The term landfall refers to the point at which the cables carrying power from an offshore wind farm reach the shore. This is where the offshore and onshore infrastructure is connected – an important step in bringing renewable wind energy into the power grid. As we make landfall, we continue to consider the local environment to avoid disturbing any sensitive cultural and historical sites.
To make the cable sea-to-shore connection, we use either open cut trenching or horizontal directional drilling (HDD) to lay the export cable under the seabed and beach area. HDD can extend anywhere from half a mile to a mile out at sea.
Step 1: From sea to shore using HDD
The first stage of making landfall is for a cable to run from the seafloor to a location a short distance inland. To put this cable in place, a hole is drilled using a technique called horizontal directional drilling.
The hole starts in a small pit behind the dunes or a beach, and is then bored using a drill rig machine. Drilling can be measured and controlled precisely, keeping it well below the surface until the drill head emerges from the seabed half a mile to a mile out to sea.
The drill head is then pulled back through the hole, bringing with it a pipe from a cable installation vessel. After this, a cable can be fed through the pipe, establishing a safe path for the wind energy to be brought ashore.
Step 2: The transition joint bay
The site of the drilling pit becomes the home of the transition joint bay. This is an underground concrete box where the cable from under the beach joins a cable leading to the onshore substation further inland.
The transition joint bay is usually constructed in a parking lot, both for ease of access, and so as not to cause any damage to the natural environment.
Since it’s housed underground, the transition joint bay is basically invisible once constructed, except for some manhole covers in case access is needed in future. We always return the parking lot to the same or better condition than we found it in, so any disruption during construction is only temporary.
Step 3: Underground transmission to the substation
The onshore substation transforms power generated by an offshore wind farm to the correct voltage before delivering it to the local grid, after which it can be sent to thousands of households and businesses.
Before building an onshore substation, we undertake extensive environmental, technical and feasibility surveys to determine the best location to build – while keeping environmental disturbance to an absolute minimum. This also involves evaluating all options to see if we can use or repurpose existing infrastructure in the area before starting from scratch.
With regards to underground transmission, the onshore substation is linked to the transition joint bay (in step 2) by a transition line (containing piping and cables).
Depending on the location of the substation the line could be several miles long. And, as with other utility pipes and cables, it tends to run lengthways underneath a road.
This means digging a trench for the pipes, encasing it in concrete, then pulling the cables through the pipes. The road will then be returned to its original state or better, with the only evidence of the transition line being the addition of manhole covers.
During construction of the line we also work with highway authorities to minimize disruption to traffic, residents and businesses, as well as to reduce dust, mud and debris.
It should also be noted that we pause onshore construction during local tourism seasons so as to avoid impacting regional economies.
Step 4: Connecting to the grid at the substation
The final stage in getting renewable power from the offshore wind farm to the distribution grid is the substation. This is where the physical connection is located, and where the current is converted to the right voltage and frequency to be fed into the grid.
The substation is the only above-ground piece of new infrastructure related to landfall, so its location must be chosen very carefully to minimize any impact on nature and on those who own and use the land. To help make this decision, we undertake extensive environmental, technical and feasibility surveys, and ongoing consultations with landowners, statutory bodies and local communities. We also undertake surveys after construction to identify any noise impacts and then take necessary measures to limit them.
Substations can vary significantly in type, size and layout. Typically, a substation site will occupy an area of six to nine acres, consisting of between two and eight buildings, up to 50 feet in height. The site includes associated roads, fences and landscaping, and may include an open yard as well as buildings.