Influence of Anisotropy on Flaw Acceptability in Spiral Welded Pipe Sections Koen Van Minnebruggen 1,* , Stijn Hertelé, Matthias Verstraete 1 , Philippe Thibaux 2 , Jeroen Van Wittenberghe 2 , Rudi Denys 1 , and Wim De Waele 1 1 : Soete Laboratory, Department of Mechanical Construction and Production, Ghent University, Ghent, Belgium 2 : OCAS NV, ArcelorMittal Global R&D, Ghent, Belgium ABSTRACT The structural response of spiral welded pipes in strain based design related projects has gained interest. The spiral weld is a potential weak link, whose structural response is influenced by directional material anisotropy introduced during the production of skelp and pipe. In this paper, Hill’s 1948 yield criterion is used to investigate the response of curved wide plate sections taken from a spiral pipe under plastic tensile deformation by means of numerical analysis. It is concluded that conducting finite element calculations of spiral welded pipes sections can yield unconservative designs, when neglecting anisotropic plastic material response. KEY WORDS: Finite Element Modeling; Spiral Welded Pipe; Curved Wide Plate; Material Anisotropy INTRODUCTION Oil and gas transportation pipelines in strain based design related projects may experience high longitudinal strains. These plastic deformations can occur during installation (e.g. offshore pipe laying), or during operation (e.g. environmental events such as soil settlements) (Kan et al. 2008, Lillig 2008). Knowledge of the structural response of pipes facing an environmentally imposed global plastic strain is of critical importance. Finite element simulations can provide cost efficient methodologies. Correct material property definitions in finite element simulation software are crucial to obtain trustworthy observations and conclusions. In a strain based design context, not only the pipe grade (particularly, the specified minimum yield strength) is important. In addition, the actual pipe stress-strain behavior containing yield strength, tensile strength, work hardening exponent and uniform elongation play key roles. From a constructional point of view, the pipe hoop strength should comply with design requirements to retain a specified internal pressure, whereas the pipe axial deformation capacity is of importance in strain based design projects. Contemporary linepipe steel with high strength, toughness and sufficient wall thickness is traditionally rolled using thermo-mechanical controlled processing technology. This production process inevitably introduces anisotropic material response in the steel. Additional heterogeneity and anisotropy can be introduced during the forming process of the pipe (Kostryzhev 2009, Bremer et al. 2008). Both influence the structural integrity of the pipeline. Recent publications mainly focus on the influence of anisotropic material properties for UOE-pipes. In this respect, Hilgert et al. (2012) concluded that anisotropic material response can have beneficial effects when compared to fully isotropic behavior. For spiral welded pipes, however, to the authors’ knowledge no in-depth study has been performed relating plastic anisotropic material response to flaw acceptability and strain capacity of the pipe. To this concern, a finite element model has been developed for the analysis of curved wide plate sections obtained from spirally welded pipes (Van Minnebruggen et al. 2013), containing a flawed helical weld and loaded in tension. It is based on a previously developed and validated model for girth welds (Hertelé et al. 2012). The curved wide plate test is commonly used to evaluate the strain capacity of girth welds since it provides a trade-off between cost and representativeness with respect to full scale dimensions. Hill’s 1948 yield criterion has been implemented in the spiral curved wide plate (SCWP) model. The adopted criterion is limited to incorporate anisotropy of yield strength, whereas linepipe steel is known to be anisotropic with respect to yield strength, work hardening, ductility and toughness (Bae et al. 2004, Fonzo et al. 2011, Venkatsuya et al. 2012). In a strain based design concept, large (plastic) strains will mainly occur in pipe axial direction. Here, the axial deformation behavior will be dominated by the stress-strain curve which incorporates yield strength, work hardening and ductility. The non- axial directions will mainly develop elastic or small plastic strains which will be dominated by the directional relative yield strength. By incorporating the stress-strain curved corresponding to the pipe axial direction and a directional anisotropy based on yield strength, a preliminary study can be performed on the influence of anisotropic material response in spiral pipe sections. Material toughness anisotropy is not considered in this paper since crack extension is not investigated. More complex anisotropic material models will be incorporated as part of future work. Different design aspects can be introduced. The constructional point of view focuses on the pressure containment capacity of a pipeline, this relates to the pipe hoop strength characteristic. A productional point of view keeps the production process in mind where a spiral pipe is formed from a skelp material with specified strength properties. Here 539 Proceedings of the Twenty-third (2013) International Offshore and Polar Engineering Anchorage, Alaska, USA, June 30–July 5, 2013 Copyright © 2013 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-99–9 (Set); ISSN 1098-6189 (Set) www.isope.org