The Delabole wind farm in Cornwall, England, was the UK’s first commercial scale wind farm, closely followed by the nearby Carland Cross wind farm. These pioneering wind turbines were small structures generating 400 kW with 34-metre rotor diameter. Nevertheless, they kickstarted a wind turbine revolution that has since expanded to an impressive 16 GW of installed onshore capacity across the UK – a figure that continues to climb.

The first wind farms were constructed around 1991. So modern wind farms can draw on more than three decades of UK development expertise in design, construction and operation. The most obvious developments were in the wind turbine design: reducing weight, improving control strategies, with better computational modelling, new manufacturing techniques and materials. All of these developments culminate in much larger wind turbines, that are currently in the region of 170-metre rotor and ~7 MW available onshore in the UK today.

Offshore, wind turbines are now offered with ~240-metre rotor diameter and 15 MW, and larger turbines are approaching due to the economic reward and competitive energy contract auctions, not to mention the sustainability benefits of fewer, larger turbines.

While this article focuses on the immediate need for life extension management of onshore wind turbines, offshore wind turbines will become the next, more challenging, topic for study, due to the complex interaction between the wind turbine, foundation, seabed and the force of tides and waves combined with wind.

What is the expected life of an onshore wind turbine?
Although less complex than offshore, onshore wind turbines by their very nature are exposed to extreme climatic conditions. They have to be designed to meet well-developed international standards to ensure they can withstand the forces they are subjected to. And they must include an appropriate safety factor, which ensures that all of that previous learning and development is incorporated into the design.

A prudent wind farm developer makes sure that the wind turbine is certified against these standards. The certification process evaluates wind turbines against many different load cases and scenarios, but importantly must guarantee that the wind turbine has a nominal design life of 20 years against a specified classification of wind conditions. Although in recent years, certified design lives have been increasing to 25 or 30 years.

As the certified design life is a ‘minimum’ threshold, and the wind conditions in situ will likely be less onerous than those used in certification, an opportunity exists to extend the safe operational life – much the same as many other industries do with their assets.

There are significant benefits of extending the life, from both a commercial perspective as you have effectively ‘paid off the mortgage’, and from an environmental perspective as green electricity is continuing to be generated with no new raw materials or carbon having to be invested.

An alternative strategy would be to ‘repower’ the wind farm and start again, reusing whatever infrastructure is available. However, as wind turbine technology has moved on, some sites simply can’t accommodate larger modern wind turbines within the site constraints.

Extending the life of a wind turbine does come at a cost though, as wind turbine lives are predominantly driven by fatigue, which is a degradation of material strength through constant load cycling. If left to run unchecked, fatigue can start to form cracks which then grow over time, leading to overall failure of the asset. Therefore, it is the responsibility of operators’ engineers to manage this degradation, ensuring the wind turbine operation is safe and structural risks are identified, managed and mitigated.

Ultimately there will be a point where the wind turbine will have to be decommissioned due to reaching the end of its structural life, maintenance becoming uneconomical, or the increasing strategic benefit of repowering. Life extension management is, therefore, a balance to ensure safe operation until decommissioning.

How is the safety of wind turbines managed? 
There are various methods engineers can employ to assist with understanding the fatigue failure risk of wind turbines to identify high-risk areas. Techniques range from theoretical, such as modelling techniques which give confidence that there is adequate operating life headroom, through to non-destructive testing (NDT) to determine if micro-cracks are forming in the structures. A rigorous maintenance regime will incorporate these types of analysis and investigation, based on risk, to ensure that the assets are serviceable.

One challenge faced by operators is the lack of availability of original detailed design information required by the analytical models, as these were not normally supplied with the wind turbines. Data such as dimensions, plate thicknesses, welds and geometry details are required. Models have to be re-developed and validated with measurement and conservative assumptions being made. This may add to uncertainty and require extra inspections, in case component life may be shorter than anticipated.

As well as the structural issues, there are other issues relating to life extension which must be managed, such as reduced reliability of equipment (rotating and electrical), as well as obsolescence of components, which must all be managed as part of an effective asset management programme.

Due to the timeframes of the first wind farms in the UK and the subsequent growth of wind turbine numbers, many operators now have assets that are just moving into life extension. The numbers of turbines needing to be managed are increasing year on year.

Therefore, the industry is learning as a whole, and international standards relating to management of life extension of wind turbines are still in draft form. Some countries are further along in the journey, such as Germany and Denmark, where there are now statutory requirements to demonstrate the integrity of the wind turbines. But no life-extension-specific statutory requirement currently exists in the UK, beyond normal ‘duty of care’ and the Health and Safety at Work Act.

The onshore wind industry in the UK does, however, have the SafetyOn forum, which runs in partnership with the Energy Institute. This forum allows onshore wind turbine operators to work together on safety initiatives. Since 2020, a Life Extension Working Group within the forum has pulled together expertise from across the industry on life extension, sharing safety concerns and approaches to life extension.

Life extension management [of onshore wind turbines] is a balance to ensure safe operation until decommissioning… The industry is learning as a whole, and international standards relating to management of life extension of wind turbines are still in draft form.

What do we need to know about onshore wind turbine life extension?
Having researched the highest-priority topics in the sector, the SafetyOn forum has funded the production of some good practice guides and resources, which are now in the process of being drafted. They are introduced below.

Aeroelastic modelling: These computational models look at the interaction between the wind and the structure to predict loading, but are very sensitive to their inputs. Most operators rely on third parties for this type of modelling. A guide on this subject will help ensure that the right data is captured and supplied to make the model as accurate as possible. It will also try to explain uncertainties in the modelling and how to reduce them.

Non-Destructive Testing (NDT): As wind turbine operators look to implement risk-based inspection regimes, this guide will cover what types of NDT are available for different parts of the wind turbine and outline the benefits and limitations of each. From external welds far up a wind turbine tower, where access may be difficult, to inspecting hundreds of blade bolts, the guide will give operators a steer as to what safe and effective methods are available for incorporation into inspection regimes.

Cross-industry signposting: Our industry is aware that there is a plethora of guidance from long-established assets that have been operating safely into life extension. This exercise will set up a website that points to useful guidance from elsewhere, such as corrosion management from offshore oil and gas, to fatigue management from naval applications. The resource links will include other Energy Institute life extension working groups.

The SafetyOn working group will continue to work on improving the understanding and safety of wind turbine life extension across the UK. It has aspirations to centralise databases of life extension failure modes and lessons learned, for the benefit of engineers looking to design their own asset management programme for life extension.

Those currently navigating the complexities of wind turbine life extension find themselves in a fortunate position. Wind turbines designed over two decades ago were typically over-engineered, incorporating levels of redundancy that allow for reasonable life extension in many cases. However, since then, computational modelling has improved, turbine control has become more site-specific and designs have become leaner to manage costs. More attention will be required earlier in modern wind turbines’ life cycle.

Fortunately, as life extension practices become more standardised, life extension can be planned for from day one. By providing appropriate information and recording accurate data over the course of the operational life, we can achieve more precise predictions of their remaining lifespan.

• Further reading: ‘Ensuring health and safety keeps pace with growth of the onshore wind sector’. As renewable sectors like the wind industry have emerged and grown, they have required new approaches to health, safety and environment (HSE) practices. Emma McIvor AMEI, Technical Manager at SafetyOn, the health and safety organisation for the onshore wind sector, gives an update on the sector and the centrality of safe operations for the industry.
• Billy Stevenson, CEO of Full Circle, a specialist wind turbine services group, examines the key role that operations and maintenance (O&M) plays in powering the UK wind sector, and the value of repair and re-use as opposed to scrappage.