Drilling is an important operation in the mining industry allowing the exploration of reserves. A major component in a drilling operation is the drill string, which provides the link between the surface of the well and the bottom hole assembly. It consists of multiple drill rods connected together by threaded joints. Drill rods commonly fail by fatigue in the pin of the threaded joint due to the fluctuating stresses they sustain.
The major loads that contribute to the fatigue failure are static tension and reverse bending. Reverse bending results from the drill rod rotating while passing through doglegs in the well trajectory. The static tension load is caused by the drill string suspending from the surface of the well and results in a positive mean value of the fluctuating stress.
The aim of this project was to investigate the fatigue resistance of tapered threaded joints of NQ drill rods under service loads, which was achieved by applying a two stage approach.
The first stage of the project was to design, construct and commission a fatigue testing facility for NQ drill rods. A four point bending arrangement was used to apply the bending load. With an electric DC motor driving the drill rod at speeds up to 1000rpm, a controlled reverse bending load results. A thrust bearing at each end of the drill rod allows a static tension load to be applied while rotating. Hydraulic actuators apply the desired loads to the drill rod and the facility is capable of applying loads significant enough to break the drill rod material, which has an ultimate tensile strength of 725MPa. To calibrate the load transmission to the drill rod, strain gauges were used to measure the axial strain on the outside surface of the drill rod under static loads.
The second stage of the project involved the fatigue testing of tapered threaded joints of NQ drill rods at different load levels. In all the tests performed, the tension load applied was 11.76MPa to minimise the number of test parameters. Tests were performed at six different bending moment loads, which determines the amplitudes of the fluctuating stress. The stress amplitudes tested ranged from 27.3MPa to 126.37MPa on the outside surface of the drill rod. A test limit was set at 3 x 106 cycles to optimise the time allocated to the test program. To ensure repeatability in the fatigue tests, the make up torque was equally and accurately applied to all the drill rods. A separate apparatus was used to apply the make up torque to the threaded joint using a dead weight suspended from a wrench.
In all the fatigue tests performed, a single mode of failure was observed in the failed specimens. All the drill rods failed or sustained damage in the form of crack propagation in the lt engaged thread of the pin joint. This suggests that this is the location of highest stress concentration for NQ drill rods with a tapered threaded joint. The lowest stress amplitude tested where complete drill rod failure was obtained was 45.84MPa. The life of this test was approximately 1 x 106 cycles.
The outcome of this project is that a unique fatigue testing facility for NQ drill rods has been constructed which is capable of simulating service loads. From the fatigue data obtained it is observed that the fatigue resistance of threaded joints, which connect drill rods together, is extremely low. This is due to considerable stress concentration present in the threaded joint and the pre-stress that results from the applied make up torque. It was also observed that over the range of load levels tested, a linear relationship existed between stress and life when the fatigue data was plotted on an S-N diagram.