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IN DEPTH: Revitalising the monopile

The European offshore wind industry’s drive into deeper waters has sparked the invention of a host of innovative foundation concepts — from “twisted” steel jackets and self-installing tripods to suction bucket technologies and concrete gravity-base structures (CGBSs).

But the go-to foundation for offshore wind turbines over the past 20 years, the monopile, is not being left in the shallows.

Designers such as Denmark’s MT Højgaard are now stretching and streamlining conventional monopiles to shoulder the next generation of supersize turbines in water depths out to some 35 metres.

Historically, monopiles have been restricted to waters of 30 metres or less. Any deeper than this and they became too unwieldy to manufacture, transport or pile in, their economics not measuring up to the jackets and new-generation CGBSs.

However, Kim Reinhard Andersen, offshore director at MT Højgaard, which has delivered well over 600 monopiles over the past 20 years, believes the industry is ready to shake off its “monopile conservatism” and refashion the workhorse design for the mid-water-depth markets ahead.

“Up to last summer, monopiles were one thing: good for up to 30 metres’ water depth and carrying a turbine of up to 3.6MW or 3.8MW. This was the standard. And that the next generation would have been bigger turbines and be in deeper water so would require that we go into jackets,” says Andersen.

“But we began thinking — and we began to see that some of our clients had been having the same thought — that it might be possible to extend the use of monopiles. So we began looking into what a bigger monopile would mean in terms of fabrication, installation and so on.”

MT Højgaard modelled monopile and jacket concepts suitable for a generic 6MW turbine against a speed-risk-cost matrix of design, fabrication, installation and operations and maintenance. It found a “huge untapped potential” for XL — extra long — steel monopiles.

Design time was “almost identical” for the two, while fabrication risk favoured the monopile. More than seven of these can currently be mass-produced in a week, compared to only one for a jacket.

On installation speed, cost and risk, monopiles won again, in MT Højgaard’s analysis. More monopiles could be shuttled out on a single vessel journey out to an offshore site and less piling work would be needed than for a three- or four-legged jacket, cutting installation costs.

The installation risk, according to Andersen, was deemed “slightly lower for XL monopiles [than jackets] as the methodologies and equipment are based on sound experience, merely scaled”, though he notes that issues of “driveability, hammer and drill availability, forces on the pile gripper and pile weight and length” will pose new challenges to offshore engineers planning wind farms with the “maxi” monopiles.

“The overall conclusion is that the benefits to be gained by utilising the experience already developed in relations to monopiles [would] lower the risk profile and hence lower the cost. When it came to jackets, more work is required in order to ensure a suitable and proven supply chain for large-scale developments.

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“It is too soon to say precisely [how the per-unit cost of an XL monopile would compare to a conventional quattropod jacket] but it would be significant given the ‘burn rate’ of lost time due to weather during installation.”

Despite its promising on-paper economics, MT Højgaard’s XL monopile concept — 70 metres long, 6.5-8-metres in diameter at the mudline and some 800 tonnes in weight — pushes the boundaries of manufacturability.

“Our sub-suppliers can do cones of eight metres’ diameter but not in cost-efficient construction — there are limits on the fabrication side at around seven metres,” states Andersen. “But there is no-one in Europe with a production line that is efficiently producing jackets in large number either. So you could say that the things that you have to overcome to produce big jackets are the same as those for big monopiles, at the moment.”

One of the key challenges in building an extended-length monopile lies in the way they respond to loads — cyclic, moment and axial — caused by the combined might of winds and waves. Andersen believes that the industry “has learnt so much in recent years” in this area that the new, extended-length designs will be more robust.

He also reckons strides need to be taken towards designs that “integrate” monopiles, transition pieces (TPs) and towers to improve the overall structure’s behaviour in the face of these forces.

“It is a lot about how you distribute the loads and weight through the whole monopile-TP-tower,” says Andersen. “To get the best efficiencies out of the XL monopile we have to look at the construction as a whole rather than as different pieces connected together to support a turbine.”

MT Højgaard is currently bidding on three projects being built in depths of at least 30 metres for a maiden outing for its XL monopile concept, including for the first phase of RWE’s 1GW Innogy Nordsee in the German North Sea.

“Each of these projects, if they had been tendered a year ago, would have been considering jackets,” says Andersen. “If the XL monopiles perform as expected, then you will be able to accommodate a bigger part of the market, but I also see jackets and gravity bases as being designs you will also need in order to fully service the market.

“When we say that we can go into as much as 35 metres [of water] it is ultimately dependent on soil conditions. You need to examine every single site to determine which is the best foundation [whether monopile, jacket, CGBS] for the job.

“The speed of adoption of new technology is so fast now. This is a new industry — 14 months is a long time. Still, [the XL monopile] is very much a [design] evolution not revolution in scaling it up.”

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