Floating platforms are a big deal for offshore wind, offering the potential to vastly expand the scope of the deeper ocean waters that will be suitable to host power-generating turbines. The National Renewable Energy Laboratory estimates that the U.S. could install up to 2 terawatts of floating wind capacity — that's more than 20 times the total amount of nuclear power installed in the country today. California is eyeing the potential for up to 10 gigawatts of floating wind in its deep coastal waters.
In addition to allowing wind farms to be built in deep waters, another benefit being promoted by the floating wind industry is the potential for cheaper operations and maintenance compared to fixed-bottom wind turbines.
The potential for cheaper repairs
The secret to that lower cost? Floating wind platforms can — theoretically, at least — be towed back to port for major maintenance and repairs, instead of forcing technicians to carry out that work on the high seas.
In practice, however, the jury is still out on how floating platforms will affect offshore wind maintenance costs. In fact, they could very well make the whole process more costly, according to a report last month by the U.K. Carbon Trust’s Floating Wind Joint Industry Project.
Rapid growth is planned for this sector, despite only one commercially operating project to date, the 30-megawatt Hywind Scotland wind farm. About 126 megawatts are scheduled to be online by the end of 2021, including the largest site to date at Kincardine, Scotland. The pipeline for demonstration projects has also expanded beyond Europe, with projects planned in Japan and the U.S.
But the report also highlights “significant technical challenges to be overcome before achieving large-scale deployment.” Solving these challenges will be the key to driving down the costs of floating wind, enabling it to compete with fixed-bottom offshore wind turbines that have already been deployed at multi-gigawatt scale in Europe and are planned in the 30 GW scale off the East Coast of the U.S. in the next 15 years.
For some types of maintenance, however, the floating turbines will need to be towed into port. This is an area in which floating wind needs to prove that the cost advantages can materialize in real-world operations. “One of the main challenges of this process is the safe detachment and storage of wet cables and mooring connections,” the report states.
The feasibility of towing floating foundations to port depends on the design of the structure. Hywind Scotland, for example, features a spar buoy design that is basically a ballasted steel cylinder extending around 260 feet into the water.
Depth makes a difference
The significant depth of water that is required to accommodate these cylinders “creates major logistical challenges for port-side maintenance,” the report finds — only a few ports in the world that have water that deep. In fact, these types of platforms are normally laid out in a horizontal position for towing from port to ocean site, and only then upended and stabilized in their final vertical position, according to the report.
Another format called the tension leg platform acts like a buoy that is held in place under the surface of the ocean thanks to taut mooring cables attached to the seabed. Tension leg platform developers include Grossmann Ingenieur Consult, Eco TLP and TLP Wind The report notes that releasing these platforms from their moorings can result in major stability challenges, “particularly during the towing operation,” when these relatively buoyant and top-heavy pieces of equipment run the risk of toppling over.
According to the Carbon Trust report, the best candidates for tow-to-port maintenance are another class of foundations, called semi-submersibles, that work on the same design principle as icebergs. Although most of the mass of the foundation is underwater, semi-subs can have relatively shallow drafts and thus can be moved around easily. Developers using the semi-submersible concept include Principle Power, Nautilus and the University of Maine.
“At this stage…other substructure types were thought to have stability issues during towing and may require further innovation to be commercially viable,” the report states.
Dockside repairs vs. towing complications
The idea of towing turbines to shore for maintenance is attractive because it would allow maintenance teams to take advantage of quayside assets such as cranes.
Many semi-submersible floating platform developers see this as a plus for maintenance. Danish floating platform developer Floating Power Plant, for instance, plans to bring turbines back to shore for major maintenance tasks, such as changing turbine blades and nacelles.
The company's “approach is to have a single disconnectable mooring point,” said CEO Anders Køhler in an email. “In this way, the platform can be disconnected and towed via cheap tugs to quayside or [a] protected area and onshore cranes can be used [for maintenance].”
To facilitate routine maintenance, Floating Power Plant’s platform design includes a crew boat landing bay where wave energy absorbers cut swells by up to 70 percent, almost doubling the amount of time during which workers can access the turbine compared to fixed-bottom offshore wind designs, he added.
The Carbon Trust’s research suggests that these kinds of on-site maintenance innovations might have greater value than tow-to-port capabilities. Keeping floating platforms stable while traveling from a wind farm to port and back is a particular challenge for tension leg platform designs, such as those being commercialized by Dutch firm Bluewater and Modec of Japan.
Just because it can move doesn’t mean it should
“In general, a floating unit performs best when it is connected to its mooring system,” said Christopher Cowland, vice president of global growth in the offshore wind division of the industrial engineering company Worley, in an email. “Does the elevated risk of a towing operation justify the benefit of maintenance at the quayside?”
A 2019 NREL study estimated annual operational expenditure costs for a Pacific Coast floating turbine would be around $130 per kilowatt of installed capacity, compared to $124 per kilowatt for a fixed-bottom unit in the North Atlantic.
The higher operating cost of floaters is mainly the result of the small size of the projects in the water today, said Magnus Ebbesen, business lead for floating wind advisory in DNV’s energy systems division, in an email.
But even once the industry matures, “we expect floating wind to have around 10 percent higher [operational costs] than bottom-fixed wind, due to additional inspection and maintenance of the floater and mooring system” that will be necessary, he said.
Furthermore, he said, floating wind’s likely location far from shore would keep operational costs higher than for fixed-bottom wind farms. In fixed-bottom offshore wind, major maintenance work is usually carried out using jack-up vessels. These massive ships are equipped with robust stilts that can be planted on the seabed and used to jack the vessel out of the water so it remains stable even in rough seas.
With jack-up vessels operating routinely in major offshore energy markets around the world, “I don’t think that floating wind has much of an upside with regards to [maintenance costs],” Ebbesen said.
“That the floater can be pulled to shore for major component replacement could potentially be a benefit in some markets. However, it is a complex operation compared to utilizing a jack-up, which is done very frequently in many markets these days."