Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 110 While the crack in the conductor grew during the simulation, a severely distorted mesh was generated. The consequence of this was that, to further extend the crack growth, some "manual" interventions were required to update the crack profile. In this study, different crack profiles were introduced to understand thoroughly the consequences of flaw extension in the conductor. The used through-the-thickness crack profile was determined analytically [16]. However, in order to investigate the effect of particularly large flaws in the structural integrity of wellhead systems (arising from a combination of variable loading conditions and aggressive environmental effects), two additional hypothetical shapes were investigated. The studied crack profiles are labelled as described in Tab. 7: Crack profiles Method of determination TW 1 analytical TW 2 analytical TW 3 analytical TW 4 hypothetical TW 5 hypothetical Table 7 : Crack-profile labels and estimation technique. Starting from the front of the initial through-the-thickness crack TW1 (Fig. 20a) the crack was extended in five steps up to a condition in which the remaining ligament of the conductor pipe was exposed only to compressive stresses [24, 25] so that any crack opening was prevented. Throughout these analyses, the crack growth in the inner wall of the casing pipe was found to be negligible. For this reason and for sake of clarity only the conductor pipe is shown in the figures. Step 1 During this step ( TW1 ), the crack grew up to reaching a configuration, where its front extended circumferentially for nearly 120° (Fig. 20b). At this crack extension, further propagation was prevented from further development due to severe distortion of the FE mesh (formation of inside-out elements) near the crack front. The initial and final crack sizes and crack extensions are summarised in Tab. 8: Conductor Casing Initial crack size [mm] 3.92 2.839 Crack extension (thickness) [mm] 36.13 3.68 Crack extension (internal surface) [mm] 42.14 3.75 Table 8 : Characteristic crack dimensions at Stage 1. Step 2 Maintaining the same position and shape of the crack front as provided at the end of Step 1, the FE mesh near it was smoothed and the simulation analysis restarted allowing the crack to grow to position TW2 as depicted in Fig. 20c. Step 3 The FE mesh at the crack front was modified as in Step 2, keeping the crack shape as calculated at the end of Step 2 to get further crack growth to position TW3 (Fig. 20d). Steps 4 and 5 Although no crack propagation was detected at the end of Step 3, due to highly compressive stresses, two further positions, TW4 and TW5 (Figs. 20e and f, respectively), were considered to estimate the capability of the conductor to bear loads in presence of extended flaws. Numerical Results: Stage 2 Similarly to Stage 1, throughout Stage 2 propagation of the crack on the inner wall of the casing pipe was found to be negligible. Fig. 21 shows the variation of stress intensity factor K with growth of the through-the-thickness crack along the circumference of the conductor pipe. Apparently, K increased as the crack grew but once the crack front partially crossed