Multivariable coordinated control of managed pressure drilling process based on comprehensive performance index
With the recent advance in high speed data communication offered by wired drill pipe (WDP) telemetry, it is now possible to design automated control systems that directly utilize downhole data (e.g. pressure) to optimize drilling procedures. This research couples drilling hydraulics, rate of penetration (ROP), and rotational speed (RPM) control into a single controller for managed pressure drilling systems. This novel multivariate controller improves drilling performance during normal drilling operations and enhances safety during abnormal drilling conditions such as unwanted gas influx situations. New advances in drilling automation have made the closed loop control of downhole weight on bit (WOB) and drill string rotational speed (RPM) possible. This study uses two feedback controllers that control the downhole WOB and RPM using surface data. A multivariate nonlinear model predictive controller (NMPC) uses downhole and surface measurements to simultaneously regulate the bottom hole assembly (BHA) pressure and maximize the ROP. For this purpose, NMPC provides the necessary set points for the WOB and RPM feedback controllers as well as manipulates the choke valve opening and pump flow rates. Controller performance is enhanced via a nonlinear estimator that works continuously online with the NMPC and provides the necessary estimated parameter values (i.e. annulus density, friction factor, and gas influx) for precise and efficient drilling control. The designed NMPC controller has a multi-priority approach which is described in the following three scenarios: (1) during unexpected gas influx, the NMPC gives priority to BHA pressure control and attenuates the influx effectively via a novel kick attenuation method that switches the control objective from BHA pressure to choke valve pressure; (2) during connection procedures when adding a new stand, ROP is stopped and the NMPC focuses on maintaining the BHA pressure constant; (3) during normal drilling operation, which involves changes in the rock formation and differential pressures, NMPC gives priority to ROP maximization while maintaining RPM, WOB, and BHA pressure within specified bounds. Preliminary results suggest that this multivariate controller for ROP and BHA pressure control decreases drilling costs, reduces operator workload, and minimizes risk significantly. Specific improvements in drilling performance include higher ROP, effective kick attenuation, and more uniform cuttings. The use of a multivariate NMPC allows for better ROP optimization and BHA pressure control than is possible with the use of two independent controllers. These benefits are demonstrated across the three scenarios mentioned above. In simulation, this technology delivers significant performance improvements during managed pressure drilling and furthers the development of auto driller systems.