Slipping turbines trigger investigation into design

OFFSHORE: The wind industry has been rocked by discovery of possible flaw in monopile foundations used by most modern offshore projects.

Laying of foundation at the Horns Rev 2 wind farm
Laying of foundation at the Horns Rev 2 wind farm

Investigations are under way into a fault in the design of some offshore wind farm foundations that have caused some to slip - potentially reducing the lifespan of the machines.

Early signs are that the problem, which has already occurred in several offshore wind farms with monopile foundations, is not related to a structural or mechanical design flaw but to a little-understood physical property.

The problem is the grouting connection used at the top of the monopile, where a transition piece slides over it. The transition piece's function is to compensate for a maximum 0.5-degree inclination that may occur when the monopile is hammered in and the high stress levels imposed by the turbine are transferred through the grouted connection. Investigations revealed that the transition pieces at a number of offshore wind farms have shifted by up to 40 millimetres.

Delicate investigation

The issue is very sensitive for all parties involved. There are legal and cost implications. Whoever is deemed the culprit will face paying for the problem to be resolved.

Many of the parties involved are tied in with non-disclosure agreements regarding their current projects. A representative of UK company Tiflex said during a discussion on networking site LinkedIn that it is offering its anti-vibration resilient seating pads to isolate the tower from the foundation. This suggests that vibrations may play a role in the problem.

Only one offshore wind expert was prepared to speak to Windpower Monthly, but he asked not to be named. The design methodology of these offshore wind turbines is inherited from the offshore oil and gas industry, he explains. It has now been found not to work properly when used on wind turbines, the source adds.

For example, the source says, the guidelines assumed that the strength of the grout connection remains static regardless of the diameter of the piles. However, it has recently been discovered that if the pile diameter increases, the strength of the grout connection reduces. In addition, the friction coefficient - a measure of friction between two surfaces - between the grout and the steel surface of the pile has been found to be only 80% of the initial assumed value. The source, however, says: "The grout quality itself is essentially OK."

Design rules

The original design rules were developed by Det Norske Veritas (DNV), one of the world's leading classification societies. It is likely that these are the chief reason for the fault. DNV's communications manager, Svein Inge Leirgulen, says that his organisation informed owners and authorities, and initiated a joint industry project in October to revise the design methodology. "New analyses and lab tests at DNV showed during early fall (autumn) 2009 that parts of the calculation basis needed to be improved," he says.

Leirgulen explains that a majority of offshore wind industry players have now teamed up in order to enhance the design methodology. Solutions have already been identified as part of the project, and are partly being implemented on existing wind turbines and new designs.

The last laboratory tests will be completed in early autumn, at which time the main findings will be made available. DNV is also updating the industry standard for design of offshore wind turbine structures based on the project outcomes.

Leirgulen stresses that the previous version of the standard was based on the best available knowledge at the time, which was accepted industry-wide. It is a regular procedure for a standard to be updated, he adds. "As new knowledge or technology is developed, the standard is updated accordingly with unified best practices from the whole industry," he says.

Learning process

The oil and gas industry has decades of experience with grouted connections for joining steel components. The wind sector has only been using monopile foundations since 2002, when the Danish 169MW Horns Rev North Sea offshore wind farm, which comprises 80 2MW wind turbines, was installed.

Dong Energy says the largest displacement among its own projects had occurred in the Horns Rev 1 wind farm, which it owns jointly with Vattenfall. It says its Burbo Bank and Horns Rev 2 wind farms were also affected. In all, Dong said 164 of its turbines in Denmark and England are affected, although it appears to be an industry-wide problem.

It is not yet known how many turbines are under threat from the monopile slippage. Nor is it clear how effective the various remedies that are being put into place will be.

In a worst-case scenario, it could extend to nearly all wind farms built on monopile foundations today - around 70% of the world's total offshore installations in terms of megawatts.

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