Assuming a design lifetime of 20 years as standard practice when designing new wind turbines, machines installed in the early 1990s will soon be reaching the end of their design lives. This means their owners need to determine whether and for how long these wind turbines can continue to operate beyond their originally assumed lifespan.
Under the right conditions, extended service life can be justified from both technical and economic viewpoints. Especially in the case of newer, larger turbines and machines used in offshore wind farms, a long lifespan is desirable because of the high initial investment cost. However, regardless of whether there is technical and economic justification for continued use, wind farm deconstruction can also run contrary to sustainability goals. Conservation of natural resources is becoming increasingly important when using alternative energies.
As the rated capacity of wind turbines has increased over the years, it tends to be small turbines with a power rating of up to 250kW that first need assessment of their suitability for continued operation. But this subject is now gaining in importance for larger wind turbines.
Many questions need to be answered when assessing a turbine's lifespan. These include whether the service life of a particular wind turbine can be assessed based on site wind data alone and if maintenance work was carried out regularly to the manufacturer's recommendations - and whether this has an impact on the original life assumptions. If re-siting a turbine, there are technical considerations that affect the installation of used wind turbines in new locations and there may be cost assumptions for new stakeholders in existing projects to be validated.
Looking for guidance
In order to find suitable solutions from a technical point of view, certification body Germanischer Lloyd (GL) began drafting a guideline on the continued operation of wind turbines.
Supported by an expert committee, GL's Guideline for Continued Operation was published in 2009. It is aimed mainly at older wind turbines that have already reached the end of their service life, although it can be equally useful when considering continued operation of turbines yet to reach this stage.
The assessment of a wind turbine for continued operation is a complex engineering task that must be executed with due regard for the special features of each individual installation.
The guideline offers two pathways for gathering the information necessary to make an informed decision on the extension of a turbine's operational life: assessment through renewed calculation (analytical method) and through inspection (practical method).
Assessment through renewed calculation
The analytical method makes new or additional calculations for the wind turbine, taking into account site-specific parameters, such as wind loads at the particular location.
In general the procedure for the analysis is the same as for the certification of a new wind turbine: the manufacturer or operator verifies the structural integrity of the turbine for extreme and fatigue loads. An accredited certification body then reviews and assesses the documentation.
Both the technical feasibility and economic viability of continued operation must be questioned. The cost of the re-evaluation itself has to be taken into account. Other considerations include whether historic data on turbine site-specific loads is available and, if this is the case, whether the available loading information is sufficient to calculate the remaining service life.
There is no universal answer to these questions as all of them have to be assessed on a case-by-case basis.
Assessment through inspection
The practical method can be used as an alternative to the assessment through renewed calculation. This method can be compared to the general inspection of roadworthiness carried out on motor vehicles.
The objective of the inspection is to assess the wind turbine's suitability for continued operation and covers examination of the machinery, including the drive train and gearbox; the rotor blades; the safety system, the tower and the foundation. In other words, the inspection looks at the condition of the entire turbine.
When completed, the expert conducting the inspection should produce a record of the results. In contrast to the analytical method, it is not possible to make a statement on the entire service life of the turbine. But the technical expert can define an interval until another inspection is required. This interval depends on the overall condition of the turbine - one year is a reasonable period of time between inspections for continued operation.
The third way
It is also possible to combine the two methods. Both must be carried out by qualified, independent experts for wind energy. The main focus of any assessment is on the structural integrity and the safety-relevant assemblies. The availability of the wind turbine and the associated economic aspects are not considered directly.
If deficiencies endanger the structural integrity of the wind turbine, or if they can be expected to result in greater damage, the technical expert would recommend taking the turbine offline for repairs. If repair is not possible or not economically justifiable, it is then recommended that the installation is dismantled completely.
In recent years the number of installed wind turbines has dramatically increased worldwide. Considering various market and environmental factors - in particular targets to reduce greenhouse gases - it can be expected that wind energy will play a major role in generation portfolios of the future. Longer service lives for wind turbines are clearly desirable.
Because the analytical method entails much more time and effort than the practical method, the assessment through renewed calculation is more costly than the assessment through inspection. Additionally, the renewed calculations also require reliable data for the actual wind load history as a basis for the fatigue verification. Such information is often not available. So, it is for these reasons it is expected that the focus will be on the practical method of assessment for service life extension. The analytical method is more likely to be employed for the verification of large wind farms and for offshore wind energy.
As described above costs for the assessment for continued operation strongly depend on every individual case and have to be analysed separately.
Klaus Hansen is a senior engineer in the civil engineering concrete group and contact person for continued operation at Germanischer Lloyd Industrial Services.