Author(s): James Stronach; Aurelien Hospital; Albert Leung; Daniel Potts; Allister Thompson
Linked Author(s):
Keywords: Numerical modelling; DSTP; Density current; Tailings management; Dilution
Abstract: Deep Sea Tailings Placement (DSTP) systems have been used at several mines to discharge diluted mine tailings at great depths. DSTP often provides considerable environmental and engineering benefits compared to traditional on-land tailings storage. The key element in the viability of a DSTP system is the ability to discharge mine tailings at sufficient depths and at such a location that a density current is formed that carries the tailings along the seabed to the abyssal ocean, and such that adequate dilution can be achieved and environmental impacts minimized. Depending on the composition of the slurry, sub-sea plumes might also form from the density current at different levels of neutral buoyancy, further diluting the effluent. The feasibility of such an outfall system involves several environmental, engineering and construction aspects. This paper first considers the engineering aspects of the outfall transporting the slurry from the on-shore mix-tank, where tailings are mixed with seawater for initial dilution, to the terminus discharge location. Then, the environmental aspects related to the behaviour of the slurry once discharged are discussed. A suite of numerical models and tools is used to design the system: an in-house hydraulic spreadsheet analyzes the pipe hydraulics; a three-dimensional hydrodynamic model (H3D) combined with two-dimensional wave models (WaveWatch III and SWAN) provides forcing on the pipe as well as hydrodynamic conditions and water properties impacting the stability of the density current; a twodimensional density current model assesses the performance of the outfall system especially with respect to plugging of the pipe at the outfall terminus, and assesses the formation and stability of the density current as it travels along the seabed. The density current model also characterized how the liquid effluent exits the density current and enters the water column as sub-sea plumes due to changes in bathymetry and water properties. Finally, the paper discusses the various steps and challenges that can be encountered in the construction of such a system. A strong focus is placed on the pipe sinking procedure for systems at 100 m depth or greater. The engineering, environmental and construction aspects of the deep-sea outfall system are illustrated through two case studies in Papua New Guinea, located in the South Pacific: the Simberi DSTP system was designed and built in 2007 and has been operational since then. The Woodlark DSTP system was designed in 2013 and the permitting process is presently underway. Challenges with these two systems arose with tropical conditions and the associated monsoon period as well as hurricane season. Tetra Tech EBA conducted the feasibility and detailed design for both outfall systems as well as the construction for the Simberi outfall.
Year: 2016