Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 115 Figure 27 : Number of cycles to achieve through-the-thickness crack in casing. Figure 28 : Stress intensity factor vs . crack depth (casing). In Fig. 28, it is interesting to notice that the magnitude of K values is now comparable to the calculated values of K for the previously investigated configurations. This behaviour clearly demonstrates that the casing bears most of the bending load. E FFECT OF L OADING n order to achieve a full understanding of the mechanical behaviour of the Subsea Wellhead System in presence of flaws and to develop the most effective damage-tolerant design, five representative scenarios were investigated under variable loading conditions. In particular, the maximum stresses in the range from 80 MPa to 200 MPa were applied. These further analyses gave the possibility to understand the effect of different bending moments on damage tolerance capability of five full-system configurations. The examined five scenarios are presented in Tab. 10. Bending moment and fatigue life cycles assessment The fatigue life, related to the five scenarios, was calculated for stress of 80 MPa evaluated on the outside surface of the uncracked conductor pipe. A similar exercise was also carried out for stress of 200 MPa. The fatigue cycles for each of the scenarios and their combinations are shown in Tab. 11 and plotted in Fig. 29. As expected, the maximum fatigue life is achieved for Scenario 2. In this case, the conductor pipe remained intact and a relatively reduced load was borne by the casing pipe. A more realistic fatigue life for the system was obtained when considering a combination of scenarios 1, 3 and 5. In Scenario 1, the inner wall crack in the conductor pipe grew into a through-the-thickness crack first and then, as I