Mapping for a renewable future

With the climate crisis continuing, nations are increasingly interested in developing clean, renewable energy. Among those energy solutions – wind.

Historically, wind energy has come from turbines built on land, but over the last decade, offshore wind – wind farms in the sea – has started to grow. “Offshore wind opens up a lot of opportunity to develop renewable energy,” says Neil Farrington, strategic offshore development manager with Celtic Sea Power, which leads the strategic development of floating offshore wind in the Celtic Sea. Indeed, 2022 saw the second-largest number of new offshore wind installations pouring energy into the grid, with growth expected to continue.

Selecting a location for an offshore wind farm requires consideration of many factors, such as the environment. The wind, including its average speed and consistency, is undoubtedly critical, but equally important are the conditions in the ocean – and the seabed.

“The [seabed] is massively important for us,” says Farrington. “It tells us which technological solutions we will need, which means we can understand the wider infrastructure and economic development we’re going to need. For example, what anchoring types are we going to use [to fix the turbines to the seabed]? Once we determine that, we can understand what port infrastructure we need. What manufacturing capabilities we have to build to make sure there's capacity to support the industry.”

A question of depth

Like any other structure built in the ocean, offshore wind turbines are attached to the seabed. Currently, offshore wind turbines are usually fixed to the seabed with a monopile or a steel jacket foundation drilled into the seabed with hydraulic hammers. The turbine is then bolted onto the foundation. Although stable, these structures are currently limited to waters up to about sixty metres in depth.

Depth has been a limiting factor for the placement of offshore wind farms, but that may soon change thanks to the development of floating offshore wind. “The platform that the turbine sits on floats on the sea surface, which is tethered to the seabed attached to a series of piles or anchors,” explains marine geologist Dr Ian Selby. “The big advantage is that you can go into deeper and deeper waters – maybe hundreds of metres deep. Given that a lot of the continental shelves around the world are deeper than sixty metres, the technology is going to be really important for a lot of countries.”

Among other roles, Selby is director of Morwind, an energy development company focusing on the area of the Celtic Sea that spreads out from Cornwall and Wales westwards towards the continental shelf. Here, depths rapidly reach 100 metres and more. Harnessing the wind in this area will invariably require a floating wind approach.

What lies beneath

While depth plays a role in the choice of technology, the physical characteristics of the seabed dictate how both fixed-bottom and floating turbines are secured in place. “Understanding the characteristics of the seabed – the sediments, the grain sizes, the vertical profile, and how these characteristics change with depth- is critical to understanding our options,” explains Farrington.

While detailed maps of the seabed depth and morphology – its configuration and processes -  can be established using technologies such as multibeam echosounder and side-scan sonar, developers require other technologies to obtain the nitty-gritty details like the composition and properties of the seabed. For example, “Vibracoring or drilling lets you recover samples of the seabed itself. Tubes recover seabed sediment and bedrock samples. For geotechnical testing, you can use a cone penetration test (CPT), a cone that you can push into unconsolidated, softer seabed. That gives you an idea of the physical properties of the seabed, for example, the bearing capacity [how much load the sediment can support]. You can also take a grab sample to collect the seabed surface sediments,” says Selby.

As a general rule, thick layers of soft marine sediments present issues for fixed-bottom turbines. Sometimes, the sediment may be too unstable to support a turbine, but specially designed foundations may make development feasible in other situations. On the other hand, bedrock and more consolidated sediments may present challenges to driving- “piles” – the large steel or concrete foundations that the turbines sit on – into the seabed.

“Anchoring system technologies for floating turbines is still evolving,” says Selby. Engineers and researchers are exploring many different systems. These include drag-embedded anchors, which are dropped onto soft sediment and then dragged so they become embedded in the sediment; suction anchors, which create a vacuum that holds the anchor in place; driven piles, which operate similarly to fixed-bottom piles; and JAVELIN anchors, which are locked inside of purpose-created boreholes.

Flat is easier

Whether building a fixed-bottom or floating wind farm in the ocean, the nature of the seabed and its morphology also need to be considered. In the Celtic Sea, for example, incisions five kilometres wide, 20 kilometres long, ridges up to kilometres wide, 300 kilometres long and up to 60 metres high, and submerged granite outcrops, such as Haig Fras, are just some of the features found in the region.

One of the components of any offshore wind farm influenced by seabed morphology relates to the cable infrastructure. “There are cables linking the various turbines and the substation, then cables linking to the shore,” Farrington explains. Developers generally like to see cables “taking the shortest and most direct route possible,” says Dr Michael Clare, leader of Marine Geosystems at the National Oceanography Centre in England. “[They] really like to see…a flat, boring seabed. They don’t like irregular terrains…where cables could get snagged.”

Morphology can also suggest other hazards that developers may wish to avoid. “Submarine canyons are deep incised valleys that are huge. The Monterey Canyon in California [USA] is one example. It is similar in [width] to the Grand Canyon, and even has meandering bends that wiggle down from coastal waters into the deep sea,” says Clare. Such canyons often host myriad marine life and sensitive habitats that developers wish to avoid, but from a hazard perspective, they “are also the focus of avalanches of sand and mud that we call turbidity currents that can move at metres per second and break any cables in their path.”

Dealing with a shifting seabed

Mudflows aren’t the only seabed changes wind farm developers need to consider. For example, “there’s a large area off the south coast of Wales with constantly moving sand, which introduces challenges,” says Farrington. All types of seabed experience the force of the ocean – currents, tides, and waves, but sandy sediments are particularly prone to being moved around. This movement can result in small ripples on the seabed, megaripples, which are larger and often very mobile, sand waves, which can reach several metres in height and several metres to kilometres in length, and finally, sand banks.

Small ripples are usually nothing to be concerned about, but megaripples and sand waves warrant consideration. “Sandwaves can move across the seabed at tens of metres a year, so one year you have a cable and other infrastructure exposed on the seabed, the next year, it might be under several metres of sand, and the year after, be exposed again,” says Selby.

Shifts in sandy sediments can present another problem for offshore wind. Scour. “When the currents flow around structures on the seabed, increased currents may scour out the seabed around it. This exposes the anchoring system for floating systems or the foundations for fixed-bottom structures, which can affect their stability,” says Selby. “If the area you’re operating in is sensitive to scouring, it is very important to understand sea processes and design your attachment system and create monitoring and mitigation plans that ensure the safety, security, and longevity of the installation.”

Protecting nature, heritage, and people 

Seabed mapping is essential for deciding many technical aspects of offshore wind, but it can also provide information about the environment and habitats - places turbines, cables, or other associated infrastructure should avoid. These can include habitats such as seagrass meadows and coral reefs that could be easily damaged or destroyed. Then there are the heritage aspects – wrecks that may have historical significance and should be protected – and may also pose a hazard to seafarers. Equally, “there are some areas of seabed where there is a lot of ordnance because if it fell onto soft mud, then it may not have exploded,” says Selby.

With many areas of the seabed poorly mapped, it is not uncommon for such aspects – as well as the details about the seabed itself – to be discovered by the various surveys done for offshore wind development. “The seabed is absolutely critical to the economic and environmental futures of nations. It’s a strategic asset, so collecting information shouldn’t just be about one project,” says Selby. Farrington concurs. “We have marine conservation areas being designated. We have loads of blue carbon projects in the works. They would all benefit from [seabed] information.”


This story was written for Seabed 2030.