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The problem considered in this paper is that of controlling downhole pressure during oil and gas well drilling, with a particular focus on handling gas kicks leading to two-phase gas–liquid flow conditions. We identify a first-order approximation to the infinite-dimensional system which captures the dominating mode of the pressure dynamics in the frequency range of interest, while the high-frequency pressure dynamics are represented by a multiplicative uncertainty. This approximation is then modified to accommodate the changes to the dynamics introduced by the two-phase flow. The linearized plant has an open-loop time constant which varies between 2 and 600 s depending on operating point and gas distribution in the well. Robust controller design is then performed using linear matrix inequalities (LMIs) via a polytopic norm-bounded description of both the high-frequency multiplicative, and the low-frequency parametric uncertainty. It is shown that, in order to achieve acceptable performance over such a large range of open loop time constants, a time-varying controller gain is required. The main contribution of the paper is to achieve this control objective systematically by formulating the control design problem as an LMI optimization problem. Then optimal solutions of the LMI problem can be obtained in polynomial time by using modern interior point method (IPM) numerical solution algorithms.

This paper presents a modification to L 1 adaptive control that allows for disturbances entering at the output and application of this control strategy to Managed Pressure Drilling with harmonic disturbances representing the heave motion of a floating drilling rig. By incorporating the disturbance at the output into the closed-loop reference model, it is shown that the L 1 adaptive control structure can be left unchanged while the original transient performance bounds are preserved. It is further shown that rejection of the output disturbance can be taken care of entirely in the filter design step of L 1 adaptive control using the internal model principle. A systematic filter design procedure based on LMIs is provided, that requires only one tuning parameter to be adjusted by the designer. The control design is applied to disturbance attenuation and set-point tracking in the so-called heave problem in oil well drilling. We present experimental results of control system tests in a medium size test facility emulating a 900 m long well.

This paper deals with predicting the occurrence of self-excited vibrations during drilling. Previous work postulates that these are due to the coupling between the distributed drill string system and the regenerative effect in the bit-rock interaction law. We use a previously developed distributed model and the linearized bit-rock interaction law to derive a graphical condition for stability based on the Nyquist stability criterion.

Keeping the pressure within predefined bounds is essential for the safety of managed pressure drilling operations, but drill string movements can induce pressure fluctuations that violate these margins. We extend previous results on disturbance rejection for a 2 x 2 hyperbolic model of the fluid dynamics in the borehole. We perform a simulation study, in which we illustrate the controller performance for heave induced oscillations and compare it to the previously available controller. The stochasticity of sea waves induces limitations on the achievable performance. Further, the controller is applied to intended string movements, where control can be used to remove conservative limitations on the speed of the movement.

As oil exploration and development costs rise, the oil industry increases its efforts to improve oil recovery (IOR) from existing fields. IOR is achieved mainly by drilling more wells, but drilling in partially depleted reservoirs is challenging due to narrow pressure margins. Offshore drilling in harsh environments, such as the North Sea, presents additional challenges, since the heaving motion from a floating rig induces large surge and swab pressures in the well. This brief suggests a remedy for this problem using automatic control of well pressure. Taking advantage of an experimental lab facility recently completed at NTNU, a model of the drilling system is developed using subspace identification methods. The model serves as a basis for state estimation and controller design using model predictive control. Applying the controller to the lab facility, pressure oscillations are suppressed by 70%-90% compared with the open-loop case, depending on the period of the heave motion. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works nb_NO

This paper presents a modification to L 1 adaptive control that allows for disturbances entering at the output and application of this control strategy to Managed Pressure Drilling with harmonic disturbances representing the heave motion of a floating drilling rig. By incorporating the disturbance at the output into the reference model, it is shown that the Li adaptive control structure can be left unchanged while the original transient performance bounds are preserved. It is further shown that rejection of the output disturbance can be taken care of entirely in the filter design step of Li adaptive control using the internal model principle. A systematic filter design procedure based on LMIs is provided, that requires only one tuning parameter to be adjusted by the designer. The control design is applied to disturbance attenuation and set-point tracking in the so-called heave problem in oil well drilling. We present experimental results of control system tests in a medium size test facility emulating a 900 m long well.

