Modelling of the drilling rates for large diameter offshore foundation piles

Abstract

In a world where the awareness of global warming and climate change increases, the shift towards sustainable energy is essential. In the upcoming years, an increasing number of offshore wind farms is expected. Over the past years, all the 'best spots' are taken and the new locations in tougher soil conditions or even rock have to be overcome. The conventional pile driving technique is a less attractive solution here, because there is a significant chance to pile buckling. Drilling is an alternative method which is able to complete these installations successful in tougher soils and rock. Furthermore, in certain regions, the rules with respect to maximum noise levels during pile driving become stricter these days and the use of noise mitigation measurements can significantly increase the costs of the installation. Drilling can be an alternative method which decreases the noise generated during the installation. The drilling phase plays a crucial role in the total drilling process, which can easily take up more than half of the total pile installation time. For installing a large amount of foundation piles, it is important, especially during the tender phase of a project, to have a good prediction of the drilling rate.
Currently, the available drilling rate models are mainly empirical and therefore tend to be specific to a certain drilling method in a particular type of rock. The goal of this thesis is to give a better prediction of the drilling rate, which is applicable for various amount of large diameter drills. In order to predict the drilling rate, a theoretical based model is developed. The model consists of an excavation and a transportation part, which can both limit the drilling rate. The excavation model determines the excavation limit of the drill. It makes a distinction between cutter heads that consist of pick points and cutter heads that consist of rollers cutters, which can be further distinguished into tooth, button or v-shaped discs cutters. In order to calculate the drilling rate, a literature study is done towards the excavation process of rock, where the existing cutting and indentation models are reviewed and incorporated within the model. The transport model determines the transportation limit of the drill. The rock chips, generated by the excavation process, need to be transported in order to continue the drilling process. In most cases, transportation of cuttings takes place by the use of an airlift system. To model the limit of an airlift system, the momentum balance over the length of the airlift pipe is solved, which defines the maximum quantity of solids that is able to be transported. The model shows a good resemblance with experimental results obtained from the raise boring industry. Furthermore, a similar trend is observed between the model and values obtained from offshore drilling manufacturers. It is now possible to give a theoretical based prediction of the drilling rate for the installation of drilled foundation piles, which is applicable for a wide range of large diameter drills.