Deformation Behavior Prediction of X80 Steel Line Pipe and Implication on High Strain Pipe Specification

The use of strain based design in pipeline technology has been widely discussed during the last decade for pipelines in harsh environment. In such cases pipelines should be designed based on strain criterion. Strain based design poses a number of challenges, particularly on pipe size and material properties. This paper presents preliminary studies on prediction of buckling strain and buckling mode for X80 high-strain line pipe by finite element methods based on full-scale test. The effects of several parameters such as internal pressure, material properties pipe size and geometric imperfection, were investigated to predict the critical strain for 48″ diameter line pipe under compression and pure bending with 12MPa internal pressure. Material parameters of a specification for high strain line pipe were analyzed to promote its application in the 2nd West-East pipeline of China National Petroleum Corporation.

Required Linepipe Properties for High Strain Applications and Possible Resolution by Pipe Manufacturing Technology

Application of strain based design to pipelines in arctic or seismic areas has recently been recognized as important. So far, there has been much study performed on tensile strain limit and compressive strain limit. However, the relationship between bending buckling (compressive strain limit) and tensile strain limit has not been discussed. A model using actual stress strain curves suggests that the tensile strain limit increases as Y/T rises under uniaxial tensile stress because a pipe manufacturer usually raises TS instead of lowering YS to achieve low Y/T. Under bending of a pipe with a high D/t, an increase in compressive strain on intrados of a bent pipe at the maximum bending moment (ε-cp*) improves the tensile strain limit because the tensile strain limit is controlled by the onset of buckling or ε-cp* which is increased by lowering Y/T. On the other hand, under bending of a pipe with a low D/t, the tensile strain limit may not be influenced by improvement of buckling behavior because tensile strain on the extrados is already larger than the tensile limit at ε-cp*. Finally, we argue that the balance of major linepipe properties is important. Efforts other than the strict demands for pipe properties are also very important and inevitable to improve the strain capacity of a pipeline.

The Development of Large Diameter and Thickness X80 HSAW Linepipe

For the requirement of 2nd West-East Pipeline Project of China, X80 large diameter & thickness linepipe with helical seam submerged arc welded (HSAW) were developed, with 1219 mm OD and 18.4 mm WT. Acicular ferrite type and super-low carbon, high Niobium chemical composition pipeline steel was adopted for the base material. The very stringent requirements at −10 °C for toughness, i.e. 220J/170J for average/minimum for pipe body and 80J/60J for average/minimum for weld and HAZ were meet successfully. The yield strength loss due to Bauschinger effect was found lower than 20MPa, which benefited. The very low residual stress level was testified by cut-ring test which cuts a section pipe about exceed 100mm long, and then cut the section apart from welds 100mm along the longitudinal direction.

Predicting the Brittle-to-Ductile Transition Temperatures for Surface Cracks in Pipeline Girth Welds: It’s Better Than You Thought

A methodology to predict the brittle-to-ductile transition temperature for sharp or blunt surface-breaking defects in base metals was developed and presented at IPC 2006. The method involved applying a series of transition temperature shifts due to loading rate, thickness, and constraint differences between bending versus tension loading, as well as a function of surface-crack depth. The result was a master curve of transition temperatures that could predict dynamic or static transition temperatures of through-wall cracks or surface cracks in pipes. The surface-crack brittle-to-ductile transition temperature could be predicted from either Charpy or CTOD bend-bar specimen transition temperature information. The surface crack in the pipe has much lower crack-tip constraint, and therefore a much lower brittle-to-ductile transition temperature than either the Charpy or CTOD bend-bar specimen transition temperature. This paper extends the prior work by presenting past and recent data on cracks in line-pipe girth welds. The data developed for one X100 weld metal shows that the same base-metal master curve for transition temperatures works well for line-pipe girth welds. The experimental results show that the transition temperature shift for the surface-crack constraint condition in the weld was about 30C lower than the transition temperature from standard CTOD bend-bar tests, and that transition temperature difference was predicted well. Hence surface cracks in girth welds may exhibit higher fracture resistance in full-scale behavior than might be predicted from CTOD bend-bar specimen testing. These limited tests show that with additional validation efforts the FITT Master Curve is appropriate for implementation to codes and standards for girth-weld defect stress-based criteria. For strain-based criteria or leak-before-break behavior, the pipeline would have to operate at some additional temperature above the FITT of the surface crack to ensure sufficient ductile fracture behavior.

