Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 117 Scenario Fatigue life (in cycles) ( N ) Stress 80 MPa Stress 200 MPa 1 2.600E+06 1.664E+05 2 1.332E+07 8.525E+05 3 1.907E+05 1.220E+04 4 2.940E+04 1.882E+03 5 1.388E+05 8.883E+03 1 + 3 2.791E+06 1.786E+05 1 + 3 + 4 2.820E+06 1.805E+05 1 + 3 + 5 2.930E+06 1.875E+05 Table 11 : Fatigue life for different scenarios. Once the casing-pipe crack became a through crack, the entire pressure barrier provided by the casing pipe disappeared. In the graph (Fig. 29), each line represents the fatigue life of the pipe-in-pipe system for a particular damaging condition for different cyclic-loading combinations. It can be clearly seen that the majority of the fatigue life is borne by the conductor pipe (Scenario 1), whereas the fatigue life represented by Scenario 2 is rather unlikely. All the other combinations are between Scenarios 1 and 2. Figure 29 : Bending moment (amplitude) vs. fatigue life for various scenarios Of paramount importance is the magnitude of the bending stress, which may lead to reaching the critical stress intensity factor. Based on fracture mechanics, the fatigue life of a component is reached when the value of the stress intensity factor exceeds the material’s critical fracture strength K c causing the component failure. Stress intensity factors K on the inner surface as the through crack propagates around the conductor (Scenario 3), are plotted as a function of loading cycles in Fig 30. The lowest curve corresponds to the load level applied in the analysis, and shows that the K value does not exceed K c (which is 7190 MPa mm 1/2 ). The K-N response was studied for the levels of bending moment multiplied by factors of 2.5, 3, 4 and 5 to generate additional curves in Fig. 30. The scale factor 2.5 gives a peak K close to K c . Higher scaling values give K values above K c . Only for comparison purposes, the analyses exceeding K c value were not prevented to complete the full crack growth simulation, albeit the critical stress intensity value was already reached.