A good quality, modern wind turbine will generally last for 20 years, although this can be extended to 25 years or longer depending on environmental factors and the correct maintenance procedures being followed. However, the maintenance costs will increase as the structure ages.
Wind turbines are unlikely to last much longer than this because of the extreme loads they are subjected to throughout their lives. This is partly due to the structure of the turbines themselves, since the turbine blades and the tower are only fixed at one end of the structure and therefore face the full force of the wind. Of course, as the wind speed increases, so do the loads that turbines are subjected to. This can reach levels almost 100 times greater than the design loads at rated wind speed, which is why many turbines are designed to shut down to protect themselves at higher wind speeds.
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One of the primary factors that determine the lifespan of a wind turbine are the environmental operating conditions faced by the wind industry. These conditions are site specific and include average wind speeds, turbulence intensities and (for offshore wind farm operators) the cyclic loading of foundations, jacket structures, and monopiles caused by waves.
In addition to these environmental factors, there are the usual concerns for any structure based around fatigue failure from use over the lifespan of the asset. These include a variety of different parts and components, from wind turbine blades to wiring and hydraulic systems.
Wind turbine blades need a special mention, as they are particularly prone to damage. As a moving component, the rotor blades are subject to higher levels of loading and fatigue, and can also suffer damage from birds or other objects striking them as well as the impact of high wind speeds or lightning strikes.
The lifecycle of a turbine can be extended through careful monitoring and maintenance. This requires the condition of the asset to be assessed and compared with the expended lifespan of the turbine, based upon the expected loads and fatigue as well as environmental factors for the wind energy site.
These assessments will determine whether continued operation is possible and when any components may need replacing to extend the life of the overall structure. This is known as a lifetime extension assessment and includes both theoretical and practical analysis, such as on-site inspections and the evaluation of design load data.
A status report will detail the maintenance requirements, from which an accurate estimate of the cost of wind turbine lifetime extension can be generated. This allows operators to determine the continued operational costs and risk of failure against the cost of replacement or even decommissioning. The report can also be used to apply for insurance policy extensions and is also often required by service providers at the end of a turbine’s design life.
As mentioned above, the actual amount of maintenance required to keep a wind power asset in operation will vary depending upon factors including specific operating conditions and the materials used. However, wind turbines generally require preventative maintenance check-ups two or three times per year. The need for these check-ups may need to increase as the turbine ages and also requires more maintenance to keep it in operation.
Offshore power generation assets face their own set of particular challenges to maintain. The challenges faced by onshore assets are often exacerbated by the offshore operating conditions while also adding their own specific problems. These challenges include corrosion, erosion and biofouling alongside the usual materials, fatigue and wind-based factors.
As the reliance on offshore renewable energy sources grows, it will become increasingly important to address these challenges to maintain operational availability.
Analytical Assessment
In order to maintain safe operation, it is important to establish the structural stability of wind turbines. Safety devices, braking systems and turbine control systems all require testing in order to verify the structural stability but there is also a need to compare the design conditions loads to the actual loads the turbine has been exposed to. This loading information can be obtained from computer simulations representing design conditions after type testing alongside environmental operating conditions.
The environmental operating conditions include site-specific wind conditions, such as average wind speeds, turbulence, and any extreme weather events. These are monitored over the previous 20 years in order to calculate estimated loads during operation. Wind farms may require each turbine to have its own set of data. This data is then assessed alongside technical documentation for the turbine. This technical documentation includes that relating to turbine construction, commissioning, operating permits, operating and yield data and wiring and hydraulic diagrams. In addition, repair, inspection and maintenance reports are also assessed. A technical report is also required to document the rotor blade condition on an annual basis.
It is the responsibility of wind farm operators to provide the relevant documents and arrange assessments on time. In some instances, it is possible to obtain replacement documentation from a manufacturer, however, if the manufacturer is no longer available it is possible to use experience to compare a turbine with others.
These analytical calculations are used to create a statement citing any immediate actions that are required for continued operation, along with those that will need to be scheduled for a later date, such as the replacement of parts or a full inspection.
All of these simulations need to be backed up by on-site inspections. This has traditionally been undertaken in-person by an inspector, but is increasingly being done remotely using robots and technologies such as the BladeSave system.
Find out more about the BladeSave Project
Physical Monitoring
The condition of a wind turbine is assessed through an on-site inspection that is informed by the analytical assessment. This allows for specific weaknesses, defects or potential problems to be checked. Physical monitoring also looks for unusual wear or damage to components and equipment. Load-bearing and safety critical components require particular attention, with some types of wind turbine having their own design flaws or production issues that could lead to premature defects.
Physical checks are performed on the turbine blades, the supporting structure and the foundation to look for signs of corrosion and cracking or to audibly listen for suspicious or unusual noises from the gear and bearing assemblies.
Significant damage can lead to the immediate shutdown of an asset, often incurring costly downtimes ahead of maintenance or repair. However, these checks tend to locate minor damage caused by corrosion, fatigue or weathering, allowing the defect to be fixed before it gets any worse.
Different parts require different levels of monitoring and maintenance, with turbine blades and cables requiring higher levels of inspection and care.
Physical monitoring also refers to monitoring the surrounding environment, and how this may influence the turbulence and wind speeds used in the analytical assessment.
The cost of operation and maintenance (also known as O&M costs) make up a sizeable proportion of the total annual costs of a wind turbine. These costs vary depending upon the age of the asset, but average out at around 20-25% of the total levelised cost per kWh produced over the lifetime of the turbine. For a new turbine, these costs may be only 10-15%, but can increase to 20-35% towards the end of the turbine’s lifecycle. Manufacturers are working on new designs to help reduce these costs by creating turbines that require fewer service visits and, consequently, less downtime.
Operation and maintenance costs cover the following expenses:
- Insurance
- Regular maintenance
- Repair
- Replacement parts
- Administration
Some of the actual costs associated with these expenses can be estimated, such as insurance and maintenance, since it is possible to obtain standard contracts covering much of a turbine’s lifecycle. However, costs for repair and replacement parts are more difficult to ascertain as they can be influenced by the age and condition of the turbine, frequently increasing as the asset ages. In addition, as very few turbines have reached the end of their life expectancy, there is little data on these costs later on the lifecycle, while many older turbines are smaller than those currently on the market.
Conclusion
Wind farm operators are faced with business decisions as their assets age – whether to continue operation, repower or to decommission. These decisions are affected by the physical condition compared to the theoretical lifetime of the turbines. On-site inspections and monitoring tools help evaluate these factors to ensure wind farms operate safely within their design lifetime. This lifetime can be extended or shortened, depending on damage caused by environmental factors and fatigue.
Certain components, such as the blades, require extra monitoring and maintenance and technologies, such as BladeSave, can simplify this process for the operator, allowing for the continual remote monitoring of wind turbine blade life.
If a wind farm is operated within the parameters of the design lifetime and conditions and maintenance is carried out regularly, they can operate beyond the design life. In many cases, the wind conditions at a site create lower loads than anticipated, meaning that turbine structures are free from significant damage. In these instances, repairs are minor and relatively inexpensive while a lifetime extension assessment could determine that a turbine can continue to operate beyond the original design life.
Wind Turbine Monitoring and Management at TWI
TWI has a wealth of experience with wind turbines, including addressing the particular challenges of offshore assets, such as the NDT inspection of offshore jacket foundations. We have also been part of the BladeSave consortium to develop a condition monitoring system for wind turbine blades and worked on the phased array ultrasonic testing of blade roots.
We provide independent expertise and advice related to materials, fabrication and inspection to offer solutions to the wind power industry and you can find out more about our services in this area here.