Offshore wind design

Efficiency – Saving steel on large windfarms

Super-streamlined monopile designs minimise steel use by doing away with welded attachments and fatigue-sensitive features. But much depends on future trends. Luke Fussell, LICenergy Deputy Head of Office, Bristol, considers pros, cons and possibilities – including life without boat landings.

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

A case where less really is more

Unnecessary steel is money lost. Extra steel adds to long-term O&M commitments. It also potentially shortens offshore turbine foundation life. Fortunately, we are now in a position to design away many traditional excesses well before primary and secondary steel begins to arrive on site.

I was kindly asked recently to present my vision to the annual IQPC Corrosion conference in Bremen of a super-streamlined monopile design that includes four key suggestions designed specifically to reduce and avoid heavy steel use: –

Bolted flange connections reducing the need for a large overlap of steel cans Gasket seals, or ‘mini-skirts’ protecting flanges out at sea without large steel skirts Thermally-sprayed metallic coating (zinc/aluminium) replacing sacrificial anodes Walk-to-work access systems instead of bump-and-jump boat landings However, these are the product of a much wider lean-steel philosophy in which we aim to pare vulnerabilities and costs down to an absolute minimum.

Twin-track approach to saving steel

In a nutshell, the concept that the team at LIC and I are working on is a two-fold approach to economic steel cutting.

The first is to reduce, or totally do away with, as many tonnes of extraneous steel as possible that to date have been accepted as an automatic part of monopile design. An important example here is the introduction of innovations that do away with the conventional steel-heavy use of grouted skirts.

The second is to systematically reduce the number of secondary welded attachments added to primary steel – ideally to the point where there are none at all!

Removing steel reduces risks

When we look at the size and remote locations of modern offshore wind farms either being built or conceived for the future, this dual-approach brings major potential benefits for developers and operators keen to keep cost profiles low and increase profits for many years to come.

By removing welded attachments, we can remove fatigue sensitive details in the primary load path. This then opens up more possibilities for structural optimisation, ultimately leading to a longer and more cost-efficient lifespan.

Turbine and market size

The importance of this area of development is best judged against market trends. Both the increasing size of turbine structures and the growing number of turbines in offshore wind farm fields under construction or on the digital drawing board for deep water metocean conditions is awe-inspiring.

The London Array’s record as the world’s largest offshore wind farm will be superseded during 2020 by the 407 km2, 1.2 GW Hornsea Project One now being constructed some 120 km off the Yorkshire coast with an estimated capacity to supply clean energy to more than one million homes. It will include 174, 800-tonne, 65m-long monopiles. With 7MW turbines and blades installed, each will stand 190m tall at maximum blade tip height.

Orsted’s plan for Hornsea Project Two could power 1.6 million homes. Hornsea Project Three is in the pipeline. In comparison, the 630 MW London Array powers the equivalent of circa 500,000 homes.

The current trend for 8MW and 9MW turbines is seeing tip heights reach circa 195m; future 15MW machines could stand at 265m. In contrast, the London Eye is 135m high and The Shard 310m. Meanwhile, onshore the German town of Gaildorf holds the record for the world’s tallest turbine to date at 246.5m.

Market share

Market share is also an important economic driver. The UK accounted for more than half of new offshore wind installed in Europe during 2017 – an estimated 53% with a net capacity of 3.15GW, according to WindPower figures. The dramatic fall in UK costs and energy prices achieved over recent years also opens up potential for more worldwide markets.

In this context, steel reduction becomes even more important.

Walking or jumping?

Depending on future developments, there are several scenarios for further streamlining and I would like to list some of the possibilities.

Walk-to-work systems that allow personnel to cross from service vessels to offshore structures via extendable bridges – rather than the traditional bump-and-jump leap of faith – are safer and less weather-dependent. However, they require a sophisticated stable vessel platform which as a rough rule-of-thumb need to be able to guarantee access for some 300 days of the year.

The larger vessels capable of supporting walk-to-work systems tend to offer a more comfortable working environment independent of weather restrictions. But large expensive vessels clearly cost much more per day than conventional, relatively-small crew transfer vessels (CTV).

On the plus side, their ability to operate safely in higher sea-states means wider installation and maintenance weather windows, and so reduces scheduling losses.

Another consideration is that larger vessels are more suited to wind farms far offshore. Also, as farm configurations expand to include potentially hundreds of turbines, having a large vessel available that can make multiple turbine visits quickly using a walk-to work system becomes more economically viable.

The counter-argument is that the sheer size and remoteness of the latest farms will lead to the installation of accommodation platforms on substations to house resident workforces. Unlike the oil and gas industry with accommodation frequently linked to individual production platforms, wind farm accommodation units will have to serve a large number of dispersed turbine sites.

It might then be more logical to use small CTVs able to nip adroitly between locations. It can also be argued that the present shortage of large vessels on the market might create a bottleneck. Meanwhile the absence of boat landings could reduce flexibility; being tied to one option means no back-up alternatives.

Autonomous options?

However, the counter-argument against this counter-argument is that the size of walk-to-work systems is coming down, opening up possibilities for the technology to be mounted successfully on smaller vessels. That would mean more and cheaper choices for developers.

Is there another even more innovative but safe and practical low-steel access solution on the horizon?

The direction of travel of autonomous systems in general now being seen across many different technologies, coupled with super-streamlining to do away with unwieldy weak points, may result in an interesting synergy.

Could totally human-free maintenance be a reality in the future?