Dynamic analysis of a drill-string under deterministic and random excitations

Qiu, Hongyuan (2014) Dynamic analysis of a drill-string under deterministic and random excitations. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (13MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Drill-strings are slender structures used to dig into the rock in search of oil and gas. Failures of drill-strings are time and money consuming and therefore the dynamics of drill-strings must be investigated and carefully controlled. In the thesis, a dynamic model of the drill-string that is suitable for predicting axial, torsional and lateral vibrations is built using Euler-Bernoulli beam theory. The drillstring is driven by a DC motor on the top and is subjected to distributed loads due to its own weight as well as bit/formation interaction. The model is axial-torsional, lateral-torsional coupled. Under deterministic excitations, the model captures stickslip behavior in drilling operation. Analysis on its negative effect on drilling performance is made, and potential mitigation measures are also discussed. In random model, the excitations to the drill-bit are modeled as combination of deterministic and random components. Monte Carlo (MC) simulation is employed to obtain the statistics of the response. Two cases of random excitation with different intensities are investigated. The results from MC simulation are compared against that from deterministic case. Secondly, the thesis focuses on the drill-string torsional vibration and its stick-slip analysis. A finite element model of the drillstring with inclusion of both deterministic and random excitations is also developed. Simulation is carried out under certain parameters and it is shown that in deterministic case the torsional vibration may behave stick-slip. With change of some parameters, bifurcation and chaos of the system are observed. In the random case, Monte Carlo simulation and path integration method are used to capture the probabilistic information of the response. The results of path integration match well to those of deterministic cases. Although there are some limitations, this thesis will help the author better understand drill-string downhole behaviors and lay a foundation for further research work.

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