Today, in search of oil resources, marginal wells with narrow pressure windows are frequently being drilled. This requires accurate and precise control to balance the bottom hole pressure (BHP) between the pore and fracture pressure of the reservoir. Managed pressure drilling (MPD) is a technique introduced to enable improved pressure control when drilling. This paper presents a cascaded control structure for BHP, choke pressure and choke position in MPD operations. Estimators for the unknown bottom hole flow and uncertain BHP are developed to improve pressure control performance. The presented method is evaluated with field operation data from drilling two 4000 m deep wells. The results show that the pressure is maintained within acceptable margins through a series of operations.

In this paper, we apply recent results on disturbance rejection in the interior domain of a class of 2 × 2 linear hyperbolic systems of partial differential equations to the problem of rejecting heave-induced pressure fluctuations at the casing shoe in managed pressure drilling. We show that a PDE model of the drilling dynamics belongs to this class of systems, and derive both state feedback and output feedback control laws for the drilling system. The performance of the control laws are demonstrated in simulations that show efficient rejection of pressure oscillations at a desired location in the well.

Automatic managed pressure drilling is an advanced pressure-control method which is intended to meet the increasingly high demands in drilling operations in the oil and gas industry. In this method, the circulating drilling fluid, which takes the cuttings out of the well, is released at the surface through a controlled choke. This choke is used for active control of the fluid pressure in the well. The corresponding automatic control system keeps the pressure at the bottom of the well at a specified setpoint despite various disturbances. One such disturbance, i.e., the vertical motion of the drill string, causes severe pressure fluctuations which need to be actively attenuated. In this paper, we present two different disturbance rejection strategies based on discretized partial differential equations for the well hydraulic system. The performance of the controllers is shown through simulations both under idealized conditions and on a high-fidelity drilling simulator.

A model-based in-/out-flux detection scheme for managed pressure drilling is presented. We apply a globally exponentially stable adaptive observer which estimates the unknown states and parameters of the hydraulic system and in particular quantifies the magnitude of the in-/out-flux and its location in the well. While the observer can be used purely as an in-/out-flux detection system, the paper also presents a simple controller that automatically and effectively stops an in-/out-flux. This novel control scheme departs from common practice for automatic managed pressure drilling, which is to regulate down-hole pressure to some predetermined set point. Experimental results and realistic simulations with a state-of-the-art simulator are provided to show the effectiveness of the method.

The focus of this paper is on the benefits of model-based control for performance of pressure control and how to enhance robustness against changes in system dynamics by use of adaptive control in managed pressure drilling (MPD) operations. In this work, we present the advantages and disadvantages with adaptive model based control for MPD operations, by implementation of a choke pressure controller. The paper will show that model-based control enables pressure control down to fully closed choke and trapping pressure without a backpressure pump. Moreover, by adding adaption to the model-based controller it is made robust to changing well parameters, recovering control performance. This is part of a pre-study and preparation for flow loop testing in Abu Dhabi Winter 2016, where performance and robustness of the controller is tested for gas influx and a variety of standpipe pump rates and pressure ranges. The purpose of the flow loop testing is to verify that the controller gives satisfactory performance and is the final test stage before the technology is ready for field use.

The two-phase dynamics of a gas-kick percolating up a vertical well, drilled using a partly evacuated riser, is described by two ordinary differential equations and algebraic relations. We model the gas as bubbles distributed with a distribution function along the well. The simplicity of the model makes it well suited for estimation purposes and controller design. We present simulation results where an unscented Kalman filter is used to estimate the current location of the gas, distribution of the gas, and rise-velocity. These estimates can be used to predict when the gas will pass the blowout preventer and reach the drilling rig.

A model describing the flow and pressure fluctuations in the bore-hole due to drillstring movement has been presented. It consists of a pair of coupled nonlinear partial differential equations modelling the distributed pressure and flow in the well, and a superposition of multiple sine waves for the disturbance. Considering only top-side flow and pressure as measurements, it is shown that the model can be represented by a linear time invariant finite-dimensional system with output delay. This result is achieved by linearization and de-coupling using Riemann invariants. An infinite-dimensional observer is designed that estimates the disturbance, and the estimate is used in a controller that rejects the effect of the disturbance on the down-hole pressure. A model reduction technique based on the Laguerre series representation of the transfer function is used to derive a finite-dimensional, rational transfer function for the controller. The performance of the full-order and reduced-order controllers are compared in simulations, which show satisfactory attenuation of the heave disturbance for both controllers.

