Installation of Large-Diameter Monopiles: Introducing Wave Dispersion and Non-Local Soil Reaction

Abstract

During the last decade the offshore wind industry grew ceaselessly and engineering challenges continuously arose in that area. Installation of foundation piles, known as monopiles, is one of the most critical phases in the construction of offshore wind farms. Prior to installation a drivability study is performed, by means of pile driving models. Since the latter have been developed for small-diameter piles, their applicability for the analysis of large-diameter monopiles is questionable. In this paper, a three-dimensional axisymmetric pile driving model with non-local soil reaction is presented. This new model aims to capture properly the propagation of elastic waves excited by impact piling and address non-local soil reaction. These effects are not addressed in the available approaches to predict drivability and are deemed critical for large-diameter monopiles. Predictions of the new model are compared to those of a one-dimensional model typically used nowadays. A numerical study is performed to showcase the disparities between the two models, stemming from the effect of wave dispersion and non-local soil reaction. The findings of this numerical study affirmed the significance of both mechanisms and the need for further developments in drivability modeling, notably for large-diameter monopiles.

A mode-matching method for the prediction of stick-slip relative motion of two elastic rods in frictional contact

“…The analysis of this dynamic problem is traditionally conducted in practice by one-dimensional models [3,4], representing the pile as an elastic rod supported by nonlinear springs and dashpots, a local soil reaction analogue, subjected to the hammer forcing. However, the aforementioned models cannot capture some physical characteristics, inherent to the system, namely the non-local soil reaction [5] and the dispersion of the stress waves propagating in the pile, which can be assumed to cause inaccurate pile drivability predictions [5,6].…”

Settlement at transition zones in railway tracks – is modelling the soil as a 2-D continuum important?

“…It is important to emphasize that by considering an elastic continuum, the reaction force becomes nonlocal in both time (history effects) and space. Other works introduce the spatial (e.g., [15,16]) and temporal (e.g., [17]) nonlocality independently. While the spatial or the temporal nonlocality can be more or less influential depending on the specific application, separating the two is not desired since frequency and wavenumber are fundamentally interlinked in waves, and separating them would be, in principle, physically incorrect.…”

Energy flux analysis for quantification of vibratory pile driving efficiency

“…For the size of the piles in the GDP campaign, plugging was not expected, based on installation tests of similar scale by Henke and Grabe [20]. An additional note, focused on offshore monopiles, is that the increase in pile diameter leads to increased radial pile motion during driving (ring frequency effect) [21], which can reduce the installation efficiency as well as affect the post-installation soil state. Evidently, further experimental tests with piles of larger diameter are required to investigate such effects in more detail.…”