Although wind turbines generally are designed for a service life of 20 years, many can continue to operate past their original design life. In fact, the lifetime of a wind turbine can often be extended by minor and low-cost repairs. This article describes the methods and results of a safety evaluation of wind turbines.
By Christian Schumacher and Florian Weber
As wind farms age, their operators face significant business decisions. Decommissioning, repowering or continued operation are the main options to be considered. Two of the key factors affecting these decisions are determining the physical condition and theoretically admissible lifetime of a turbine. Physical condition can be evaluated through on-site inspections. The theoretically admissible lifetime is usually determined by means of a comparison of site-specific and design loads.
A work group created by the German Wind Energy Association has defined the basic principles of the lifetime extension assessment of a wind turbine, in order to ensure sustainable use of wind energy. This work group — formed by manufacturers, technical experts, operators, legal experts and representatives from the authorities — was assigned the task of specifying the technical requirements necessary for ensuring the safe operation of a wind turbine after the end of the design lifetime.
Principles of a lifetime extension assessment
To evaluate the current condition of a turbine, the lifetime extension assessment is usually performed during the last year of validity of the operating permit. If divestment of the turbine is being considered, or in the case of medium-term budget planning, it may be preferable to carry out the analysis at an earlier stage. Preliminarily results are normally arrived at without a physical inspection and indicate whether continued operation is feasible and when specific components are likely to need replacing. These findings may be incorporated into a lifetime extension assessment at a later point in time.
The assessment to determine whether a wind turbine may operate beyond its design life consists of two parts, conducted in parallel. Experts in the analytical and practical evaluation provide each other mutual assistance during the entire process. After the analytical evaluation and on-site inspection, a status report is drawn up specifying the requirements needed for lifetime extension. For instance, repairs or precautionary replacements of the bolted connections of the rotor blade are often necessary, as these are usually the first elements to reach their design load limits. Thus, an accurate financial estimate of the potential costs involved in a lifetime extension can be generated. The results of the assessment provide valuable input for weighing opportunities and efforts for continued operation and are important in assisting wind farm operators in their decision-making process.
Structural stability of a turbine
One critical factor in the safety evaluation process is establishing the structural stability of a wind turbine. The tests required to verify structural stability are mainly focused on the load-bearing components, from the rotor blades to the foundation, as well as the safety devices, braking systems and turbine control systems. The actual loads to which a turbine has been exposed during its operational lifetime need to be calculated and compared with loads resulting from design conditions. This information is obtained from computer simulations that reflect design conditions after type testing, as well as environmental operating conditions. Furthermore, an on-site inspection of the turbine is performed.
Environmental operating conditions include site-specific wind conditions. Data documenting average wind speeds, turbulence intensities and extreme wind events for the previous 20 years need to be quantifiable in order to calculate loads for the period of operation. This calculation is based on operating data and data from the anemometer on the nacelle. Should this data not be known for the entire period, other data sets (i.e. reanalysis data) are used to perform long-term extrapolation. In the case of a wind farm with a variety of capacity additions, turbulence is calculated individually for each turbine as well as for each of the windfarm layouts during the design lifetime.
Environmental conditions and required documents
In the analytical assessment, the potential duration of continued operation is calculated based on turbine technical documentation, as well as the environmental operating data. Wind farm operators are responsible for arranging the assessment on time and for presenting the relevant documents. Required documentation includes information relating to turbine construction and commissioning; the operating permit of the turbine; repair, inspection and maintenance reports; operating and yield data; and wiring and hydraulic diagrams. In addition, a technical report is required documenting the conditions of the rotor blades, carried out within the last year of operation.
It is not unusual for technical documentation to be incomplete. Missing certificates and technical documents can be obtained from the manufacturer. This includes documentation from the construction and commissioning phases. However, if a turbine manufacturer is no longer available, comparisons with other turbines and assessments based on previous experience may be used to bridge the knowledge gaps.
