Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 118 Figure 30 : Variation of K with cycles for different load cases . C ONCLUSION n analytical and numerical study based on the LEFM theory was carried out for the girth weld (although the weld was not explicitly modelled) of the subsea wellhead system. Analytical studies were performed either to establish necessary parameters (e.g. initial crack dimensions) or to validate the finite-element results. The combined study allowed a thorough assessment of the overall behaviour of the wellhead system under cyclic loading conditions for different flaw extensions and various stress ranges. It was demonstrated that most of the load was borne by the conductor pipe even for relatively extended flaws while the crack in the casing pipe underwent a virtually negligible crack growth in the fully coupled system with a kinematic constraint. Furthermore, this study allowed the evaluation of an adequate modelling strategy (e.g. kinematic coupling), which will help pipeline engineers to assess accurately structures with flaws and, in particular, compound systems. The effect of welding residual stress [27, 28] is also important for subsea wellhead systems, especially for low-toughness steels, where a high tensile residual stress can provide a significant part of the crack driving force. Therefore, although this study proposed an efficient approach to the structural integrity assessment of subsea wellhead systems, the future work will contemplate the direct modelling of girth welds and the effect of residual stresses, induced during the welding process, on crack initiation and propagation. R EFERENCES [1] DNV-RP-C203, Fatigue design of offshore steel structures, Det Norske Veritas (2005). [2] Robelius, F., Giant oil fields-the highway to oil: Giant oil fields and their importance for future oil production, PhD thesis, Uppsala University, (2007). [3] King Lim, T., Tellier, E., Howells, H., Wellhead, Conductor and Casing Fatigue-Causes and Mitigation, In: Deep Offshore Technology (DOT), Perth, Australia, (2012). [4] Det Norske Veritas, Well Fatigue Analysis Method, Report for JIP Structural Well Integrity, Report No. 2011-0063, Revision 01, (2011). [5] Evans, J., An Evaluation of the Fatigue Performance of Subsea Wellhead Systems and Recommendations for Fatigue Enhancements, In: Offshore Technology Conference (OTC), Houston, USA, (2011). [6] Buitrago, J., Baxter, D., Hudak S., High-cycle and low-cycle fatigue resistance of girth welds in sour service, In: Proceedings of 27th International Conference on Offshore Mechanics and Arctic Engineering, Estoril, Portugal, (2008) 57545. [7] Chen, X., Qiao, G., Han, X., Wang, X., Xiao, F., Liao, B., Effects of Mo, Cr and Nb on microstructure and mechanical properties of heat affected zone for Nb-bearing X80 pipeline steels, Materials and Design, 53 (2014) 888– 901. A