Quality assurance and scour protection at the Amrumbank West offshore wind farm

Geotextile sand containers were used as an alternative to a granular scour-protection system for the Amrumbank West wind farm in Germany. There were two decisive arguments in favour of the “Soft Rock” solution: The sand containers required only a third as much of typically used rock material, and in the construction schedule, their installation is logistically uncoupled from the process of driving the foundations.

The Amrumbank West offshore wind park is situated 35 km northwest of Heligoland and 37km west of Amrum; it extends over an area of 34km². Here, E.ON is installing a total of 80 Siemens 3.6 MW-class wind turbines. The wind farm is scheduled to gene-rate a total of 288 Megawatt, which is enough to power 300,000 households.

Geotextile scour protection requires two-thirds less material than rock scour protection

The foundations for the wind turbines are situated at a water depth of -19 to -24m LAT (Lowest Astronomical Tide / sea-map zero) and are constructed as monopiles (“single posts”) with a maximum diameter of 6m. A double-layer scour-protection system is used to optimize the length of these piles, and this was originally de-signed as a rock system with a total thickness of 2.4m. But, with a thickness of only 0.8m, the geo-textile alternative consumes significantly less material with the same functionality.

Cutting tests at Hanover University

To assess the influence of the geotextile scour protection on the driving work, cutting tests were carried out prior to the tender procedure at the Institute of Geo-technical Engineering of the Leibniz University Hanover. The testing was focused on the execution of the lower end of the monopile and on the behaviour of the sand container during driving. It was observed that the monopiles penetrate the scour protection purely under their own weight (there is no cutting action), and that they pull the sand containers into the sea bed along with them. The sand remains inside the sand containers, since the geotextile does not necessarily tear until it is beneath the sea bed and is then trapped in the monopile/soil shear interface.

Filling of the sand containers on site

Additional advantages of the sand containers become evident during construction. Since the fill material, sand, is available locally, transport costs can be significantly reduced.

The Naue Secutex® Soft Rocks are 1.45m x 2.38m in size and are made from Secutex®, a specially bonded staple-fibre nonwoven made into containers using a high-performance sewing yarn. Starting in October 2012, Naue produced 50,000 sand containers and delivered these to Rømø in Denmark. There, the contractor Peter Madsen Rederi A/S (PMR) filled each container with 1,350 kg North Sea sand, a degree of filling of 80%. PMR developed a special filling line with a capacity of 500 bags per day.

Time-independence of installation steps

Perhaps the most significant advantage of the geotextile sandbags became evident when installation logistics were considered. The rock scour protection would require the filter layer to be placed immediately before the monopiles were driven and the top layer shortly thereafter. The additional sand containers and the second layer were installed. After a third survey, sand containers were placed where necessary to ensure the two-layer coverage with a total thickness of approximately 0.8m.

Inspection with underwater camera

To complement the GPS seabed survey in assessing the quality of the scour protection, eight locations were inspected using an underwater camera and the layer thicknesses verified.

During foundation construction (beginning in December 2013), it was observed that the monopiles penetrated the scour protection purely under their own weight, as had been indicated in the cut-ting tests.

A camera inspection is now planned to indicate the condition of the sand containers after the driving operation. In addition, the possible placing of additional sand containers in the vicinity of the piles is easily available if needed.