Flow Assurance of Vertical Solid-Liquid Two Phase Riser Flow During Deepsea Mining

Abstract

Abstract Growing economies and world population make the step to deep sea mining forfulfilling our needs for food and raw material inevitable. Deep sea mining typically consists of excavation of material at the sea bottom, vertical transport of material to the sea surface and further processing. Oneof the most promising concepts for vertical transport from large water depths(over 1000 m) is hydraulic transport: transport of solid material in an upward water flowthrough a riser. This paper describes part of a PhD study to flow assurance invertical hydraulic transport (VHT). Excavated material has varying sizes, shapes and densities: relative velocitiesof particles during hydraulic transport therefore vary as well. Typicalmaterials are manganese nodules, phosphorous nodules and chips of SeafloorMassive Sulfide deposits. Particles heavily interact, e.g. collisions, overtaking andclustering. Worst case is riser blockage due to large clusters packingtogether, which immediately stops the mining operation. Nodules have a roundedshape, while chips of SMS are unspherical. Literature on VHT up to date isconcerned with spherical particles (spheres, nodules), while SMS cuttings arebecoming more important. Therefore, this paper extents research to thetransport of unspherical particles and consequences thereof for riser blocking. To assess flow assurance of VHT, a numerical model has been set up, presentedin this paper along with simulation results. The model is experimentallyvalidated, methods and results are discussed in this paper. Experiments focuson transport velocities, transport regimes and formation of concentration peaks over longdistances. The information can be used in the hydraulic design of riser systemsfor solid-liquid flow. Simulations show that, depending on inflow conditions, development of highlyconcentrated clusters is a large risk. Experiments indicate large solids arelikely to be transported in the plug flow regime. The combination of clusterdevelopment and highly un-spherical particles being transported in the plug flow regimeincreases the risk of riser blockage. Inflow conditions therefor should beengineered carefully to ensure a safe and reliable operation. Guidelines forinflow conditions are given.