Influence of initial soil state and installation parameters on the lateral behaviour of vibratory-driven monopiles in sand

Abstract

To support offshore wind turbines (OWT), monopiles are currently the most frequently used foundation method. These monopiles are open-ended steel tubes with a diameter of 5 - 12 m, which are most often installed in the seabed by means of hydraulic impact hammer ingusually mounted on a specialised ship. This method is produces high noise levels and releases shock waves into the water, which can damage the sea life’s hearing or even outright kill them. Additional measures can be taken to reduce noise emissions and adhere to strict environmental regulations written for marine biology protection, but they are costly and slow down projects significantly. There are other installation methods which have the potential to be more silent but are not well understood yet from a noise generation, drive ability, and lateral behaviour point of view. These are aspects that will be investigated within the SIMOX Joint Industry Project (JIP). One of these methods is vibratory installation.
A monopile used as a foundation for an OWT will experience a multitude of loads during its service life, the most important of which are the lateral loads. While the behaviour of impact hammered piles under lateral loading has been researched extensively, the behaviour of vibratory driven piles is still relatively unknown. The influence of different parameters used during installation (frequency, penetration speed)and other conditions (wall thickness and soil conditions) on that behaviour must be understood to be able to accurately predict how a vibrated monopile will react to both cyclic and monotonic loading.

The present research explores the behaviour of monopiles under lateral loading and the impact different installation parameters have on it. To achieve this, a laboratory testing campaign was carried out with model piles. The purpose of these tests is to produce qualitative results to be used in following field-testing campaigns within SIMOX. Piles were installed with differing installation parameters in sand beds with different density. These piles were then subjected to initial monotonic loading, followed by cyclic loading, and then monotonic loading again. The data obtained during the experiments was then interpreted and analysed. By comparing the results as well as measurements taken during and after installation, conclusions are made concerning installation parameters and other factors that may play a role on the lateral behaviour of monopiles.

The interpretation focused mainly on pile head displacement during initial lateral loading depending on penetration speed and frequency. The loading tests have shown that impact hammered piles under went lower displacements than vibrated piles after being loaded laterally. When solely considering vibrated piles, piles with a larger wall thickness showed lower displacements in general than thin-walled piles.

Frequency and penetration speed were found to play a role in the lateral behaviour of monopiles. In the experiments considered for this thesis, it seemed in dense sand crane-controlled piles showed lower displacements than free-hanging piles. In medium dense sand, lower penetration speed led to lower displacement during loading, but more research is needed on this topic to be able to formulate clear conclusions on the exact role of each installation parameters. The experiments also show an interesting phenomen on regarding measured soil elevation that might link compaction around the monopile to lower lateral displacements. The difference in elevation before and after installation seemed to correlate with lateral displacements. In general, piles with larger compaction around the pile displaced less during initial loading.

In conclusion, this paper provides a range of observations regarding the impact of installation parameters and other conditions such as wall thickness and sand density on the behaviour of monopiles under lateral loading, as well as offering a comparison with impact hammered piles. Recommendations and suggestions are given for further research and for the field testing experiments, so that the analysis made here may be used to predict the lateral behaviour of vibrated monopiles more accurately.