Rock Cutting for an Undercut Anchor Geometry

Abstract

The expansion of floating offshore wind projects into deeper waters with seabeds composed of hard rock requires alternatives to conventional drill-and-grout anchoring systems. Grouted anchors require offshore mixing of grout, transportation of the grout to the seabed and sufficient curing time before the anchor can become operational. To overcome these limitations, a groutless undercut anchor has been proposed as an alternative. This concept relies on the mechanical interlock between the anchor and surrounding rock, eliminating the need for grout and thereby reducing both installation time and the number of offshore operations. To achieve this interlocking mechanism, a belled geometry must be drilled at the base of the borehole using a technique known as ‘underreaming’. However, the understanding of the rock cutting process for this reaming operation remains limited.

This thesis presents the development of an analytical modelling framework, based on a conceptual reamer design, to simulate and evaluate the rock cutting process during underreaming. A kinematic model is formulated to describe the combined rotational and radial expansion motion of the conceptual reamer. This is coupled with a brittle tensile failure-based cutting model to predict cutting forces and to characterise the rock excavation mechanism. The model simulates different cutting regimes: crushing, relieved cutting, and unrelieved cutting based on tool spacing and the interaction of breakout geometries between adjacent cutting teeth. Two spacing configurations were considered in the simulations: equal and optimal tool spacing. The performance of the reamer is assessed for varying operational parameters on both configurations in (homogeneous) brittle rock. The limitations of the model are the exclusion of shear and ductile (cataclasis) failure mechanisms. Furthermore, the modelled cutting forces were underestimated and approx. 24% lower compared to experimental results. Lastly, the cutting depth is determined from the (prescribed) tool motion and thus not constrained by the operational torque or thrust, which are instead modelled as demands.

The operational parameters investigated were the rotational speed (rpm) and expansion rate ( φ ̇) of the reamer. Analysis showed that the spacing-to-depth ratio of the cutting teeth varies along the cutter arm. Lower values were obtained at the lower part, and higher values for the upper part. Decreasing rpm and increasing φ ̇ decreased this ratio for both configurations. Under identical conditions, the optimal tool spacing configuration showed lower specific energy, despite a small reduction of the production compared to the equal tool spacing configuration. Furthermore, both rpm and φ ̇ were found to influence the cutting production and specific energy. Lower rpm increased the cutting depth per rotation increased, promoting relieved cutting when tooth spacing remained the same. A similar effect was observed with increasing φ ̇. Further load analysis revealed that torque and thrust demand for the optimal tool spacing configuration were consistently lower under varying operational conditions.

Findings of this thesis provide a first estimation of how a conceptual reamer can be analysed by integrating a kinematic framework with established rock cutting theories and experimental observations, thereby offering insight into the cutting process during reaming.