Severe slugging in an offshore riser pipeline imposes a major challenge to production and flow assurance in the oil and gas industry. Riser-top-valve choking has shown effectiveness in eliminating severe slugging. However, most manual-choking and active-control techniques were tuned by trial and error, resulting in an operation at a smaller-than-necessary valve position for a stable-flow condition. This imposes unnecessarily high backpressure on the riser pipeline, which leads to reduction in production. One way to overcome this problem is to design the active-control system to operate at a large valve position. However, at such an operating point, the riser-pipeline system is naturally open-loop unstable associated with severe slugging flows.
In this work, an approach to tune a robust proportional-integralderivative (PID) slug controller at an open-loop unstable condition is proposed. First, at an open-loop unstable operating condition, a reliable linear model is derived from the nonlinear simplified riser/separator model (SRSM) developed in previous work. Then, a robust stabilizing PID controller is designed on the basis of the linearized model.
The controller was successfully applied to a 2-in. laboratory riser at Cranfield University and an 8-in. generic industrial riser system modeled in the commercial multiphase-flow simulator OLGA. Simulation on the industrial riser system shows that the proposed approaches not only can eliminate severe slugging but also can increase oil production. It also shows that the percentage improvement in oil production compared with manual choking will increase as the well pressure declines. This means that adopting active slug control is even more beneficial for mature oil fields than for relatively new fields.
The result is very significant for mature fields that are susceptible to severe slugging and low oil production because of declining reservoir pressure.