Pipe Stress Research Articles

Failure mechanism of a coil type crude oil heater and optimization method

Wax deposition reduces the transportation capacity of oil pipelines and brings major safety hazards to oil production. Heating crude oil with a coil type heater can prevent wax deposition. To study the failure mechanism of the connection between the coil pipe and the gathering pipe and then find optimization methods, the microstructures of the welded joint are observed by scanning electron microscopy, and the mechanical properties of the materials are tested. A mechanical model of the interaction between crude oil and coil pipe is constructed for simulation calculations. The relations between mechanical properties and structural parameters are obtained, and the influence of inclination angle of coil pipe on heat transfer performance is found opposite to that on static performance. Based on the comprehensive analysis of heat transfer rate and stress-strain of multi groups of coil pipes, the methods to optimize the overall performance of the coil heater from the aspects of material, structure and processing technology are proposed. The maximum equivalent (von-Mises) stress and shear stress of bell end coil pipe are reduced by 14.81% and 3.22% respectively, and the heat transfer rate is increased by 7.36 kW compared with that of the original structure under the same working condition.

Allowable stresses related to crack growth stability and leak-before-break for circumferentially cracked pipes governed by limit loads: Part I, Pipes subjected to tensile loading

When cracks are found in high toughness ductile pipes governed by limit loads at in-service inspections, and the applied stresses at the cracked locations are less than the allowable stresses, the cracks are allowable. The allowable stresses are generally determined in combination with failure stresses and safety factors. Failure of pipes with circumferential part-through cracks subjected to tensile loading occurs in a two-step sequence: first there is the onset of crack penetration, and then there is the break after the crack penetration. The Local Approach of Limit Load Criteria, which is based on the net-section stress approach, can estimate both crack penetration and break stresses for high-toughness ductile pipes. Using the Local Approach of Limit Load Criteria, allowable tensile stresses based on crack penetration stresses are determined by means of safety factors for plant service level conditions. The allowable stresses decrease with increasing crack angles and depths. The characters of the allowable tensile stresses are found to vary with changing sizes of crack angles and depths. These different characters involve the presence/absence of two parameters: leak-before-break (LBB) and crack growth stability. The crack size boundary on LBB vs non-LBB can be expressed by appropriate equations containing crack depths and angles. In addition, the boundaries of crack growth stability/instability are also expressed by crack depths and angles. These boundaries are taken into consideration during non-destructive inspection and crack growth evaluation.

Bi-objective design-for-control for improving the pressure management and resilience of water distribution networks

This paper investigates control and design-for-control strategies to improve the resilience of sectorized water distribution networks (WDN), while minimizing pressure induced pipe stress and leakage. Both evolutionary algorithms (EA) and gradient-based mathematical optimization approaches are investigated for the solution of the resulting large-scale non-linear (NLP) and bi-objective mixed-integer non-linear programs (BOMINLP). While EAs have been successfully applied to solve discrete network design problems for large-scale WDNs, gradient-based mathematical optimization methods are more computationally efficient when dealing with large search spaces associated with continuous variables in optimal network control problems. Considering the advantages of each method, we propose a sequential hybrid method for the optimal design-for-control of large-scale WDNs, where a refinement stage relying on gradient-based mathematical optimization is used to solve continuous optimal control problems corresponding to design solutions returned by an initial EA search. The proposed method is applied to compute the Pareto front of a bi-objective design-for-control problem for the operational network BWPnet, where we consider reopening closed connections between isolated supply areas. The results show that the considered design-for-control strategy increases the resilience of BWPnet while minimizing pressure induced leakage. Moreover, the refinement stage of the proposed hybrid method efficiently improves the coarse approximation computed by the initial EA search, returning a continuous and even Pareto front approximation.

Mechanism of Casing String Curvature Due to Displacement of Surface Strata

One of the main problems of well operation is the risk of uncontrolled leakage of hydrocarbons into the environment. This problem is especially relevant for the long-term operation of wells. The idea for this study was inspired by a real industrial problem that the authors of the article were involved in solving. At several operating gas wells, an abnormal slope of the production tree occurred, which raised the question of the safety of their further operation. An analysis of known studies and current regulatory documents did not allow us to assess the safety of using a gas well based on the measured kinematic parameters of production tree deviations. A mathematical model for the deformation of a package of casing strings when the surface layer of the rock is displaced is developed in the article. A boundary value problem is formulated for differential equations of pipe bending on an elastic foundation. Based on the results of solving this problem, an unambiguous relationship was established between the maximum bending stress in the surface pipe and the angle of inclination of the production tree. The quantitative characteristics of the connection depend on the geometric and mechanical properties of the pipes and on the thickness and mechanical parameters of the formations. It was established that the existing inclination of the production tree can be achieved due to the beginning of the plastic bending of the surface pipe under the slickenside, which does not exclude the exhaustion of the safety margin of the surface pipe and indicates the possible operation of the casing string in a pre-emergency state. In general, the obtained results develop analytical approaches to assessing the behavior of underground structures of a production well in unstable soil bodies.

