Why structural stability is key to life extension

When wind turbines approach the end of their design life, an assessment for lifetime extension determines whether a turbine is suitable for continued operation. Christian Schumacher and Florian Weber of certification body TÜV SÜD outline the process and how operators can prepare.

An inspector calls… An on-site inspection is a key part of deciding suitability for lifetime extension (pic: Andreas Nilsson)
An inspector calls… An on-site inspection is a key part of deciding suitability for lifetime extension (pic: Andreas Nilsson)

When a wind turbine reaches the end of its design life, the risk of any damage affecting safe operation as a result of material fatigue increases.

But this does not automatically mean that the turbine must be decommissioned and dismantled.

In fact, even after the end of the manufacturer’s stated service life — usually 20 years — many turbines still have safety reserves that can be drawn on for lifetime extension.

German wind energy association BWE formed a lifetime extension working group, bringing in turbine manufacturers, wind-farm operators, expert assessors, representatives from official authorities and legal experts.

The objective was to define the technical requirements for the safe and economically viable continued operation of wind turbine. The critical factor is the turbine’s structural stability.

Checks required to verify this mainly focus on load-bearing components from the rotor blades to the foundation, as well as the safety functions, the control system and the braking systems.

To work out whether a turbine is suitable for lifetime extension, the actual stress to which it has been exposed during its operational life must be established, using computer simulations reflecting the design conditions after type testing, as well as the real-life conditions at the site.

An on-site inspection of the turbine condition is also carried out.

The resulting report provides information about the conditions under which lifetime extension will be possible, such as repairs or replacing bolts as a precaution, enabling an accurate estimate of the potential costs involved.

This analysis is also worthwhile in cases where the extended lifespan would be relatively short if no further action were taken, as it gives the operator an overview of the opportunities and risks involved in continued operation.

Knowledge is power

The assessment is made up of two parts: a physical inspection and data analysis. The first part looks at whether the overall technical condition of the turbine allows continued operation, based on an on-site inspection by an expert.

The analysis part takes the machine’s technical documentation, together with the wind, weather and operating data, as a basis for calculating how long the turbine could potentially continue to run.

Wind-farm operators are responsible for arranging the inspection in a timely fashion and ensuring the necessary preconditions are met.

The experts need all the necessary information and documentation as a basis for their analyses.

This includes the operating permits for the turbine, documents concerning its construction and commissioning, all operating and yield data, maintenance, repair and inspection reports, and wiring and hydraulic diagrams.

In addition, an expert report carried out within the past 12 months on the current condition of the rotor blades is required.

Experience shows that technical documents are often incomplete, with important items missing, particularly documentation from the construction and commissioning phases.

The turbine manufacturer may be able to replace design plans, documents and certificates if necessary.

Other important information for the lifetime extension assessment includes data concerning wind conditions for the turbine’s operating life to date.

Data on average wind speeds, extreme wind events and turbulence intensities — sourced from operating data and the anemometer on the nacelle — from the previous 20 years needs to be taken into account to accurately calculate loads and stresses for the period.

If this data is not available for the full period, other data sets, such as reanalysis data, are used to perform long-term extrapolation.

For turbines on wind farms that may be subject to yet-to-be-specified capacity addition, turbulence is calculated individually to take the conditions at each turbine location into account.

The assessment process

Prior to their site visit, the experts analyse the information and data already available.

Weather and performance data, technical documentation and maintenance, repair and inspection reports are evaluated to give an initial impression of the turbine’s service history.

The turbine can then be examined for specific weaknesses and defects, and any special features considered for further assessment.

This analysis of existing data forms the basis for the next steps of the assessment. Specialists in the practical and analytical parts of the assessment work in parallel and provide mutual assistance.

A statement is produced describing any measures immediately required in order for operation to continue and specifying the remaining time until design loads are reached.

All relevant load-bearing elements and components contributing to the structural stability of the turbine are examined during both parts of the assessment.

These are the support structure — the tower and foundation; connecting elements such as screws and bolts; all load-bearing parts of the nacelle; the drive shaft; the hub and the rotor blades; and all braking systems and turbine safety functions.

During the on-site inspection, the experts examine maintenance logs and compare the documentation with the in-situ condition of the turbine, looking for corrosion, visible cracks and any unusual noises in the gearbox or other gear and bearing assemblies.

Close attention is paid to any known issues that may be characteristic of the same or similar turbine types.

A critical aspect of the assessment concerns the condition of the main elements, such as rotor blades, gearbox, supporting structures and foundations.

In a worst-case scenario, the discovery of severe damage that materially impacts on structural safety will result in an immediate shutdown of the turbine being advised.

