Drill-strings are slender structures used to drill the oil-well in searching for oil andudgas. Failures of drill-strings cause the loss of time and money and therefore predictingudfatigue damage induced by vibrations is of benefit. In this thesis, fatigue calculationudof drill-strings is conducted in both time domain and frequency domain, consideringudaxial and torsional vibrations.udIn time domain analysis, the stress time histories at any specific location of the drillstringudare obtained from a finite element model. Both deterministic and randomudexcitations are taken into account. Then using a rainflow counting method, the damagingudstress cycles are extracted. Based on the linear cumulative damage law, fatigueuddamage is then calculated and fatigue life of the drill-string is therefore predicted.udResults under both deterministic and random excitations are compared against eachudother.udTime domain fatigue analysis gives accurate predictions at the cost of long calculatingudtime. For the sake of time-saving, a method in frequency domain is developed.udThe drill-string vibration model is built using continuous parameter method. Powerudspectral densities (PSD) of the random excitation are assumed. In the process to gainudexpected damage, equivalent stress spectra are calculated based on Mocha’s random fatigue failure criterion. Then the expected fatigue damage and predicted life areudestimated by a spectral method: Dirlik’s method.udFurther, the drill-pipe threaded connection, which is used to connect numerous drillpipesudcollars to form a drill-string, is investigated. With the help of ANSYS workbenchud15.0, a standard 4.5” API line pipe threaded connection model is built. Randomudexcitation is considered mainly due to the unevenness of rock formation being drilled.udFirstly, a static stress analysis is conducted considering "make-up" and "tensile load"udsteps. Then modal analysis and random vibration analysis are conducted, assumingudacceleration PSD as input. Finally, based on three-band technique, the fatigue damageudis calculated.udComputer simulations are run and results are given by the above three fatigue damageudcalculation methods. Results show the critical positions of both the drill-string andudthe threaded connection, where attention should be paid to by the manufacturers andudthe drilling operators.udAlthough some limitations exist, this thesis proposed two ways to evaluate fatigueuddamage of a drill-string. The threaded pipe connection, which is a complex structure,udis also analyzed. Further research work will aim at the validation of the simulations.
展开▼