This paper presents a switched control scheme for regulation of the annular pressure in a well during drilling. The main feature is automatic kick attenuation while drilling reservoir sections. A simple mathematical model is presented that captures the dominant phenomena of the drilling system and forms the basis for observer and control design. A switched pressure control algorithm for feedback control of the choke valve opening and the back-pressure pump is proposed, and asymptotic stability properties are established. Kick attenuation is guaranteed for a set of reservoir models, including the most common ones. As part of the control scheme, novel observers for estimation of the flow rate through the bit and kick detection are developed. Simulation results obtained with a high fidelity drilling simulator are presented to demonstrate the effectiveness of the proposed switched control scheme.

This paper addresses dynamic positioning of surface vessels moored to the seabed via a turret-based spread mooring system, referred to as position mooring. Different options relevant for construction of automatic control schemes for the thruster system are first discussed based on consideration of relevant loss functions. In particular, the role of structural reliability criteria applied to the mooring system is addressed. A control algorithm based on such reliability criteria is introduced. The structural reliability measure becomes an intrinsic part of the controller, automatically adjusting the allowed motion limits based on current weather conditions and structural properties of the mooring lines. The performance of this control system is subsequently demonstrated both by numerical simulations and laboratory experiments on a model vessel.

This paper addresses the dynamic positioning of surface vessels moored to the seabed via a turret based spread mooring system, an operation referred to as the position mooring. While the mooring system keeps the surface vessel in place most of the time, thruster assistance is needed in severe weather conditions to avoid mooring line failure. Traditionally, this is done by keeping the vessel within a predefined geographical region. We present a conceptually new controller for position mooring operations. By using a structural reliability measure for the mooring lines, the new controller protects the mooring system whenever needed as a result of severe weather conditions and high environmental loads. This is done by maintaining the probability of mooring line failure below a preset value. In particular, the excessive use of thrusters caused by conservatively defined safety regions in conventional PM systems is avoided, giving a fuel optimal operation. The feasibility of our controller is successfully verified in laboratory experiments.

This paper addresses dynamic positioning of surface vessels moored to the seabed via a spread mooring system, referred to as position mooring. In normal weather conditions, the mooring system constrains the vessel and the controller applies thruster force for motion damping and heading control, only. When environmental loads due to wind, waves, and current increase, thruster assistance is required also for positioning in order to avoid damage to the mooring lines. While traditional position mooring systems apply thruster force based on constraining the vessel to lie within a predefined geographical region, the controller designed in this paper instead employs structural reliability measures for the mooring lines to restrict movement. These structural reliability measures become an intrinsic part of the controller, enabling it to automatically adjust the admissible region for the vessel in the presence of changing winds, currents, and waves.

Floating marine structures are frequently kept in position by means of mooring lines in addition to a thruster system. Various types of control schemes for the thruster system are first investigated based on a simplified response model. In particular, the role of structural reliability criteria applied to the mooring system is investigated. Subsequently, a more refined control algorithm based on such reliability criteria is introduced. The performance of this control system is demonstrated by numerical simulations.

In this paper, a model of a futuristic fish farming structure is developed and problems related to interconnected marine structures and strategies for configuration control are studied. A control system is designed that: (1) ensures limited loading of the mooring system; (2) keeps the chain of surface modules aligned transversely to the incoming current, and; (3) ensures positive strain in the connectors between the modules. Control actuation is by means of a thruster mounted on the first module, and a hydrofoil mounted on the last module. The performance of the control system is demonstrated by simulations, and evaluated by a structural reliability criterion referred to as the delta index.

This paper illustrates the potential of nonlinear model-based control applied for stabilization of unstable flow in oil wells. A simple empirical model is developed that describes the qualitative behavior of the downhole pressure during severe riser slugging. A nonlinear controller is designed by an integrator backstepping approach, and stabilization for open-loop unstable pressure setpoints is demonstrated. The proposed backstepping controller is shown in simulations to perform better than PI and PD controllers for low pressure setpoints, and is in addition easier to tune. Operation at a low pressure setpoint is desirable since it corresponds to a high production flow rate.

