Bottomhole Pressure Estimation and L 1 Adaptive Control in Managed Pressure Drilling System Zhiyuan Li ∗ Naira Hovakimyan ∗∗ Glenn-Ole Kaasa ∗∗∗ ∗ Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign (e-mail: li64@illinois.edu) ∗∗ Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign (e-mail: nhovakim@illinois.edu) ∗∗∗ Statoil Research Center, Porsgrunn, Norway (e-mail: gkaa@statoil.com) Abstract: This paper designs an integrated estimator and L 1 adaptive control scheme to address the two main challenges involved in the Managed Pressure Drilling (MPD) system: first, the bottomhole states are updated at a low rate, which can be viewed as unmeasured and thus need to be estimated in real time; and second, the drilling process is subject to uncertainties including unknown parameters (e.g., frictions, densities), unmodeled actuator dynamics and noise, which require a robust adaptive controller for control of the bottomhole pressure. The estimator provides fast estimation of the bottomhole pressure and flow rate, based on the available measurements from the top-side. The L 1 adaptive controller drives the bottomhole pressure to the desired value following a reference model. We also provide a solution to handle the input delay. The design is based on a recently developed nonlinear drilling model. The results demonstrate that the L 1 adaptive controller has guaranteed performance bounds for both the input and the output signals of the system while using the estimation of the regulated outputs. Simulations that include different operational conditions verify the theoretical findings. 1. INTRODUCTION During well drilling, a fluid circulation system is used to maintain the pressure profile along the well. The drill fluid (usually called mud) is pumped into the drill string, which is a structure of a series of connected pipes. The fluid then flows down to the drill bit, sprays out through the bit, circulates back up the annulus, and finally exits through a choke valve. The pressure balance between the well bottom hole and the reservoir is critical to the well drilling system (Stamnes, 2007). It is desired to keep the bottomhole pressure in some safety margin: if the bottomhole pressure is too low (under-balanced), a kick incident could happen, which can lead to an influx oil/gas while drilling; on the other hand, if the pressure is too high (over-balanced), the well could be fractured and the mud will be lost. The safety margin is narrow especially in deep water and some matured well. The managed pressure drilling (MPD) is a technology to control the bottom hole pressure precisely, the advantage of which includes the capability to drill the otherwise undrillable well, reduced non-productive-time (NPT), fluid loss and influx, more productivity of the well, and reduced drilling hazards, etc. The basic principle of MPD is to seal the annulus top and use the chock opening and an additional back-pressure to control the bottom hole pressure and compensate for annular pressure fluctuations. A simplified schematic diagram of an automated MPD system is shown in Fig.1. A description of the standard setup of an automated MPD system can be found e.g. in (Riet et al., 2003). One of the main challenges of MPD control is that the mea- surements from the bottom hole (flow rate and pressure, etc.), if possible, are transmitted by telemetry and updated at a low rate. For the purpose of control, these bottom hole states need to be estimated in real-time. Another challenge is the uncertainty Fig. 1. MPD drilling process in the model for the bottomhole, due to uncertainties in the friction, density and mud compressibility parameters, as well as the unmodeled dynamics in the actuator. Moreover, the model parameters are subject to significant changes during the drilling process. Proceedings of the 2012 IFAC Workshop on Automatic Control in Offshore Oil and Gas Production, Norwegian University of Science and Technology, Trondheim, Norway, May 31 - June 1, 2012 ThC1.1 Copyright held by the International Federation of Automatic Control 128