Abstract Summary
Offshore wind is a primary source of renewable energy. The international sustainability targets - set in view of the energy transition - require a significant increase of offshore wind capacity. This leads to ever increasing size of offshore wind turbines (OWTs), water depth of installation and distance to shore. As a consequence, engineering challenges continuously arise in the design and installation of OWTs and innovative solutions are required to accommodate the growing offshore wind demand. Currently, OWTs are primarily supported by bottom-fixed foundations – in particular, monopiles correspond to over 80% of installed OWTs up to date in Europe. Monopile installation is mostly performed by means of impact hammers. However, impact piling raises major environmental concerns related to underwater noise emissions, which can be only partially mitigated by employing costly sound-proofing measures. As a result, alternative installation techniques that are environmentally friendly and high-performing are vital for the offshore wind industry. Vibratory pile driving methods pose an efficient alternative to impact piling and have been widely employed in onshore applications. However, their deployment in the offshore environment remains limited. To further boost the potential of vibratory techniques, the Gentle Driving of Piles (GDP) was developed and successfully tested throughout an extensive experimental campaign in 2019. This technique is based on the combination of high-frequency torsional and low-frequency vertical vibrations. The installation tests carried out in the preceding field campaign showcased the potential of the GDP method in terms of installation performance and significantly encouraged the further development of the method. In this paper, the introduction of new and unique features in the context of vibratory and GDP techniques is presented, along with relevant experimental results. Specifically, a lab-scale shaker that accomplishes frequency-amplitude decoupling of the input has been designed, engineered and manufactured; frequency-amplitude coupling is a common constraint in vibratory devices operating based on the counter-rotation of eccentric masses. The present shaker consists of a main optimized aluminium block that connects three linear hydraulic actuators, two positioned horizontally and one positioned vertically, in order to generate torsional and vertical vibrations. The actuators are position-controlled during installation, and their amplitude and frequency are variable and independent to each other. In this experimental campaign, the new lab-scale GDP shaker is used to install piles into a conditioned soil and the respective results are discussed. Particular emphasis is placed on the effect of the different shaker settings on the installation performance, which is studied by means of direct shaker (input) and pile measurements.
