Performance of an auger dredging cutter for subsea dredging and mining

Increasing use of deep-water dredger and miner vehicles has been anticipated for resource collection, engineering construction and environmental protection. Among existing deep-dredging equipment designs submersible dredgers have advantages over others such as surface designs. Considering the limitations of commercially available and conceptualised submersible dredgers, a different type - the 'Swimming Remotely Operated Submersible Dredger' (SROSD) is conceptualised in this research. In its working mode, it imitates a walking motion by using spuds that are also used for station keeping during dredging. When required, the vehicle can swim by means of vector thrusters. The vector thrusters also help in position-keeping and motion-control during swimming. To offset high forces generated during excavation of hard materials, spuds, variable buoyancy tanks and control planes are included as secondary station-keeping devices. The general arrangement and the sub-systems of the conceptualised vehicle are described. For swimming and control in a submerged condition, the vehicle is ballasted to a neutrally buoyant condition. Balancing of the excavation forces is necessary for efficient production and this is a design problem with submersible dredgers. A transverse axis auger cutter capable of surgical cutting with low force fluctuations is used in the design. A theoretical model of the cutting forces in vertical and horizontal directions generated by this type of cutter is proposed. An experiment was designed to measure the cutting forces generated by this cutter. Comparisons between the power requirements derived from the experiment and the theoretical model were done to validate the model. The measured experimental power requirements were found to be within the calculated power ranges for dredging loose to slightly compacted sand in similar operating conditions. In a further experiment to assess the spillage from the cutter in operation, near and far field turbidity generation by the auger was measured by using turbidity sensors. Measurements of turbidity were taken to indicate the environmental impact of the auger. It was observed that the turbidity increase was dependent on the relative position between the probes and the auger and its shroud. It is hoped that, through this work, a new design of vehicle, along with the reaction force analysis of the excavation and the turbidity measurements will significantly contribute to the evolution of existing deep-dredging and mining equipment leading to improved efficiency, increased mobility and position control while minimising environmental disturbance.