The ability to perform accurate pore pressure and reservoir inflow estimation during a kick incident is necessary, particularly when drilling in formations with narrow pressure margins. Currently available techniques for pore pressure estimation and reservoir characterization either rely on empirical correlations requiring access to well logging data and other petrophysical information, or require downhole pressure sensing and advanced flow metering capabilities. This paper introduces a model-based estimation technique which uses surface measurements commonly available in a Managed Pressure Drilling (MPD) system, coupled with a simplified transient two-phase model. This model is capable of representing essential dynamics during a gas kick with reduced computational overhead, but without sacrificing significant modeling accuracy. First, the model is validated in a gas kick scenario against experimental data, showing good agreement between key measured parameters and the model predictions, and thereby justifying the model applicability to field operations. Next, data generated from a commercial simulator test case is used to evaluate the proposed estimation methodology. The estimated pore pressure and reservoir productivity are close to their respective values from the commercial simulator, and the flow out rate and surface back-pressure predicted by the simplified two-phase model yield very good match against the simulator results. © 2016 Elsevier B.V. All rights reserved. This is the authors' accepted and refereed manuscript to the article. Locked until 31 May 2018 due to copyright restrictions

This paper illustrates the potential of nonlinear model-based control applied for stabilization of unstable flow in oil wells. A simple empirical model is developed that describes the qualitative behavior of the downhole pressure during severe riser slugging. A nonlinear controller is designed by an integrator backstepping approach, and stabilization for open-loop unstable pressure setpoints is demonstrated. The proposed backstepping controller is shown in simulations to perform better than PI and PD controllers for low pressure setpoints, and is in addition easier to tune. Operation at a low pressure setpoint is desirable since it corresponds to a high production flow rate. The simulation results are presented to illustrate the effectiveness of proposed control scheme.

Floating marine structures are considered which are kept in position by means of mooring lines and a thruster system. Various types of control schemes for the thruster system are first investigated based on a simplified response model. In particular, the role of structural reliability criteria applied to the mooring system is investigated. Subsequently, a more refined control algorithm based on such reliability criteria is introduced. The performance of this control system is demonstrated by numerical simulations.

We design a leak detection system consisting of an adaptive Luenberger- type observer based on a set of two coupled one dimensional first order nonlinear hyperbolic partial differential equations governing the flow dynamics. It is assumed that measurements are only available at the inlet and outlet of the pipe, and output injection is applied in the form of boundary conditions. Heuristic update laws for adaptation of the friction coefficient and leak parameters are given, and simulations demonstrate their ability to detect, quantify and locate leaks. Particular attention is given to time-varying boundary conditions, such as during pipeline shut-down. A scenario consisting of leak detection followed by pipeline shut-down during which the leak is accurately quantified and located is successfully simulated for both liquid and gas systems.

Real-time estimation of annular pressure profile and formation pressure is crucial for the execution and planning of a well control operation, especially when drilling formations with narrow pore and fracture pressure margins. A simple transient multi-phase simulator, capable of accurately representing gas and liquid dynamics while minimizing complexity and computational requirements, is highly desirable for real-time kick mitigation and control applications. Such a simulator is presented here in the form of a coupled ODE-PDE model composed of a first order ODE and a first order hyperbolic PDE. This model is shown to retain the dominating two-phase dynamics encountered during gas kick incidents. As a particular application, we demonstrate the use of the model in design of switched control algorithms for kick handling in a Managed Pressure Drilling setting. A Recursive Least Squares algorithm is employed for estimation of unknown model parameters.Copyright © 2015 by ASME

