Abstract:

To improve the safety risk prediction accuracy of injection-production string, a joint simulation analysis method was used to perform finite element verification of the fatigue crack propagation process of straight-notch compact tensile specimens. Then, a finite element model of fatigue crack propagation of injection-production string with external surface cracks in gas storage was proposed. Moreover, the injection-production strings' fatigue crack propagation behavior was analyzed under alternating pressure loads. The results indicated that the specimen simulations were consistent with the fatigue crack test results, indicating that the joint simulation method had high accuracy in fatigue crack propagation analysis. During the fatigue crack propagation of the injection-production string, the larger the initial crack's length-to-depth ratio or the higher the stress ratio, the faster the crack propagation rate with a critical minimum crack length of 4 mm. Under the same number of pressure load cycles, the larger the initial crack's circumferential angle, the longer the surface crack propagates, and the critical minimum circumferential angle was 45°. The crack consistently propagated along the axis of the pipe string regardless of the initial crack's circumferential angle, and the unstable propagation length was 52 mm. During alternating injection and production in gas storage, the amplitude of the alternating pressure load should be properly controlled to avoid the initiation and propagation of fatigue cracks.

Key words: alternating pressure load, underground gas storage, injection and production pipe column, fatigue, crack propagation