A Novel Approach to Subsea Multiphase Solid Transport

Transportation of multiphase reservoir fluid through subsea tiebacks has gained considerable attention in recent years especially in the deep offshore and ultra deep offshore environments where there is increasing pressure on the operators to reduce development costs without compromising oil production. However, the main challenge associated with this means of transporting unprocessed reservoir fluids is the need to guarantee flow assurance and optimise production. Solids entrained in the fluid may drop off and settle at the bottom of horizontal pipe thereby reducing the space available to flow and causing erosion and corrosion of the pipeline. The problem has been largely attributed to insufficient flow velocity among other parameters required to keep the solids in suspension and prevent them from depositing in the pipe. The continuous changing flow patterns have introduced additional complexities dependent on gas and liquid flow rates. Acquisition of experimental data for model development and validation in multiphase flow has been largely focused on single and two phase flow. This has impeded our understanding of the behaviour and associated problems of three phase or four phase (oil, water, gas and solid) in pipes. The result is inappropriate solid transport models for three phase and four phase. In order to bridge this gap, the Well Engineering Research group at Robert Gordon University has initiated a project on integrated multiphase flow management system underpinned by comprehensive experimental investigation of multiphase solids transport. The project is aimed at developing precise/accurate sand transport models and an appropriate design and process optimisation simulator for subsea tiebacks. In this paper, the physics of the multiphase transport models being developed is presented. The models will allow for the prediction of key design and operational parameters such as flow patterns, phase velocity, pressure gradient, critical transport velocity, drag & lift forces, flow rate requirements and tiebacks sizing for transient multiphase flow. A new multiphase flow loop is being developed which will be used to generate experimental database for building and validating the theoretical models for use in a proposed integrated simulator for deepwater applications.