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 105 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 can undergo arbitrary large displacements as absolutely rigid body, but its flexible displacements due to elastic deformations are small. A method of floating frame of reference for flexible bodies and component mode synthesis is used for modeling beams dynamics. Parameters of the coupling force elements are calculated automatically based on stiffness and inertia characteristics of the connected beams.
This paper discusses the development of the rigid-flexible multibody system for modeling drillstring dynamics and the influence of model parameters on simulation accuracy and calculation time.
A close match is shown between theoretical and numerical results for static and buckling problems as well as resonant frequency values. Several transient drillstring dynamics problems are analyzed for wellbores with uniform diameter. Examples of the analysis of resonant conditions during drilling planning stage are also presented. It is also shown how transient time domain analysis can provide further insights into lateral and torsional vibrations, whirl behavior, and effect of local wellbore curvatures on the drillstring performance.