Research of Pipe Cutting and Support in the Pipe-Roof Pre-Construction Method

The studies on the steel pipe cutting and support stage in the pipe-roof pre-construction method (PPM) are rare, and their design lacks relevant standards, which is relatively conservative. Based on the Xinleyizhi Station of the Shenyang Metro construction case, MIDAS/GTS is used to study the stress of the original jacking steel pipes, supporting column, and surface subsidence under the influence of different construction parameters in the steel pipe cutting and support process in the PPM. The measured data are consistent with the numerical simulation results, which verifies the reliability of the numerical simulation results. With the increase of the distance between support columns, the maximum Mises stress of the original jacking steel pipe, the axial force of the support column, and the ground settlement gradually increase in a specific range. This indicates that the original design scheme can be optimized.

Safety assessment of allowable through-wall crack angles for pipes subjected to tensile loading

Fully plastic collapse stresses for high toughness ductile pipes with circumferential cracks subjected to tensile loading can be predicted by means of Limit Load Criteria based on the net-section stress approach. The Limit Load Criteria are provided by the ASME Code Section XI. Allowable membrane stresses for part-through cracks were determined by plastic collapse stresses in combination with safety factors. The allowable stresses decrease with increasing angles of the part-through cracks. When crack angles are large, the allowable stresses of the part-through cracks are larger than the collapse stresses of through-wall cracks. For such large cracks, allowable stresses greater than the collapse stresses cause instability, and are thus detrimental to pipe integrity, especially in thin-wall pipes, even though the stresses are allowed. In order to avoid the anxiety associated with this, it is necessary to establish maximum allowable crack angles. This paper proposes maximum allowable crack angles for allowable stresses based on the collapse stresses of through-wall cracks. The maximum allowable crack angles can be expressed by equations for various service level conditions.

Stability of Allowable Flaw Angles for High Toughness Ductile Pipes Subjected to Bending Stress in the ASME Code Section XI

Abstract Allowable stresses for pipes with circumferential flaws are provided by the ASME Code Section XI. The stresses for the high toughness pipes are determined by fully plastic collapse stresses and safety factors. The plastic collapse stresses are estimated by limit load criteria, which are also provided by the ASME Code Section XI. When applied stresses of the pipes at the flaw locations are less than the allowable stresses, the flaws are acceptable for the end-of-evaluation period. The allowable stresses are categorized for various service-level conditions of the plant operation. When pipe walls are thin, part-through flaws can easily develop into through-wall flaws, and the likelihood of coolant leakage is high. The ASME Code Section XI provides final allowable flaw angles of through-wall flaws that are a limit on the length of part-through wall flaws for thin-walled pipes. The final allowable angles are currently applied to pipes in order to maintain structural tolerance if the part-through flaws become through-wall flaws. To ensure that this stability is not compromised, plastic collapse stresses for through-wall flaws are combined with the allowable stresses. However, the final allowable angles of through-wall flaws are not identified for thin-walled pipes. This paper compares plastic collapse stresses of through-wall flaws and allowable stresses of part-through flaws for pipes. The comparison of these stresses is used to derive the final allowable angles of through-wall flaws. The angles can be expressed either in the form of exact solutions or as conventional options that are appropriate for various service-level conditions.

Theoretical Study on Thermal Hydraulic Expansion Process of Stainless Steel Lined Clad Pipe

Abstract The stainless steel lined clad pipe (SSLCP) can be widely used in different fields. Industrial SSLCP are generally manufactured by the cold hydraulic expansion (CHE) method which requires high hydraulic pressure and good sealing property. The interlayer bonding force is difficult to obtain by the CHE when there is little difference between the yield strengths of inner and outer pipes. Furthermore, the hydraulic expansion process nearly cannot be achieved when the yield strength of inner pipe is higher than that of outer pipe. The thermal hydraulic expansion (THE) method can overcome these difficulties. In this paper, the graph method was adopted to analyze the THE principles for “inner strong and outer weak (ISOW)” and “outer strong and inner weak (OSIW)” pipes. The effective hydraulic expansion criterion conditions of SSLCPs were proposed and can be used as basis of clad pipe material matching and forming process selection. Through the stress and strain analysis of inner and outer pipes during THE process, the deformation coordination conditions of elastic deformation and thermal deformation of the inner and outer pipe were established. The correlation between the residual contact pressure p*c, the hydraulic expansion pressure pi, and the effective temperature difference ΔTe were derived. The calculation formula of the maximum expansion pressure pimax and the minimum expansion pressure pimin was obtained. The maximum heating temperature of outer pipe was also derived. Furthermore, the finite element analysis (FEA) method was adopted and simulated results verify the feasibility and applicability of the theory study.

