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The discrete element method (DEM) has been used successfully to simulate rock failure. However, when considering the deformation of fluid-saturated rock, the DEM lacks the contribution of the fluid pressure. The presence of a (pore)-fluid can affect the effective stress state, which might result in a change in some of the mechanical properties of the rock, e.g. strength and modulus. Additionally, saturated rock is more susceptible to strain rate effects. An extension to DEM is presented in which the effective stress theory is embedded by coupling the DEM with a pore pressure diffusion process. This is achieved by interpolating the discrete properties of the DEM to a continuum by using a smoothed particle approach (SP). The model is able to predict the strengthening/weakening of a rock with respect to the amount of fluid drainage that has been allowed. To demonstrate the validity of the model, several simulations of tests under different loading conditions are conducted and the results are compared with experimental data from literature. The agreement between theory and experiment is very good. [ABSTRACT FROM AUTHOR]

In deep sea mining, the vertical transport of excavated material from the seafloor to a vessel at the sea surface is a key process. Stationary flow is preferred, and blockage of the riser would terminate the entire operation. For a blockage to occur there needs to be accumulation of material, the formation of a solid plug, and the plug needs to exert sufficient friction on the riser wall. The formation of a blockage is a complex chain of events, described in detail in this paper. The hypothesis presented in this paper on the formation of plugs and on the conditions for a blockage to occur is checked with the results of a unique experiment. We developed a test setup in which the conditions for riser blockage are enforced. We were able to conduct a reproducable blockage experiment, which shows that the mechanism presented in this paper is a very accurate description of the blockage process. To ensure safe operations and high production levels, the transport system needs to be designed for maximum flow assurance. Knowledge of the mechanism and conditions for riser blockage can be used in the design of feeding systems and strategies for the vertical transport system. [ABSTRACT FROM AUTHOR]