor composition. The seafloor has diverse sediment types. In some locations, rock outcroppings and large boulders dominate while other locations have smaller sediment sizes like gravel, sand, or silt. In many locations, there is a mixture of sediment types. Export cables are easier to bury in finer grained sediments than in hard bottom areas. Additionally, hard bottom areas can provide important fish spawning and feeding grounds.
Examples of Ongoing Offshore Wind Transmission Studies that Will Inform Siting
While finding a completely “risk-free” route is highly unlikely, careful and coordinated planning is necessary for advancing offshore wind energy transmission infrastructure in a way that minimizes risk to the environment and existing ocean users. Federal and state agencies have efforts already underway in support of just this.
One example is the Atlantic Offshore Wind Transmission Study (AOSWTS), led by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy (EERE) Wind Energy Technologies Office (WETO) with analysis conducted by the National Renewable Energy Laboratory (NREL) and the Pacific Northwest National Laboratory (PNNL). The AOSWTS seeks to evaluate multiple pathways to achieving offshore wind goals through coordinated transmission solutions along the U.S. Atlantic Coast from Maine to South Carolina in the near-term (by 2030) and long-term (by 2050).
This comprehensive transmission analysis will compare the costs and benefits of multiple transmission buildout scenarios (e.g., interstate, inter-regional transmission topologies, including meshed networks and backbones) under various combinations of electricity supply and demand while considering reliability, resilience, and environmental and siting constraints associated with ocean co-use. The objective of the Study is to elucidate and evaluate multiple pathways for OSW deployment across the Atlantic coast in support of the national goals.
Additional examples of ongoing offshore wind transmission studies:
Spatial Data Product Needs to Help Inform Transmission Siting in Federal Waters
The Mid-Atlantic Committee on the Ocean (MACO) is the ocean planning committee of the Mid-Atlantic Regional Council on the Ocean (MARCO). MARCO is the Mid-Atlantic Regional Ocean Partnership. Recognizing the important role spatial data plays in early planning for offshore wind transmission siting and initial desktop studies, MACO’s Offshore Wind Regional Collaborative (OWRC) convened an Offshore Wind Energy Transmission subcommittee that worked to identify the spatial data products and needs that will be most important to transmission siting in federal waters. The subcommittee included representatives from the Department of Energy, National Oceanic and Atmospheric Administration, United States Army Corps of Engineers, New York Department of State, New Jersey Department of Environmental Protection and Delaware Department of Natural Resources and Environmental Control.
The OWRC work group identified the spatial datasets most important to transmission siting in federal waters. From this the subcommittee prioritized data products to be considered by the MARCO Board to add to the Mid-Atlantic Ocean Data Portal. By highlighting these data product needs, OWRC members and other partners may be able to more quickly enhance existing or develop new or expanded spatial data products to ensure that offshore wind transmission siting continues to be informed by the best available science and data.
A study area was created based on where transmission cables in federal waters are anticipated to be sited over the next 5-10 years. The study area boundaries followed the 3 nautical miles state/federal waters boundary on the landward side, the 2,600-meter bathymetry contour on the seaward side, and the federal Outer Continental Shelf (OCS) administrative boundary between New York and Rhode Island to the north, and between Virginia and North Carolina to the south. This area included active offshore wind projects; however, the subcommittee's assessment is meant to be applied to new lease or call areas, not proposed projects.
The subcommittee identified and prioritized 33 transmission siting challenges within those boundaries. Each challenge was ranked by subcommittee members as “high, medium, or low,” based on how important or significant the challenge is anticipated to be in the process of siting transmission cables for offshore wind energy. Then, a desktop review of the Portal was conducted to evaluate the availability of information related to each challenge. Challenges that were assigned high importance by subcommittee members and low data availability on the Portal will lead to recommended data products for the Portal.
The subcommittee’s analysis showed the Portal already contains a significant amount of high-quality spatial data needed to avoid or minimize transmission siting challenges in federal waters. Out of the few challenges that ranked high importance with medium or low data availability, most can be easily addressed. For example, the U.S. Army Corps of Engineers Placement Areas layer shown above was added to the Portal to display sites at sea and along the coast that the agency relies on to deposit materials from channel dredging and other important public works projects. These results will inform the OWRC and Portal work plans, including future outreach and coordination efforts as well as data product development.
You can learn more about MACO’s OWRC work group initiatives and MARCO’s State initiatives on our website. The Portal contains thousands of map layers and additional informational materials to explore, including a Current Agency Actions and Public Comment Opportunities page that tracks offshore wind activities in the region.
To stay up to date on events and notifications you can sign up for MARCO’s quarterly newsletter and view our events calendar. You can also click here to sign up for updates on the latest Portal data additions and tool enhancements.
This story was produced by the Mid-Atlantic Regional Council on the Ocean, Delaware Department of Natural Resources and Environmental Control, Monmouth University Urban Coast Institute, National Oceanic and Atmospheric Administration, New Jersey Department of Environmental Protection, New York State Department of State, U.S. Army Corps of Engineers, and U.S. Department of Energy.