Deep Sea Mining: An Adhesion Problem

Abstract

The clay adhesion phenomenon has been problematic in multiple engineering projects involving soft cohesive soils. In this dissertation, the adhesive characteristics of offshore high plasticity clay will be analyzed in support to deep sea mining applications. Adhesion of clays are particularly relevant in 2:1 swelling clays, where the magnitude of the force is highly dependant on the clay properties itself, working conditions and the soil engaging component surface material. The most prevalent force in offshore saturated conditions arises from the suction forces between micro-channels in the clay and the soil engaging component. Methods to reduce this adhesive force were assessed, ranging from solutions such as vibrations and electro-osmosis, to modifying the soil engaging surface to break continuous contacts between the clay and component. The natural adhesion of two typical offshore clays were experimentally determined by interface shear tests for the shear adhesion and pull-out tests for tensile adhesion. Interfacial shear strength was measured for typical materials found in the offshore industry, that being steel, aluminium, rubber and a polymer coating. Results from the tests showed that materials do have an impact on adhesive stresses. Similarly, pull-out tests showed this variation pattern. The influence of pull-out rates and contact time on adhesive tensile strength were also assessed and showed to increase with faster rates and longer contact times, respectively. In application to a typical rotational cutterwheel in deep sea mining, a scaled test-setup of a rack of cutter-teeth was dragged through the test clay to acquire horizontal cutting forces with the utilization of the experimentally found natural adhesion in the interface tests. The found adhesion value for the test clays were utilized to validate the cutting clay model from "The Delft Sand, Clay & Rock Cutting Model" (Miedema, 2019). Practical results from the scaled experiment seemed to coincide relatively well with the theoretical model. It is therefore recommended to further investigate the use of experimental means to determine adhesion of clays to develop an even more accurate empirical model.