Operating conditions for offshore wind turbines are both a blessing and a curse. Stronger, steadier winds mean higher and more constant electricity output than can be achieved onshore. But maritime conditions throw up their own problems.
The search for economies of scale is driving a trend to develop larger machines with longer blades that behave very differently from their shorter counterparts. Stronger winds at sea mean higher loads on the turbines, and the effect of waves hitting turbine towers is also recognised as an area that calls for urgent attention.
Wind turbulence within the wind farm, which affects turbine efficiency, combines with salinity and high moisture in the air to make corrosion a threat to towers, machine house walls and blades. Electrical contacts also feature on the long list of elements that need adapting to the offshore environment.
Experts are often divided on the best approach. One contentious area of offshore machine design is the location of transformer and converter in the turbine. "From a structural point of view, it is not desirable to have these heavy components at the tower top. However, there are electrical arguments for not locating them at the tower base," says Ed Norton of consultants GL Garrad Hassan’s turbine engineering department.
Offshore blade design, too, is changing fast. In the past, blades were virtually rigid structures. But today’s rotor diameter sizes in excess of 150 metres could reach 240 metres for the 10MW+ offshore turbines envisioned for the future. "These blades are far more flexible than originally thought, to the extent that they can even touch the tower and therefore require a new aero-elastic design," says Haico van der Heijden, business development manager at the wind energy unit of Energy Centre Netherlands (ECN). He says the challenge is to tackle these aspects early in the design process, and to assess how the technical problems created can be mitigated by electronic control of turbine operation.
Another grey area is blade tip speed and its implications for noise constraint, according to Norton. High blade tip speed makes rotors more efficient but increases noise levels, he explains.
Installation presents considerable challenges. If large rotors could be installed blade-by-blade, this would simplify the issue — not least because individual blades can be stacked far more efficiently on an installation vessel. "However, there are difficult hurdles to overcome relating to the huge imbalanced mass of a part-constructed rotor in terms of the extreme torque this imposes on the drive train," Norton adds.
The impact of waves has until recently been largely neglected in offshore turbine design. Most research and development work has focused on wind loads on the turbine. But waves hitting the tower can cause significant extra loads on the nacelle. This spring, ECN and fellow Dutch company Marin began a one-year project to investigate this phenomenon, especially for turbines installed near shore. "Waves tend to get bigger where the water depth changes quickly, which is often closer to land," observes van der Heijden.
There is growing awareness that the spatial arrangement of turbines in offshore wind farms also affects their ability to withstand maritime wind conditions. For ease of maintenance, offshore turbines are usually tidily installed in lines. Although the first line gets the full blast from the dominant wind direction, failures tend to occur more often in the second row. "This is not fully understood but is connected with the wind turbulence caused by the first row of turbines," says van der Heijden.
In a bid to influence these wake flows, ECN is now experimenting with altering control of the yaw — which keeps the turbine pointed into the wind — and pitch of the blades into or out of the wind. Such changes may reduce the efficiency of turbines in the first row but steer the wake to cause less vibration in row-two turbines, creating an overall positive effect. According to van der Heijden, optimising the whole wind farm in this way can increase total efficiency by 1–2% — a significant amount in this sector — while at the same time reducing the load on all turbines.
After working on wake theory for several years, ECN is testing its findings in practice, initially on a row of five Nordex 2MW turbines onshore at the ECN Wieringerwerf test field. Experiments will follow later this year in a scaled wind farm comprising ten 10kW machines surrounded by a raft of measurement equipment. "The landscape is flat enough to mimic conditions at sea sufficiently for the research work," says van der Heijden.
Anti-corrosion technology for offshore foundation structures is a key area being tackled under the three-year Kowind programme launched in May 2012 by German research institute Fraunhofer IWES and partners, including chemical company Evonik and offshore foundation manufacturing company Weserwind.
Corrosion protection is currently not designed to last for the expected 20- to 25-year lifetime of the turbines. Companies are developing a thick film coating to lengthen the lifetime of foundation structures and reduce manufacturing and maintenance costs.
During transport, mechanical loads can damage the corrosion protection, meaning that treatment is needed onsite. "This kind of work is difficult and costly because it has to be carried out at sea," points out Hanno Schnars, head of the Kowind project. Conventional coating with epoxy resins is time consuming, he adds, as it requires several layers, each having to harden first.
In future, steel foundation components are to be coated with thermoplastic, which should provide a maintenance-free protection layer for more than 25 years. According to Schnars, Kowind aims significantly to lengthen inspection intervals, cut manufacturing costs for foundation structures and bring down operation costs by reducing maintenance.
Improving offshore wind turbine support structures is important "to improve technical availability, reliability and economy of offshore wind", according to Fraunhofer IWES. A new test centre, expected by 2014, will examine various foundation structures and their components. A "geotechnical test pit" will be built to investigate, among other things, the load-bearing and deformation behaviour of a single foundation pile or group of piles. Interaction between structure and soils saturated with water will also be assessed.
The optimisation potential for offshore turbines is enormous, ranging from individual components to the complete nacelle, from software to foundation structures, and extending to project management of vessels, installation and maintenance teams. The supply chain, and timing of manufacture and delivery, also has a role.
But perhaps the most important area is the difficulties of accessing remote offshore sites. It may prove desirable for maintenance and component replacement to be carried out by means other than large jack-up vessels taking to the sea, says GL Garrad Hassan’s Norton. "Ideally, we would have smaller crane systems not reliant on the largest jack-up vessels, together with a sufficiently open architecture of the wind turbine structure to facilitate swapping of components."
As offshore wind energy climbs its steep learning curve, it remains unclear where the main weak points lie. Currently, primary problems are damage to gear systems and generators, and faults with pitch and control systems, according to Fraunhofer IWES. But generalisations are not yet possible, the institute warns, "because reliable conclusions on offshore turbine reliability are only possible after many years of operation".