The offshore wind farms standing proud in the North Sea are in danger of tumbling down unless developers start using a new way of anchoring them to the seabed.
By Jason Deign in Barcelona
Gaze at an offshore wind farm and it is hard not to be moved by the might and power of it all. Unfortunately for the industry, however, at least one element of many offshore installations does not have all the might and power it should… and its effect is moving in a more worrying way.
In a September 2010 Europe wind energy advisory note, IHS Emerging Energy Research alerted: “At approximately 600 of Europe’s installed 948 offshore turbines, dissolved grouting has shifted turbines within monopile foundations.
“Retrofitting these could cost developers as much as EUR€120,000 per turbine, equalling up to €9.6 million at larger projects such as Horns Rev, although future projects using monopiles are not assumed to be affected.”
The problem stems from the way marine turbines are fixed onto their monopile foundations. The monopile is a massive hollow steel rod that gets hammered up to 40 metres into the seabed.
A tapered, hollow transition piece is placed over the top of the monopile like a glove, and the turbine tower is then plugged into the top of it. To prevent the transition piece from moving, the gap between it and the monopile is filled with grout. And that is where the fault has occurred.
“A challenge may occur if the transition piece is settling on the pile such that it rests on temporary supports required for the installation phase,” says Svein Inge Leirgulen, spokesperson for DNV, a risk-avoidance foundation that looked into the problem in 2009.
“These supports are not designed to transfer bending moment from wind loading. Due to the dynamic loading from the wind, fatigue cracks can initiate and propagate into the main steel structure. How severe it is depends on loading and utilisation with respect to capacity.”
This is not the first time monopiles have caused problems. The IHS note points out that: “In 2009, the Greater Gabbard project was struck by quality issues in the welding of its monopile foundations, supplied by China’s ZPMC.”
However, the Greater Gabbard fault was an isolated case. When it comes to grouting, though: “In theory all towers that are designed as cylindrical-shaped grouted connections without shear keys may be affected,” says Leirguien.
By the time the IHS note was published, “almost everything was a monopile,” says Marc Muhlenbach, the company’s Europe wind energy advisory analyst.
And the other offshore foundation options, including tripods and gravity foundations, would likely require a similar sort of grouted connection. Nevertheless, there are ways to deal with the grouting problem.
Says Leirguien: “Where cylindrical-shaped grouted connections are already used offshore and where the design is such that settlement may be expected, additional support arrangements should be considered. A repair will imply installation of alternative supports.”
He explains that this requires welding inside the towers before the supports can be installed. “There are already supports developed that are tested and considered to be reliable,” he adds.
So far the high cost of repairs “seems somewhat manageable, unless every last monopile has a problem like this,” Muhlenbach says. “It’s a teething issue.”
And for new developments, DNV, working with 11 offshore wind power players in a joint industry project, seems to have the answer: change the shape of the grouted section of the monopile so it forms a cone.
This concept is already being put into use in developments such as the 1GW, 177-turbine London Array Limited (LAL) installation that began in the UK Thames Estuary this month with backing from DONG Energy, E.ON and Masdar.
“LAL is adopting a conical solution to the problem of grout failures, in conjunction with DNV,” confirmed the company’s media manager Joanne Haddon. “During the development stage of the project, LAL considered monopiles, jackets and gravity foundations.
“Monopiles were more suitable and generally cheaper than jacket foundations [a lattice tower designed for deeper installations] for all bar some 50 deep water foundations, for which we have invited tender proposals.
“Gravity foundations were discarded because of the environmental impact of their footprint and cost; they would also not be suitable for the deep water foundations.”
Haddon adds that monopiles have been used as turbine foundations for all the projects in the Thames Estuary to date, including Kentish Flats, Gunfleet Sands, Thanet and Greater Gabbard, and LAL will be installing measuring systems to monitor its turbines.
Provided the DNV concept works, grout problems should thus become just another chapter in the growing volume of wind energy industry learning. “It doesn’t seem like it’s been much of a hang-up,” Muhlenbach concludes.
And for potential offshore players it will be a valuable lesson in the challenges ahead.
Speaking for Spain’s wind energy industry, which is not planning to venture offshore until next year, Emilien Simonot, project engineer at the wind energy industry association Asociación Empresarial Eolica, says: “We are learning from the experience of others.”
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