Optimizing The Installation of steel, Open-Ended Piles Through Impact Hammering For Offshore Applications

Abstract

As a result of population growth and economical prosperity, energy consumption has been on the rise for decades. Present-day projections predict the continuation of this trend with the rapid industrialization of former second world countries. Together with the rise of energy demand, incentives fo the scientific community to quantify the environmental impact of the ever increasing need for energy have gained momentum. It now is clear that continued use of carbon-based energy sources will have a catastrophic, irreversible impact on the global climate. The urgency of this message, which is supported by nearly the entire scientific community, was finally heard in 2012 when the Paris Agreement was drafted. Nearly collectively, the world’s countries are committing themselves to start the transition to durable sources of energy. One of the most promising sources of energy to facilitate the aforementioned transition, is the wind. Europe specifically is home to large patches of sea, which are ideally suited for the construction of offshore wind farms. Due to high construction costs, these endeavors out at sea were, until recently, heavily dependent of governmental support. However, due to advances in technology, wind frams have become profitable enough to be realized without governmental grands. Of the current offshore wind farms, the majority of the budget is allocated to the foundation design, construction and ultimately installation. Monopiles are convincingly the most common foundation type found. Although alternatives under development, it is unlikely for the popularity of this simplistic foundation will diminish in the near future. Especially, as the hollow, large diameter, hollow, steel profiles are finding their way into other foundations types, in example tripods. The installation of the monopiles offshore is mostly done through costly operations involving large hydraulic hammers. Prior to installation, drivability analyses are commissioned which determine the required hammer capacity. The rather simplistic software (in terms of soil representation) used to conduct these calculations, offers limited room for the optimization of the installation process. On the other hand, several full scale experiments have demonstrated that clever manipulation of driving parameters, specifically: (I) hammer weight; (II) driving frequency; (III) falling height/impact velocity; can significantly benefit installation times. This leaves a huge potential for cost savings (several millions EUR) per farm and forms a prime opportunity to stimulate the transition of offshore wind energy towards the mainstream. However, no consensus has been reached on the dynamic processes which positively contribute to the drivability of monopiles, let alone how these processes can be consciously induced in the subsoil. This research sets out to, by means of a parametric experimental study in the centrifuge, evaluate the effects of changes in driving parameters on driving time. Three hypothesis has have been drafted in an attempt to explain the higher efficiency piling operations employing HiLO (high frequency, low falling height) techniques instead of conventional driving, namely: (I) aggravated friction fatigue along the shaft due to an increased number of load cycles as a result of frequency increase; (II) Less dynamic soil resistance following from lower impact velocities, yielding the more efficient usage of available piling energy; (III) accumulation of excess pore water pressures, which reduce the effective stress regime surrounding the pile and thereby benefit piling rates. Through 24 centrifuge experiments, the aforementioned hypotheses are evaluated. During the experiments the effect of changes in driving parameters in monitored. Moreover, water pressure sensors mounted both on the pile shaft and inside the surrounding soil body record the soil response during driving. Results indicate that the dynamic installation of open-ended tubular piles in sandy soil, characterized by a high Rd (¼80%), is associated with the development of excess pore water pressures at larger radial distances from the pile due propagation of seismic waves. However, unlike similar experiments of samples with a lower Rd , the generated excess pore fluid pressures are limited in their magnitude as the soil exhibits no contraction to aid further generation. Moreover, closer to the pile, a transition towards a dilative soil regime is observed, where the increase of driving frequency is arguably related to the accumulation of tensile pore fluid pressures along the shaft, which negatively affects pile drivability. Results indicate the aforementioned adverse effect is partially compensated through the use of a heavy hammer due to subtle difference in soil-structure interaction related to the different geometry of the hammer. Consequently, it seems that HiLo driving is not a technique which guarantees better drivability under all circumstances. Hence, in the quest for optimum drivability, the prevailing soil conditions should play a decisive role in the selection of the best suited pile-hammer combination and driving technique.