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  • Structural Integrity Assessment of C-Mn Pipeline Steels Exposed

    to Sour Environments

    Colum Holtam

    TWI Ltd Granta Park Great Abington Cambridge CB21 6AL UK

    Centre for Innovative and Collaborative Engineering (CICE) Loughborough University Loughborough Leicestershire LE11 3TU UK

  • STRUCTURAL INTEGRITY ASSESSMENT OF C-MN

    PIPELINE STEELS EXPOSED TO SOUR ENVIRONMENTS

    By Colum Holtam

    A dissertation thesis submitted in partial fulfilment of the requirements for the award of the degree Doctor of Engineering (EngD), at Loughborough University

    [April 2010]

    by Colum Holtam (2010)

    TWI Ltd Granta Park Great Abington Cambridge CB21 6AL UK

    Centre for Innovative and Collaborative Engineering (CICE) Loughborough University Loughborough Leicestershire LE11 3TU UK

  • Acknowledgements

    i

    ACKNOWLEDGEMENTS This EngD research project was funded by the Industrial Members of TWI, as part of the Core Research Programme and the Engineering and Physical Sciences Research Council. The author would like to acknowledge the support of his academic supervisors at Loughborough University, Dr. Ian Ashcroft and Professor Rachel Thomson and in particular the technical guidance from his industrial supervisor and former colleague Dr. David Baxter. Further thanks go to Richard Pargeter, Technology Fellow for Ferritic Steels and Sour Service at TWI, for his continuing interest in this area of research.

  • Abstract

    iii

    ABSTRACT Oil and gas fields can contain significant amounts of hydrogen sulphide and the behaviour of C-Mn pipeline steels exposed to sour environments (i.e. those containing water and hydrogen sulphide) continues to be one of the most active areas of research in the oil and gas industry. This project is aimed at improving the procedures used to assess the significance of flaws in offshore pipelines and risers operating in such environments. Experimental work has focused on examining the behaviour of C-Mn pipeline steel in a sour environment with respect to both static and fatigue crack growth behaviour, for which there is a paucity of data. In particular, the critical influence of crack depth on the crack growth rate has been studied, in order to ensure that test methods and assessment procedures used in industry are appropriately conservative. Under cyclic loading conditions, an environmental crack depth effect has been demonstrated, whereby, shallow flaws appear to grow faster than deeper flaws at the same (low) value of

    K. The observed behaviour is believed to be dominated by bulk hydrogen charging, i.e. hydrogen charging by absorption from the external surfaces of the specimen rather than at the crack tip, and a lower concentration of hydrogen exists in the centre of the specimen than at the edges. The novel data generated have been applied to real-life pipeline defect assessments to demonstrate the influence of the observed crack growth rate, with a view to developing an improved assessment method. Example engineering critical assessments have been performed for circumferential surface-breaking girth weld flaws located on the internal surface of a typical steel catenary riser, operating in a sour environment and subject to vortex induced vibration fatigue loads. Companies operating in the oil and gas sector will derive benefit from this research programme through the application of new validated test methods and the development of improved in-service assessment procedures.

    KEY WORDS Defect assessment, fatigue, pipeline steel, sour service.

  • Preface

    v

    PREFACE The research presented within this thesis was conducted in partial fulfilment of the requirements for the award of an Engineering Doctorate (EngD) degree at the Centre for Innovative and Collaborative Engineering (CICE), Loughborough University. The EngD is in essence a PhD based in industry, designed to produce doctoral graduates that can drive innovation in engineering with the highest level of technical, managerial and business competence. This EngD research project was sponsored by TWI Ltd, and the Engineering and Physical Sciences Research Council (EPSRC). The EngD is examined on the basis of a thesis containing at least three (but not more than five) research publications and/or technical reports. This discourse is supported by five technical publications, located in Appendices A to E.