A decision then has to be made whether it is financially viable to replace or repair the faulty main components so as to enable lifetime extension.

However, such major faults are the exception. More frequently, inspections uncover small faults caused by material fatigue or weathering.

Damage such as corrosion, flaking or peeling of protective coatings, or worn cables can usually be remedied with simple repairs.

Rotor blades are often subject to small cracks, erosion or flaking in the protective coat, and must be repaired in almost all cases in order to protect the long-term condition of the turbine.

During the on-site inspection, the experts also look for changes in the environment in the immediate vicinity of the turbine since its commissioning.

If the wind farm has been expanded or turbines have been built on nearby sites, these additions may cause changes in wind conditions that could materially change the loads acting on the turbine under inspection; this must be taken into account in turbulence calculations.

New neighbours… The effects of any capacity additions at the wind farm or adjacent sites are taken into account (pic: Windpark Druiberg)

In the analytical part of the assessment, the experts calculate the end-of-life date of the relevant parts of the turbine by comparing actual loads and stresses to which the turbine is exposed against those in the manufacturer’s design conditions for which the turbine was originally envisaged.

The results of the on-site inspection are also included in these calculations.

By using software-based models that compare actual wind conditions at the site against the wind conditions factored into the original design, combined with their experience, the experts can produce precise calculations of the service-life reserves of a turbine and reliably estimate the extended period.

Their assessment also provides clear indications of precautionary replacement times for specific components to ensure safe and reliable extended operation.

Experience from lifetime-extension assessments carried out by German certification and testing body TÜV SÜD shows that most turbines can continue operating safely for several years after a few minor repairs.

For example, the bolts attaching the rotor blades to the hub are often the first to reach their design-load limits.

During turbine operation they must withstand the continuously changing bending moment from the weight of the blade through each rotation, which is largely independent of wind conditions at the site.

Replacing these bolts is a relatively straightforward procedure that only requires a short shutdown.

As far as the operator is concerned, this is almost always a cost-effective measure that will enable a longer lifetime extension for the turbine.

Planning ahead

A lifetime-extension assessment is normally performed in the last year of operation covered by the operating permit to ensure it addresses the up-to-date condition of the turbine.

However, it may be useful to do it earlier if a sale of the turbine is being considered or to be factored into medium-term budget planning.

In these cases, the assessment for lifetime extension can be performed without an on-site inspection.

The result provides an estimate, based on calculations, of whether continued operation is possible and when specific components are likely to need replacement.

The results of this preliminary forecast can also be incorporated into the actual lifetime extension assessment.

TÜV SÜD’s experience in the field shows that most wind turbines still have service life reserves even after reaching the end of their design life. Wind conditions at the site may involve lower loads than originally factored into the design.

The turbine’s supporting structure is often free from significant damage, so that it only needs relatively minor, inexpensive repairs to allow lifetime extension.

The lifetime-extension assessment also allows operators to map out a realistic picture of the repair and maintenance costs for the remaining life of their turbine, and take this into account in their service strategy.

The inspection report is usually also required for extension of insurance policies or for collaboration with service providers after the end of the turbine’s service life.

Turbine operators may submit the report to approval authorities as proof of suitability.

Growing segment — Thousands of turbines to leave support system

The assumed loads factored into the turbine design by the manufacturer are based on a defined service life for the machine.

All components related to the turbine’s operation, safety and construction, plus all load-bearing parts, are designed, built and dimensioned to withstand foreseeable loads and stresses caused by wind, weather and operation for the duration of this period.

This is usually 20 or 25 years — provided the specified maintenance activities are completed, regular inspections and testing are performed, and any faults are immediately remedied.

If this is the case, operators can rely on the structural stability of their turbine for the defined period.

The design service life and period of lifetime extension are used to calculate the total service life.

Registrations recorded under Germany’s Renewable Energy Sources Act, known as EEG, showed that at the end of 2016/start of 2017, more than 1,200 wind turbines in the country had been in operation longer than 20 years, meaning their lifetimes had been extended.

From the end of 2020, the first generation of German onshore wind farms will no longer receive EEG subsidies.

Market prices will apply for their output, which will play a major role when operators decide on the viability of lifetime extension.

Between 2019 and 2024, the decision of whether to decommission, repower or continue operation will apply to around 1,500 to 2,000 turbines each year, falling to an estimated 1,000 turbines a year from 2024.

Repowering will not always be possible, particularly given restrictions such as the mandatory distance required between the turbine and the nearest housing.

In such cases, lifetime extension could be a particularly attractive option.

Christian Schumacher and Florian Weber work in the wind turbine certification and assessment department of TÜV SÜD

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