Physical inspection of a turbine
The physical condition of a turbine is assessed through an on-site inspection during the practical part of the lifetime extension evaluation. Prior to the on-site testing and inspection of the turbine, the information and data already available are analyzed. Technical documentation and reports, as well as weather and performance data, are examined so that the turbine can be checked for specific weaknesses and defects.
The objective of the physical assessment is to document any damage or unusual wear and tear to the turbine’s components and equipment. Load-bearing and safety-relevant components are examined in detail. Maintenance records are checked, and the turbine condition is compared with the technical documentation. In particular, inspectors search for signs of corrosion, visible cracks and suspicious noises in the gearbox or other gear and bearing assemblies. Also, a detailed investigation is carried out for weaknesses or flaws associated with a particular type of wind turbine, such as known shortcomings in the quality management during specific production periods or certain components or design flaws that lead to premature defects.
The conditions of the main elements of the turbine — i.e. the rotor blade, the supporting structure and the foundation — are carefully evaluated. The immediate shutdown of a turbine is recommended in the event of significant damage that would compromise the safety of continued operation. In most cases, however, the damage discovered is relatively minor and caused by corrosion, weathering and material fatigue. For instance, rotor blades or cables frequently need maintenance. Close attention is also paid to any changes in the surrounding environment of a wind farm. Expansions in neighboring sites must be taken into account in turbulence calculations.
In the analytical part of the lifetime extension assessment, operating loads are compared with design loads. The results of the physical inspection are considered in these calculations. Fatigue loads are simulated using software-based models that take into account site-specific wind conditions as well as design conditions. All load-bearing components contributing to the structural stability of the turbine are examined: the tower and foundation, screws and bolts, load-bearing parts of the drive train, the hub, the shaft, the rotor blades, braking systems and the safety functions. This report specifies the remaining time until design loads are reached. Based on the calculations, a statement is prepared highlighting immediate measures required for continued operation, as well as measures that become necessary at defined points in time, like exchange of parts or individual inspection strategies.
As long as wind farms are operated within their design lifetime and the design conditions are not exceeded, a defined safety level against the occurrence of damages caused by material fatigue is maintained. TÜV SÜD’s experience shows that many turbines still can be operated beyond their design lives while not crossing this safety level. In many cases, wind conditions at the site result in lower loads than originally planned. Thus, the supporting structure of the turbine is often free from significant damage and the necessary repairs are generally minor and cost-effective. A lifetime extension assessment determines whether continued operation is possible and assists managers and operators when planning for the future of their assets.
The results of a lifetime extension assessment can also be used to plan maintenance shutdowns and to forecast the costs that are likely to be incurred during the remaining lifetime of a turbine. This assessment is also recommended when applying for extension of insurance policies and is generally required by service providers after the end of the design life of the turbine.
Christian Schumacher and Florian Weber are with TÜV SÜD Industrie Service GmbH.
Design service life
The assumed loads a manufacturer factors into turbine design are based on a defined service life for the wind turbine. All operational, safety and construction relevant components and load-bearing parts of the turbine are designed, built and dimensioned to withstand foreseeable loads and stresses caused by wind, weather and operation for the length of this period. This design service life is usually 20 or 25 years, provided the specified maintenance is completed, regular inspections and testing are performed, and faults are immediately rectified.
The design service life and period of lifetime extension are used as a basis for calculating the total service life. Registrations undertaken within the terms of the German Renewable Energy Sources Act (EEG) showed that at the end of 2016/start of 2017, more than 1,200 wind turbines in Germany had been in operation for over 20 years, i.e. their lifetime had been extended.
Germany’s first wind turbines to be placed into operation will no longer receive EEG subsidies from the end of 2020. Thus, market prices will have a major impact on turbine operators’ decision-making process over the viability of lifetime extension. Between 2019 and 2024, the decision to decommission, repower or continue to operate will apply to about 1,500 to 2,000 turbines every year, decreasing to about 1,000 turbines per year from 2024. It is likely that repowering will not always be possible, particularly given the mandatory distance required between a turbine and the nearest residential area. In such cases, options for lifetime extension will be particularly attractive.