High-pressure Gas Transmission Pipelines Research Articles

Study on Coercivity-Stress Relationship of X80 Steel under Biaxial Stress

Buried pipelines are prone to deformation under surface displacement, pressure occupation, and/or other actions, resulting in stress concentration. Conducting regular stress detection is an important approach to reducing pipeline accidents. This study performed experiments on measuring the coercivity stress of X80 steel, the most commonly used material for high-pressure large-diameter gas transmission pipelines. The coercivity of the specimen under different stress combinations was measured using the DC-driven coercive force measurement system, and the variation of coercive force with biaxial stress was also analyzed. The modified coercive force-stress expression of the generalized Hooke’s law was proposed; the model for the coercive force-stress relationship of X80 steel under the biaxial stress state was established based on this expression. The study indicates that the coercive force method is reliable for accurately measuring the stress of pipeline steel and evaluating the safety margin of in-service pipelines.

Assessment of in-service degradation of gas pipeline steel taking into account susceptibility to stress corrosion cracking

Stress Corrosion Cracking (SCC) is often identified as one of the predominant failure causes of high-pressure gas transmission pipelines. Long-term operation of pipelines causes degradation of mechanical properties of pipeline steel, including a significant decrease in brittle fracture resistance and resistance to SCC. For oil and gas pipeline steels, there are regulatory requirements for impact toughness, disregarding steel condition, namely as-received or serviced. However, SCC resistance of pipeline steels, being very important for structural integrity, is not regulated. In the present work a new method to evaluate in-service degradation of pipeline steels, taking into account increasing susceptibility of operated metal to SCC, was developed. To ensure a safe operation of gas pipelines, it is suggested that the minimum allowable value of SCC resistance characteristics, in particular, the threshold of J-integral based stress intensity factor for SCC Jscc, should be defined and regulated. It is quite difficult to input these characteristics into regulatory requirements, therefore the developed method is based on modification of regulated limit values of impact toughness for as-received and serviced metal conditions separately, taking into account increasing susceptibility of serviced steel to SCC. The method was applied to evaluate in-service degradation of API 5L X52 ferrite-pearlite pipeline steels. The developed method considers different SCC susceptibility of as-received and serviced pipeline steels and shows there are limitations in regulating the same limit value of impact toughness for as-received and serviced pipeline steels.

An integrated risk assessment of onshore gas transmission pipelines based on defect population

The aim of this research is to develop an original probabilistic method for an underground onshore gas transmission pipelines maintenance risk estimation. The method considers a population of the anomalies detected by an in-line inspection . In the methodology, features are considered as a family with certain defect depth and shape distribution derived from the repeated in-line diagnostics of a steel pipeline under operation. A growth rate in the radial direction is determined based on a defects size change between two or more inspections. The number of non-detected hypothetical anomalies were taken into account due to probability of detection and sizing uncertainties by means of a non-perfect inspection device. Metal losses of the pipe wall are assessed by a fracture mechanics method based on a limit load theory and considered as a leak or rupture. Gas pressure distribution and temperature along the pipeline are calculated in steady state conditions. Particular attention was paid to the evaluation of the probabilistic pipeline failure consequences to the surrounding area with parameters as population density and road traffic taken into consideration. Although the presented risk estimation is universally applicable, the novel methodology was applied to the high pressure gas transmission pipeline situated in the area similar conditions typical for Europe. The failure probability calculations are carried out for different pipeline modes of the fluid transportation. A significant connection of the risk evaluation with pressure distribution along the pipeline was observed based on the differences in risk distributions between Summer, Autumn and Winter operational modes.

Fiscal and Economic Impact Analysis of Proposed Nexus Natural Gas Pipeline on the City of Green, Ohio: A Case Study

In order to provide a sustainable outlet for natural gas harvested from the fracking fields of eastern Ohio, in 2015 Nexus announced plans to build 250 miles of high-pressure natural gas transmission pipeline that would run from the source into Michigan, ultimately to Ontario, Canada. The pipeline route passes through the fast-growing City of Green, Ohio, and could disrupt development plans there. The proposed path would cause the City of Green to disproportionately bear the burden of anticipated economic losses and reduction in tax revenue associated with the pipeline. Despite substantial front-loaded ad valorem taxes paid by the pipeline utility, over a 50-year period, the pipeline is projected to cause NPV fiscal losses of over $52 million, largely from foregone property and income taxes for future development.

Influence factors of X80 pipeline steel girth welding with self-shielded flux-cored wire

For girth weld of high-pressure oil and gas transmission pipeline, there is impact toughness values deviation phenomenon with self-shielded flux-cored semi-automatic welding technology. The macro-images, microstructure and mechanical performance of girth welding joint have been investigated by OM, SEM, TEM. The results show that there are several factors of impact toughness unqualified values of weld joints, such as welding heat input, coarse grain zone and a chain of M–A organisations, Al2O3 and Zr Precipitates particle sizes and distribution et al., which are the main unqualified reasons of welding impact toughness of the semi-automatic self-shielded core butt welding process of X80 pipeline steel.

