Pipe Toughness Research Articles

Study on crack tip opening angle evolutions during dynamic crack propagation of pipelines

AbstractCrack tip opening angle (CTOA) has been used to describe the arrest toughness of high‐grade pipelines. However, its evolutions and influencing factors during pipeline crack propagation are unclear. In view of this, this paper reproduces the full‐scale test using a numerical simulation method that is proven to be reliable. On this basis, the evolutions of CTOA during the dynamic crack propagation of the pipeline are systematically studied, and the effects on CTOA values are clarified. The results show that the critical CTOA of a pipeline is independent of the design factor, the pressure decompression, and the large change in the crack velocity and is also not sensitive to the pipe strength, pipe wall thickness, and the inertial effect of soil, but only related to the pipe diameter and pipe toughness. The above conclusions have laid a foundation for the subsequent establishment of the arrest control method for natural pipelines based on CTOA.

A new model for predicting the decompression behavior of CO2 mixtures in various phases

Abstract The pipeline transportation has been considered as the best way to transport pressurized CO2 and plays an important role in Carbon Capture and Storage (CCS) technology. The risk of ductile fracture propagation increases when a CO2 pipeline is ruptured or punctured, and CO2 decompression behavior must be determined accurately in order to avoid the catastrophic failure of the pipeline and to estimate the proper pipe toughness. Thus in this work, a new decompression model based on GERG-2008 equation of state was developed for modeling the CO2 decompression behavior. And for the first time, a relaxation model was implemented to calculate the sound speed in two-phase region. The model predictions were in excellent agreement with experimental ‘shock tube’ test data in the literature. Furthermore, via modeling, it has been demonstrated how impurities in the CO2 and initial temperatures would affect the CO2 decompression wave speed in various phases. The results obtained show that the effects of these factors on supercritical and gaseous CO2 mixtures are absolutely different while liquid CO2 mixtures behave very similarly when compared to supercritical CO2 mixtures, which indicate that the toughness required to arrest fracture propagation is highly based on the initial phase states of CO2 fluid.

A CFD decompression model for CO2 mixture and the influence of non-equilibrium phase transition

Carbon Capture and Storage (CCS) is widely seen asan effective technique to reduce what is perceived as excessive CO2 concentration in the atmosphere. This technique includes transporting CO2 from source point to the storage site, usually through high-pressure pipelines. In order to ensure safe transport (i.e. to prevent the contents from being released into the atmosphere), it is important to estimate the required pipe toughness in the design stage. This requires an accurate prediction of the speed of the ‘decompression wave’ in the fluid, which is created when the high-pressure fluid escapes into the ambient. In this paper, a multi-phase Computational Fluid Dynamics (CFD) model is presented to simulate the decompression of high-pressure pipelines carrying CO2 mixtures. A ‘real gas’ Equation of State (EOS), the GERG-2008 EOS, is incorporated into the CFD code to model the thermodynamic properties of the fluid in both liquid and vapour states. The non-equilibrium liquid/vapour transition is modelled by introducing ‘source terms’ for mass transfer and latent heat. The model is validated through simulation of a ‘shock tube’ test. A ‘time relaxation factor’ is used to control the inter-phase mass transfer rate. The measured decompression wave speed is compared with that predicted using different values of the time relaxation factor. It is found that the non-equilibrium phase transition has a significant influence on the decompression wave speed. Also, the effects of delayed bubble formation and of various impurities on the decompression wave speed are investigated.

Decompression Modelling of Pipelines Carrying CO2-N2 Mixture and the Influence of Non-equilibrium Phase Transition

The Carbon Capture and Storage (CCS) is the technology that has been proposed to reduce high concentrations of CO2 in the atmosphere. This method involves transporting the CO2 to the storage site, usually in pipelines. In order to ensure safe transport, the required pipe toughness must be accurately estimated. This, in turn, requires an accurate estimate of the decompression wave speed in the fluid. In this paper, a multi-phase Computational Fluid Dynamics (CFD) model is presented to simulate the decompression of a CO2-N2 mixture in a pipe. A real gas EOS (GERG-2008) was incorporated into the CFD code. The non-equilibrium liquid/vapour transition was modelled by introducing source terms for mass transfer and latent heat. The model is validated through simulation of a ‘shock tube’ test. The effects of the non-equilibrium phase transition on the decompression wave speed are discussed.

Experimental and numerical study of steel pipe with part-wall defect reinforced with fibre glass sleeve

The paper presents numerical and experimental burst pressure evaluation of the gas seamless hot-rolled steel pipe. The main goal was to estimate mechanical toughness of pipe wrapped with composite sleeve and verify selected sleeve thickness. The authors used a nonlinear explicit FE code with constitutive models which allows for steel and composite structure failure modelling. Thanks to the achieved numerical and analytical results it was possible to perform the comparison with data received from a capacity test and good correlation between the results were obtained. Additionally, the conducted analyses revealed that local reduction of pipe wall thickness from 6 mm to 2.4 mm due to corrosion defect can reduce high pressure resistance by about 40%. Finally, pipe repaired by a fibre glass sleeve with epoxy resin with 6 mm thickness turned out more resistant than an original steel pipe considering burst pressure.

