Fatigue performance of OCTG threaded connections subjected to the cyclic rotational dogleg bending events during casing installation and well construction has become more and more crucial in horizontal well tubular design as its lateral becomes longer and longer for improving the well productivity. There are different methods for fatigue evaluation of structural components, such as full-scale fatigue testing and fatigue evaluation using analytic means (S-N fatigue, strain-based fatigue life, fatigue crack growth and fracture mechanics, and so on). In this paper, we implement fatigue crack growth and fracture mechanics method together with full-scale resonant bending fatigue testing for evaluating fatigue performance of OCTG connections.
The fatigue evaluation procedure consists of the following steps: First, nonlinear elastic-plastic FEA model is utilized to simulate the connection makeup to its desired makeup torque (makeup position); Second, linear elastic FEA is performed to compute the alternating stress (i.e. fatigue driving stress) and static mean stress due to makeup and/or other external constant loads; Third, fatigue crack growth of a pre-existing crack-like surface circumferential flaw at critical locations is performed for selected dogleg severity (DLS), e.g.10, 15, 20, 25, 30, and 35 deg/100ft, etc. Finally, the connection fatigue curve in terms of fatigue life (number of cycles to fatigue failure,Nf) versus DLS (in deg/100ft) is constructed.
Material parameters used in the fatigue crack growth model were calibrated to a single set of full-scale connection fatigue-to-failure tests for different DLS values on a single connection product (i.e., same OD, wall, grade, and connection design). By using this calibrated fatigue analysis model, we are able to achieve excellent agreement on the connection fatigue life for a broad range of DLS magnitudes (from 10 to 35 deg/100ft) between the analysis and the actual test results for several different connection products, i.e., different OD, different wall thickness, and different connection designs (API buttress-type thread, proprietary wedge thread, etc.). Moreover, the model is also able to predict the critical location at which a through-wall crack would develop and cause leak that are consistent with what were observed in the full-scale connection fatigue tests.
Connection fatigue evaluation by means of fatigue crack growth and fracture mechanics presented in this paper is a value-added tool to the full-scale connection fatigue testing since it is extremely time and cost effective. The fatigue analysis tool can be used to calculate the fatigue life of any threaded OCTG connections subjected to cyclic loading (e.g., rotational dogleg bending, frac cycles, and so on). For instance, it can be very beneficial for assessing fatigue performance of a new connection product design and development, especially when the connection is intended to be used in the horizontal wells with long lateral for extended reach.