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Modeling and control of drillstring dynamics for vibration suppression
Drill-string vibrations could cause fatigue failure to downhole tools, bring damage to the wellbore, and decrease drilling efficiency; therefore, it is important to understand the drill-string dynamics through accurately modeling of the drill-string and bottom-hole assembly (BHA) dynamics, and then develop controllers to suppress the vibrations. Modeling drill-string dynamics for directional drilling operation is highly challenging because the drill-string and BHA bend with large curvatures. In addition, the interaction between the drill-string and wellbore wall could occur along the entire well. This fact complicates the boundary condition of modeling of drill-string dynamics. This dissertation presents a finite element method (FEM) model to characterize the dynamics of a directional drill-string. Based on the principle of virtual work, the developed method linearizes the drill-string dynamics around the central axis of a directional well, which significantly reduced the computational cost. In addition, a six DOF curved beam element is derived to model a curved drill-string. It achieves higher accuracy than the widely used straight beam element in both static and dynamic analyses. As a result, fewer curved beam elements are used to achieve the same accuracy, which further reduces the computational cost. During this research, a comprehensive drill-string and wellbore interaction model is developed as the boundary condition to simulate realistic drilling scenarios. Both static and dynamic analyses are carried out using the developed modeling framework. The static simulation can generate drill-string internal force as well as the drilling torque and drag force. The dynamic simulation can provide an insight of the underlying mechanism of drilling vibrations. Top drive controllers are also incorporated as torsional boundary conditions. The guidelines for tuning the control parameters are obtained from dynamic simulations. Drill-string vibrations can be suppressed through BHA configuration optimization. Based on the developed modeling framework, the BHA dynamic performance is evaluated using vibration indices. With an objective to minimize these indices, a genetic algorithm is developed to optimize the BHA stabilizer location for vibration suppression. After optimization, the BHA strain energy and the stabilizer side force, two of the vibration indices, are significantly reduced compared to the original design, which proves the BHA optimization method can lead to a significant reduction of undesirable drilling dynamics. At the end of this dissertation, reduced order models are also discussed for fast simulation and control design for real time operationMechanical Engineerin
EXPERIMENTAL AND NUMERICAL STUDIES OF DRILL-STRING DYNAMICS
A drill string is the transmission component of rotary drill-rig system used for mining petroleum and natural gas resources. The drill-string system is essentially a long slender structure whose length can be in kilometers. Additionally, the drill-string is subject to discontinuous forces from interactions with the wellbore, which can cause erratic torsion
oscillations and stick-slip motions. Here, a unique scaled experimental apparatus has been constructed to understand the dynamics of one section of the drill-string subjected to stick-slip interactions with an outer shell. In both the experimental and modeling efforts, the drill-string system is studied as a slender rod with large discs on either end, with the bottom disc being enclosed within a shell, which is representative of a borehole. The experimental setup allows for studies of stick-slip interactions between a drill-string like system and an outer shell, unlike the prior studies. A series of careful experiments are conducted with special attention to parameters such as the drive speed, the mass
imbalance, and the nature of contact between the bottom disc and the outer shell. The experimental results indicate that the rotor motions can be divided into different phases, with each phase being characterized by its own unique features that include bumping, sticking, slipping, and rolling characteristics. In order to gain insights into the drill-string dynamics, reduced-order models have been developed inclusive of a novel drill-string wellbore force-interaction model that can account for stick-slip behavior. Both the experimental observations and model predictions are found to be in agreement, in terms of the system dynamics. Furthermore, parametric studies have been conducted and the
findings are presented in the form of experimental and numerical simulation results, and the qualitative changes observed in the dynamics are discussed. These findings suggest that the drill-string curvature and contact friction plays an important role in determining the present of erratic motions. This dissertation effort provides clues to how the drive
speed can be used as a control parameter to move the system out of regions of undesired dynamics and how the drill-string motions can be influenced to keep them close to the borehole center
Nonlinear Model and Qualitative Analysis for Coupled Axial/Torsional Vibrations of Drill String
A nonlinear dynamics model and qualitative analysis are presented to study the key effective factors for coupled axial/torsional vibrations of a drill string, which is described as a simplified, equivalent, flexible shell under axial rotation. Here, after dimensionless processing, the mathematical models are obtained accounting for the coupling of axial and torsional vibrations using the nonlinear dynamics qualitative method, in which excitation loads and boundary conditions of the drill string are simplified to a rotating, flexible shell. The analysis of dynamics responses is performed by means of the Runge-Kutta-Fehlberg method, in which the rules that govern the changing of the torsional and axial excitation are revealed, and suggestions for engineering applications are also given. The simulation analysis shows that when the drill string is in a lower-speed rotation zone, the torsional excitation is the key factor in the coupling vibration, and increasing the torsional stress of the drill string more easily leads to the coupling vibration; however, when the drill string is in a higher-speed rotating zone, the axial excitation is a key factor in the coupling vibration, and the axial stress in a particular interval more easily leads to the coupling vibration of the drill string
Underreamer mechanics
In the oil and gas industry, an underreamer is a tool used to extend and enlarge the diameter of a previously-drilled bore. The problem proposed to the Study Group is to obtain appropriate mathematical models of underreamer dynamics, in forms that will lead to feasible computation. The modes of dynamics of interest are torsional, lateral and axial.
