Subsea Excavation of Seafloor Massive Sulphides

This paper discusses the excavation phase of seafloor massive sulphide mining. The excavator is part of an overall mining system that will also include a vertical riser to lift the excavated material to the surface, and a shipboard dewatering plant to minimize the loss of fine particles back into the water. External variables affecting the excavator performance include rock properties and terrain. Given a set of rock properties and a desired production rate, we can choose a cutter type and we can estimate the required forces, torques, and power requirements. Typically we can use marinized versions of existing land-based rock cutters for this application. However, we need to pay particular attention to the design of the cutterhead assembly - we need to control the flow field so that we can lift excavated material into the riser and not leave it on the bottom or lose it into plumes. Rock cutters can be deployed in multiple modes, i.e. transverse cutting, sumping, and trenching. The design of the excavator as a system needs to consider the cutting mode and the method of advance of the excavator platform. Examples of excavator advance methods include "lawn mowing", where the cutterheads continuously excavate as the machine advances, "scything", where the cutterhead takes horizontal swaths as the excavator advances, and "open pit mining" where the machine excavates a steep face in front of itself. Each of these modes has advantages and disadvantages in terms of machine size, wear, and time efficiency. Finally, the interface to the riser is important in terms of decoupling vessel and riser motions from the excavator and whether or not the excavator can be mated and unmated to and from the riser while on the bottom.