Offshore turbine design maximises cost reduction

Next year, one company plans to complete an installation at an offshore wind project in northern Denmark that will demonstrate a novel hybrid substructure that could drastically reduce the costs associated with foundations and their installation — typically 20-30% of a project.

Although wind turbine foundations — the substructures that link tower to earth — come in a variety of designs, a handful have dominated the market for offshore wind installations, and a much wider array of concepts are under study or in demonstration.

According to the U.S. Department of Energy National Renewable Energy Laboratory's (NREL) 2014-2015 report on offshore wind technologies, three foundation structures have captured the majority of the offshore wind market — monopiles, jackets and gravity bases. NREL reports that monopiles have been the dominant substructure, with 6.746 MW installed by June 2015, at 75% market share. Jackets held 10% and gravity bases 8%.

The monopile is a steel tube that is driven into the seabed and to which the tower is attached. Jacket foundations have three or four smaller-diameter poles that are connected with lattice work and attached to small piles that are driven into the seabed. A gravity-based foundation is typically made out of concrete and buried in the seabed.

According to NREL, monopiles are expected to continue their market dominance, but jackets could gain market share based on their suitability to deeper water sites. While monopiles were originally preferred for shallower sites, improvements in their design have pushed their application beyond original expectations.

According to NREL, the deepest application of monopile foundations to date will be at Veja Mate — a 402 MW project in the German North Sea that will use 6 MW Siemens wind turbines in 40 meter depths. Construction on Veja Mate began in April and is expected to be completed by the end of next year.

At the Nissum Bredning Vind project in northern Denmark, Siemens will test a new foundation design intended for deep-water applications and that the company expects will cut foundation costs significantly.

Nissum Bredning Vindmøllelaug and developer Jysk Energi placed the winning bid in the Danish Energy Agency's tender for construction of the 28-MW pilot project in northern Denmark. Siemens will supply its 7-MW direct drive wind turbine for the project, which is situated just inside Nissum Bay on the east coast of Denmark.

Jesper Møller, head of offshore concepts and solutions, Siemens Wind Power and Renewables Division, describes the new foundation concept that Siemens will use for the project as a hybrid foundation that brings together the benefits of gravity foundations and jacket foundations — what they are calling a gravity-jacket foundation.

Siemens has been looking for a way to take advantage of the cost-effectiveness of that foundation for offshore wind projects. The benefits are highlighted when using a bucket foundation interface — or suction caisson, an anchor system for the turbine that resembles an overturned bucket that is placed in the seabed. Water is suctioned out of the bucket to decrease the pressure and sink the bucket into the seabed.

According to Møller, there is an overall benefit to having the heavy foundation on top of the bucket so it can start the suction process. In designing the hybrid foundation, Siemens looked at the price of steel and concrete.

"There aren't that many different jacket systems for offshore wind turbines yet — although we expect that will increase — but if you compare the cost for one of these relatively complex steel transition pieces (TP) with the cost of the concrete TP that we have designed so far, we see that the concrete TP is roughly half the price of a steel TP, even at realistically low steel prices," Møller said.

The findings of that cost-comparison were "an eye opener," he said. Siemens' goal is to reduce the levelised cost of energy (LCOE) for offshore wind projects within the six years from 2014 to 2020 by 35%.

"Wind turbines count for 27% of the LCOE and contributed well to cost reduction, but we know what we need to do to get the last 10% with the other levers, and foundations' share in LCOE is still 18%" Møller said. "We believe 40% cost reduction is absolutely reasonable on the foundation, including installation."

Møller noted that the Nissum Bredning site is not optimum for the test of these new foundations because it is a very shallow site — between 2 and 6 meters of water. Jacket foundations are most suited to deeper waters.

According to Møller, Siemens believes that monopiles are most cost-effective for offshore projects up to a certain water depth — though there is no fixed depth where one system is better than the other because it depends on the environment, such as wave height, wind, and soil conditions.

"We expect that larger turbines, maybe much larger than the ones we have today, will have much better cost-out potential if they are on jackets in the future; we have learned that this concept seems to scale very well," he said. "So that's why we are doing what we can to learn as much as possible on jackets even though it's a shallow site."

Siemens expects to begin turbine installation next summer, and commissioning is scheduled for 3Q17. The North Sea is one of the most active offshore wind power development regions in the world, and it could become even more active.

This year, the nine North Sea countries signed a declaration that seeks to facilitate further cost-effective deployment of offshore renewable energy — specifically wind. Here's a look at the status of five offshore wind projects in the North Sea.