Six new ways to float skyscraper-sized wind turbines

By means of Emma C. Edwards, University of Oxford

Yes, you read that right – float. You’ve probably seen an offshore wind turbine before, but you’re probably looking at a “fixed” turbine – that is, one that sits atop a foundation drilled into the seabed. For the new frontier of offshore wind power, the focus is on floating wind turbines. In this case, the turbines are supported by floating structures that swell and sway in response to waves and wind and are attached to chains and anchored to the seabed.

It has become central to the sector for the simple reason that most of the wind blows over deep water, where building fixed platforms would be too expensive or impossible. Designing these new floating platforms is a real engineering challenge, and is my focus academician RESEARCH REVEALS.

These wind turbines are huge, reaching up to 240m in height – about the size of a skyscraper. Because they are so tall, strong winds far above the sea tend to make the turbine want to tilt, so platform designs focus on minimizing this tilt while still being cost competitive with other forms of energy. .

There are more than 100 ideas for platform designs, but we can group them mostly into the following six categories:

1. Spar

Spars are narrow, deep platforms with weight added to the bottom to counteract wind forces (this is called “ballast”). It is usually easy to do because it usually has only one cylinder.

However, they can extend 100 meters or more underwater, which means they cannot be deployed in normal ports that are not deep enough. Specialist installation procedures are required to install the turbine when the platform is towed in deep water.

2. Barge

Barges are wide, shallow platforms that use buoyancy away from the center of the structure to counteract the wind forces on the tower. Since they usually extend less than 10 meters underwater, they do not require any specialized deep-water docks or installation containers.

However, this can be difficult to do because the platform is usually a single, large unit with a complex shape.

3. Tension-leg platform

Tension-leg platforms, or TLPs, use tight mooring lines to connect the platform to the seabed and stop the turbine from tilting to the wind.

These platforms are usually smaller and lighter than other types, making them easier to fit into a standard dock. In addition, their “footprint” on the sea floor is small due to tight lines.

However, platforms are generally unstable until attached to their mooring lines, meaning a special towing and installation solution is required.

4. Semi-submersible

Semi-submersibles consist of three, four or five connected vertical cylinders, with the turbine in the middle or on top of one of the columns. The platform uses buoyancy off center (like a barge) and ballast at the base of each column (similar to a spar).

Like barges, semi-submersibles do not require special tow-out equipment and work for a wide range of water depths. Manufacturing is another challenge.

5. Combination-type

The four categories above are the more “traditional” platforms, influenced by their predecessors in the oil and gas industry. Since the 1960s, floating platforms have meant that large oil rigs can access deeper areas of water (the deepest being over 2,000m). Most of these deepwater oil rigs are semi-submersibles, anchored to the seabed with chains, or TLPs, which are connected to the seabed with tight cables.

Recently, there is a trend of platforms that are more specialized in floating air. Specifically, some use a combination of stability mechanisms, taking advantage of each of the previous designs.

For example, “lowerable ballast” platforms are like traditional semi-submersible or barge platforms, but with weight hanging from spiky cables.

During the installation of the turbine in the port and tow-out, the weight is raised, so that a traditional (non-deep) port can be used and no special equipment is needed. At the installation site, the weight is lowered and the platform gets more stability from the lower center of mass.

Other designs take advantage of the stability benefits from tight mooring lines (similar to a TLP) but are designed to be stable during tow-out and therefore do not require a special container installation. For example, the image below shows the X1 Wind platform:

Rigid mooring lines are attached to a column, which is installed first. The rest of the platform, which is stable on its own, is then towed and connected to the pre-installed column with mooring lines. The platform uses the added stability from mooring lines but without the instability common to TLPs.

6. Hybrid platforms

These platforms add another type of renewable energy, usually a wave energy converter. This increases the total amount of energy generated, and reduces costs as power cables, maintenance and other infrastructure can be shared.

A wave energy converter also reduces the movement of the platform, which in turn increases the power performance from the turbine.

Room for improvement

Four floating offshore wind farms have already been built, the largest of which opens in 2023 off the coast of Norway. Two of these farms use the Hywind spar design and two use the The WindFloat is semi-submersible.

There are 18 other platform designs to reach sea trials, including at least one in each of the categories described above. Some have plans to build floating farms in the next few years, and more early-stage designs have plans to deploy their own prototype devices in the near future.

Interestingly, the platforms actually differ in design. After many years, wind turbines have mostly combined the three-bladed design you see today, but there is still no such meeting of a consensus “best” floating platform. This suggests that significant improvements are still possible, especially in terms of reduced mobility and reduced costs.

Don’t have time to read about climate change as much as you’d like?

Get a weekly roundup in your inbox instead. Every Wednesday, The Conversation’s environment editor writes Imagine, a short email that goes deeper than just one climate issue. Join the 30,000+ readers currently subscribed.

Emma C. EdwardsCareer Development Fellow in Engineering, University of Oxford

This article was reprinted from The Conversation under Creative Commons license. Read the original article.