Floating to the Future: The Rise of Offshore Wind

Floating wind turbines are, on a basic level, exactly what they sound like. As opposed to fixed-bottom turbines, which rely on contact between the seabed and a foundation structure, floating turbines rely on waterborne platforms of different types to remain stable. They are typically tethered to the seabed with cables which ensure stability while also allowing them to move with the swells of the sea to prevent damage. Like fixed-bottom turbines, the current they generate travels through subsea cables back to the grid.

The primary benefit of this technology lies in the power generation potential of ocean winds. Offshore wind speeds are consistently much higher than onshore, where they face more friction from natural and artificial obstructions such as mountains, trees, and buildings. While technically viable offshore turbine sites have a yearly average wind speed threshold of 7 m/s (at a 100-metre elevation), we still find that ocean winds in key project areas regularly exceed this amount, climbing as high as 11 m/s.  

Stronger winds mean more rotational force, which allows offshore turbines to generate electricity more efficiently than onshore turbines relative to their size. But this benefit is true of offshore wind technology generally, not just floaters. So, why choose them when building a project?  

According to analysis carried out by the Energy Sector Management Assistance Program (ESMAP), 115 countries share a technically extractable offshore wind resource of just over 71,000 GW. Various regions across the continents have pushed hard to seize on this potential, including but not limited to Scandinavia, the UK, USA, and Taiwan, all of which continue to pursue ambitious offshore wind capacity goals despite ongoing regulatory and supply chain challenges.

However, only around 20,000 GW of that total potential is suited to fixed-bottom turbines, which typically are only viable in water up to 50 metres deep. In developing a region’s offshore wind industry, poor seabed conditions and water depth are the biggest hurdles to overcome. This is exemplified by the cases of California, which has a seabed consisting of  a complex arrangement of highly turbulent water masses, and Japan, which is at a geographical disadvantage due to its coastal water depth featuring sheer drops immediately offshore and frequent earthquakes.  

This is why the rest of the world’s untapped wind resource will need to be captured with floating offshore wind turbines. From a materials perspective, they are limited by the length and strength of tethers rather than enormous monopiles, jacket, and tripod structures, opening the door to deepwater wind farms situated past water depths of 50 metres: even down to 200 metres or beyond. 

Floating wind foundations are the key differentiator between turbine platform types, with a variety of designs built around two central stabilising mechanisms: ballast and buoyancy. Reliance on buoyancy is of course essential for all floaters, as the upward pressure exerted by the water on the immersed structure must be enough to keep it from sinking, but the way that stability is ultimately achieved is where these mechanisms differ.

In buoyancy systems, reserve buoyancy, which is the excess of buoyant force over the weight of the turbine and foundation, serves to keep the tethers under tension. Typically, the floating structure itself does not contain permanent ballast and is designed to be light and use less steel. Tension Leg Platforms (TLPs), for example, are highly stable, but their vertical and rotational stiffness under large dynamic loads can lead to tether damage.

Ballast is the weight added to the structure to increase its stability or modify its position. By placing significant amounts of ballast at the base of the floating platform, engineers can adjust the turbine’s centre of gravity to a lower level relative to the centre of buoyancy. This carefully planned distribution of weight enhances the structure's stability and reduces the chance of it overly pitching or rolling in the water. Semi-submersible and especially spar-buoy platforms extend more deeply into the water and use ballasts to resist wave and wind forces, and are generally lower cost than TLP, but semi-submersibles are the cheaper and more popular option as they can work in any water depth beyond 40 metres.