Summary While casing-heading instability in single gas lift wells has at- tracted a lot of attention, gas-distribution instability in dual gas lift wells has not. In this paper, we present a simple, nonlinear dy- namic model that is shown to capture the essential dynamics of the gas-distribution instability despite the complex nature of two- phase flow. Using the model, stability maps are generated showing regions of stable and unstable settings for the production valves governing the produced flows from the two tubings. Optimal steady-state production is shown to lie well within the unstable region, corresponding to a gas distribution between the production tubings that cannot be sustained without automatic control. A simple control structure is suggested that successfully stabilizes the gas-distribution instability in simulations and, more impor- tantly, in laboratory experiments. fact that under certain operating conditions, it is impossible to sustain the feed of injected gas into both tubings. Instead, all the injected gas will eventually be routed through one of the gas lift valves. As a consequence, the second tubing produces poorly or not at all, decreasing the total production substantially. There are few reports, if any, on automatic control of dual gas lift wells, although Boisard et al. (2002) briefly mentions an application. In this paper, we present a simple, nonlinear dynamic model that captures the essential dynamics of the gas-distribution insta- bility. It is an extension of the model for a single gas lift well presented in Eikrem et al. (2003) and Aamo et al. (2005). Using the model, we generate a stability map for a single-point dual gas lift well, and present a control structure for stabilizing the system at open-loop, unstable setpoints. The performance of the controller is demonstrated in simulations using the model, but more impor- tantly, stabilization is also achieved in laboratory experiments. This paper is organized as follows. In the Mathematical Model section, a nonlinear dynamic model applicable to dual gas lift wells is presented, followed by a discussion on instability mechanisms and the generation of stability maps in the Instability Mechanisms and Control section. The stability analysis is based on computing eigenvalues for the linearized model, accompanied by simulations using the nonlinear model. The proposed control structure is pre- sented in the Automatic Control segment of that section, and ex- perimental results using a gas lift laboratory located at the U. of Technology-Delft are shown in the Laboratory Experiments sec- tion. The paper ends with discussion and conclusions in the Con- clusions section. Mathematical Model

We design a leak detection system consisting of an adaptive Luenberger- type observer based on a set of two coupled one dimensional first order nonlinear hyperbolic partial dierential equations governing the flow dynamics. It is assumed that measurements are only available at the inlet and outlet of the pipe, and output injection is applied in the form of boundary conditions. For the linearized model without friction and leak, exponential convergence of the state estimates to the plant state is shown by Lyapunovs method. The observer design is performed for the continuum model, ensuring that convergence properties established theoreti- cally are not an artifact of the method of discretization. Heuristic update laws for adaptation of the friction coe! cient and leak parameters are given, and simulations demonstrate their ability to detect, quantify and locate leaks.

Unstable regimes occurring for multiphase flow in vertical risers have successfully been stabilized using conventional linear control techniques. However, these control systems rely on downhole measurements which are at best unreliable, if at all available. In this paper, we design a nonlinear observer for the states of the multiphase flow that relies on topside measurements only, and apply it to estimate downhole pressure for feedback control. A key feature of the design is that it exploits the structure of the model to obtain robustness with respect to the internal flows in the system. Combining the nonlinear observer with conventional PI control of the downhole pressure, we demonstrate in laboratory experiments the potential for increasing production from gas-lift wells by stabilizing the multiphase flow.

Unstable regimes occurring for multiphase flow in vertical risers have successfully been stabilized using conventional linear control techniques. However, these control systems rely on downhole measurements which are at best unreliable, if at all available. In this paper, we design a nonlinear observer for the states of the multiphase flow that relies on topside measurements only, and apply it to estimate downhole pressure for feedback control. A key feature of the design is that it exploits the structure of the model to obtain robustness with respect to the internal flows in the system. Combining the nonlinear observer with conventional PI control of the downhole pressure, we demonstrate in laboratory experiments the potential for increasing production from gas-lift wells by stabilizing the multiphase flow.

Unstable flow regimes occurring for multiphase flow in gas-lift wells have successfully been stabilized. using conventional linear control techniques. However, the control systems rely on downhole measurements which are at best unreliable, if at all available. In this paper, we employ a nonlinear observer for the states of the multi phase flow in the gas-lift well. The observer relies on topside measurements only, and downhole pressure for feedback control is estimated. Combining the non linear observer with conventional PI control of the downhole pressure, we demonstrate in laboratory experiments the potential for increasing production from gas-lift oil wells by stabilizing the multiphase flow.

In this paper a model of a futuristic fish farm is developed. Problems related to configuration control of interconnected marine structures are studied. Configuration control refers to positioning of individual modules or the entire structure as a whole. The structure is a chain of surface vessels, moored to the seabed via a mooring system. We design a control system that: 1) Ensures limited loading of the mooring system; 2) Keeps the chain of surface vessels aligned transversally to the incoming current, and; 3) Ensures positive strain in the connectors between vessels.

