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Offshore Wind Journal

'Loads and rust', offshore wind’s drivers of life expectancy

Mon 02 Jul 2018 by David Foxwell

'Loads and rust', offshore wind’s drivers of life expectancy

The lifetime of some offshore windfarms could be greater than the 25 years commonly assigned to them, which could make them even more attractive to investors, particularly as projects get under way in southeast Asia.

Given adequate design margins and sufficient TLC during the operation and maintenance phase, offshore windfarms could continue producing power for longer in future.

Already accepted as a viable, secure asset class, they could become an even more attractive asset for institutional investors if they produce more energy over longer periods, boosting revenues and profits.

But there’s a catch. With the industry focusing laser-like on cost reduction in the last five years developers have revisited assumptions about design margins – many imported wholesale from related sectors – and have realised that margins could be reduced. That might make new windfarms less expensive but reduce the likelihood that they can be extended.     

That was the message coming from speakers at Global Offshore Wind 2018 in Manchester in June, among them Everoze partner Colin Morgan. Mr Morgan told delegates that, when you boil it down, there are two classes of component that determine the lifetime of an offshore windfarm: the primary structure, the lifetime of which is largely determined by fatigue life, and the rest, the replaceable parts, including the generator.

As Mr Morgan’s colleagues at Everoze have noted before, notwithstanding the wind turbine itself, foundations are a key driver of what can be achieved for life extension. As he told delegates at the RenewableUK event, one type of component drives lifetime (the foundation) and one drives opex (the turbine). If you want to reduce opex you have to focus on the turbine, but if you want to extend the lifetime of an offshore windfarm, you need to focus on the primary structure.

So, what are the drivers of fatigue for the bit of a turbine that’s stuck in the ground and how might we make life extension possible? They include loading from the turbine itself, the material (soil type) in which the foundation is embedded and things like driving damage (incurred when the pile on which the turbine sits is driven into the seabed).

So far, said Mr Morgan, the news about foundations has been pretty good, mainly due to the fact there has been a lot of margin in the design of structures. “Rust is your enemy. If you also get the corrosion design right, the load margin is an extension option,” Mr Morgan told delegates.

As Everoze put it in a recent blog, simplistically, the biggest impact on life extension will be the loads experienced by a foundation during operation compared with what was assumed for design. Where the operational loads are significantly lower – either through conservative turbine loads or an overestimate of environmental loads – there is a margin which will increase the fatigue life of the structure thereby allowing for a life extension.

Equally, said Everoze, while ground conditions are usually developed from site-specific measurements, the extent of these measurements can lead to conservatism in foundation design, increasing steel thickness and embedment depth. “On the flip-side, get the soils interpretation wrong and you may be piling-the-heck out of foundations during installation and eating into their fatigue life,” it said.

You won’t be surprised to know that data is really important.  

If you are an investor, therefore, you really need to know about the history of a project, how was it installed, has it suffered from scour or water ingress potentially leading to internal corrosion? What level of monitoring is in place and is the inspection and monitoring campaign fit-for-purpose to support engineering integrity assessments for life extension? Have the wind turbines/foundations been instrumented to assess operation loads versus design loads, thereby allowing operator to quantify any load margin and re-estimate fatigue life? Have adequate foundation installation records been kept to assess piling damage?

Coming back to the point I raised above, as Everoze said, projects in construction/development have been subject to improvements in design approach and now carry significantly less margin in material than early projects. If a foundation was designed for a 25-year, can it really be pushed to 30 or more?

So, there’s a potential trade-off, it seems. Get the design margins and environmental assessment just right, and you could reduce costs during the construction phase and allow for lifetime extension. Go too far in the direction of reducing margins and you might reduce the lifetime and attractiveness of your asset to an investor.

Interestingly, Mr Morgan told the conference, the opportunities for lifetime extension could be greater in southeast Asia, where offshore wind is beginning to take off. This is because loads in the region are likely to be lower, because of more compliant soils. Here, he said, the lifetime of a windfarm might not be determined by fatigue but by how well you deal with corrosion. In warmer water – where the corrosion regime could quite different from the colder waters of the North Sea – I imagine that could well be the case. 

Thinking about the potentially adverse effects on the lifetime of a foundation when it is driven into the seabed by a pile hammer raises another interesting thought. For the time being most offshore turbines are founded on monopiles installed in this way, but suction buckets and suction bucket jackets are beginning to enter service, and they don't need to be rammed into the seabed. Quite the opposite in fact.

Then there’s floating offshore wind. In this market, nothing is driven into the seabed, or to put it another way, there is no single point of fatigue attached to the seabed. Looking at it that way, nothing on a floater is irreplaceable. Floating offshore wind could therefore be quite an asset class for investors: far offshore, where floaters are installed, the winds are more reliable and capacity factors higher, and everything can be replaced. What’s not to like about that?

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