Offshore Wind Energy (OWE) Transmission
While turbines are a significant aspect of offshore wind facilities, they aren’t the only critical infrastructure required for the creation of offshore wind energy (OWE). Transmission – the process that enables the transfer of energy from turbine arrays via submerged cables to land-based collection and distribution sites – is also an important piece of the OWE puzzle. Collaborative dialogue and learning among government, private sector, academic, advocacy, and community entities focuses on exploring transmission solutions that support effective and efficient energy delivery while maintaining and protecting the ocean environment and resources arena.
Cable Route Planning
Successful cable installation starts with science-backed route planning and risk mitigation and involves federal and state processes. Routes for the cable corridor are determined based on the results of surveys and studies that identify geophysical and geotechnical conditions and the hazards that may influence where a cable can be safely and effectively buried. (Source)
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The assessment uses publicly available and client-supplied data to develop several cable corridor alternatives with the fewest sea bed hazards, fewest potential impacts to interested and affected parties and the least risks. This assessment specifically targets the avoidance (or minimized interaction) of the following features or conditions:
shipping channels
dredged areas
dumping grounds (active or historic)
known fishing grounds
hang areas (shipwrecks)
areas with potential for unexploded ordnances
existing and planned seabed structures
areas of shoals or ledges
strong currents
protected areas of environmental of cultural importance
unsuitable areas (steep slopes, boulder fields)
Based on these preliminary results, the developer will commission hydrographic surveys to more closely investigate corridor suitability and help complete detailed Cable Burial Risk and Feasibility Assessments.
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This assessment determines the minimum recommended burial depth (Depth of Lowering, DOL) using a standardized risk-based methodology. The outcomes are:
A recommended minimum DOL at each point along the cable route.
A target DOL: a contractor aims to hit this depth to allow some margin of error (ex. backfilling before cable is lowered).
A trench depth: this is how deep the burial tool has to go to achieve the target DOL. The tool may be specified based on this depth.
The Target DOL must be realistic and optimized to achieve the following objectives:
Mitigate threat to the cable from natural processes and external aggression (fishing, mobile sediment)
Reduce potential environmental and social impacts of exposed cables.
Allow for a wide array of tools during cable installation.
Ensure that the cable is not over-buried which could reduce the power carrying capacity.
Ensure access to the cable for maintenance.
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The composition of the seabed is the subject of a key dataset required for cable route planning. Early planning is based on publicly available data or data obtained by other seabed users. However, once a preliminary route has been identified, high resolution geotechnical (composition feature, such as soil descriptions) and geophysical surveying (structural features) is conducted for site-specific data required for permitting and design.
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The commercial and recreational fishing industries are a major consideration in the planning of cable routes. Engagement early in the process is key to effectively mitigating fishing impacts.
There are two primary considerations for cable route planning with respect to fishing interest:
Identify and avoid heavily fished grounds during the beginning of the planning process.
Develop appropriate mitigation focused on gear-types used and seabed composition, such as penetration depth of scallop dredges.
Cable Permitting
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Federal Permitting
While the Construction and Operations Plan, submitted to BOEM, is the main permitting document required for cable installation, there are guidance and regulatory requirements from a variety of other sources, including the U.S. Army Corps of Engineers (USACE), the International Cable Protection Committee (ICPC), the North American Submarine Cable Association (NASCA), the Carbon Trust, the Bureau of Safety and Environmental Enforcement (BSEE), and the American Wind Energy Association (AWEA), among others.
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State Permitting
State requirements regarding cable spacing and burial depth vary. New Jersey, for example, has a minimum burial depth requirement in waters of 5 ft (1.5 m), while New York does not have an explicit minimum burial depth requirement, but its agencies are charged with protecting water dependent uses. In Rhode Island, state agencies, such as the Coastal Resources Management Council and the Energy Siting Board, have jurisdiction over cable activities in state waters.
One of the major concerns about OWE development is the potential impacts of cables on wildlife and our marine ecosystems.
Types of Cables
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Array Cable
Inter-array cables connect the individual wind turbines in an offshore wind farm to a central point, such as a substation or offshore platform. They are typically made of copper or aluminum conductor, and are surrounded by insulation and an outer sheath. Inter-array cables may be used for both HVAC and HVDC systems.
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Export Cables
Export cables are used to transport the power generated by an offshore wind farm to an onshore substation. They are typically larger in diameter and capacity than inter-array cables and can be either HVAC or HVDC depending on the distance and power capacity of the wind farm.
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HVDC
HVDC (high-voltage direct current) cables are used for longer distances and higher capacity wind farms. They have a lower transmission loss than HVAC cables and can carry larger amounts of power over greater distances. HVDC cables typically consist of a conductor made of aluminum or copper, and are surrounded by insulation and a metallic shield.
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HVAC
HVAC (high-voltage alternating current) cables are typically used for shorter distance and lower capacity wind farms. They are made up of a copper or aluminum conductor, and are surrounded by insulation and an outer sheath. The maximum distance for HVAC cables is generally around 80 km.
Methods of cable installation vary based on the type of cable (Array vs Export), type of permitted installation (simultaneous lay and burial vs pre/post lay), and conditions of the seabed (sediment type, geospatial conditions).
Cable Installation
Once the offshore wind transmission cables make landfall, they are brought ashore via horizontal directional drilling, where they are connected to onshore cables via a transition joint bay. Once the cables are connected, they run underground to an onshore substation where the power generated offshore is connected to the local transmission grid.
Onshore Transmission
The Joint State Innovation Partnership
In response to the DOE Grid Innovation Program, Connecticut, Maine, Massachusetts and Rhode Island, with support from New Hampshire and Vermont, have proactively come to gather to plan, identify, and select an initial portfolio of one or more high voltage (HVDC) transmutation lines, and associated onshore system upgrades. The goal is to facilitate offshore wind development, improve grid reliability and resilience, foster regional innovation, and advance diversity, equity and inclusion while investing in job growth and quality.
Resources about offshore wind transmission.