Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 97 Assessment of structural integrity of subsea wellhead system: analytical and numerical study A.R. Maligno Institute for Innovation in Sustainable Engineering, University of Derby, Quaker Way, DE1 3HD Derby (UK) R. Citarella Department of Industrial Engineering, University of Salerno, Salerno (Italy) V.V. Silberschmidt Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough (UK) C. Soutis School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester (UK) A BSTRACT . Subsea wellhead systems exposed to severe fatigue loading are becoming increasingly a significant problem in offshore drilling operations due to their applications in wells with higher levels of pressure and temperature, situated at larger depths and in harsher environments. This has led to a substantial increase in the weight and size of offshore equipment, which, in combination with different loading conditions related to the environmental factors acting on the vessel and riser, has greatly increased the loads acting on subsea well systems. In particular, severe fatigue loading acting on the subsea wellhead system was detected. For this reason, a combined analytical and numerical study investigating the critical effect of crack depth on the overall structural integrity of subsea wellhead systems under cyclic loading was carried out. The study is based on a Linear Elastic Fracture Mechanics (LEFM) approach. K EYWORDS . Fatigue crack growth; Zencrack; Defect assessment; Girth welds; Steel pipelines. I NTRODUCTION owadays, offshore oil and gas exploration move into deeper waters and harsher environments. Technological advancements have enabled exploitation of previously inaccessible reservoirs, thanks to such important advancements as the improved ability to find offshore reservoirs and the capacity to drill deviated wells [1, 2]. Simultaneously, new technology makes extraction of more hydrocarbons and gas from existing wells possible, and many of them are now more than twenty years old. At the same time, economic demands imply a prolonged use of the existing welds. Therefore, subsea wells that are used for longer periods are likely to experience dangerous levels of accumulated load cycles. As a result, regulatory bodies are seeking assurances that the well-system conditions and integrity are being N