Abstract Summary
Presently, over 80% of the offshore wind turbines in Europe are founded on monopiles. These large tubular substructures are commonly installed via impact hammer. During impact piling, a hammer is mounted on the pile top and the applied pulses progressively drive the pile into the seabed. Notwithstanding the simplicity and robustness of this method, impact piling poses alarming problems related to pile structural damage and significant underwater noise emissions, which are harmful to aquatic species. With a view to improve monopile installation in terms of performance and environmental aspects, alternative techniques are investigated. Vibratory pile driving is used onshore for decades and possesses advantageous features such as high installation speed and low axial pile stresses. In offshore monopiles, vibratory installation is hindered by the incompleteness of available field observations and knowledge gaps related to drivability, energy efficiency and lateral pile response. To further boost the performance of vibratory methods, the Gentle Driving of Piles (GDP) has been proposed and tested successfully by TU Delft. The GDP method is based on simultaneous application of low-frequency axial and high-frequency torsional vibrations, with a view to enhanced installation performance and reduced underwater noise emissions. In this paper, the focus lies in the study of the GDP method with the aid of field data and numerical modelling. Medium-scale field tests have been executed at Maasvlakte II site, at the port of Rotterdam, in which the proof of concept of the method was achieved. In these tests, piles were installed by means of GDP and conventional installation techniques, in order to facilitate their comparison. Furthermore, a 3-D axisymmetric model is presented for the analysis of vibratory and GDP installations. In particular, the model is comprised by a thin cylindrical shell (pile), a linear elastic layered half-space (soil) and a history-dependent frictional interface. The coupled non-linear pile-soil problem is solved by a hybrid frequency-time approach based on sequential application of the Harmonic Balance Method (HBM). Comparison of numerical results against field data testify the model validity for the study of pile installation and showcase its predictive potential. Both axial vibratory driving and GDP are analyzed in terms of installation performance, induced ground motion and energy efficiency, with the aid of the benchmarked model. Conclusively, the effect of the high-frequency torsional excitation is realized, with friction force redirection being the major mechanism that leads to superb installation performance.