The field of flow control has picked up pace over the past decade or so, on the promise of real-time distributed control on turbulent scales being realizable in the near future. This promise is due to the micromachining technology that emerged in the 1980s and developed at an amazing speed through the 1990s. In lab experiments, so called micro-electro-mechanical systems (MEMS) that incorporate the entire detection-decision-actuation process on a single chip, have been batch processed in large numbers and assembled into flexible skins for gluing onto body-fluid interfaces for drag reduction purposes. Control of fluid flows span a wide variety of specialities. In Part I of this tutorial, we focus on the problem of reducing drag in channel and pipe flows by stabilizing the parabolic equilibrium profile using boundary feedback control. The control strategics used for this problem include classical control, based on the Nyquist criteria, and various optimal control techniques (H2, H-Infinity), as well as applications of Lyapunov stability theory.

We present a simplified drift-flux model for gas-liquid flow in pipes. The model is able to handle single and two-phase flow thanks to a particular choice of empirical slip law. A presented implicit numerical scheme can be used to rapidly solve the equations with good accuracy. Besides, it remains simple enough to be amenable to mathematical and control-oriented analysis. In particular, we present an analysis of the steady-states of the model that yields important considerations for drilling practitioners. This includes the identification of 4 distinct operating regimes of the system, and a discussion on the occurrence of slugging in underbalanced drilling.Copyright © 2014 by ASME

In this paper, we develop a new finite element model for a cable suspended in water. Global existence and uniqueness of solutions of the truncated system is shown for a slightly simplified equation describing the motion of a cable with negligible added mass and supported by fixed end-points. Based on this, along with well known results on local existence and uniqueness of solutions for symmetrizable hyperbolic systems, we conjecture a global result for the initial-boundary value problem. The FEM model for the cable is assembled to give a model of a multi-cable mooring system, which, in turn, is coupled to a rigid body model of the floating vessel. The result is a coupled dynamical model of a moored vessel, which can be applied to applications such as turret-based moored ships, or tension leg platforms.

As oil exploration and development costs rise, the oil industry increases its efforts to improve oil recovery (IOR) from existing fields. IOR is achieved mainly by drilling more wells, but drilling in partially depleted reservoirs is challenging due to narrow pressure margins. Offshore drilling in harsh environments, such as the North Sea, presents additional challenges, since the heaving motion from a floating rig induces large surge and swab pressures in the well. The paper suggests a remedy for this problem using automatic control of well pressure. Taking advantage of an experimental lab facility recently completed at NTNU, a model of the drilling system is developed using subspace identification methods. The model serves as a basis for state estimation and controller design using model predictive control. Applying the controller to the lab facility, pressure oscillations are suppressed by 70-90% compared to the open-loop case, depending on the period of the heave motion. © 2016. This is the authors’ accepted and refereed manuscript to the article. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

OSCAR, an integrated system of models and databases, has undergone a sensitivity analysis as part of a calibration and testing program. A total of 48 simulations was performed in this sensitivity study, which focused on selected parameters that may directly or indirectly influence environmental evaluations: spreading, response actions, and threshold concentration for biological effects. The main conclusions from the study are as follows:The spreading coefficient, K, is a key parameter in calculating areal coverage and mass balance.The number of particles used to represent an oil slick has a minor influence on the mass balance, to the extent that it has an influence on the simulation of response actions.In calm weather, the effectiveness of mechanical cleanup only slightly affects the total amount of oil collected, but is decisive for the rate at which oil is collected. At higher wind speeds, the total amount of oil collected becomes a strong function of effectiveness.The amount of oil that is chemically dispersed by helicopter is more dependent on the delay due to trips for refueling and refilling than on the effectiveness of the dispersant.Oil spill response actions from boats may be sensitive to the number of particles used to represent the surface slick, depending on the search strategy specified. Response from aircraft is not sensitive to particle number.In the simulations performed here, the threshold concentration for biological effects only influences the fraction offish eggs and larvae in the interval 0 to 40 ppb-hours. The sensitivity study has shown the model to be numerically robust. The importance of specifying realistic search strategies for response vessels has also been made clear.

