Offshore wind design

Lifecycle- design for wind farm life extension

Ageing offshore wind farms and oil & gas assets share many common problems. However, fast-rotating turbines present unique challenges. New digital tools are now helping to extend and maximise their working lives.


( Text content written by Twenty6 for Luke Fussell, LIC Energy )

Young but mature

Ten years ago, it would have been naive to call the offshore wind industry ‘mature’. What a difference a decade makes! A dramatic fall in energy costs over the last two and half years means that the sector today can quite reasonably be described as very mature economically. It stands on its own unsubsidised feet in open markets. However, we can go further.

The average cost of offshore wind energy between competitive Contracts for Difference (CfDs) auctions in February 2015 and September 2017 fell by an average of 47%, according to figures quoted by RenewableUK last year.

This isn’t by chance. An awful lot of hard work and technical development lies behind its success.

But where do we go from here? How can we continue to push and keep prices down with increasing numbers of offshore turbines that are already probably past their best?

As I was able to explain at the IQPC Corrosion Conference in Bremen in March 2018, the seemingly unstoppable development of digital technology is providing us with an advanced modelling tool to optimise the real working life of sophisticated turbine structures.

‘Digital twinning’ to eke out and extend the lifespan of wind farms conceived, manufactured and installed a decade ago is being pioneered by a new generation of innovative engineers to create a reliable and safe picture of how many cost-effective years busy working turbines have left.


Tricky challenge

One definition states that life extension is that it is the, ‘… study of slowing down or reversing the processes of ageing to extend both the maximum and average life span’. That actually refers to human life. However, the principles applied to wind farms are similar, with the proviso that out at sea life extension is more difficult than it might at first appear.

One administrative difficulty is that when original consent was granted for most of today’s offshore structures a maximum turbine size was specified. Installing larger units now would involve completely new environmental impact studies. New cable infrastructures to carry higher currents could also be needed. Therefore, scraping and replacing older projects is not particularly feasible.

However, there are precedents that we can draw on from other sectors.

Companies such as our own Danish-based parent, LICengineering, have accumulated many decades of ageing offshore oil & gas asset expertise. By adding our own offshore renewables design and engineering experience, LICenergy UK’s Bristol-based team is able to transfer important petroleum sector know-how to offshore wind installations.

But while there are useful precedents, there are also major differences that the LICenergy team is well acquainted with.


Engineering chalk and cheese

Wind structures are dynamic. In general, they are more sensitive to fatigue. As racehorses rather than workhorses, wind farms are actually designed for a high-speed life with a relatively swift payback over a short period. Their margins and business models are different, with a design ethos based on optimisation.

Inevitably, the increasing cost of inspections, increasing risks of failure, plus uncertainties surrounding of what practical life is left in them, are issues for all ageing offshore structures.

But in the wind sector the challenge is often compounded by the same basic turbine structure design being duplicated hundreds of times across a single wind park. This may bring economy of scale benefits. However, manufacturing requires its own optimisation. The result can be a very complex problem that is not straightforward to untangle.


Growing importance

Why is this important now? At the close of 2017, there were 4,149 grid connected turbines in 11 countries across Europe – equating to 15,780 MW. By 2020, this is expected to exceed 25 GW. To illustrate the growth rate, in 2009 there were 828 turbines accounting for 2,056 MW.

So, what happens at the moment? An example is Vinedeby in Denmark, the world’s first offshore wind farm. Installed with 11x 450 kW turbines in 1991, it was decommissioned after a successful 25-year life in 2017.

In the future, decommissioning will increasingly be seen as a poor first option. Decommissioning means game over! The second conventional option, refurbishing the asset with shine new technology, is expensive. But the third way is life extension.

Typically, by the time the natural frequency has changed in, say, monopile foundations, water has already entered where it shouldn’t and it is too late. In parallel, a good proportion of primary structure is challenging or impossible to inspect. Regular sub-sea welding inspections on wind farm structures are very costly. So, is there another route?

The safe way that we use to leverage the life left in a structure is by understanding what actual loads it has experienced. This calls for some form of regular local monitoring and accurate structural modelling.

Cue everyone’s favourite buzzword – digital twinning.


Digitisation

You can’t attend an offshore wind conference these days without hearing ‘digitalisation’ mentioned about 100 times! Also known as ‘digital twinning’, this means creating computer models of structures but using real-world measurements to load them accurately, with data updated regularly.

To acquire this data, sensors must be installed on foundations with load information fed back to shore.

The result is the most accurate assessment possible of how much design life has been used up from the ‘real’ loading regime. The technique can be applied down to an individual turbine level, which gives a fine resolution across an entire wind farm.

Fortunately, cautious engineers have bequeathed us a major advantage.

Because traditional designs are inherently conservative to ensure safety and guarantee that a structure survives for its whole required life – typically 25 years – in practice structures are not loaded as highly as their designs might indicate. This is particularly true of new technology.

Therefore, at the end of the design life period, there is usually still safe operating life ‘left in the tank’. Digital twinning means that this safe remaining life can be quantified. The result can be a huge cost saving.


Not the only way forward

While modern digital twinning methods are a very handy and welcome modern design and maintenance tool, it remains true that many North Sea oil rigs have operated safely for more than 20 years longer than their design life without them.

Regular inspection and monitoring methods can be used successfully to extend asset life safely, such as acoustic emission monitoring of critical welds and also natural frequency analysis for jacket structures.

What is on offer now is the ability to work closer to the margin with more accuracy, accountability and peace of mind. This is essential to an industry that has already proved the advantage of chasing down incremental cost improvements.

We are going to hear much more about design for wind farm life extension in the future!