Modelling and measurements for the assessment of a full scale pipe bend test

Simon D Smith ¹ , Henryk G Pisarski ¹ and Cosmas Vlattas ²

¹ TWI Ltd, Cambridge UK
² Saipem UK Ltd, Surrey UK

Paper presented at ISOPE 2007, Seventeenth International Offshore and Polar Engineering Conference, Lisbon, 1-7 July 2007.

Abstract

There is a significant need for well documented full scale tests to provide verification results for Strain Based Design (SBD) methods. The current paper presents some results from a full scale pipe bend test. The pipe contained agirth butt weld with a surface crack located at the edge of the weld. A nominal strain of around 1% was applied. Strain and mouth opening measurements were made. The crack was broken open after the test to reveal the extent of ductilecrack growth. The full scale test was analysed using FEA and SBD methods.

KEY WORDS: FEA; Full Scale testing; Tearing; Strain Based Design; Pipe bending; Crack.

Introduction

Sub sea pipelines are frequently laid in a manner that imposes significant bending strains on the pipe. In some cases this has lead to failure of the pipe even before any operational loads are applied to it. Such failures could havebeen prevented if accurate methods of assessment were adopted. These are presently being developed for pipe-lay.

There has been a significant amount of research into assessment methods for structures experiencing nominal stresses that are below yield strength. Codes have been developed to assess flaw tolerance in the weakest parts of thestructure, like welds for example. Welds can contain flaws and regions of lower fracture toughness than the parent materials. The codes have recognised this and provided rules for flaw assessment. However, most of the codes are notsuitable for significant applied strains (above the nominal material yield strain).

The current paper describes a test conducted on a typical pipe length with a weld at the centre and a surface-breaking defect in the weld. The weld was a typical pipe-line girth butt weld, and the applied load levels caused nominalstrains at the crack to reach 1%, or well over the nominal yield strain of 0.2%. The pipe did not fail, so fractography was undertaken to measure the amount of ductile tearing that occurred during the test. The result of thefractography was compared with the expected tearing based upon separate small scale test results. Further synthesis using finite element analysis (FEA) was done to provide information about the deformations of the pipe during thetest.

Pipe bend test

The pipe was 24in diameter with a wall thickness of 24.5mm. The specimen was 6m long with the weld located at the centre of its length. The weld was made with the Passo, automatic GMAW process. CO ₂ shielding was used on the root pass, and 70%CO ₂ , 30%Ar for the filling and capping passes. The weld was made into a steep sided machined V preparation in approximately 8 passes, using Redaelli RMS wire. All passes were made with a heat input in the range 0.6-0.9kJ/mm exceptthe root where the heat input was 0.4-0.5kJ/mm.

A misalignment of the pipe axes was deliberately made across the weld. The misalignment was greatest at a location designated 6 o'clock on the pipe, and a flaw up to 3.3mm deep by 162mm surface length was introduced at this locationoff the weld centre line as shown in Fig.1. The crack was manufactured by use of a 0.15mm wide slitting wheel. The average misalignment at the crack was 1.7mm. A peak misalignment of 3mm was measured. The crack location, misalignment and the weld geometry areshown schematically in Fig.1.

The applied strain, ε, assumed in the above equations was the nominal value measured across the crack in the test, multiplied by a strain concentration factor (εCF) associated with the pipe misalignment. A value of 2.7 was chosen from the strain distribution across the weld near the crack, Fig.6 (the nominal strain was 0.7% and the strain at the location of the crack was about 1.9%). It appears that the PD 6493:1980 formula is generally conservative, at least until a strain of about 0.7%, but the ETM method ismore accurate at lower strains. The effect of residual stresses has been ignored because the work is presently concentrating on the results at large strains, when residual stresses will have been redistributed by mechanical loading. Itis not difficult to suggest how residual stresses and residual stress redistribution could be incorporated into this method.

The εCF was provided by FEA and includes the effect of misalignment and mismatch. It is anticipated that the future use of methods like this would rely ontabulated strain concentration factors for practical pipe geometries and material properties. For example, the strain distribution across an under matched weld is shown in Fig.8.

