Offshore monopile failure - a solution may be in sight

OFFSHORE: A remedy may have been found for the design flaw that is present in a large number of offshore wind farms. But its implementation may force the sector to look at how it works, writes Windpower Monthly technical writer Eize de Vries.

The recently completed monopile filling process at the Dutch OWEZ wind farm
The recently completed monopile filling process at the Dutch OWEZ wind farm
The number of offshore wind farms to be affected by the foundation problem that causes the monopiles to slip has risen to 10, according to well-placed sources. However, some relief could be on the way.

The first successful remedying action has been completed this June in the Netherlands on the 108MW OWEZ offshore wind farm jointly owned by Royal Dutch Shell and Swedish utility Vattenfall.

It is just as well. Figures presented during EWEC 2010 indicate that monopile foundations represented 88% (MW basis) of all offshore wind turbine installations in 2009.

To underline this, another statistic showed that ending 2009 showed that of the 830 turbines (2GW) operating in European waters, 65.2% of this capacity were built on monopile foundations.

Alternative monopile concepts

Semi-standard monopile substructures consist of a pile driven into the seabed and a transition piece (TP) that slides loosely over the top and to which the tower is mounted. The circular gap between pile and TP is filled with a special concrete (grout) creating the bonding.

Corrective monopile concepts have been developed for two wind farms. The 60MW Scroby Sands (UK) wind farm features a solution without TP and instead a tower-mounting flange welded to the pile top.

Monopile slippage was first officially reported for Egmond aan Zee (OWEZ) during last year autumn. It is, according to Dutch experts, the first wind farm where the problem has been successfully tackled. Designed by Dutch civil engineering contractor Ballast Nedam, it comprises a TP that fits into the pile with a grout connection connecting the two components. This design strategy specifically aims at materials (steel) optimisation by reducing the surface area exposed to wave loading.

Ballast Nedam has also filled all 36 foundations from inside the turbine towers with concrete.

This is conducted up to a certain level whereby the TP's steel bottom section is over a short distance submerged into the liquid concrete. After the curing process adequate loads transfer between TP and pile has been re-established.

The filling operation itself took several weeks and was completed by mid June 2010, said Shell press spokesperson André Romeyn. Experts are now monitoring the end result. The operation itself was conducted from a floating (moving) vessel equipped with an advanced Ampelmann access system.

Straightforward solution 

Monopile foundation expert Jan van der Tempel is Assistant Professor Offshore Engineering at the Delft University of Technology in the Netherlands. Commenting on the monopile foundation slippage issue, he highlighted difficulties in replicating the solution on other affected projects.

He said: 'The specific OWEZ design enabled a relatively straightforward remedying solution. But, a similar remedying method cannot simply be repeated for monopile foundations comprising an external TP for structural reasons. Alternative remedying solutions involving welding will prove complex and costly. The grout can hamper such operations and the typical 50mm to 80 mm steel wall thickness of the piles and TP provides a real challenge."

Van der Tempel in this respect pinpoints the unwillingness of the wind industry to exchange information of information. He uses the Scroby Sands and OWEZ monopile designs as an example. He said: "Both solutions have been tried only once and I would like to know why? Do they for instance not function according to expectations well, or is the cost price an issue?"

He believes that part of the answer can be attributed to the conservative, risk-avoiding manner in which offshore wind farms are financed and insured. The focus on track-record leads to a preference for tried and tested solutions, explaining why the majority of offshore wind farms are so similar. Simultaneously, he argues, the offshore wind industry should become less conservative and instead more geared towards innovation.

TP, a time for re-evaluation?

In parallel with these observations Van der Tempel says that the original concept that led to the use of a TP as monopile foundation element needs re-evaluation: "A TP's main function is to compensate for a maximum 0.5-degree inclination from vertical position after pile ramming. However, multiple results clearly show that today the piles are usually hammered almost vertically into the seabed. Taking that into consideration, it might be an idea to eliminate that initial TP function completely.'

Van der Tempel further refers to a 2002 study he conducted as part of a Dutch R&D programme. It analysed track record experiences and up-scaling potential of a so-called slip-joint connection between wind turbine foundation and tower, as an alternative to traditional flange-type bolted joints.

These slip-joints comprise two parts. The first is a top section with a slight outer coning, and the second a section with a matching internal coning surface. A firm connection is achieved by 'dropping' the upper part from a brief height onto the lower section.

He explains: "This solution incorporated in about forty 750kW former WindMaster turbines has, for a long period, proven to be fast, reliable and cost-effective. Our research indicated that the slip-joint design is at least scalable to 5 – 6MW power rating, including offshore applications. It can potentially offer a superior alternative for the current TP's other main functions, like J-tubes fitting, access ladders and boat landing vertical tubing."

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