Strain-Based Design Of Pipelines
October 8, 2003Project No. 45892GTHStrain-Based Design of PipelinesSubmitted to:U.S. Department of Interior, Minerals Management ServiceHerndon, VAU. S. Department of Transportation, Research and Special ProgramsAdministrationWashington, DC
ReportProject No. 45892GTHonStrain-Based Design of PipelinestoU.S. Department of Interior, Minerals Management ServiceHerndon, VAU. S. Department of Transportation,Research and Special Programs AdministrationWashington, DCOctober 8, 2003William MohrEWI1250 Arthur E. Adams DriveColumbus, OH 43221
ContentsPageExecutive Summary . ivAbbreviations and Definitions List. vi1. 0 Introduction . 11.1 Introduction to Strain-Based Design. 11.2 Use of Strain-Based Design . 31.3 Observed Problems. 31.4 Code Provisions Related to Strain-Based Design. 42.0 Estimation of Maximum Longitudinal Strains . 62.1 Displacement Control as it Differs from Load Control . 62.2 Limit State Definitions. 72.3 Installation Strains . 82.3.1 Cold Field Bending . 92.4 Environmental and Operational Conditions. 102.4.1 Buried Pipelines. 102.4.2 High Temperature and Pressure . 102.4.3 Offshore Environmental Loading Conditions . 112.4.4 Deepwater Offshore Loading Conditions. 122.4.5 Arctic Onshore Environmental Loading Conditions . 122.4.6 Arctic Offshore Environmental Loading Conditions . 133.0 Pipeline Resistance to Compressive Axial Strain . 143.1 Critical Strain as an Appropriate Parameter. 153.2 Plain Pipe Data. 163.3 Girth Weld Effect . 163.4 Information from Weld Fracture Studies. 173.5 Effect of Internal Pressure. 183.6 Effect of External Pressure. 193.7 Y/T Ratio . 203.8 Yield Strength as a Separate Parameter. 213.9 Summary . 234.0 Factors in Choosing Material Based on Tension Stress-Strain Behavior . 244.1 Choosing Maximum Y/T Ratio. 254.2 Choosing Elongation-to-Failure Limits for Pipe Material . 254.3 Choosing Elongation-to-Failure Limits for Weld Material . 254.4 Choosing Minimum and Maximum Weld Metal Strengths. 264.5 Remedial Measures for High Y/T . 274.6 Remedial Measures for Undermatched Weld Metal. 274.7 Tensile Testing . 274.7.1 Tensile Testing of Weld Metals. 285.0 Optimizing Pipe Material for Strain-Based Design: Tension and Compression Strain . 295.1 Range of Materials . 305.2 Corrosion Protection and Weight Coating . 306.0 Prevention of Strain Localization around Girth Welds . 316.1 Strain Concentration at the Girth Weld. 326.2 HAZ Softening . 33i45892GTH/R-3/03
Contents (Continued)Page6.3 Wall Loss and Corrosion . 346.4 Dents and Gouges . 346.5 Concrete Weight Coating or Thermal Insulation Coating . 356.6 Misalignment Across Girth Welds . 357.0 Qualification of Pipe . 367.1 Qualification of Base Pipe Material . 367.2 Qualification of Weld Seam in Pipe . 367.3 Qualification of Welding Procedures . 377.3.1 Batch Testing of Welding Materials for Girth Welds . 377.3.2 Resistance to Weld Cracking. 377.4 Resistance to Strain Aging . 387.5 Toughness Requirements . 398.0 ECA Methods. 408.1 BS 7910:1999. 408.2 DNV 2000. 418.3 API 1104 Appendix A . 418.4 CSA Z662-M1999 Oil and Gas Pipeline System . 428.5 EPRG . 428.6 ASME BPV Section XI. 428.7 WES TR2808 . 438.8 Experience with ECA Applied to Strain-Based Design. 438.9 Safety Factors for ECA. 448.10 Peak Strains . 449.0 Multiple Loading Cycles . 459.1 Methods of Accumulating Strain in Cycles . 479.2 Safety Factors on Strain-Based Fatigue . 489.3 Ratcheting . 4910.0 Probabilistic Methods. 5011.0 Criteria for Full-Scale Testing . 5012.0 Current Research and Development . 5213.0 Recommendations . 5314.0 References. 55Appendix A - Finite-Element Models and Tests for Strain-Based Design with Low-StrengthRegions Near WeldsAppendix B - Data for Analysis of Critical Strain for Pipes in CompressionAppendix C - Guidance Document on Strain-Based DesignTablesTable 1.Table 2.Examples of Pipelines that have Used Strain-Based Design . 63Effects of Pipe Mechanical Properties on Axial Strain to Failure . 63ii45892GTH/R-3/03
Contents (Continued)PageFiguresFigure 1.Figure 2.Figure 3.Figure 4.Figure 5.Figure 6.Figure 7.Figure 8.Figure 9.Figure 10.Example of Moment Curvature Curve Determined under Displacement Control . 64Ratcheting Effect of Cycles with Plastic Strain and Asymmetric Stress . 64Critical Buckling Strain for Plain Pipe in Bending . 65Critical Buckling Strain for Plain and Girth-Welded Pipe. 65Critical Buckling Strain for Pressurized Plain Pipe . 66Critical Buckling Strain for Pressurized Girth-Welded Pipe. 66Critical Buckling Strain for Pressurized Girth-Welded Pipe with New PressureCorrection. 67Effect of Work-Hardening Ratio on Critical Buckling Strain. 67Effect of Yield Strength on Critical Buckling Strain. 68Definition of Skeleton Strain . 68iii45892GTH/R-3/03