Residual Stress Evaluation by X-Ray Diffraction and Hole-Drilling in an API 5L X70 Steel Pipe Bent by Hot Induction

The API 5L X70 steel is used in high-pressure gas transmission pipelines. Because of this, knowledge of presence of residual stress and their magnitude is important to assess the material integrity in service. For the pipeline manufacturing, tubes need to be curved which is often made using the hot induction bending process. This process can introduce different residual stress depending of tube position. For this research, in order to evaluate the residual stress, was used an API 5L X70 tube that was previously curved by hot induction process. Samples were taken from the extrados, intrados, neutral line and straight section of the curved tube. Residual stresses were studied by two conventional methods: X-Ray Diffraction (XRD) and Hole-Drilling, which are destructive and non-destructive methods, respectively, in order to assess their qualitative responses. Each of these methods presents particular methodologies in sample preparation and material analysis, but also they differ in factors such time consumption and cost of the analysis. The qualitative responses obtained by the two different methods were comparable and satisfactory and pointed out the existence of a compressive residual stress state in steel pipe.

Gas pipeline preferential site selection occurrence for elemental sulphur & other particle matter formation & deposition

Field observations of “elemental sulphur” deposits within high-pressure natural gas transmission pipeline systems have prompted investigations into the study of preferential site selection process for the formation and deposition of these sulphur based contaminants. These deposits can cause complete blockage of the gas stream path. There have not been any known studies into why this preferential site selection process exists, and therefore it is the aim of this research to investigate plausible reasons by investigating affected pipeline systems.The results from the studies, which have been validated against known “elemental sulphur” affected transmission pipeline sites, confirm that the flow dynamics at the pipeline “T” junction could be a major contributing factor to the preferential site selection process. The paper discusses the results.

A Study on Natural Gas Decompression Behavior

This article deals with decompression wave propagation in high-pressure gas transmission pipelines. A decompression wave calculation model called Decomwave including a united two-phase sound velocity model was proposed and programmed and can be used for poor or rich gas and has shown good results with experimental data. Under certain operating conditions of a gas transmission pipeline, the effects of gas composition, initial pressure, and temperature on decompression wave behavior were analyzed using Decomwave.

IMPACT FRACTURE BEHAVIOR OF X80 PIPELINE STEEL

Pipeline steels have been widely used in a long-distance transportation of large amounts of crude oil or natural gas under high pressure due to their high transportation efficiency, low energy loss and production cost. For high pressure gas transmission pipelines made from high-strength steels an important problem is to know their fracture behavior in a long running process. In this paper, fracture toughness of X80 pipeline steel was measured by V-notch Charpy impact test at −20 and −60 . OM, SEM, EDS and EBSD were used to analyze its fracture mechanism. Experimental results show that the maximum impact load is slightly influenced by temperature, but with the decrease of temperature, crack forming work, crack propagating work and energy absorbed by crack arresting decrease significantly. During fracture process, the intensive tensile stress in the sample would induce the deformation bands formed around the fracture surface and grains elongated along the direction vertical to the main crack, but original austenite grain boundaries are hardly deformed, it would cause stress concentration and then produce intergranular fracture. Generally, brittle second phase particles could act as crack sources under intensive internal stress. In the crack propagation process, grains near the main crack are deformed and elongated tempestuously, resulting in their breaking and new grains with high angle grain boundary (HAGB) formed. Therefore, the grain size decreases and the fraction of HAGB increases in the crack propagation region. Temperature has great influence on the plastic deformation behavior of pipeline steel during facture process. At −20 , the tested steel has good plasticity and the grains are refined during deformation, but at −60 , they are difficult to

Transient flow in natural gas pipeline – The effect of pipeline thermal model

One-dimensional, nonisothermal gas flow model was solved to simulate the slow and fast fluid transients, such as those typically found in high-pressure gas transmission pipelines. The results of the simulation were used to understand the effect of different pipeline thermal models on the flow rate, pressure and temperature in the pipeline. It was found that simplified flow model with steady-state heat transfer term overestimates the amplitude of the temperature fluctuations in the pipeline. This result indicated that unsteady heat transfer model with the effect of heat accumulation in the surroundings of the pipeline should be used to calculate the gas parameters at locations of interest within high-pressure gas transmission pipelines.

Recoverable Energy in Natural Gas Pressure Drop Stations: A Case Study of the Khangiran Gas Refinery

The natural gas which is supplied to the Khangiran gas refinery comes from a high-pressure natural gas transmission pipeline and passes through a metering and reducing station, sometimes referred to as the pressure drop station. The gas at this reduced pressure enters the refinery for internal uses. It is at the pressure drop station that a lot of energy is wasted, as the pressure of the natural gas is reduced by an expansion-type ball valve. But, the design of the ball valve does not allow any of the energy from the compressed natural gas to be recovered, as it regulates from a higher pressure to a lower pressure. If this energy loss can be captured, it can be converted into electricity. In this study, a comprehensive program has been carried out to record and measure natural-gas properties and flow rates at Khangiran pressure drop station for a year. Based on these data and an accompanying exergy analysis, the amount of available work that can be produced from capturing natural-gas pressure exergy from the station has been calculated. To produce work, a system which consists of preheating and a turbo expander has been proposed and studied. The effect of preheating temperature and turbo expander exergetic efficiency on amount of obtainable work has been studied. The results show that the amount of available energy can meet all electrical demand of the refinery.