Decompression wave speed in CO2 mixtures: CFD modelling with the GERG-2008 equation of state

The development of CO2 pipelines for Carbon Capture and Storage (CCS) raises new questions regarding the control of ductile fracture propagation and fracture arrest toughness criteria. The decompression behaviour in the fluid must be determined accurately in order to estimate the proper pipe toughness. However, anthropogenic CO2 may contain impurities that can modify the fluid decompression characteristics quite significantly. To determine the decompression wave speed in CO2 mixtures, the thermodynamic properties of these mixtures must be determined by using an accurate equation of state. In this paper we present a new decompression model developed using the Computational Fluid Dynamics (CFD) package ANSYS Fluent. The GERG-2008 Equation of State (EOS) was implemented into this model through User Defined Functions (UDF) to predict the thermodynamic properties of CO2 mixtures. The model predictions were in good agreement with the experimental data of two ‘shock tube’ tests. A range of representative CO2 mixtures was examined in terms of the changes in fluid properties from the initial conditions, with time and distance, immediately after a sudden pipeline opening at one end. Phase changes that may occur within the fluid due to condensation of ‘impurities’ in the fluid were also investigated. Simulations were also conducted to examine how the initial temperature and impurities would affect the decompression wave speed.

Structure and properties of peroxide crosslinked polyethylene tubing after drawing

Abstract Crosslinked high-density polyethylene piping is currently in use for carrying pressurized hot water, chemicals, and waste. Hot and cold drawing processes, above and below melting, were studied and compared aiming at improving the pipe's toughness, strength, and creep resistance. Initially, the tensile behavior as function of temperature was investigated, and a transition from a ductile to an elastomeric behavior was noticed within the melting range, along with a decrease in strength. Based on these results, and on differential scanning calorimetry analysis, representative temperatures were selected for cold drawing (below the peak melting temperature) and for hot drawing (above the peak melting temperature). A drawing machine was constructed that enabled drawing at elevated temperatures, followed by immediate cooling. The cold-drawn samples are characterized by a transparent neck and a uniaxial c-axis orientation parallel to the drawing direction, as revealed from wide-angle x-ray diffraction. Ho...

Fracture Control Testing and Acceptance Criteria for Line Pipe

Summary A model specification for imposing mill test and acceptance criteria for fracture control is discussed. The specification applies to line pipe with wall thickness up to 1.5 in. (38 mm) intended for use in cross-country pipelines at temperatures between 32 and 250 deg. F (0 and 121 deg. C). Methods for establishing minimum acceptance levels (MAL) for Charpy V-notch specimens and drop-weight tear test specimens are described.Fracture controls are established to ensure crack arrest; methods for establishing acceptance criteria are based on equations modified from relationships found in Ref. 1.API grades B through X-70 are covered. Fracture control criteria are established separately for four service classes. Introduction Fracture control for pipelines has been of major interest to the industry for more than 25 years. This interest was prompted by the occurrence of pipeline fractures in which uncontrolled crack propagation, in lengths up to 3,000 ft (914 m), was observed.Initially, interest centered on the development of methods to prevent brittle fractures. More recently, interest has shifted to the prevention of uncontrolled ductile fractures. In both cases, the objective of studies funded by the industry and pipe vendors was to find out why such fractures occurred and to develop design and metallurgical methods of prevention.Study and development have been worldwide. While the causes of uncontrolled fractures now are understood reasonably well, controversies still exist about the best methods to be adopted for cost-effective fracture control. Nevertheless, results are sufficiently detailed to allow us to begin developing engineering standards and specifications to impose fracture controls on pipelines. These standards and specifications must take into account relevant design and metallurgical factors. A model specification was developed incorporating these factors; the specification is provided in the Appendices. Background Relevant literature is extensive and is not subjected to review in this paper. It is sufficient for the purpose of this paper to state that an extensive literature review effort was made and that, where possible, a conservative approach was used in those areas still subject to controversy. Key literature is cited in Refs. 1 through 3.The principles behind pipeline fracture control are straightforward - one must either incorporate crack stoppers in the pipelines or have line pipe toughness adequate to stop a fast-running crack. For a long pipeline, crack stoppers are impractical and, further, complicate construction. The preferred method is to specify adequate toughness; in most cases this can be accomplished by design and metallurgical controls at little or no extra cost.Both brittle and ductile fractures must be provided for. Brittle fracture can be eliminated by requiring a low brittle/ductile transition temperature. This sometimes is done by specifying Charpy impact energies, but a more reliable method is the use of a drop-weight tear test. The latter method is used in the proposed model specification.Much of the more recent industry work addresses the problem of specifying ductile fracture arrest in line pipe using Charpy impact toughness. JPT P. 527＾