This paper describes the empirical approach used in the development of the IKU oil-weathering model (IKU-OWM). Weathering data from field trials with experimental oil spills during the past 15 years have provided the basis for the algorithms used in the model. These data, combined with a standardized laboratory study of each specific oil, including a bench-scale (“stepwise”) weathering investigation, a chemical dispersibility investigation, and a meso-scale flume basin weathering investigation, form the basic input to the model. The IKU-OWM has been extensively tested and verified with results from full-scale field trials with experimental oil slicks in the North Sea in 1994 and 1995. This paper presents the correlations between the oil-weathering values predicted by the model and the ground-truth data obtained from the field trials. To obtain reliable weathering predictions, it is essential to have good laboratory weathering data of specific oils. The IKU-OWM is now extensively used in Norway to predict the changes in oil properties due to weathering under user-specified conditions. Predictions provided by the IKU-OWM enable oil spill personnel to estimate the most appropriate “window of opportunity” for using chemical dispersants in various spill situations. Prespill scenario analysis with the IKU's oil spill contingency and response (OSCAR) model system, of which the IKU-OWM is one of several components, has become an important part of contingency plans and of contingency training of oil spill personnel at refineries, oil terminals, and offshore installations in Norway.

We apply a PDE observer to a hydraulic system to detect and quantify an in- or outflux. We show that the PDE model of the hydraulic system can be expressed as a 2×2 linear hyperbolic system of PDEs with spatially varying coefficients coupled with an ODE at the inlet boundary that models the in- or outflux. Using the method of backstepping, we design an observer which is exponentially stable at the origin in the ℒ2-norm while relying on measurements taken at the outlet boundary, only. Simulation results verify the validity of the observer.

The periodic heave movement of a floating drilling rig is transferred to an elastic pipe suspended in a drilling well. The pipe oscillates up and down in the well with the period of the heave motion pushing the mud in the well and exciting pressure fluctuations in the mud in the pipe. These dynamics are described by six coupled first-order linear partial differential equations of hyperbolic type. By using the Laplace transform, the system is solved in the frequency domain and the resulting irrational transfer functions, from the periodic heave disturbance and the controlled choke flow to the distributed pressure in the annulus around the pipe, are found for different boundary conditions. We show that the response can be improved by choosing an impedance matched boundary condition which can be achieved without the use of active control. We also derive the response when a theoretical `perfect feed-forward control law', which yields full attenuation of the effect of the disturbance at a single point in the well-annulus, is applied. The limitations of this `perfect point control' when considering an extended section of the well is shown.

A model-based in-/out-flux detection scheme for managed pressure drilling is presented. We design a globally exponentially stable adaptive observer which estimates the unknown states and parameters of the hydraulic system and in particular quantifies the magnitude of the in-/out-flux and its location in the well. While the observer can be used purely as an in-/out-flux detection system, the paper also presents a simple controller that automatically and effectively stops an in-/out-flux. This novel control scheme departs from common practice for automatic managed pressure drilling, which is to regulate down-hole pressure to some predetermined set point. Simulations are provided to show the effectiveness of the method.

Managed pressure drilling is an advanced pressure control method which is intended to meet increasingly high demands in drilling operations in the oil and gas industry. In this method, the circulating drilling fluid, which takes cuttings out of the well, is released at the surface through a controlled choke. This choke is used for active control of the fluid pressure in the well. The corresponding automatic control system keeps the pressure at the bottom of the well at a specified set-point despite various disturbances. One of such disturbances, vertical motion of the drill string, causes severe pressure fluctuations which need to be actively attenuated. In this paper we present two different disturbance rejection strategies based on discretized partial differential equations for the well hydraulic system. The performance of the controllers is shown through simulations both under idealized conditions as well as by simulations on a high fidelity drilling simulator.

The objective of Managed Pressure Drilling (MPD) is to enable precise control of the downhole pressure during drilling and completion operations. An essential part of an automated MPD control system is the downhole pressure estimator, usually referred to as the hydraulic model, which in most cases is the limiting factor for achievable accuracy of the system. In this paper we present a new design of an adaptive observer for online estimation of the downhole pressure in the well. Unlike conventional hydraulic models, which are based on complex partial differential equations, the adaptive observer is based on a simplified lumped model of ordinary differential equations, combined with online adaptation that tunes uncertain model parameters utilizing topside measurements. We demonstrate that the novell adaptive observer design captures the important hydraulics of MPD operations with high accuracy, and fundamentally improves the convergence rate of the parameter estimaties by using delayed observers. The results are demonstrated on field data from an offshore MPD operation.

