In order to face the more inclement conditions encountered offshore, the industry needs to take the reliability of offshore wind turbines to a new level.

Wind turbine system reliability is considered to be a vital component in any wind energy project as the undesired level of the same can directly affect both the project’s revenue stream through increased operation and maintenance (O&M) costs and reduce availability to generate power due to turbine downtime.

According to Sandia National Laboratories, the acceptance of wind-generated power by the financial and developer communities as a viable enterprise is influenced by the risk associated with the capital equipment reliability; increased risk, or at least the perception of increased risk, is generally accompanied by increased financing fees or interest rates.

According to Roger Hill, Principal Member off the Technical Staff, Wind Energy Technology Department, Sandia National Laboratories, reliability is the probability that a system or component will perform a prescribed function under stated conditions for a specified period of time.  

“However, in wind plants operation, what is most important is that the turbines and systems be available for energy capture and delivery to the maximum extent that is economically feasible.  Obviously, there are going to be failures of components that will cause turbine outages.  When this happens a two-fold penalty is incurred; energy production is lost and repairs and/or replacements must be made.  Revenue is lost and expenses are incurred.  The trick is to anticipate, budget, and marshal resources for the repairs so that they can be made as rapidly as possible so that return to service can be made,” said Hill, who is scheduled to speak during Wind Energy Operations and Maintenance Summit USA, to be held on April 1-2 in Dallas, Texas this year.

Designers need to look at reliability 

Offshore turbines have technical needs not required of onshore turbines due to the more demanding climatic environmental exposure offshore.

The reliability of large modern onshore wind turbines is improving but in order to face the more inclement conditions encountered offshore, where the wind energy harvest will be more substantial but where access is limited, it is necessary to increase reliability still further.

Overall, the industry acknowledges that designers need to look at Reliability  - reduce the number of repairs, Redundancy – reduce the number of turbine visits required, Maintainability – reduced frequency and facilitate shorter job times utilising automated systems and condition monitoring, Remote Functionality – increasing functionality of the SCADA system and turbine control system to reduce frequency of human intervention at site.

In an interview with www.windenergyupdate.com last year, Jonathan Duffy, offshore wind generation manager, Airtricity, had referred to reliability as the most critical factor in sustaining the efficiency of offshore wind turbines, which are bigger than onshore turbines, to take advantage of the steadier offshore winds and economies of scale.

In an analysis (the paper investigated the reliability of more than 6000 wind turbines in Denmark and Germany, ranging in size from 300-1800 kW, over 11 years and particularly the reliability of subassemblies in up to 650 wind turbines in Schleswig Holstein in Germany) out last year, the most unreliable subassemblies were identified by studying the reliability of different wind turbine concepts with relevance to offshore application. It was also shared that Mean Time to Repair (MTTR) is also important.

It pointed out that that direct drive wind turbines do not necessarily have better reliability than geared drive turbines but do have better potential to improve their reliability with time. Wind turbine generators and converters are both achieving reliabilities considerably below that of other industries but the reliability of these subassemblies is improving with time.

The findings were as follows:

• The sub-assemblies with the highest failure frequencies are, in descending order of significance:
  • Electrical system
  • Rotor (i.e. Blades & Hub)
  • Converter (i.e. Electrical Control, Electronics, Inverter)
  • Generator
  • Hydraulics
  • Gearbox
• Larger wind turbines have a lower reliability than smaller ones.
• Technological advances in wind turbines variable speed and pitch-control have conferred reliability improvements on wind turbines with time.
• Wind turbines direct drive and geared generators exhibit higher failure frequencies than generators in other industries, during the initial phases of operation.

The failure frequency of direct drive generators is generally higher than that of geared generators. This disparity may be due to the much larger number of coils used in the direct drive machine, the larger diameter of the machine and the fact that it is not a standard machine produced in large numbers. The failure frequency of the direct drive generator could be improved by replacing the field coils by permanent magnets.

·          Wind turbines direct and indirect drive converters exhibit higher failure frequencies throughout their operation than converters in other industries. Direct drive wind turbines are not necessarily more reliable than geared wind turbines.

·          Aggregate failure frequencies of generators and converters in direct drive wind turbines are greater than the aggregate failure frequency of gearboxes, generators and converters in geared for the reduction of failure frequency by the elimination of a gearbox is a substantial increase in the failure frequency of electrical-related subassemblies.

·          However, the MTTR of electrical-related subassemblies is lower than the MTTR of gearboxes. This suggests that an all electric, direct drive wind turbines may ultimately have an intrinsically higher availability than an indirect drive wind turbines.

Quantifying O&M costs over a period of time

Providing an insight into how to quantify O&M costs over a period of time, be it for expansion of existing project or setting up a new project, Sandia National Laboratories’ Hill said if we eliminate the regular costs such as forecasting, taxes, lease payments and such, and focus on manageable costs of preventative and corrective maintenance, then it is strongly suggested that a history be maintained of all components that need maintenance, repair, replacement, calibration, testing, or any other type of action that either takes equipment out of service (loss of availability) or requires some form of repair (and expense).  

“Reliability theory provides for this in the metrics of mean time between failure (MTBF) and mean time to repair (MTTR).  It is recommend that reliability tracking use these metrics and compare calculated to measured availability of the plant,” said Hill, adding that reliability is a technical discipline in itself and commercial software models and consultants are available.

Wind Energy Operations and Maintenance Summit USA

A session, titled `Identifying wind turbine reliability issues across the industry matters’, will be conducted as part of Wind Energy Operations and Maintenance Summit USA, to be held on April 1-2 in Dallas, Texas this year.

For more information, click here: http://www.windenergyupdate.com/omusa/programme.shtml

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Contact: Tom Evans by email tom@windenergyupdate.com