Raju Gandikota

A Multibody System Approach to Drill String Dynamics Modeling

The selection of optimal operational parameters for drilling oil and gas wells is a complex dynamic problem that depends on multiple parameters. Numerous physical and mechanical processes such as rock cutting, friction, hydraulics, and different modes of vibrations, occur during drilling, which should be accounted for in numerical models. It is widely accepted that bottom hole assembly (BHA) vibrations are the primary source of drilling equipment premature failure. Over the last 30 years, progress of computational sciences has enabled the use of numerical simulations of drillstring dynamics as a useful tool to understand and mitigate sources of harmful vibrations. The majority of these models have been based on nonlinear finite elements. There are several significant limitations with this approach, including an extremely high number of degree of freedom (DOF) required to represent geometries with 10 5 ratio of axial to lateral dimensions and also the complexity of modeling variable contacts in bifurcating systems. While it is relatively new for simulating drilling dynamics , the advantage of the proposed rigid-flexible multibody system approach has been proven for modeling complex dynamic systems in other industries. Using a rigid-flexible multibody system approach to analyze dynamic effects both in frequency and time domains, dynamic modeling of BHA and drillstring is proposed. Drillstring is simulated as a set of uniform flexible beams connected via linear viscous-elastic force elements. Each beam

Stick-Slip Model for PDC Bits Accounting for Coupled Torsional and Axial Oscillations

Drilling with PDC bits can cause severe torsional and axial oscillations. These oscillations are accompanied by periodic sticking of the bit followed by accelerated rotation. The so-called “stick-slip” increases bit wear and fatigue and causes premature failure of BHA and drillstring components. It is well known that these torsional oscillations are nonlinear and self-induced. The present study investigates the coupling between axial and torsional oscillations.The cutting process is based on the Detournay model, which provides for the effect of the bottomhole pressure and the local pore pressure. The axial stiffness of the drillstring is taken into account with the axial motion equations coupled with the torsional equations, in contrast to previous models where axial equations were considered independently. Axial oscillations are allowed to occur even when the bit is in the stick phase. The new model also includes bit “bouncing” when it loses contact with the bottomhole. The equations are solved by time integration. By results of the analysis of transient processes the spectral density is determined.The objective of the paper is to improve understanding of stick-slip oscillation nature and assess the contribution of parameters that influence their intensity.The study includes the effect of the rotor rpm, intrinsic specific energy of rock, number of PDC blades, wear flat length of blades, etc.Results of the study will help drillers to select and change drilling parameters more efficiently to reduce severe stick-slip oscillations.© 2014 ASME

Flexible Multibody Approaches for Dynamical Simulation of Beam Structures in Drilling

Two approaches for simulation of dynamics of complex beam structures such as drill strings are considered.In the first approach, the drill string is presented as a set of uniform beams connected via force elements. The beams can undergo arbitrary large displacements as absolutely rigid bodies but its flexible displacements due to elastic deformations are assumed to be small. Flexibility of the beams is simulated using the modal approach. Thus, each beam has at least twelve degrees of freedom: six coordinates define position and orientation of a local frame and six modes are used for modeling flexibility.The second approach is dynamic simulation of the drill string using nonlinear finite element model. The proposed beam finite element uses Cartesian coordinates of its nodes and node rotation angles around axis of Cartesian coordinate system as generalized coordinates. The nonlinear finite element is developed based on method of large rotation vectors. Rotation angles in the nodes can be arbitrary large.Equations of motion of beam structure are derived in the paper. The number of degrees of freedom is decreased by factor two as compared with the modal approach. Thereby, computational efficiency under simulation of dynamics of long drill strings is considerably increased.The features of creating the models and numerical methods as well as results obtained by applying both approaches are discussed in the paper.Copyright

Buckling Analysis of Drill Strings in Inclined Wellbores Using the Explicit Finite Element Method

Understanding drill string buckling behavior is a significant challenge to the petroleum industry. In this paper, the explicit finite element method implemented in Abaqus software is employed to study the buckling of drill strings for inclined straight wellbores. Classic solutions for the critical buckling length of self-weighted columns as well as critical buckling load for drill pipe inside inclined wellbores are compared to explicit FEA and accurate results are provided by the finite element based predictions. The effect of different inclination angles and string effective weight due to the buoyancy effect has been studied and the results for sinusoidal and helical buckling are compared to analytical results and experimental data in the literature. The theoretical predictions for different inclination angles agree with the simulations. Theoretical buckling load of inclined drill strings approaches zero by decreasing the effective weight of a floating drill string. However, the results of finite element simulations show that significant buckling load would still exist for very low drill string effective weight. These results are confirmed by experimental results provided by other researchers. Overall, the efficacy of using explicit finite element methods to model drill string buckling behavior is demonstrated.Copyright