  • Symbols and Definitions

    vii

    SYMBOLS AND DEFINITIONS Symbol Definition Units a Flaw depth mm acal Flaw depth used for calibration of direct current potential drop

    system

    a0 Critical crack size (as defined in BS 7910) or average original crack length (as defined in BS 7448-1)

    mm

    B Specimen thickness mm C Constant in Paris fatigue crack growth law d Distance between current input and crack plane in direct current

    potential drop system mm

    D Diffusivity cm2s-1 da/dt Rate of crack propagation with time mms-1 da/dN Rate of crack propagation per cycle mmcycle-1 f Distance between wire and crack plane in direct current

    potential drop system mm

    fSCC Factor of Safety (as defined in BS 7910) with respect to Stress Corrosion Cracking (fSCC > 1.0)

    FQ Force (as defined in BS 7448-1) N J J-integral; a line or surface integral that encloses the crack front

    from one crack surface to the other, used to charaterise the local stress-strain field around the crack front (as defined in BS 7910)

    Nmm-1

    JISCC Threshold J-integral for stress corrosion cracking Nmm-1 k Calibration coefficient K or KI (Applied) stress intensity factor Nmm-3/2

    MPam0.5 KIC Material fracture toughness or critical stress intensity for failure

    under mode I static loading conditions Nmm-3/2 MPam0.5

    KISCC Threshold stress intensity factor for stress corrosion cracking Nmm-3/2 MPam0.5

    Kmax Maximum value of stress intensity factor during the fatigue cycle

    Nmm-3/2 MPam0.5

    Kmin Minimum value of stress intensity factor during the fatigue cycle

    Nmm-3/2 MPam0.5

    K Applied stress intensity factor range Nmm-3/2 MPam0.5

    KTH Threshold stress intensity factor range Nmm-3/2 MPam0.5

    Keff Effective stress intensity factor range Nmm-3/2 MPam0.5

    Kr Fracture ratio of applied elastic K value to material fracture toughness (as defined in BS 7910)

    Nmm-3/2 MPam0.5

    KIH Measure of material fracture toughness for embedded or external flaws in hydrogen charged material

    Nmm-3/2 MPam0.5

    KQ

    Provisional value of KIC (as defined in BS 7448-1)

    Nmm-3/2 MPam0.5

  • Structural Integrity Assessment of C-Mn Pipeline Steels Exposed to Sour Environments

    viii

    Lr Ratio of applied load to yield load (as defined in BS 7910) Mk Stress concentration factor at weld toe m Exponent in Paris fatigue crack growth law N Fatigue life (i.e. the number of cycles to failure at a certain

    stress range)

    R Stress ratio (ratio of minimum to maximum stress in any fatigue cycle)

    S Span between outer loading points in three point bend tests (as defined in BS 7448-1)

    mm

    v Instantaneous voltage volts vcal Voltage used for calibration of direct current potential drop

    system volts

    W Specimen width mm Y Geometrical stress intensity factor correction (as defined in BS

    7910)

    Crack tip opening displacement or CTOD (as defined in BS 7448-1)

    mm

    (Applied) stress Nmm-2 MPa

    SCC Threshold stress for SCC Nmm-2 MPa

    (Applied) stress range Nmm-2 MPa

    TH

    Threshold stress range

    Nmm-2 MPa

    Y or YS Yield strength (e.g. 0.2% proof strength) at the temperature of the fracture test

    Nmm-2 MPa

    Decay constant AUT Automated Ultrasonic CT Compact Tension CTOD Crack Tip Opening Displacement DCB Double Cantilever Beam DCPD Direct Current Potential Drop EAC Environment Assisted Cracking ECA Engineering Critical Assessment EDM Electrical Discharge Machining FAD Failure Assessment Diagram FCGR Fatigue Crack Growth Rate FFS Fitness-For-Service HAZ Heat Affected Zone HIC or HPIC Hydrogen (Pressure) Induced Cracking LEFM Linear Elastic Fracture Mechanics NDT Non-destructive Testing PWHT Post Weld Heat Treatment SCC Stress Corrosion Cracking SCR Steel Catenary Riser SENB Single Edge Notched Bend SENT Single Edge Notched Tension

  • Symbols and Definitions

    ix

    SMYS Specified Minimum Yield Strength SOHIC Stress Oriented Hydrogen Induced Cracking SSC Sulphide Stress Cracking SSR Slow Strain Rate TDP Touchdown Point UTS Ultimate Tensile Strength VIV Vortex Induced Vibration

  • Table of Contents

    xi

    TABLE OF CONTENTS Acknowledgements.................................................................................................................... i Abstract .................................................................................................................................... iii Key Words ............................................................................................................................... iii Preface ....................................................................................................................................... v Symbols and Definitions ........................................................................................................ vii Table of Contents .................................................................................................................... xi List of Figures ........................................................................................................................ xiii List of Tables..................................................