Developing the Vibro-drill

Developing the Vibro-drill: A feasibility study into an improved installation method for offshore foundation piles

Arntz, Ben (TU Delft Mechanical, Maritime and Materials Engineering)

Metrikine, Andrei (mentor)
Hoving, Jeroen (mentor)
Pisano, Federico (mentor)

Delft University of Technology

Offshore and Dredging Engineering

2018-01-25

To meet the demand for renewable energy sources, large numbers of offshore windfarms are planned to be constructed in the near future. The majority of the foundations for these offshore wind turbines are large-diameter monopiles. The driving of monopiles into the seabed is conventionally done using a hydraulic hammer, this process is commonly known as piling. Piling causes high noise levels that are damaging to marine life, for example it is known to cause permanent hearing loss to marine mammals. Also smaller fish are known to be affected by this noise. Piling additionally causes fatigue damage to the pile during the installation process requiring over dimensioned monopile foundations.

A potential solution to these problems is currently being developed by GBM Works; To install a monopile foundation, a set of vibrating elements is installed at the bottom of the monopile that reduce the resistance of the soil at the tip of the pile. Water is injected upwards on both sides of the pile wall to lubricate the pile-soil interface. In this way, the resistance of the soil is temporarily reduced to such an extent that the monopile foundation penetrates the soil to target depth under its own weight, thereby greatly reducing noise levels and fatigue damage.

To determine the feasibility of this installation method, an experimental study has been performed, where a prototype of one oscillating element was constructed and attached to a steel pile. Tests with this prototype were performed at the Maasvlakte, where very dense sand is found with cone resistance values of up to 30 MPa in the test depth range. Multiple experiments were conducted in which the amplitude and frequency of the vibrations were varied. Tests were done for a large range of variables in attempt to optimize the penetration speed. Additionally, based on the test results, a semi-empirical model was built to predict the installation speed of the prototype pile and to identify the influence of the different variables.

The experiments show that it is possible to penetrate the prototype pile to its full penetration of 4.5 meters into the soil. Depending on the properties of the applied oscillations and soil conditions, the penetration rates are found between 1 and 5 cm/s.

To determine the feasibility of reaching penetration depths that are similar to those of offshore foundation piles, i.e. 30 to 40 meters with larger diameter monopiles, additional experiments are required .The noise levels during the experiments were significantly lower than the noise levels for conventional piling. However, as sound in air propagates differently than in an underwater environment underwater measurements are required to further assess the noise levels of this new installation method for monopile foundations.

Foundations
Offshore
Noise

http://resolver.tudelft.nl/uuid:5809ce3f-f82b-4422-9f24-29a0330df999

2019-01-25

Student theses

master thesis

© 2018 Ben Arntz