Offshore wind design

Windpower’s November offshore fabrication conference in Brussels considered gravity bases, eternally-youthful monopiles and the growing success of floaters. However, my biggest takeaway was that with a boost to overcome their key weakness, jackets could still impress the market.

( Text content written by Twenty6 for Danny Bonnett, LIC Energy )

A gathering of some many experts was bound to raise many interesting issues. among them that monopiles and the exciting ‘young pretender’ of floating structures have obvious and expanding wind turbine markets.

But, it was also clear that heavy, expensive jackets are struggling. Which is a shame because I think jackets still have unique benefits that could see them make a major comeback to deep, remote offshore waters where the jacket’s ability to support large turbines technically and economically is increasingly being needed.

Relatively low hydrodynamic loads, form-flexibility, broad stable base widths and strong bracing configurations are just a few of their many strengths. And as Professor Fergal Brennan reminded us in Brussels, jackets can be inspected, cleaned and repaired remotely with ROVs. They have a high damage tolerance resulting from structural redundancy, something monopiles are distinctly short of!

But jackets do have an Achilles Heel. In a nutshell, the availability of very high quality steel plate means that we use fabricated steel pipes for all members which have to be joined to form nodes (braces and chords) at a point where the geometry typically creates significant stress concentrations. Rather confusingly, this configuration is often idealised as a pin, even though it is actually formed of pipe members from 1m to 2m in diameter.

An even more negative factor is that the way platforms are currently designed and built means that this is a hard-to-reach welded joint at the very point where the prising forces between members are greatest. Even if we tried hard, it would be difficult to design a more challenging structure!

However, my team believes that this is by no means the end of the story. Innovative alternatives that I describe later are quite feasible and possible. These could resolve the node problem and see the advantages of jackets returned to main stream offshore wind turbines markets.

Brussels

Windpower’s conference was clearly extremely worth attending, with a spectrum of clients, consultants and suppliers. Importantly, it identified major trends. Monopiles reduce the need for vessels. Seatower explained their crane-free gravity foundation concept. Foundation structures recently installed by EDF at Blyth were discussed.

There were also updates from around the world on floaters. Note the square moon pool concrete floater that can be seen at: – offshorewind.biz, plus the project’s recently announced successful trial at: – gicon-sof.de

A recent high-profile event was announcement was that Hywind is now up and generating.

But jackets are obviously in the Doldrums. Recent news of Bifab’s cash crisis could have many causes. The jacket market is without doubt in a hard place at the moment. Compared to monopiles, jackets are labour intensive. Though they vary, a typical monopile cost is £1,500 per tonnes for primary steel. Jackets can exceed this by a factor of three or four.

Redefining jacket design and construction

Even so, designers work hard to make jacket structures perform under fatigue loads. And hats off to the fabricators who do an amazing job putting designs into practice, given the difficultly of accessing jacket joints. This often involves working in tight congested spaces, sometimes upside down, many tens of metres off the ground!

Various approaches have been taken to circumvent this problem. A number of parties have looked into using castings – which are very expensive. Fatigue codes (most people believe) are also pretty conservative. The Carbon Trust and Belgian testing house, OCAS, are trying to resolve this with the JaCo joint-industry project

To develop a better understanding of fatigue performance, the JaCo project is testing full-size jacket nodes made from existing manual and novel automated welding processes. With improved standards, it estimates that a 10% weight reduction can be achieved if the fatigue resistance is enhanced by 10% to 20% via optimised design.

OCAS’ unique in-house techniques use a high frequency resonant rig to test welded joint resilience (fatigue performance) much faster than was possible before, potentially allowing the industry to strip out some of that conservatism in the years to come.

More innovation needed

But I still think more is needed to keep the jacket in clear water and believe that this will come from the emerging field of additive manufacture.

As the size of what we can ‘3D print’ increases, the possibilities for offshore wind are coming into greater focus. The dominant advantage of future 3D printed jacket nodes will be that they can divert stress concentrations away from vulnerable welds.

In addition, you will gain the metallurgical ability to write the recipe for your ideal material. This revolution will open a new chapter for designers who will have the chance to think about their most favourable compromise in terms of material performance. For example, they may choose resilience and corrosion resistance over ultimate strength.

Cranfield University is already doing a great deal of work in this area. The strategic implications are far-reaching. Jacket weights should reduce. Governing loads will be defined by extreme wind and wave, but no longer fatigue. Corrosion protection will be simplified. Life extensions will be defined the on the basis of continuing usefulness, not theoretical fatigue life.

A whole new world could be on the cusp of opening up for jackets.