Ductile Fracture Characterization of an X70 Steel: Re-Interpretation of Classical Tests Using the Finite Element Technique

The crack arrest capacity of a linepipe is one of the most important material parameter for such components. In current design codes, it is expressed as the energy absorbed by a CVN impact test. This prescribed impact energy for a given pipeline is typically between 50 and 120J, depending on the grade of the material, the pressure and the dimensions of the pipe. The continuous improvement of steel production has lead to the situation that the impact values achieved in standard pipeline steel production are much larger than 200J for the base material. The question of the significance of these high impact energies can be raised, particularly considering that no correlation has been found between CVN values and crack arrest properties of very high strength materials (X100–X120). In this investigation, instrumented Charpy tests and notched tensile tests were performed on an X70 material. The same tests were also simulated using the finite element method and the Gurson-Tvergaard-Needleman damage model. The combination of supplementary experimental information coming from the instrumentation of the Charpy test and finite element simulations delivers a different insight about the test. It is observed that the crack does not break the sample in 2 parts in ductile mode. After 6–7mm of propagation, the crack deviates and stops. The propagation stops when the crack meets the part of the sample becoming wider due to bending. Finite element simulations proved that it results in a quasi constant force during a displacement of the hammer of almost 10mm. The consequence is that more than 25% of the energy is dissipated in a different fracture mode at the end of the test. Finite element simulations proved also that damage is already occurring at the maximum of the load, but that damage has almost no influence on the load for two-thirds of the displacement at the maximum. In the case of the investigated steel, it means that more than 27J, as often mentioned in standards for avoidance of brittle failure, are dissipated by plastic bending before the initiation of the crack. From the findings of this study, one can conclude that the results of the Charpy test are very sensitive to crack initiation and that only a limited part of the test is meaningful to describe crack propagation. Therefore, it is questionable if the Charpy test is adapted to predict the crack arrest capacity of steels with high crack initiation energy.

Finite Element Modelling of Volumetric and Shear Ductile Micro- and Macro-Fracture Processes Under Long Time Loading

Submarine and onshore pipelines transport enormous quantities of oil and gas vital to the economies of virtually all nations. Any failure to ensure safe and continuous operation of these pipelines can have serious economic implications, damage the environment and cause fatalities. A prerequisite to safe pipeline operation is to ensure their structural integrity to a high level of reliability throughout their operational lives. This integrity may be threatened by volumetric and shear ductile micro- and macro-fracture processes under long time loading or continuous operation. In this paper a mathematically consistent damage model for predicting the damage in pipeline structures under tensile and shear loading is considered. A detailed study of widely used damage models (e.g., Lemaitre’s and Gurson’s models) has been published in the literature. It has been shown that Gurson’s damage model is not able to adequately predict fracture propagation path under shear loading, whereas Lemaitre’s damage model (Lemaitre, 1985) shows good results in this case (e.g., Hambli 2001, Mkaddem et al. 2004). The opposite effect can be observed for some materials by using Gurson’s damage model in the case of tensile loading (e.g., Tvergaard and Needleman 1984; Zhang et al. 2000; Chen and Lambert 2003; Mashayekhi et al. 2007) and wiping die bending process (Mkaddem et al. 2004). Therefore, the mathematically consistent damage model which takes into account the advantages of both Lemaitre’s and Gurson’s models has been developed. The model is based on the assumption that the damage state of materials can be described by a damage tensor ωij. This allows for definition of two scalars that are ω = ωkk/3 (the volume damage) (Lukyanov, 2004) and α = ωij′ωij′ (a norm of the damage tensor deviator ωij′ = ωij −ωδij) (Lukyanov, 2004). The ω parameter describes the accumulation of micro-pore type damage (which may disappear under compression) and the parameter α describes the shear damage. The proposed damage model has been implemented into the finite element code ABAQUS by specifying the user material routine (UMAT). Based on experimental research which has been published by Lemaitre (1985), the proposed isotropic elastoplastic damage model is validated. The results for X-70 pipeline steel are also presented, discussed and future studies are outlined.

SAWL 485 for 48” Offshore Application in Thickness up to 41 mm

Vyksa Steel Works (VSW), part of United Metallurgical Company (OMK), has manufactured a trial batch of large diameter pipes for subsea pipelines in accordance with the DNV-OS-F101 standard and the specification of the Nord Stream project. The plates were produced by Dillinger Hu¨tte (DH). The batch included 1,220 mm (48″) diameter pipes of steel grade SAWL 485 (X70) with a wall thickness of 33 mm and 36 mm. All the requirements were met and OMK/VSW became Russia’s and the CIS’s first qualified producer of subsea pipes in accordance with DNV-OS-F101. In order to meet these high-class property requirements for thick wall pipes a successful development program was performed. The development program is outlined and the test results are explained. As a further consequence of the successful qualification work VSW became one of the two suppliers for the world’s largest and first 48″ diameter pipe subsea pipeline project (Nord Stream). Pipes will be supplied for the most sophisticated segment with wall thicknesses of 30.9 mm, 34.6 mm and even 41.0 mm. Results of manufacturing procedure qualification testing (MPQT) and start of production are presented.

