Improvement Fatigue Performance of Threaded Drillstring Connections by Cold Rolling
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The research work presented in this thesis is concerned with analytical, numerical and experimental studies of the effect of cold rolling on the fatigue behaviour of threaded drillstring connections. A comprehensive literature study is made of the various effects on the fatigue behaviour of residual stresses introduced by mechanical deformation of notched components. Some of the effects studied are cyclic hardening behaviour after prestraining, cyclic creep, fatigue initiation in prestrained materials, short cracks and crack growth models including crack closure. Residual stresses were introduced in the surface of a smooth pipe by a rolling device to simulate a cold rolling process and verify the calculated residual stresses by measurements. Strain hardening and contact algorithm of the two bodies were incorporated in the FE analyses. Two significant errors were found in the commercial software package for residual stress evaluation, Restan v. 3.3.2a also called SINT, when using the Schajer method. The Schajer algorithm is the only hole-drilling algorithm without theoretical shortcomings, and is recommended when measuring large residual stress gradients in the depth directions. Using the Schajer method solved by in-house Matlab-routines good agreement between measured residual stress gradients and residual stress gradients from FE analyses was found. Full scale fatigue tests were performed on pipes cut used drillstrings with notches of similar geometry as threads used in drillstring connections. The simulated threads consisted of four full depth helix notches with runouts at the surface. The pipe threads were cold rolled and fatigue tested in a full-scale four-point rotating bending fatigue testing rig. The test results showed that cold rolling had an effect on the crack initiating period. A major part of the fatigue life was with cracks observed at the notch root, but due to the increased fatigue crack propagation resistance the final fracture initiated at pits inside the pipe. Therefor, an optimisation of the roll geometry and rolling parameters was not possible. However, a significant fatigue life improvement was achieved. Based on experiments, a roller with similar profile as the thread root is recommended. A rolling force of maximum 20 KN is recommended to minimise the possibility of damaging the thread profile. Shallow cracks were observed typically when 5% of the fatigue life had expired. Re-rolling after 50% of expected improved fatigue life, when also short cracks were observed in the notch roots further increased the fatigue improvements. Pretensioned small steel specimens with a notch were used to simulate cold rolled threats. The specimens were fatigue tested in tension with minimum load close to zero. Pretensioning increased the fatigue life form approximately 50 000 cycles to an infinite number of cycles. In these test non-propagating cracks of typically 0.4 mm length were found. The benefit from pretensioning gradually disappeared with increasing mean stress. FE analyses indicated that an almost instant relaxation of residual stresses to a level with no monotonic strain hardening from preloading would take place when cycled to moderate mean stress. Cycled at low mean stress, an instant relaxation of the surface layer was found in analysis. All observations from notched pretensioned fatigue specimens were in good agreement with the available literature. However, preloading was found to be strain rate dependent in tests where a pretension load held for 2 minutes gave a longer fatigue life than a sinusoidal loading-unloading cycle performed over a one minute interval. Strain hardening was found not contributing to the fatigue life improvement, whereas the polishing effect from improved surface quality after cold rolling increased the fatigue initiation period. However, residual stress and subsequent early crack closure was the dominating effect at moderate cyclic mean loads. The material data required to perform FE fatigue simulation studies of a full threaded cold rolled coupling incorporating make-up torque, include cyclic stress strain behaviour at various amplitudes and mean stress caused by various degrees of prestraining. Such data are not readily available today, and are only possible to obtain in carefully planned and executed experiments. Also, 3D FE model required for cold rolling analysis is extremely CPU time consuming. Consequently, cold rolling simulations could not be successfully implemented in this work. One of the main conclusions from this work is that drillstring connections will respond differently to thread rolling at the pin or box. A significant improvement in the fatigue life of box threads from residual stresses is expected mainly from increased resistance to crack propagation. However, the compressive residual stress is sensitive to overloading in compression, and the improvement from residual stress depends strongly on the mean stress (or R-ratio). At values of R of approximately 0.6 or higher the beneficial of rolling therefore tends to disappear. At the critical locations of the pin, which are the last engaged thread or the stress relief groove, the effect of residual stresses introduced by rolling is therefore likely to be severely reduced by the high mean stress imposed during make-up of the connection. However, a beneficial effect of rolling is expected to remain due to improved surface condition and due to a possible effect of strain hardening. The net results of these factors on the fatigue performance of actual drillstrings can only be determined in full scale rotating bending tests.