Discrete Element Modelling: The influence of High Hydrostatic Pressure on the Cutting Processes of Hard Rock

Discrete Element Modelling: The influence of High Hydrostatic Pressure on the Cutting Processes of Hard Rock

Parasie, N.

Alvarez Grima, M. (mentor)
Van Baars, S. (mentor)
Dijkstra, J. (mentor)
Van Rhee, C. (mentor)
Ngan-Tillard, D.J.M. (mentor)

Civil Engineering and Geosciences

Geotechnology

2009-08-25

Seafloor Massive Sulfide (SMS) contains high levels of metals such as copper and gold. Water depths of 2000 up to 3000 meter make mining of SMS a challenge. Despite this, mining industry encourages research into feasible extraction methods of SMS, pushed by the high metal prices nowadays. This research focuses on the influence of hydrostatic pressure on the cutting process of hard rock. A 2D numerical model of the cutting process is created using discrete element modelling (DEM). The software package Particle Flow Code 2D (PFC2D) from Itasca Consulting Group is used. The rock material creation in PFC2D consists of reproducing numerically the physical UCS, biaxial and Brazilian tests executed on the benchmark material. SMS is the rock of interest for deep-sea mining, nevertheless Langmeil Sandstone is chosen as benchmark material. This consideration is taken because SMS samples are rare and no strength test results obtained on SMS samples are published. In addition the high heterogeneity of SMS makes their numerical modelling difficult. It was not possible to come up with a singular numerical rock sample matching the mechanical properties of the Sandstone for a large range of confining pressures (0 to 40 MPa). Therefore two samples were created: the first sample is valid for unconfined tests, the second sample is valid for confining pressures between 20 and 40 MPa. It appeared that the material unconfined compressive strength and elastic constants are independent of the particle size. The transition point from brittle to ductile failure is simulated successfully for the second rock sample. Three different cutting scenarios are studied: cutting of dry rock without hydrostatic pressure, cutting of dry rock under hydrostatic pressure and cutting of saturated rock. The boundary particle method is an algorithm capable of simulating hydrostatic pressure. This method applies a force similar to the hydrostatic load on each boundary particle of the sample. The method simulates the transition between brittle and ductile rock cutting successfully, by increasing the effective stress, while fluid and pore pressures are absent. The transition from brittle to ductile cutting is reached at a hydrostatic load of 20 MPa for the Langmeil sandstone. The cutting force (450 kN) obtained by the numerical model for the unconfined calibrated rock is compared with existing semi-empirical models. Goktan’s (1995) model (300 kN) provides the best match , Evans’ (1961) model (75 kN) underestimates the required cutting force. The horizontal cutting force increases with increasing hydrostatic pressure. The increase of hydrostatic load is mainly transferred into an extra load on the cutting tool in horizontal direction. The specific energy calculated from the cutting force obtained by the numerical model (4.9 MJ/m3) is in good accordance to the values for sandstone found in literature (5.5 MJ/m3). The influence of the tool shape, the cutting velocity and depth of cut on the cutting process of dry rock is modelled. The chisel pick tool shows to be more efficient than the pick point tool for shallow water depths. The difference in efficiency between these tools becomes smaller with increasing hydrostatic pressures. Measurement circles in PFC2D are able to register porosity changes during the cutting process. These measurements are used to estimate pressure differences in the crushed zone. This method provides a good qualitative insight in the development of pore pressures during the cutting process. Useful quantitative values were not obtained. The measurement circle method indicates that cavitation will not occur for high hydrostatic loads such as 20 or 30 MPa, which implies that the required cutting force will continuously increase for an increasing cutting velocity. The main recommendations are introducing the Biot poroelastic equations and heterogeneity into a 3D discrete element model. Quantitative estimations for the specific energy and pore pressures during the cutting processes should be obtained. Execution of laboratory tests of rock cutting under varying hydrostatic pressures is of most importance to check the performance of the numerical rock cutting models.

DEM
discrete element modelling
rock cutting
dredging
particle flow code

http://resolver.tudelft.nl/uuid:bcf5c5d8-e00e-430a-9684-1938a356cf0b

2012-08-24

Student theses

master thesis

(c) 2009 Parasie, N.