A Non-Linear Fracture Assessment Procedure for Pipeline Materials with a Yield Plateau Tomasz Tkaczyk * Offshore Engineering Division Technip Westhill, Aberdeenshire, UK Noel P. O’Dowd Department of Mechanical and Aeronautical Engineering University of Limerick Limerick, Ireland Kamran Nikbin Department of Mechanical Engineering Imperial College London London, UK Brett P. Howard Offshore Engineering Division Technip Westhill, Aberdeenshire, UK ABSTRACT An accurate defect assessment procedure is needed to ensure in- tegrity of girth welded steel pipelines while avoiding unnecessary repairs. An integral part of such a procedure is an estimation scheme for the crack driving force. In a recent paper, a mod- ified reference stress method has been proposed by the authors for the assessment of elastic-plastic pipes with surface breaking defects. For materials with continuous yielding the method has been shown to provide accurate estimates of crack driving force. However, for materials with a yield plateau (L¨ uders plateau), the description of the evolution of the crack driving force is less accu- rate. In this work, a method for the estimation of crack driving force (phrased in terms of the J -integral) for surface breaking pipeline defects, applicable to materials with a yield plateau, is proposed. The method is validated by comparing the predictions with the results of three-dimensional finite-element analysis of a surface cracked pipe under tension and bending loading. INTRODUCTION The reel-lay method is a cost efficient alternative to the S-lay and J-lay methods (Kyriakides and Corona, 2007) for installation of small to medium size steel offshore pipelines (up to approx. 0.5 m in diameter). During reeling a long section of pipe (up to several kilometers in length) is spooled onto a large diameter reel, situated on a vessel while docked at a spoolbase (see Fig. 1). The pipe is installed by unreeling the pipeline once the vessel has arrived at the destination site. In addition to a faster installation process, the * Address all correspondence to this author. Reel Pipe Aligner Straightener Figure 1. Schematic of Reel Vessel. quality of the pipeline construction is also enhanced, compared to conventional installation techniques, by the use of on-shore weld- ing and inspection under controlled conditions. However, reeling operations induce plastic strain in the pipeline (up to 2.3% for an 18” (457 mm) diameter pipe). For a pipe of diameter D, with a bending radius R, the maximum bending strain, , (assuming pure bending) is, = D/2 R + D/2 (1) An accurate defect assessment procedure will ensure integrity while avoiding unnecessary repairs. An integral part of such a pro- cedure is a method to calculate the crack driving force, typically phrased in terms of the J -integral or crack tip opening displace- Proceedings of the Nineteenth (2009) International Offshore and Polar Engineering Conference Osaka, Japan, June 21-26, 2009 Copyright © 2009 by The International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-53-1 (Set); ISSN 1098-618 100
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A Non-Linear Fracture Assessment Procedure for Pipeline Materials with a Yield Plateau
Tomasz Tkaczyk∗Offshore Engineering Division
TechnipWesthill, Aberdeenshire, UK
Noel P. O’DowdDepartment of Mechanical and
Aeronautical EngineeringUniversity of Limerick
Limerick, Ireland
Kamran NikbinDepartment of Mechanical Engineering
Imperial College LondonLondon, UK
Brett P. HowardOffshore Engineering Division
TechnipWesthill, Aberdeenshire, UK
ABSTRACTAn accurate defect assessment procedure is needed to ensure in-tegrity of girth welded steel pipelines while avoiding unnecessaryrepairs. An integral part of such a procedure is an estimationscheme for the crack driving force. In a recent paper, a mod-ified reference stress method has been proposed by the authorsfor the assessment of elastic-plastic pipes with surface breakingdefects. For materials with continuous yielding the method hasbeen shown to provide accurate estimates of crack driving force.However, for materials with a yield plateau (Luders plateau), thedescription of the evolution of the crack driving force is less accu-rate. In this work, a method for the estimation of crack drivingforce (phrased in terms of the J-integral) for surface breakingpipeline defects, applicable to materials with a yield plateau, isproposed. The method is validated by comparing the predictionswith the results of three-dimensional finite-element analysis of asurface cracked pipe under tension and bending loading.
INTRODUCTIONThe reel-lay method is a cost efficient alternative to the S-lay andJ-lay methods (Kyriakides and Corona, 2007) for installation ofsmall to medium size steel offshore pipelines (up to approx. 0.5 min diameter). During reeling a long section of pipe (up to severalkilometers in length) is spooled onto a large diameter reel, situatedon a vessel while docked at a spoolbase (see Fig. 1). The pipe isinstalled by unreeling the pipeline once the vessel has arrived atthe destination site. In addition to a faster installation process, the
∗Address all correspondence to this author.
ReelPipeAligner
Straightener
Figure 1. Schematic of Reel Vessel.
quality of the pipeline construction is also enhanced, compared toconventional installation techniques, by the use of on-shore weld-ing and inspection under controlled conditions. However, reelingoperations induce plastic strain in the pipeline (up to 2.3% for an18” (457 mm) diameter pipe). For a pipe of diameter D, with abending radius R, the maximum bending strain, ε, (assuming purebending) is,
ε =D/2
R+D/2(1)
An accurate defect assessment procedure will ensure integritywhile avoiding unnecessary repairs. An integral part of such a pro-cedure is a method to calculate the crack driving force, typicallyphrased in terms of the J-integral or crack tip opening displace-
Proceedings of the Nineteenth (2009) International Offshore and Polar Engineering ConferenceOsaka, Japan, June 21-26, 2009Copyright © 2009 by The International Society of Offshore and Polar Engineers (ISOPE)ISBN 978-1-880653-53-1 (Set); ISSN 1098-618
100
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