Offshore wind design

Grouted connection – will it live to fight another day

The offshore wind turbine market is moving towards ever-larger 12MW, and even 15MW generators, to match the sector’s rapid growth. But what kind of connection will be used between foundations and transition pieces?

( Text content written by Twenty6 for Diego Cocco, LIC Energy )

Big is now beautiful

General Electric’s 1st March 2018 announcement that its new Haliade-X 12MW wind turbine generator (WTG) could be on the market just a few years from now is confirmation of the sector’s continuing move towards increasingly large offshore wind turbine generators (WTGs).

The GE machine, now the world’s largest offshore wind turbine, is designed to produce 45 percent more energy than any other available offshore wind turbine and is capable of generating up to 67 GWh annually.

The obvious follow-up question that springs into my mind is will monopiles continue to be the most cost-effective foundation solution with lowest operational risks for the impressive upcoming generation of highly-innovative WTGs?

And if they are, as I suspect they will be, will the connections between foundations and transition pieces be flanged or grouted?

Monopile magic

Let’s look at monopile foundations first. Their flexibility keeps on surprising us, and, as I have already mentioned above, they are highly-valued for the low risks they pose on projects. Easy to manufacture, transport and install, they are popular from a scheduling and supply-chain perspective when compared to jacket foundations.

Their limit envelop is being stretched continuously, to the point where engineers are now less shy about discussing the idea of monopiles of more than 2,000 tonnes with diameters of up to 10m. There is also a growing movement within the industry to keep on optimising and extending monopile design parameters as an alternative to trying to beat the restrictions of jacket foundations.

Making connections

With monopiles holding on to their supremacy, connections are the next question. The grouted connection was quite popular among the first wind farms. However, a series of cases of grout failure due to errors in design codes during the early years damaged the reputation of this type of connection.

Shear keys and conical grouted connections were introduced as design improvements to keep grouted connections in the market. But in the hotly-contested race to reduce energy costs, developers and engineers were pushed towards ever more robust solutions. Flanged connections joined the running and are currently leading the competition.

Flanged connections today

Flanged connections are now well-respected and in fairly common use throughout the offshore wind sector. Where there are grey areas, more conservative design seems to offer a satisfactory solution. However, O&M procedures have also proved to be more involved than expected.

While engineers and owners tackle these issues, the next question is whether flanged connections can be scaled up to meet the development of next generation WTGs?

Bolt connection size, flange forging thickness, installation procedure, long-term preload relaxation and O&M procedures are among the main points being discussed.

One side of the argument suggests that advances in manufacturing and monitoring technologies may actually make a pivotal difference in helping the flanged connection to move the ahead in the market. The alternative case is that the simplicity, robustness and scalability of the grouted connection is now being viewed with increasing interest by contractors.

Open debate

The debate is still open and the ‘flanged versus grouted’ contest is far from over. What is recognised, however, is that the flanged connection must leap forward if it wants to keep pace with market growth.

Other forms of connection are being considered, such as the double slip joint and the blue-wedge connection. Interesting though they are, they are not yet seen as being on a par with flanged connection developments.

However, they could be symptomatic of the market’s need for change.

Watch this space to see what happens next!