This paper presents a switched control scheme for regulation of the annular pressure in a well during drilling. The main feature is automatic kick attenuation while drilling reservoir sections. A simple mathematical model is presented that captures the dominant phenomena of the drilling system and forms the basis for observer and control design. A switched pressure control algorithm for feedback control of the choke valve opening and the back-pressure pump is proposed, and asymptotic stability properties are established. Kick attenuation is guaranteed for a set of reservoir models, including the most common ones. As part of the control scheme, novel observers for estimation of the flow rate through the bit and kick/loss detection are developed. Simulation results obtained with a high fidelity drilling simulator are presented to demonstrate the effectiveness of the proposed switched control scheme.

This paper presents a nonlinear observer-based control scheme to stabilize the downhole pressure and eliminate riser slugging. A simple empirical model is developed that describes the qualitative behavior of the downhole pressure in case of severe riser slugging. A new nonlinear adaptive observer is developed for state estimations with the new parameter adaptation law. The controller is designed by an integrator backstepping approach to stabilize the downhole pressure and achieve asymptotic tracking. The stabilization in the unstable region is demonstrated by the proposed control. For the design and implementation of the controller, no knowledge is assumed on the system parameters. It is shown that the proposed controller not only can guarantee asymptotic stability, but also transient performance.

In this paper, we present an algorithm for optimizing reservoir production using smart well technology. The term smart well is used to indicate an unconventional well equipped with down hole inflow control valves (ICVs) and instrumentation. This additional instrumentation extends the degree of freedom in the field production planning, since production can be efficiently distributed on the different well segments available. By proper utilization of the ICVs through optimal production planning, an increased oil recovery for the reservoir can be expected.We propose a method for optimal closed-loop production known from control theory as model predictive control (MPC). A commercial reservoir simulator, ECLIPSE, is used for modeling and predictions. MPC is chosen for its ability to provide an optimal solution for the constrained multivariable control problem. To compute the optimal ICV settings, we propose using a nonlinear MPC (NMPC) application, which can handle the severe nonlinearities found in reservoir models. The NMPC uses a single shooting multi-step quasi-Newton (SSMQN) method to solve the optimization problem. As the term multistep suggests, this is an iterative method which solves a sequence of quadratic problems (QPs) in each time step.We apply our method to a benchmark reservoir model with multiple geostatistical realizations. This model has already proven potential for increased oil recovery by using optimization techniques. We show an even additional increase over the former approach in production totals, using the SSMQN method, with as much as 68% increase in one case, and 30% on average compared to a reference case.

In this paper a reduced order observer that adapts to unknown friction and density, and estimates the bottomhole pressure in a well during drilling, is presented. The design is based on a newly developed third order nonlinear model with a nonlinear output equation containing a product between an unknown parameter and unmeasured state. Based on a Lyapunov approach the pressure estimate is shown to converge to the true pressure under reasonable conditions. Application of the observer to real data from a North Sea oil well demonstrates promising behaviour.

This paper addresses dynamic positioning of surface vessels moored to the seabed via a turret based spread mooring system, referred to as position mooring. The controller utilize a reliability index to determine the actuator force needed to operate safely. The structural reliability measures become an intrinsic part of the controller, automatically adjusting the allowed geographical region based on current weather conditions and structural properties of the mooring lines. The performance of the controller is demonstrated through laboratory experiments on a model vessel named CyberShip III.Copyright © 2007 by ASME

This paper proposes a state feedback design method for attenuating severe slugging in multiphase flow pipeline systems. The feedback is designed based on the input-output linearization method, and incorporates the saturation effect on the input. The designed feedback can suppress the slugging phenomena provided some sufficient conditions are satisfied. Finally, checking the conditions lead to the selection of the variable which is `more relevant' to be controlled.

The present paper is concerned with floating marine structures that are kept in position by means of mooring lines in addition to a thruster system. Different options relevant for construction of automatic control schemes for the thruster system are first investigated based on a simplified response model. In particular, the role of structural reliability criteria applied to the mooring system is investigated. Subsequently, a more refined control algorithm based on such reliability criteria is introduced. The performance of this control system is demonstrated by numerical simulations.Copyright © 2006 by ASME