In January 2014, six companies got together to test the use of vibration piling as a quieter, faster way to sink monopiles into the seabed. The results aren’t finalised yet, but what’s the current status?

RWE Innogy, E.ON, Dong Energy, Vattenfall, EnBW and Bilfinger Marine and Offshore. When a line-up of companies like this gets together to support an innovative project, you would expect them to make quite an impact. In the case of this pilot project, however, the situation is quite the opposite.

WHERE’S THE WATER?

The scene is Cuxhaven in Lower Saxony, Germany. Here, monopile foundations are being sunk into the ground using the simple method of vibrating the sandy earth so that it liquefies, allowing the heavy foundation to ‘fall into’ the space created. Granted, there’s no seawater in sight, but the sand density and groundwater conditions of the site is identical to much of the North Sea’s bed, making it the perfect place to conduct tests.

The tests involve, on the one hand, installing three 21m-long, 4.3m-diameter monopiles into the ground with vibration piling. On the other, a control group of three identical monopiles is being sunk using traditional hammering. This allows results to be compared in an accurate and
meaningful manner.

The project team aims to understand a number of important parameters. However, the main aim is to use lateral load tests to determine if the foundations installed completely using vibration are just as stable under lateral loads as their conventional counterparts.

EXECUTION AND ANALYSIS

The six piles were successfully installed in June and July 2014 and the lateral bearing capacity of the piles has been determined through lateral load tests thereafter. The three pairs of hammered or vibrated piles were each submitted to loading cycles using a 20MN test rig. To do this, the Technical University of Braunschweig installed measurement and monitoring equipment and recorded the results. Final analysis
of the results is currently underway and is expected to be finalised in April, allowing the project partners to aim for offshore execution in the near future.

BIG BENEFITS

If all goes according to plan, the six companies hope to be able to take a major step forward in wind farm construction, removing the need for additional noise mitigation devices, slashing piling time by over 50 percent, and optimising the pile installation sequence. There’s an  additional advantage too, in that a lower level of fatigue is induced in the steel during installation, meaning that the material’s structural
integrity is preserved to a greater degree.

CARBON TRUST SUPPORT

The project has the funding support of UK-based Carbon Trust’s Offshore Wind Accelerator Project, which has covered a large portion of the costs. RWE Innogy is the project lead and holds the overall responsibility for the project. Bilfinger Marine and Offshore, as well as being a
project partner, is the main engineering and installation contractor. Finally, the project was made possible through significant industrial in-kind contributions from Steelwind Nordenham, IHC Merwede and PVE Diesko.

PVE Diesko

For the test site, Dutch-based PVE  Diesko used the 500M vibrator and hydraulic power package with 12 vibrating elements. First, a crane lifts the vibrator to the top of the pile. The 60-ton machine (conventional hammers can be four times as heavy) is held in place on the pile by four clamps, each developing some 300 tons of pressure. The soil around the pile is then liquefied, allowing penetration.

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