PO065

Vibration installation and removal of a monopile

Jan van der Tempel, Robert Hasselaar, Thijs Kamphuis
DOT, Delft, The Netherlands

Abstract

Foundation installation for offshore wind turbines is mostly done using hydraulic impact hamers. Recent advances in legislation have put restrictions on noise emisions to reduce chance of negative effect on under water animals. This resulted in turn in a large number of noise mittigation tools. They work, but cost money and time.

Vibration installation is common practice for many onshore and marine applications. But for some reason, the offshore wind industry is not yet embracing this technology. A test in North Germany seemed to show positive results, but the data is still being studied: 2 years and counting.

For the development of the DOT turbine, a 4m diameter monopile was installated by a floating spread on an onshore location. The pile was lifted and tilted by a special vibro block with lifting and tilting capabilities. The pile was vibrated into sandy soil to a full penetration depth of 15m. Extensive soil analysis, vibration analyses and installation measurements were done and processed.

After 6 months of turbine testing, the pile was removed with the same tool. Also this process was monitored and measured.

The campaign was a success and delivered valuable insight in operational issues, soil-structure interaction and a proof that whatever is vibrated in, can be vibrated out.

Method

For the DOT500 test, a 4m diameter monopile was constructed. On site, CPTs were made to assess the soil properties. These were used to assess the required penetration depth of the pile and to make an installation prediction for the vibro tool. Because the location of the test was unknown when ordering the steel, and one of the options was a location next to the quay, in the water, the pile was longer than required for the onshore location. This gave the option to test penetration with the vibro tool to deeper depth of 15m, where 8m was sufficient for turbine stability.

The installation was monitored and an in-place measurement was done when the installation was complete. In a similar way the removal was monitored.

Results

The results include insight on the installation procedure for lifting and tilting a monopile from horizontal to vertical and then virbrating it in. The operation was performed from a shear-leg barge. Initial inclination was over 2.5 degrees, which was corrected by retracting the pile and correcting. The final inclination was within 1 degree, which was the required tolerance for the test.

Installation time was much longer than anticipated due to the compactness of the soil (recently reclaimed new land).

Extraction was done in only 12 minutes.

Valuable insight was gained in calculation methods and validation with the real operation.

Conclusions

The conclusion of the paper are that vibration installation and removal can be done swiftly and efficiently and the operation can be well predicted by existing analysis methods. For this test, the bearing capacity was sufficient, paving the way to use this type of installation method for offshore use. With a floating crane barge verticality within 1 degree can be reached.

Objectives

Delegates will get a first-hand view of the installation method and removal of the monopile. They will also learn about the analysis relating to measurements.