Subsea Separation and Water Injection at Luno II - Conceptual Design and Control Assessment

Abstract

Subsea separation and water re-injection give rise to production from oil fields earlier considered economically unfit and give a longer production life to existing fields. This thesis addresses the design of a subsea separation process where the water will be re-injected into the reservoir as pressure support. The separators have been sized, together with an evaluation of factors influencing the separation. An analysis of possible future field tie-ins has also been conducted. Furthermore, a design of a basic control system was proposed and verified through dynamic simulations in the chemical simulation software, Aspen HYSYS. The design basis for this thesis was the oil field Luno II. The separation design proposed was two gravity separators in series where the first will do the rough separation and the second will polish the water to its required purity. The first separator was sized to have a 3 meters diameter and a 15 meters length. A conclusion could not be made with regards to the size of the second separator as the purity of water did not vary with changing separator size, which is due to using a feed fraction carry over for this separator. The suggested control model has been tuned, and the system process variables show resilience to realistic disturbances. However, the water purity varies substantially with small variations in process variables.