This report describes some initial models, two of which are developed in more detail: one for the propagation of torsional waves along the drill string and their reflection from contact points with the well bore; and one for the dynamic coupling between the underreamer and the drill bit during drilling
An inverse problem via cross-entropy method for calibration of a drill string torsional dynamic model
International audienceModel selection and parameter identification are challenging tasks in drill string dynamics due to the high degree of nonlinearity that abound the diverse and complex mechanisms involved. This work explores the application of a stochastic metaheuristic procedure for parameter identification over the torsional mode of drill string vibration. A proposed model is calibrated with data from a validated experimental setup , adjusting stiffness, damping and friction parameters. The resulting simulations display a reasonable fit with almost ideal correlation coefficients even in the presence of stick-slip. The optimization strategy is compared with an Genetic Algorithm, revealing significantly greater efficiency and showcasing how the cross-entropy method may be a viable tool in the demanding context of drill string modeling
Non-smooth dynamics modeling of drill-string systems in heterogeneous formations
An extension of the drill-string dynamics model for drilling in heterogeneous rock formations is presented to study the heterogeneity effect on both the axial and torsional dynamics
Hydraulic Effects of Drill-String Tool Joint and Rotation on Annular Flow Profile and Frictional Pressure Loss Using ANSYS-CFX
In oil and gas well drilling, inaccurate estimation of drilling parameters can affect the predictions of annular flow profile and frictional pressure loss along the wellbore which can result in hole problems, such as hole erosion due to high annular fluid velocity, and inadequate drill cuttings transport, well control issues such as kick or lost circulation. Drill-string tool joints alter the annular geometry, when coupled with drill-string rotation, they affect the annular flow profile and frictional pressure loss by causing turbulence, fluid acceleration/deceleration or changing the drilling mud apparent viscosity. As the oil and gas industry moves towards deeper wells, drilling operation uses more drill-string tool joints, the additional frictional pressure loss can be significant, up to 30% of the total frictional pressure loss. Therefore, there is a need to better understand the effects of drill-string tool joint and pipe rotation on annular flow profile and frictional pressure loss. The objective of this study was to analyse individually and collectively the hydraulic effects of drill-string tool joints and rotation on annular flow profile and frictional pressure loss. The scope and methodology of this research involved Computational Fluid Dynamics (CFD) approach, with ANSYS-CFX (in ANSYS 15) as the analysis system, where a CFD model with an optimum mesh size was created and validated against previous experimental data, where frictional pressure loss values were compared
A Method for the Design and Optimization of Nonlinear Tuned Damping Concepts to Mitigate Self-Excited Drill String Vibrations Using Multiple Scales Lindstedt-Poincaré
In downhole drilling systems, self-excited torsional vibrations caused by the bit-rock interactions can affect the drilling process and lead to the premature failure of components. Especially self-excited oscillations of higher-order modes lead to critical dynamic loads. The slim drill string design and the naturally limited drilled borehole diameter limit the installation space, power supply and lead to numerous potentially critical self-excited torsional modes. Consequently, small and robust passive damping concepts are required. The variety of possible downhole boundary conditions and potential damper designs necessitates analytical solutions for effective damper design and optimization. In this paper, two nonlinear passive damper concepts are investigated regarding design and effectiveness to reduce self-excited high-frequency torsional oscillations in drill string dynamics. Based on a finite element model of a drill string, a suitable minimal model based on the identified critical mode is generated and solved analytically using the Multiple Scales Lindstedt-Poincaré (MSLP) method. The advantages of MSLP compared to conventional MS methods are shown for this example. On the basis of the analytical solution, parameter influences are determined, and design equations are derived. The analytical results are transferred to self-excited drill string vibrations and discussed using time domain simulations of the drill string model
Stability Analysis of the Rotary Drill-String
Oil and natural gas are major energy sources for modern society. A rotary drilling system is the best known technology to extract them from underground. The vibration and stability of drilling systems have been studied for decades to improve drilling efficiency and protect expensive down-hole components. It is well known that severe drill-string vibrations are caused by many different loads: axial loads such as the hook load and the self-weight of the drill-string, end torques applied by the surface motor and restrained at the bit, the inertial load caused by whirling, the fluid drag force, and the contact force between the borehole wall and the drill-string. The drill-string is usually subjected to a complex combination of these loads.
The motivation for this dissertation is the need to understand the complex vibration states and the stability of the drill-string in order to better control its constructive and destructive potential. A mathematical model is proposed to describe the steady-state stability of a long, vertical, rectilinear drill-string. The model accounts for a complex combination of constant and variable loads that affect the behavior of drill-strings. The first critical values of these loads and the corresponding mode shape are obtained by the analytical method and the Rayleigh-Ritz method. COMSOL and ABAQUS are used to validate the numerical results for the cases without analytical solutions. With these results, we see that the Rayleigh-Ritz method gives accurate results and is a good way for us to understand more deeply the dynamics of the drilling process and predict the instability of the drilling system
Modeling and Control of Axial and Torsional Stick-Slip Oscillations in Drill String
A drill string is the transmission component of rotary drill-rig system used for mining petroleum and natural gas resources. The drill string system is essentially a long slender structure whose length can be in kilometers. Additionally, the drill-string is subjected to discontinuous forces from interactions with the wellbore, which can cause erratic torsion oscillations and stick-slip motions. Throughout this report, all the information that is needed to execute this project will be explain in chapter 1 which is introduction that consist of background of the project, the problem statement of the project and the objectives of doing this project. The literature review of this project will be explained after the introduction. All the studies about this project in order to gain insights into the drill string dynamics and reduce order model have been stated in the literature review. In chapter 3, the methodology in doing this project will be explained which consists of project flow, Gantt chart and the key milestone of doing this project. Furthermore, the results and discussion during this Final Year Project have been concluded and the findings are presented in chapter 4 in this report. These findings provide the simulation code in forms of MATLAB code. This report effort provides clues to how the drive speed can be used as a control parameter to move the system out of regions of undesired and how the drill-string motions can be influenced to keep them close to the borehole center
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