Procedures for the strain based assessment of pipeline dents Dauro Braga Noronha Jr. a , Ricardo Rodrigues Martins a , Breno Pinheiro Jacob b, * , Eduardo de Souza c a PETROBRAS Research and Development Center, Brazil b COPPE/UFRJ, Post-Graduate Institute of the Federal University of Rio de Janeiro, Civil Engineering Department, Brazil c PETROBRAS Exploration and Production Division, Brazil article info Article history: Received 25 August 2009 Received in revised form 3 March 2010 Accepted 4 March 2010 Keywords: Pipelines Dents Strain-based assessment abstract A criterion for strain-based assessment of dents in gas pipelines has been recently proposed in the ASME B31.8 code. This work initially presents a critical review of the equations for estimating strains presented in Appendix R of that code. Next, a procedure based on B-spline curves is presented that interpolates dent geometry from data measured by in-line inspection (ILI) tools and evaluates strain components. The paper proceeds with studies to assess the influence of the ILI tool resolution and positioning on the estimation of the circumferential bending strain. Finally, the evaluation of longitudinal membrane strains is assessed by taking two alternative definitions for dent length that enters in the formula proposed in Appendix R of the aforementioned code, and comparing with results provided by Finite Element anal- yses. The comparison suggests that directions on how to measure the dent length might be incorporated in a revision of the ASME B31.8 Code. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Motivation: significance of dents in pipeline incidents It is well known that mechanical damage on pipelines (such as dents and gouges) causes local stress and strain concentration, and therefore may have adverse effects on the pipeline integrity. According to the Office of Pipeline Safety of the U.S. Department of Transportation (DOT), mechanical damages are the cause of about 28% of the incidents reported from 1985 to 2003 [1]. Data from the same survey revealed that 83% of the oil pipeline leakages occurred at the time the damage was introduced, while 17% caused delayed failures. In gas pipelines the figures are similar, respectively 90% and 10%. Further analyses of records of pipeline incidents caused by mechanical damage that did not lead to immediate failure indicated that about 6% occurred in a very short time (hours or days) after the damage, whereas 7% failed due to increase in internal pressure. The remaining 87% non-immediate incidents were caused by the development of secondary problems such as corrosion, stress corrosion cracking, puncturing by rocks and fatigue due to pressure fluctuation. It is noteworthy that rock dents, although not usually considered as a main threat to pipeline integrity, were responsible for 19% of the non-immediate incidents [1]. Based on these statistical results, one can conclude that delayed pipeline failures caused by dents and mechanical damage represent a significant part of the reported incidents. Hence, the development of tools capable of identifying such defects and the establishment of criteria to properly assess their severity are present concerns of the pipeline industry. 1.2. Context: definition of pipeline dents A dent may be formally defined as ‘a depression, which produces a gross disturbance in the curvature of the pipe wall, caused by contact with a foreign body’ [2]. Dents can be classified in terms of the severity of the changes in the curvature of the pipe wall. Accordingly, a smooth dent is a dent that results in a smooth change in the curvature of the pipe wall. A smooth change in curvature may be defined when the radius of curvature of the sharpest part of the dent is more than five times the wall thickness. A smooth dent that contains no wall thickness reductions (such as a gouge, crack or corrosion) and does not change the curvature of an adjacent girth or seam weld is referred to as a plain dent [2]. A dent can be defined further based on whether it is free to move under influence of the internal pressure. A dent that is pre- vented from rerounding under pressure is called a constrained dent. This is the case of the so-called rock dents, caused by settle- ment of the pipeline onto rocks or other type of support. On the * Corresponding author. Centro de Tecnologia Bloco B sala B-101, Cidade Uni- versitária, Ilha do Fundão, Caixa Postal 68.506, 21945-970 Rio de Janeiro, RJ, Brazil. Tel.: þ55 21 2562 7385; fax: þ55 21 2259 4271. E-mail addresses: dnj@petrobras.com.br (D.B. Noronha Jr.), ricardo.r.martins@ petrobras.com.br (R.R. Martins), breno@coc.ufrj.br (B.P. Jacob), edusouza@ petrobras.com.br (E. de Souza). Contents lists available at ScienceDirect International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp 0308-0161/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2010.03.001 International Journal of Pressure Vessels and Piping 87 (2010) 254e265