ENECO: Update on the grouting issue of Princess Amalia Wind Farm

On 1 July the Dutch offshore wind farm Eneco Luchterduinen became fully operational. On 21 September this milestone will be celebrated with an official opening ceremony in IJmuiden, the port town from where the main construction activities have taken place and from where the wind farm will be maintained throughout its lifespan. The wind farm is Eneco’s second offshore wind farm in Dutch waters.

At the same time the Dutch utility has something else to celebrate on the Princess Amalia Wind Farm, their first wind farm in Dutch waters, which, although operating since 2008 has been facing problems with the grouting.

At a press conference at their office in IJmuiden, a spokesman from Eneco explained how they tackled this issue. In 2008 the industry was alarmed when at the Dutch Egmond aan Zee offshore wind farm detected a settlement on one of the turbine foundations. The reason is a flaw in the grouting. Grout is a volume stable cement-like packing between the transition piece (TP) and the monopile which ensures optimal horizontal positioning of the complete turbine (and distributes vertical load). Since the use of the TP – Monopile-combination with a grouted connection was the standard at that time it was reason enough for the industry to set up a Joint Industry Project who would look at possible solutions to ensure that the grouting problem would not lead to long term problems, and avoiding even the smallest chance of the turbines collapsing.

The grouting concept used at offshore wind farms at that time was based on the oil and gas industry and DNV GL certified. While perfectly suitable for the oil and gas industry, the situation at Egmond aan Zee demonstrated that it might not be sufficiently robust for wind turbine foundations where some stresses, such as those caused by the movement of the turbine itself, might be affecting the grout connection over time.

As a result, DNV GL withdrew the grouted connection certification. The Joint Industry Project came up with a few possible solutions to prevent further inclination of the foundations at affected wind farms. In 2011 Eneco decided also to check the foundations at their Princess Amalia Wind Farm and performed a zero-measurement inside the confined space of each of the foundations. A second inspection followed in 2012, showing an average 1 to 2mm (annual) inclination. After the design of a solution, Eneco executed a pilot repair case on one foundation to be followed by all the remaining foundations during the next few years. A project that would turn out to involve a total investment of around € 47 million.

Bearing solution

Eneco looked at the ideas from the Joint Industry Project and decided to design a bearing connection between the inner wall of the TP and the inner wall of the top of the monopile, there where the upper rim of the monopile meets the TP. The company chose a concept with rubber bearings from an American company which was then designed for use at the wind farm. The rubber bearing components were manufactured in Asia.

The mitigating solution consists of two steel blocks that are welded, one to the inside wall of the TP and one to the inside wall of the monopile. These are then connected by 2 steel plates and in between a rubber bearing. The elements are then tightened using a hydraulic pump.

Working process

In workable weather conditions, five vessels carrying nine teams of in total sixty technicians and supervisors transferred to the wind farm each day. The repair work was performed by Fabricom Offshore Services. Before starting the work offshore all welders and other technicians had to go through extensive pre-testing on mock-ups to ensure that the work would be done correctly offshore.

These technicians would have to fix 24 bearing connectio ns in each foundation involving 1,800k of material in total. However each component or tool did not exceed 25k therefore complying with the manual lifting health and safety standards for the technicians.

Repair work was not able to start straight away as the area concerned was sealed off after installation work. The man hole had to be opened first followed by ventilating the air in the confined space and putting lights into place. To create the optimal work environment a platform, which was at the level of the TP connection had to be lowered into position.

Once a working area, which complied with the ARBO rules for confined spaces, was created the technicians could then start their procedures. Next, the areas where the steel blocks would be connected had to be cleared and levelled. The first steel block could now be welded against the inner wall of the TP.

A positioning frame was then used to make sure that the second steel block was placed and welded correctly against the inner wall of the monopile. Now the steel plates and the rubber bearing could be positioned between these blocks. Magnetic Particle Inspection was used to ensure the welding activities were performed properly.

On average the teams would be working on up to 12 TPs each day. The total work involved in completing one foundation was approximately 21 days. In the meantime the bearing solution received certification approval by DNV GL. In 2014 the remaining 59 turbines were inspected. The modifications on 21 TPs were completed that year, and after the winter the work continued in April this year, on the remaining TPs.

By early September all repair activities had been successfully completed and now Eneco can truly celebrate their two fully operational Dutch offshore wind farms. Eneco no longer uses grouted connections, having moved to flanged foundations bolted to the tower on the Eneco Luchterduinen wind farm.

Sabine Lankhorst

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