Bottomhole Pressure Control Of Managed Pressure Drilling Based On L₁ Adaptive Algorithm

During the process of managed pressure drilling?MPD?,the control effect of classical algorithm can not adapt the needs of drilling with narrow margins because of unknown parameters,unmodeled states,complex condition and high requirement of bottomhole pressure control.Therefore,an advanced control algorithm for accurately controlling bottomhole pressure and flow has a great deal of engineering meaning for MPD system.First,a nonlinear dynamic model is proposed in this paper which reflects the dynamic characteristics of MPD.And L₁ adaptive controller is designed to control the bottomhole pressure.For the unmeasurable state and nonlinear part in the model,state estimator and radial basis function neural network are designed respectively for state estimation.Finally,L₁ adaptive controller is simulated under normal drilling,making connection and kick.Simulation results under these situations show L₁ adaptive controller can control the bottomhole pressure and flow accurately and rapidly.In the current MPD model,the opening of choke is used as control signal and the relationship is simplified between the flow through choke and the choke opening.However,the real characteristic of choke is much more complex and the parameters of the choke model are difficult to be decided.Then the uncertainty of model is more serious and the difficulty of algorithm implementation is increased because of the simplification of model.In this paper,the choke pressure is directly used as the control signal,and the dynamic model of MPD is improved.Meanwhile,the output of adaptive law is used as the set point of the backpressure control loop.Therefore,an improved L₁output feedback adaptive controller is proposed.Under the situation of normal drilling,making connection and kick diction,the improved L₁ adaptive algorithm is simulated and the results show the improved algorithm can keep the bottomhole pressure and flow stability and be more consistent with the requirement of practice drilling.During making a trip and connection,the traditional MPD device compensates the pressure through external backpressure pump,which causes low productive efficiency and high productive cost.The automatic diverter is designed to replace the backpressure pump and integrated into the MPD device.Thus the drilling process can be simplified and non-productive time can be decreased.Finally,the efficiency of drilling production can be improved as a whole.