Correction factors for safe performance of API X65 pipeline steel Sayyed H. Hashemi * Department of Mechanical Engineering, the University of Birjand, P.O. Box 97175/615, Birjand, Iran a r t i c l e i n f o Article history: Received 5 September 2008 Received in revised form 23 January 2009 Accepted 23 January 2009 Keywords: Correctio n factor API 5L X65 Instrumented Charpy test Initiation energy Propagatio n energy a b s t r a c t Prediction of required Charpy energy for fracture arrest is vital for safe performance of gas transportation pipelines. This is commonly estimated through failure models calibrated in the past on fracture data from combined Charpy tests and full-thickness burst experiments. Unfortunately , such pipeline failure models are unable to correctly predict the minimum arrest toughness of thermo-mechanical controlled rolled (TMCR) steels. To refine the existing failure models, different empirical adjustments have been proposed in rece nt years. In this paper, simi lar correction factors were derived from fracture information ofinstrumented Charpy impact tests on API X65 steel. The contribution of different fracture mechanisms ofimpa ct test specime ns was determi ned through energy parti tioni ng anal ysis . Parts of the ener gy contribution were correlated then to the source of uncertainty observed in similar experiments. The applied technique was similar to that of previous studies on X70 and X100 steels, and proved to be encouraging in giving consistent results compared to available test data. 2009 Elsevier Ltd. All rights reserved. 1. Introduction More than half the world’s hydrocar bon resourc es are located in remote areas, far from major markets. Long-distance high-pressure pipelines are used to transport natural gas from production sites to consumers. The materials used initially for pipeline networks were X52 in 60s, X60–X65 in 70s, and X70 and X80, with X70 domi- nating,in rec ent years [1]. The inc rea sing demand in energymar ket has led the ind ustr y towards the use of higher -gra de pipe line stee ls (AP I X80 and abo ve) . The se pip eli ne mat er ials are fabr icated through TMCR processing, and provide higher capacity for trans- portation of richer gases. Such gas pipelines work at harsh envi- ronme nts und er high int ern al pressures (up to 80% of the ir minimum speci fied yield str eng th; MS YS). A ma jor concern currently limiting the more extensive use of these high-strength pipeline steels is the lack of accurate propagation/arrest prediction models to assess the pipeline resistance against ductile fractures [2–6], see Fig.1 . Thes e frac tures are initia ted most ly from exc ava tors (third party impact; gouge and dent), weld defects or corroded spots (due to loss of wall thickness), and might cause catastrophic results, e.g., injury or fatality of humans and the loss of national assets. The safety of gas pipelines against such fractures was originally evaluated by absorbed energy in impact testing of pipeline mate- rials. From this, the issue of dominant brittle type fractures at the earl ier stag es of gas pipe lining was address ed simp ly thro ugh selecting pipeline steels with low DBTT (ductile-to-brittle transi- tion temperature) [7] . The DBTT can be characterised either from shear fractures on the broken surfaces of drop weight tear test (DWTT ) spec imens (typically 85% duct ile shear appeara nce), or from absorbed energy of Charpy samples tested at low tempera- tures. Fracture data from these laboratory specimens and failure information from complementary full-scale burst tests was used later to cons truc t failur e mode ls of ductile frac tures [8]. These models correlate the pipe geometry (wall thickness and outside diame ter) and its loa ding cond ition s (from inter nal pres sure ) to the Cha rp y fractu re ene rgy of pip eli ne mat erial. Alt hou gh such pip eline failure models worked well for low-grade pipeline steels, they are unable to cor rec tly pre dict the minimu m arrest tou ghness ofmod ern (TMCR) ste els . To overc ome thi s pro ble m, dif fer ent emp irical adj ust men ts to exist ing failure models (in ter ms ofcorrection fact ors) have been propo sed in rece nt year s. In this pape r, an experimen tal appr oac h for deriv ing simila r correctio n factors from Charpy impact test is given. The fracture processes in imp act tes ting of Cha rpy sample s (ma de fro m gra de API X65 1 pipeline steel) are closely monitored. As the conventional Charpy experiment has only one output (i.e., the overall fracture energy), an instrumen ted Cha rp y rig is us ed. Full fai lur e inf ormation (includin g yieldi ng, fractu re ini tiation and max imu m loa d) is *Tel.: þ98 561 2502516; fax:þ98 561 2502515. E-mail address: [email protected]1 The API X65 steel together with X70 are main pipe materials used for spiral pipelines (with a range of outside diameter from 508 mm to 1422 mm) and largely utilised in gas transportation networks in Iran. Contents lists available at ScienceDirect International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp 0308-0161/$ – see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2009.01.011 International Journal of Pressure Vessels and Piping 86 (2009) 533–540