Executive SummaryIn recognition of the increasing trend toward strain-based design of pipelines and the need forbasic guidance on strain-based design, the Minerals Management Service (MMS) and theOffice of Pipeline Safety (OPS) co-funded EWI to provide a general guidance on strain-baseddesign for pipelines both for the on-shore and off-shore environment. The resulting guidancecan be found in this report.Special consideration has been given to the choice and qualification of pipe material, the choiceand qualification of girth welding procedures and the demonstration that both pipe and weldareas have sufficient strain capacity to meet the requirements of the design.The current use of strain-based design has many project-specific components. This limits theability of a “cookbook” approach where each step can be laid out as part of common designsequence to apply to all areas of pipe strain-based design. This situation would indicate thattaking the current state-of-the-art methods and creating a code or standard would be ineffectiveat covering the range of needs for future pipeline designs. Yet, because there are manychoices that are part of a particular pipeline strain-based design, the availability of guidance andrecommended practices can help simplify the design and qualification process for manypipelines. Going forward with this approach, the guidance provided in this report couldprofitably be taken forward by the industry into, for instance, an API-recommended practice.The primary areas where strain-based design will be used are in design of reeled laying ofoffshore pipelines, in thermal design of arctic pipelines, in design of types of offshore pipelaysystems, in design and assessment of pipelines in areas with significant expected groundmovement, and in high-temperature and high-pressure HT/HP pipeline designs.Pipeline may also have some applications of strain-based design where cyclic loadings causeoccasional peak stresses above the pipe yield strength. Here, the cyclic lifetime assessment isimproved by using strain ranges for the cycles, instead of stress ranges.Past design practices have asked designers to determine whether a particular loading was “loadcontrolled” or “displacement controlled” without any other possible choices. Designers todayneed to recognize that there are a range of intermediate cases between full load control and fulldisplacement control. The behavior of the pipe, particularly its buckling resistance, can changesignificantly depending upon the designer’s choice of the appropriate intermediate case fordesign.iv45892GTH/R-3/03
Guidance on local buckling compression resistance of pipelines appears to be well foundedwhen using the critical strain. Some changes are recommended here to account better for theeffect of internal pressure on the resistance to local compression buckling. The additionalstrains that can be achieved under partly or fully displacement-controlled loading can providesignificant additional capacity.The methods for assessing tensile failure resistance of pipelines by engineering criticalassessment (ECA) become fewer when the plastic strain exceeds 0.005 (0.5%) and fewer stillas the strain increases to 0.02 (2%) or more. These ECA methods are used to demonstrate thesizes and types of imperfections that can remain in pipes and welds for high-strain service.Further study is needed on the effect of pressure, internal or external, on the tensile failureresistance of girth welds in pipelines. Models and experiments done for this project haveindicated an important effect of strain concentration around welds with mismatched areas underinternal pressure.Methods of assessing cycles of loading that include plastic strain are available. But the limitednumber of tests on which they are based may mean that these methods are conservative formany pipeline design situations to which they might be applied.Design of pipelines to resist ratcheting has become more important recently because of thermalcycle effects on high-temperature pipelines and flowlines. As for other types of cyclic loading,the current design methods are relatively conservative, but have been shifting to allow morecycles of plastic strain.v45892GTH/R-3/03
Abbreviations and Definitions LBWMkMMSOPSR6SAWAmerican Petroleum InstituteAustralian Pipeline Industry AssociationAmerican Society for Testing of MaterialsAutomated ultrasonic testingBoiler and Pressure VesselBritish StandardCanadian Standards AssociationCrack-tip opening displacement (a measure of toughness)Diameter-to-thickness ratiodet Norske Veritas (Norwegian ship and equipment classification society)Electron beam weldedEngineering critical assessmentExtra-low interstitialEuropean Pipeline Research GroupElectric resistance welded (a solid-state weld process used to join the edges ofa single piece to make pipe)Edison Welding InstituteFailure assessment diagramFusion lineGrain-coarsened heat-affected zoneGas tungsten arc weldingHeat-affected zone (area adjacent to a weld affected by the weld’s heat)High-frequency weldedHigh temperature and high pressureA measure of toughnessA measurement of toughness appropriate to ductile crack growthCritical stress intensity factor (a measure of toughness)Laser weldedStress concentration due to local weld shape that changes through the partthicknessMinerals Management ServiceOffice of Pipeline SafetyA fracture assessment technique developed for the British utility industrySubmerged arc weldingvi45892GTH/R-3/03