Failure of pipe joints during hydrostatic testing

Large diameter butt-weld tee-joints (30-in. × 18-in.) on high-pressure gas-transmission pipeline exhibited leakage during the hydrostatic testing that was carried out to testify the integrity of the pipeline after its commissioning. The leakage-failure comprised the formation of small cracks at the neck-region of the tee-joints. A detailed examination showed that the cracking was preceded by plastic yielding, and that at these locations the wall-thickness was lower than even the minimum specified. A further examination of selected sections obtained from the failed-tees revealed that the tees had actually been ‘fabricated’ by butt-welding a small length of 18-in. diameter pipe onto a partially-formed neck on the 30-in. diameter pipe. And also, that the weld-filling of this butt-weld was then ground-off to smooth-out the curvature at the neck-region, or more likely, to conceal the weld and thereby the ‘faulty’ manufacturing procedure. An outcome of this procedure was that the wall-thickness at the neck-region which was lower (even to start with) than the minimum specified, had been further reduced by the grinding of the weld-filling. It was also concluded that a tee-joint of the required/specified design and dimensions could not be possibly produced by this method of ‘fabrication’.

A Model to Predict the Characteristics of Fires Following the Rupture of Natural Gas Transmission Pipelines

The gas industry has an excellent safety record in operating high pressure transmission pipelines. Nevertheless, it is important that pipeline operators have an understanding of the possible consequences of an accidental gas release, which may ignite, in order to help manage the risks involved and to develop appropriate standards and design codes for pipeline operations. The most serious type of hypothetical incident that could affect a high pressure gas transmission pipeline, involves a full pipe rupture. This paper describes the extension and validation of a physically-based phenomenological model of jet fires for predicting the size and position of the fires resulting from underground gas pipeline ruptures and to predict thermal radiation levels around such fires. The application of the complete model to underground pipeline ruptures has been made possible by the development of a crater source sub-model which provides suitable input parameters to the fire structure calculation. Predictions of the complete model have been compared with the results from two full-scale pipeline rupture tests, which has demonstrated the performance of the model for a range of realistic scenarios. The work was undertaken by an international collaboration of gas companies, as part of a programme of research to obtain information on the consequences of accidental gas releases from transmission pipelines.

Stress Corrosion Cracking Characteristics of a Range of Pipeline Steels in Carbonate-Bicarbonate Solution

Abstract Stress corrosion cracking (SCC) characteristics of a range of pipeline steels immersed in a carbonate-bicarbonate solution were studied in terms of the deleterious effects of small-amplitude cyclic loading on threshold stress, together with the increase of crack nucleation and the decrease of average crack growth rates with increasing test times. Data were reported on conditions for coalescence or otherwise of adjacent cracks in terms of spatial separation. The time dependence of crack nucleation rates and crack growth in laboratory tests varied with stress. The laboratory rates for crack nucleation and growth likely are applicable in high-pressure gas transmission pipelines in service, but values of the constants in the power law expressions for those rates differed for service conditions. The data showed service condition calculations can be guided by empirically assigning ranges of values for the constants by imposing boundary conditions that relate to service experience. Ranges of values also...

Stress corrosion cracking of high-pressure gas transmission pipelines

Stress corrosion cracking of high-pressure gas transmission pipelines

Stress corrosion cracking of high-pressure gas transmission pipelines

Stress corrosion cracks that have been found on the outside surfaces of gas pipelines in the USA have been characterized by a branched, intergranular path and a black Fe3O4 deposit on the crack surfaces. An ongoing research programme sponsored by the Pipeline Research Committee of the American Gas Association has revealed much about the causes of the cracking and about possible preventive measures. It is believed that most, if not all, of the cracks have formed in a sodium carbonate—sodium bicarbonate environment in which a critical balance of activity and passivity has been achieved. Laboratory experiments have shown that the narrow range of potentials in which cracking is possible and the rate of crack growth can be correlated with the anodic polarization behaviour. Temperature has a strong effect on the rate of crack growth and on the probability of establishing appropriate electrochemical conditions for cracking. The occurrence of cracking has not been related to any unusual or specific composition of steel but modification of steel processing might eventually offer a practical way of preventing the problem. The function of stress is primarily to produce a rate of straining sufficient to continually expose fresh metal at the tip of the crack. A large number of conceivable ways of avoiding stress corrosion cracking are being investigated. For existing pipelines, lowering the discharge temperature at compressor stations and hydrostatic retesting are being considered. Additional alternatives for future pipelines include modification of the steel and its surface condition, improvement of the coating, and the use of inhibitors.