Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 98 correctly supervised, particularly with regard to fatigue, fail-safe design as well as analytical and numerical methodologies for their prediction [1-6]. Circumferential girth welds are a critical location for the structural integrity of pipelines and risers, and are produced using mechanised welding processes. Welding involves the heating of metal to its melting temperature followed by rapid cooling. As the weld metal cools it contracts, and the cooling rate influences a type of microstructure [7-10], mechanical properties [11,12] and a level of residual stresses [13] in the welds. Besides, flaws are likely to occur during the welding process. Fracture mechanics-based structural integrity assessments, most commonly referred to as either engineering critical assessments (ECAs) or Fitness-For-Service (FFS) assessments have found widespread acceptance to deal with such problems over the years [1-6]. The nuclear and offshore oil and gas industries are the main drivers behind the development of formal FFS procedures. Structural integrity assessments [14, 15] can be used at the design stage, to estimate the maximum flaw size that will not grow to intolerable levels during the life of the component, or to assess defects grown after some time in service. Such information about defect tolerance relies on the availability of representative and reliable experimental data, on which any defect-assessment calculation is based. This study aims at developing a fracture mechanics-based model to study and elucidate the effects linked to crack depth and shape as well as crack propagation in the subsea wellhead systems under cyclic loading conditions. In particular, this research focuses at simulations of crack propagation in a conductor/casing pipe girth welds and at understanding the impact of flaws on the drift-off/drive-off capacities and on the residual fatigue life of the wellhead system. In order to determine the maximum allowable flaw size and to simulate crack propagation under cyclic loading, the in-house software Zencrack was used. The validity of this software has been widely assessed in several research and industrial projects [16, 17]. The wellhead system was calibrated to fit the S-N curve of Quality Category Q2 as described in BS 7910 Code [18], which is equivalent to BS 7608 Code design class E [19]. W ELLHEAD SYSTEM typical configuration of a wellhead system is shown in Fig. 1. In this study, a simple two-pipe wellhead system comprising a conductor pipe (diameter 36". thickness 1.5”) and a surface casing pipe (diameter 22", thickness 1.0”), with a rigid lock wellhead, was considered. The girth weld for both the conductor and casing pipes to be investigated is class E type following BS7910 code. The effect of stress concentration or misalignment was not considered in this study. Both the conductor and the casing are considered fixed at the base, and a constant-amplitude cyclic bending moment was applied at the top end, with a crack plane situated half way along the length (Fig. 2). (a) (b) Figure 1 : Typical wellhead system: (a) schematic presentation [5]; (b) detail. No presence of cement in the annular spacing between the conductor and the casing pipe or the surrounding soil was assumed. The crack geometries and the applied load, used in this study, are symmetric with respect to the YZ plane (Fig. 2). Hence, it was proposed to use half models for all analyses taking advantage of this symmetry. Material properties for X80 steel used for the casing and X65 steel for the conductor are summarised in Tab. 1. A

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