Modeling and CMOD Mapping of Surface-Cracked Wide Plates

Curved wide plate (CWP) tests are frequently used to measure the tensile stress and strain capacity of pipeline girth welds. The parameters affecting the CWP measurement include specimen geometry and cooling setups. High-quality data is obtained when valid test conditions are confirmed. Crack mouth opening displacement (CMOD) is often measured in CWP tests. CMOD is a direct indicator of the amount of deformation at the cracked plane. It is an indirect indicator of the crack driving force (CDF) imparted on the crack. For a given test geometry and material, certain relationships can be derived between the measured CMOD and the more conventional representation of crack driving force, such as CTOD (crack tip opening displacement) and J-integral. Such relationships are a key element in fracture toughness testing standards. This kind of relationship is also particularly useful in strain-based design where CWP specimens are used for strain capacity and flaw growth prediction. In this paper finite element (FE) analysis is first used in modeling CWP testing conditions for X100 specimens with girth weld flaws to validate the test conditions. A novel approach called CMOD mapping is then developed to characterize the flaw behavior which, by making a direct use of CMOD test data from the CWP tests, is used to estimate the crack growth in the CWP. Finally analysis of strain limits using crack driving force (CDF) for the CWP specimens is also given by comparing experimental test data and FE estimation.

An Upheaval Buckling Limit State Function for Onshore Natural Gas Pipelines

The reliability-based design and assessment (RBDA) methodology has gained increasing acceptance in the pipeline industry, largely due to a multi-year PRCI program aimed at establishing RBDA as a viable alternative for the design and assessment of onshore natural gas pipelines. A key limit state of buried pipelines that operate at elevated temperatures is upheaval buckling. The elevated temperatures generate large compressive axial forces that can cause Euler buckling susceptibility. The tendency to buckle is increased at vertical imperfections (i.e. a series of cold formed bends) that primarily occur due to topography. Upheaval buckling in itself is not an ultimate limit state but can lead to high strains, local buckling, high cycle fatigue, expensive remediation measures, and even loss of pressure integrity. The critical forces at which upheaval buckling occurs for typical hill-crest type imperfections present in onshore pipelines cannot be readily predicted using analytical methods. A parametric study is therefore undertaken using non-linear finite element analyses to generate a matrix of upheaval buckling responses. The critical force for the onset of upheaval buckling is then developed using a series of empirical relationships to capture the influences of all key parameters. An upheaval buckling limit state function is subsequently developed by comparing the critical buckling force with applied compressive force, which is a function of operating pressure and temperature differential between the operating and tie-in conditions. The limit state function can be readily implemented in a reliability analysis framework to calculate the pipeline failure probability due to upheaval buckling.

The Effect of Hoop Stress on the Burnthrough Susceptibility During In-Service Welding of Thin-Walled Pipelines

Most companies control the risk of burnthrough by prohibiting welding on pipelines with wall thicknesses below a specified thickness. This is a safe approach but the risk of burnthrough depends not only on the wall thickness, but also on the welding parameters and the operating parameters of the pipeline which include pressure. It is generally acknowledged that the hoop stress caused by pressurizing the pipeline has a relatively minor effect on the risk of burnthrough since the size of the area heated by the welding arc is small. While this has certainly been shown to be true for thicker materials, previous research has shown that the pressure can have a dramatic effect on burnthrough risk for thinner materials. The objective of this project was to further investigate the effects pressure and hoop stress has on the burnthrough risk of welding onto thin-walled pipelines in service. For circumferential welds, pressure and wall thickness determine the burnthrough risk and pipe diameter appears to have no effect. The failure mechanism for circumferential welds is consistently a burnthrough. For longitudinal welds, pipe diameter does appear to affect burnthrough risk even though the effect appears to be secondary to pressure and wall thickness. The pipe diameter is believed to be more influential for longitudinal welds because of the larger area of heated material that is exposed to the hoop stress. Also, the results indicate that the magnitude of the hoop stress has a direct effect on the failure mechanism for longitudinal welds (i.e., burnthrough or weld centerline cracks). For longitudinal welds, the failure mechanism is commonly burnthrough for welds made onto pipes with a hoop stress below 30% specified minimum yield stress (SMYS) which indicates that the internal pressure of the pipe is the main driving force for failure. Longitudinal welds made on pipes which are experiencing hoop stress above 30% SMYS commonly fail by weld cracking. It is important to note that even though pressure does have an effect on the burnthrough susceptibility of welds made on thin-walled pipelines, pressure only becomes a factor for welds made at heat input levels in excess of what is predicted safe by thermal analysis modeling.