Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 107 Figure 12 : Detail of crack path. F ULL - SYSTEM STUDY he undertaken preliminary crack-propagation analyses on the full model (combining the conductor and casing pipes), demonstrated that the crack in the conductor pipe grew much faster than that in the casing pipe. It was found that the inner-wall crack in the conductor pipe grew to become a through-wall-thickness crack and further, grew nearly 120 o wide in the cross-section of the conductor pipe before the inside-wall elliptical crack in the Casing pipe showed any substantial growth. A three-stage crack-propagation strategy was therefore adopted to evaluate the cumulative fatigue life of the system and of the individual pipes. Three-stage Analysis Strategy The three stages of the used strategy are:  Stage 1: Application of the fatigue cyclic load (bending moment) to the combined model with the initial elliptical cracks both in the casing and conductor pipes; then the crack in the conductor pipe was allowed to grow until it broke through the wall thickness.  Stage2: Continuation of Stage-1, until the crack in the conductor stabilised.  Stage 3: Continuation until the inner-wall crack in the casing pipe became a through-wall-thickness crack. Stage1 Crack-propagation analysis in Stage 1 was performed on the combined conductor/casing system described above with initial semi-circular cracks in both pipes (Fig. 13). The FE model and the location of the cracks are displayed in Fig. 14. Figure 13 : Stage-1 crack configuration. T