SCFSENBSMYSTMCPUOEWESY/TStress-concentration factorSingle-edge notch bendStandard Minimum Yield StrengthThermomechanical-controlled processingU’ed, O’ed and Expanded (a description of a pipe making process where theplate is rounded into a U shape, then an O shape and the expanded to thecorrect diameter)Japan’s Welding Engineering SocietyYield strength-to-tensile strength ratiovii45892GTH/R-3/03
DefinitionsStrain-Based Design – This is a design method that places a limit on the strains at the designcondition rather than the stresses.Load Control – Load control describes a situation where the combination of load anddisplacement is controlled by the load variable, where a change in shape will not change theload.Displacement Control – Displacement control describes a situation where the combination ofload and displacement is controlled by the displacement variable, where a change in load willnot change the shape.Reeling – Reeling is a part of a pipeline installation procedure where the pipe is fabricated intoa long section, wrapped around a circular reel, transported to the laying site, and then unwoundfrom the reel.S-Lay – This is a type of offshore pipe laying method where the pipe above the water surface isbasically horizontal and has an S-shape below the water surface.J-Lay – This is a type of offshore pipe laying method where the pipe above the water surface isbasically vertical and has a J-shape below the water surface.Wrinkling – Wrinkling is the formation of ridges and troughs in the pipe wall, which is often thevisible consequence of local buckling.Upheaval Buckling – This is a buckling mode of offshore pipelines where the pipe locallyleaves the supporting seafloor and forms an upward kink.Pull Tube – A pull tube is a tube with at least one bend through which an offshore pipeline ispulled to connect it with a structure such as a platform.Poisson Loadings – These loadings are loadings induced by the Poisson effect, where a bodyloaded in one direction will change shape in the perpendicular direction if it is not restrained.This loading occurs in buried pipelines where the hoop stress from pressure loading induces aPoisson loading to keep the pipe the same length.viii45892GTH/R-3/03
Overbend – This is the upper part of the suspended pipeline between the layship and theseafloor during S-lay that is curved concave downwards.Sagbend – This is the lower part of the suspended pipeline between the layship and theseafloor that is curved concave upwards.Field Bending – Field bending is an on-shore pipeline practice where a pipe bending machineis used
S-Lay – This is a type of offshore pipe laying method where the pipe above the water surface is basically horizontal and has an S-shape below the water surface. J-Lay – This is a type of offshore pipe laying method where the pipe above the water surface is basically vertical and has a J-shape below the water surface. Wrinkling
For measuring the strain in three different directions strain rosettes are used. Strain rosettes are three strain gages positioned in a rosette-like layout. Therefore by measuring three linearly independent strain in three direction, the components of the
Strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length, as shown in Figure 1. Figure 1. Definition of Strain Strain can be positive (tensile) or negative (compressive). Although dimensionless, strain
Given: a strain gage subjected to in-plane strains referenced to the a-t coordinate system: εa, εt, γat Define three types of strain gage strain sensitivities Sa axial strain sensitivity ( R/R)/ εa St transverse strain sensitivity ( R/R)/ εt Ss shear strain sensitivity ( R/R)/ γat
A strain gauge, in mechanical term, is a device for measuring mechanical strain. However, in instrumental term, it is generally taken to mean the electrical resistance strain gauge, and as the name implies, the strain gauge is an electrical conductor whose resistance varies in proportion to the amount of strain in the device. It is thus .
2.5 State of Strain at a Point 73 2.6 Engineering Materials 80 2.7 Stress–Strain Diagrams 82 2.8 Elastic versus Plastic Behavior 86 2.9 Hooke’s Law and Poisson’s Ratio 88 2.10 Generalized Hooke’s Law 91 2.11 Hooke’s Law for Orthotropic Materials 94 2.12 Measurement of Strain: Strain Rosette 97 2.13 Strain Energy 101 2.14 Strain Energy in Common Structural Members 104
Rosette Strain Gages aA strain gage only measures strain in one direction aTo get principal strains, it is necessary to use a strain rosette aA strain rosette is a cluster of File Size: 273KBPage Count: 13
strain gauge is just like the wire strain gauge except the wire is now a printed circuit that is etched out of foil. Figure 1.3 is a picture of a foil strain gauge4. Figure 1.3 – Foil Strain Gauge4 1.3.1 Resistive Theory Both the wire and foil strain gauges work
Strain Gauges The strain gauge is a device commonly used in mechanical testing and measurement. The most common gauge, the bonded-resistance strain gauge, consists of a grid of very fine foil or wire whose electrical resistance varies linearly with the strain applied to the device. When using a strain gauge, you bond the strain gauge to the .
