How to get the best offshore wind turbine reliability

George Marsh

Part 3. Offshore turbines need to be more reliable than their onshore counterparts. How is this creating divergence in approach as the offshore turbine industry evolves?

About: part 3 of a multi-part article looking at the development of offshore wind turbines as companies look to different approaches to improve reliability.

Part 1 - Outages offshore are more critical than those onshore due to problems of access at sea, limited weather windows during which maintenance work can be carried out, and the sheer size of offshore machines...read more.

Part 2 - the drive for fewer components..read more.

Reduction vs. redundancy

Part count reduction is not always the best strategy because there are various trade-offs to consider. For instance, the higher reliability expected of direct drive must be traded off against increased generator diameter and weight, along with greater nacelle size and windage. Then again, the higher efficiency of PMGs has to be balanced against their need for full power converters.

An alternative strategy for augmenting reliability, in some ways diametrically opposed to part count reduction, is redundancy – meaning the duplication of critical systems or components so that single failures can be tolerated. This may be highly effective, especially when allied with remote monitoring so that any failure is registered. However, duplication inevitably adds to part count so, for this strategy to pay off, its benefit to reliability (strictly speaking availability since failures are tolerated rather than eliminated) must outweigh the adverse effects of having more components. Since the redundant half of any duplicated system is generally on light ‘standby’ duty most of the time and will be less subject to wear, this can indeed be so, but the issue must be decided at the initial design stage.

Finnish company WinWind builds high reliability into its 3 MW rated WinWinD 3 turbine by having two 1.5 MW converters running independently in parallel, which means that if one converter fails, the turbine will still be operational with the remaining unit. The converters’ cooling systems and auxiliaries are also independent and redundant.

A hybrid halfway house

A ‘halfway house’ between geared and gearless solutions, avoiding the extreme characteristics of both, is the hybrid system. A single stage of gearing (sometimes two) is combined with a generator having an intermediate number of poles with a rotor of commensurate diameter. As a result, the generator and gearbox are of approximately the same size, a dimensionally more balanced arrangement than the full no-gearbox solution. These elements can be housed in relatively compact nacelles. The simplified gearbox has fewer parts and should therefore be more reliable than the standard three-stage model. Should the need for replacement nevertheless arise, it is also lighter and more compact, and therefore easier to manage.

An early realisation of the hybrid concept was the Multibrid system. Multibrid, notable as a direct drive pioneer, held that low generator rotational speed and a reduced number of rotating parts are key to reliable and economical operation offshore. Integration of the rotor bearing, gear system and generator within the same housing led to a compact installation as well as short load paths. The company also used redundancy for filters and other critical components, plus IP54 environmental protection in its synchronous PMG-based turbine designs. Multibrid, now part of the French Areva Group, incorporated the technology in its M5000 5 MW offshore machine.

Another type of hybrid drive is the multiple generator system, as exemplified in Clipper Wind's 2.5 MW Liberty design. In this system, a single-stage gearbox drives four PMGs, thereby spreading the loading on the main drive gear and achieving a drivetrain that has considerable failure tolerance thanks to its four generators.

Conventional dominance

Despite the inherent advantages of direct drive and hybrid systems, it seems that the conventional turbine format will continue to dominate for some time to come, both on- and offshore.

Siemens’ wind energy competitor Vestas Wind Systems is cognisant of this and, more conservatively, has adopted conventional drive arrangements for its V165 6 MW offshore turbine now being developed. Vestas believes that required levels of offshore reliability can be achieved through careful detailed design of existing drive components, including gearboxes. Discussing the background to the new turbine, Finn Strøm Madsen, until recently President of Vestas Technology R&D, explains:

“We actually kept all options open from the start, running two separate parallel R&D development tracks, one focusing on direct drive and the other on a geared solution. It became clear that if we wanted to meet customer expectations about lowest cost of energy and high business case certainty, we needed an optimum combination of innovation and proven technology. So we chose to go for a medium-speed drivetrain solution. Offshore wind customers do not want new and untested solutions. They want reliability and certainty, and that is what the V164-7.0 MW gives them.”

A number of analysts agree with this point of view. Birger Madsen, Director of wind industry forecaster BTM Consult ApS, has said of direct drive:

“The technology is relatively unknown and there is the risk that something will surprise you. We have limited experience so far with direct drive machines offshore. While the technology eliminates gears that are a major cause of outages in current turbines, the novelty may be its main drawback.”

Nevertheless, many in the industry expect that, as experience offshore grows, the direct drive or hybrid approach will prove its economic worth. However, there are other technology contenders too.

Gearless gearing

One of these provides gearing without gears. Instead of being a number of fixed ratios, gearing can be varied continuously in a smooth progression if fluidic systems are adopted. DeWind, the US-based subsidiary of Daewoo Shipbuilding and Marine Engineering, has picked up on this having adopted the Voith variable speed transmission system, well established in marine and industrial applications.

A big advantage of DeWind's WindDrive system, which combines a fluidic torque converter with a planetary gear system, is that it does not need power conversion electronics. This is because the ‘gearbox’ output shaft can be set to any speed required and can therefore be used with a synchronous generator such that no electrical adjustment is necessary. Another plus for the fluidic system is that the fluid effectively decouples the input and output shafts from each other, thereby preventing transmission of vibration and torque spikes to the generator.

Axis change

An intriguing and radical possibility for offshore wind is that present horizontal axis wind turbines (HAWT) might not be the best answer and that ultimately, the vertical axis wind turbine (VAWT) might prevail. A leading proponent of VAWT technology is Theo Bird, who started the UK company Wind Power Ltd to revisit the concept. Bird argues that having a rotor that rotates parallel with the ground/sea surface avoids the reversal of gravitational load that occurs at every revolution of a conventional turbine rotor, creating fatigue in blades, bearings, shafts etc. VAWTs, he says, would therefore be more reliable and require less maintenance.

Further benefit should result from the ability of a Wind Power Aerogenerator to accommodate wind from any direction without the need for a yawing mechanism. Bird says that US wind researchers who worked on original vertical axis projects have always regarded the technology as great to work with at sea because it can be big, tough and easily managed.

Recently the UK Government-backed Energy Technologies Institute (ETI) investigated a conceptual VAWT as part of a drive to bring down the cost of offshore wind power from about double that of onshore wind currently, to parity by 2020. The universities of Sheffield, Cranfield and Strathclyde also took part, together with research organisation QinetiQ and others.

The proposed Nova (Novel Offshore Vertical Axis) machine features an innovative 100 m high V-shaped rotor rotating around a vertical axis located at low level along with all rotating machinery. The ETI's Grant Bourhill says a 5-10 MW Nova would be more reliable and require less maintenance than an equivalent modern turbine and would also be cheaper to build because it can be assembled from the base of the machine rather than at the top of a tower. There are hopes that a Nova demonstrator can be installed offshore by 2015 so that its economic and technical characteristics can be explored.

The question now is: Which of these technologies will dominate, if any?

About George Marsh: Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.

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Wind power  •  World Future Energy Summit