Authors Gianluca Mannucci and Giuseppe Demofonti from the Centro Sviluppo Materiali SpA, Rome, Italy, examine the Control of ductile facture propagation in X80 gas linepipe. As they comment, the increasing need to transport large quantities of gas from reservoirs located in remote areas to the consuming markets is pushing the development of a significant number of new long-distance, high-pressure gas transmission pipelines. In order to be cost effective, these lines should be operated at high pressure (>10 MPa) and constructed using large-diameter, high-grade steel linepipe such as API 5L X80 or greater. In addition, it is expected that these lines will convey rich gases – that is, gases rich in heavier components – as well as having to cross remote areas characterised by significant environmental constraints such as very low operating temperatures.
Ductile-fracture propagation control for these new lines is of primary importance, since a failure could cause a long and costly service delay. The forecast operating conditions (high grade, large diameter, high pressure, rich gas, low temperature) are such that the determination of minimum toughness requirements for arresting a propagating fracture is not a trivial issue.
The paper reviews the state-of-the-art about ductile-fracture control for X80 linepipe, looking at the available predictive methods, standards’ provisions, and the published full-scale burst-test database; it also mentions specific aspects such as the influence of separation on the fracture surface and the difference between the behaviours of spiral- and longitudinal-welded pipes. Finally the alternative to the linepipe material’s self-arrestability by the use of an external mechanical device – the so-called crack arrestor – is discussed.
The theme of pipeline defects is continued by Professor Guy Pluvinage et al. from the Fiabilité Mécanique, Conseils, Silly sur Nied, France with their Assessment of a gouge, a dent, or a dent plus a gouge, in a pipe using limit analysis or notch fracture mechanics. The paper describes the methods of assessment for gouges, dents, and combined gouge and dent defects. Due to the fact that these defects induce elastoplastic failure, the assessment methods are based on limit analysis or notch fracture mechanics, and both provide similar safety factors.
The authors point out that these types of defects cannot be considered as crack-like defects and treated by classical fracture mechanics because this procedure is inappropriate and much too conservative. It is preferable to use a failure criterion based on ductility. In this case, the increase of fracture resistance due to strain hardening during dent formation also needs to be taken into account.
The issue of Landslide risk assessment for pipeline systems in mountainous regions is reviewed by Dr Alfred M Pettinger and David W Sykora of Exponent Failure Analysis Associates, Natick, MA, USA. The authors describe the use of a generalised matrix method to evaluate the risk of landslides imposing external forces on a buried pipeline system traversing typically mountainous areas. The procedure has been applied in details on the Camisea pipeline system in Peru, a 700 km buried pipeline system extending from the Amazon Basin, over the Andes Mountains, to the Pacific Ocean. The specific objective was to establish a baseline risk assessment using information that could be readily measured and collected by field maintenance personnel over several hundred kilometres of the pipeline’s alignment. Once the baseline assessment was established and the areas of highest risk identified, a more reliable means of quantifying likelihood was applied to those areas in order to make sound decisions on remediation and subsequently to update the risk assessment.
Although the focus of this paper is on the evaluation of the likelihood of occurrence of a landslide, considerations in assessing severity rankings in a sparsely populated but environmentally and socially sensitive mountainous region are also discussed.
