Flow Assurance for Deepwater Mining

The flow assurance aspects of all subsea projects have a major contribution to the pipe design, field layout, choice of lifting equipment (subsea-pump or air lift), power requirement and system topside as well as subsea operability. In the context of deepwater mining, the main task is to estimate the pressure drop in the flowline and riser as well as the liquid-solid or gas-liquid-solid flow regime. These are needed to guarantee the production target at steady state and to guarantee the system operability in transient mode at shut-down and system start-up cycles. Current knowledge of solid transport has successfully characterized pneumatic and hydraulic transport of fine particles that are close to powders, sand, or small gravel. Semi-empirical theories based on concepts such as isolated or hindered slip velocities, mixture properties, and in-situ concentration have emerged to analytically resolve the flow behavior in vertical, horizontal, and inclined pipe. The context of deepwater mining pushes these theories beyond the existing application cases due to the significantly larger particle size combined with the riser configuration. The riser configuration includes a wave shape to accommodate vessel motions due to environmental loads and excursion requirements to follow the subsea excavation tool. This paper starts with the state-of-the art papers review on hydraulic lifting fundamentals to the need of using computational fluid dynamics (CFD) for understanding transient behavior and pressure and power prediction for the wave shaped riser. This paper further proposes coupling of CFD and the discrete element method (DEM) for improving computational efficiency and accuracy of the computational results.