A Design Approach for "Self-Lifting" Method to Eliminate Severe Slugging in Offshore Production Systems

Abstract

Abstract Recently, exploitation of offshore petroleum reservoirs has moved to ever increasing water depths. Production from fields in water deeper than 1800 m is now a reality. The use of long deep-water risers that conduct production from multiple wellheads on the sea floor to the surface predisposes the system to severe slugging in the riser for a wide range of flow rates and seabed topography. Considering the length of the deep-water risers, the problem is expected to be more severe than in production systems installed in shallower waters. Severe slugging could occur at high pressure, with the magnitude of the pressure fluctuations so large as to cause a shorter natural flow period with subsequent consequences such as premature field abandonment, loss of recoverable reserves and earlier-than-planned deployment of boosting devices. In this study, a novel idea to lessen or eliminate severe slugging in pipeline-riser systems has been investigated. The principle of the technique is to transfer the pipeline gas to the riser at a point above the riser-base. The transfer process will reduce both the hydrostatic head in the riser and the pressure in the pipeline, consequently lessening or eliminating the severe slugging by maintaining the steady-state two-phase flow in the riser. In this paper a steady state model is presented. It can be used as a design tool for the self-lifting concept by determining the operation envelope for successful self-lift operation given the location of the gas takeoff or injection points; it can predict the acceptable range of pressure losses in the gas bypass for continuous steady-state operation of the self-gas-lift system. The model was found to perform very well when compared to the acquired experimental data.