Discussion

A Strain Based Design (SBD) assessment is based on applied strain, material toughness and the correct prediction of the crack tip driving force. The full scale test described here has addressed each these components of SBD. The CTODcrack tip driving force was measured during the test. The measured driving force can therefore be directly compared with the material toughness as measured by small scale test specimens. The comparison permits a direct prediction ofthe crack growth. The crack growth prediction based upon small scale tests was significantly larger than the actual crack growth in the full scale test. The difference was attributed to the higher constraint that exists in the smallscale test specimen. An improved prediction was made by conversion of the high constraint bend result into a low constraint toughness curve using the methods similar to those discussed by ^{[Wang et al, 2004]} . Clearly there is a significant need to match the constraint of the small scale test specimens with the constraint that exists at cracks in welds in pipes. For installation of subsea pipelines involving plastic straining,current practice ^{[DNV 2006]} is to use single edge notch tension (SENT) specimens to derive fracture toughness since this is better at replicating the crack tip constraint conditions for a flaw in a pipe compared with the deeply notched bend specimen ^{[Pisarski, 2002]} . The bending loads applied to pipe line during laying operations and during in-service movement on the seabed tend to apply predominantly tensile stresses in the whole section containing the flaw. Membrane loads produce lowconstraint loading of cracks. However, higher constraint could exist if the crack was located in an under matched weld metal at the weld root. Also, biaxial loading conditions when the pipe is subjected to axial strain combined withinternal pressure or severe misalignment can cause high constraint. The SBD procedures should therefore consider the full likely loading of a flaw when recommendations of toughness testing procedures are made.

The crack tip driving force in the full scale test was assessed in three ways. The value was measured and also predicted by FEA and estimated from the applied level of strain. It was found that the crack driving force could beaccurately predict using a strain based method provided that the strain distribution at the weld was correctly included. A misalignment was deliberately made at the weld, and this misalignment caused a strain concentration at thelocation of the crack. The crack driving force was accurately predicted when the nominal strain and a strain concentration factor were used to determine the local strain at the defect. The driving force was then determined using someextremely simple formulae (see Eq.1 and Eq.2). The simplicity of these formulae is attractive and suggests that SBD for pipe lay could be straightforward.

Methods of strain based defect assessment already exist ^{[BSI 1980 and Schwalbe 1998]} and these have been proven to be satisfactory for the present test specimen. The method does not need a stress versus strain curve. The predicted crack driving force is a linear function of the applied strain in the high strainregime. This simplifies the process of crack driving force calculation. The simple equations mean that accurate defect assessments could easily be done without the need for advanced software tools to determine the elastic-plastic cracktip loading. More work is needed to confirm this assessment and to develop an overall procedure. For example, a plastic collapse calculation would be needed.

Conclusion

The full scale test has shown that pipe line flaw assessment methods should use toughnesses that are representative of the membrane tension that is applied on the tensile side of a pipe in bending.

Simple crack driving force prediction methods have been assessed and shown to be suitable when the results were compared with the full scale test. The method is based upon the local strain at the flaw, including any local strainconcentrations. The results suggest that existing strain based crack driving force procedures could provide the basis for SBD.

Reference

BSI: 'Guidance on some methods for the derivation of acceptance levels for defects in fusion welded joints', BSI PD 6493:1980.

DNV-RP-F101, January 2006. Fracture control for pipeline installation methods introducing cyclic plastic strain.

Pisarski H G and Wignall C M (2002): 'Fracture toughness estimation for pipeline girth welds', Proc of IPC 2002, Calgary, Canada, Oct 2002, paper IPC 02-27094, ASME.

Schwalbe K-H, Zerbst U, Kim Y-J et al (1998): 'EFAM ETM 97 - The ETM method for assessing the significance of crack-like defects in engineering structures, comprising versions ETM 97/1 and ETM 97/2', GKSS 98/E/6,1998.

Wang Y Y, Horsley D J, Cheng W, Glover A, McLamb M and Zhou J (2004): 'Tensile strain limits of girth welds with surface breaking defects, Part 2: Experimental correlation and validation', Pipeline TechnologyConference, May 2004, Ostend Belgium