Fatigue estimation of drill-string and drill-pipe threaded connection subjected to random loadings

Zheng, Jiahao (2015) Fatigue estimation of drill-string and drill-pipe threaded connection subjected to random loadings. Masters thesis, Memorial University of Newfoundland.

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Abstract

Drill-strings are slender structures used to drill the oil-well in searching for oil and gas. Failures of drill-strings cause the loss of time and money and therefore predicting fatigue damage induced by vibrations is of benefit. In this thesis, fatigue calculation of drill-strings is conducted in both time domain and frequency domain, considering axial and torsional vibrations. In time domain analysis, the stress time histories at any specific location of the drillstring are obtained from a finite element model. Both deterministic and random excitations are taken into account. Then using a rainflow counting method, the damaging stress cycles are extracted. Based on the linear cumulative damage law, fatigue damage is then calculated and fatigue life of the drill-string is therefore predicted. Results under both deterministic and random excitations are compared against each other. Time domain fatigue analysis gives accurate predictions at the cost of long calculating time. For the sake of time-saving, a method in frequency domain is developed. The drill-string vibration model is built using continuous parameter method. Power spectral densities (PSD) of the random excitation are assumed. In the process to gain expected damage, equivalent stress spectra are calculated based on Mocha’s random fatigue failure criterion. Then the expected fatigue damage and predicted life are estimated by a spectral method: Dirlik’s method. Further, the drill-pipe threaded connection, which is used to connect numerous drillpipes collars to form a drill-string, is investigated. With the help of ANSYS workbench 15.0, a standard 4.5” API line pipe threaded connection model is built. Random excitation is considered mainly due to the unevenness of rock formation being drilled. Firstly, a static stress analysis is conducted considering "make-up" and "tensile load" steps. Then modal analysis and random vibration analysis are conducted, assuming acceleration PSD as input. Finally, based on three-band technique, the fatigue damage is calculated. Computer simulations are run and results are given by the above three fatigue damage calculation methods. Results show the critical positions of both the drill-string and the threaded connection, where attention should be paid to by the manufacturers and the drilling operators. Although some limitations exist, this thesis proposed two ways to evaluate fatigue damage of a drill-string. The threaded pipe connection, which is a complex structure, is also analyzed. Further research work will aim at the validation of the simulations.

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