Assessment of the Response of Offshore Monopile Foundations to Vibratory Hammer Action

Abstract

The demand for wind energy has led to the need for larger offshore wind turbines, which require larger monopiles for support. Vibratory pile driving is a common technique for installing these monopiles, but current engineering models are designed for smaller diameter monopiles and may not be applicable to larger ones. Additionally, the vibratory action produced by counter-rotating eccentric masses in the hammer can cause unintended bending motions in the pile if there is a slight misalignment. This study aimed to investigate the effects of gear misalignment on the response of monopiles under vibratory action, including these imperfections.

To achieve this, a hammer model was created and coupled with a pile model and a simplified soil reaction. A parametric study was conducted to evaluate the pile response to the modeled vibratory hammer with imperfections over a range of specified variables. The transfer of force from the hammer to the pile and the range of driving frequencies that activate the isolation springs in the vibratory hammer were examined, as well as the effect of misalignments on the power consumption of the vibratory hammer and the bending displacements caused by these misalignments.

The study found that there was no significant impact on axial vibrations as a result of misalignment of rotating masses.

The force transfer ratio from hammer to pile is 0.5 to 0.7 for smaller diameter piles, and it is important to include force transfer ratios in simple engineering models during the initial design phase of pile drivability prediction models when the decision is made not to model the hammer.

The study also showed that modeling the hammer-pile-soil system with shell elements instead of 1D rod elements yields similar trends in the responses, but the absolute response values of large diameter piles with simple 1D rod elements leads to an underestimation of the response in the lower driving frequency range and an overestimation in the higher driving frequency range.

Even if as much as 50% of the eccentric masses are misaligned, the impact of bending vibrations on the power consumption of the vibratory hammer is insignificant. However, bending displacement can become substantial when 6% of the eccentric masses are misaligned. Smaller diameter piles are more susceptible to vibrations at lower frequency ranges, while larger diameters are impacted in higher frequency ranges.

Only large diameter monopiles are susceptible to axial vibrations near their natural frequencies, while all diameter piles are affected by bending vibrations near their natural frequencies.

Isolator springs are effectively activated in the region of higher driving frequencies (> 28 Hz) for large diameter monopiles. In the case of small diameter piles (< 3 m), the relative motion between suppressor housing and pile head remained above unity for all driving frequencies, meaning poor activation of the isolator springs.