Influence of welding sequences on induced residual stress and distortion in pipes

A three-dimensional (3D) finite element (FE) model based on thermal-elastic-plastic approach in SYSWELD® was employed to investigate the effects of welding sequences on induced residual stresses and radial distortion in AISI 316L stainless steel welded pipes. To mesh the model, the HYPERMESH® processor was utilized and the model was then exported to SYSWELD® for further thermo-mechanical analysis. The Goldak's double ellipsoidal moving heat source model was employed in SYSWELD® to obtain the volumetric heat flux density in the Gas Metal Arc Welding (GMAW) simulation process. A neural-network program was suitably trained and used to predict the parameters of Goldak's model. The obtained numerical models using predicted Goldak’s parameters were then successfully validated by experimental results in terms of size of welding pool and the temperature distribution. Four different welding sequences (concepts) were investigated and compared: three quarters and one full circumferential welding. These welding sequences were anticipated to create the least residual stress and distortion in pipe welding. The results demonstrate that the welding sequences significantly influence induced residual stresses and distortions in magnitude and in distribution. For the full circumferential welding concept, which was proposed in this study, the residual stresses and distortion present more uniform distribution along the circumferential direction than those in quarter welding sequences. The magnitude of welding distortion was also significantly lower in the full circumferential concept when compared to the quarter welding concepts. The FE results of residual stresses in the full circumferential welding sequences were accurately validated by experimental tests.

The Influence of the Performance Parameters of Bimetal Composite Pipe on the Forming Efficiency

In order to solve the problem of low efficiency in the mechanical drawing process of bimetal composite pipe, a dynamic model of the forming process of bimetal composite pipe is established based on the principle of virtual work and the elastic-plastic deformation criterion. The influence of the material properties of the inner pipe and the outer pipe on the forming success rate is studied. The residual stress of the inner pipe and the outer pipe after forming is taken as the main evaluation object, and the elastic modulus and yield of the material are systematically analyzed The influence of strength on residual stress. The results show that the ratio of the elastic modulus of the inner tube to that of the outer tube is more than 0.6 times of the yield strength when the bimetal composite tube is formed, and the materials with large elastic modulus and small yield strength of the inner tube and small elastic modulus and large yield strength of the outer tube should be selected as far as possible. At last, the theory of material parameter proportion for the successful forming of inner tube and outer tube is put forward. The research conclusion is reasonable and universal, which provides a scientific basis for the selection of composite tube materials, and has engineering practical significance for improving the forming efficiency of bimetal composite tube.

Thermal Fatigue of Pressurized Water Reactor Coolant System Loop Drain Lines Due to Outflow Activities

Abstract Thermal cycling induced fatigue is widely recognized as one of the major contributors to the damage of nuclear plant piping systems, especially at locations where turbulent mixing of flows with different temperature occurs. Thermal fatigue caused by swirl penetration interaction with normally stagnant water layers has been identified as a mechanism that can lead to cracking in dead-ended branch lines attached to pressurized water reactor (PWR) primary coolant system. Electric Power Research Institute (EPRI) has developed screening methods, derived from extensive testing and analysis, to determine which lines are potentially affected as well as evaluation methods to perform evaluations of this thermal fatigue mechanism for the U.S. PWR plants. However, recent industry operating experience (OE) indicates that the model used to predict thermal fatigue due to swirl penetration is not fully understood. There are limitations with the EPRI generic evaluation. In addition, cumulative effects from other thermal transients, especially those resulted from outflow activities, may also contribute to the failure of reactor coolant system (RCS) branch lines. In this paper, we report direct OE from one of our PWR units where thermal fatigue cracking is observed at the RCS loop drain line close to the welded region of the elbow. A conservative analytical approach that takes into account the influence of thermal stratification, in accordance with ASME section III class 1 piping stress method, is also proposed to evaluate the severity of fatigue damage to the RCS drain line, as a result of various transients, particularly the transients from outflow activities. Finally, recommendations are made for future operation and inspection based on results of the evaluation.

Digital Engineering for Integrated Modeling and Simulation for Building-Piping Systems Through Interoperability Solutions

Designing piping systems for nuclear power plants involves engineers from multiple disciplines (i.e., thermal hydraulics, mechanical engineering, and structural/seismic) and close coordination with the contractors who build the plant. Any design changes during construction need to be carefully communicated and managed with all stakeholders in order to assess risks associated with the design changes. To allow the quick assessment of building and piping design changes through a streamlined building-piping coupled analysis, this paper presents a novel interoperability solution that converts bidirectionally between building information models (BIMs) and pipe stress models. Any design changes during construction that are shown in an as-built BIM are automatically converted into a pipe stress model. Any further design changes due to building-piping interaction analyses are converted back to the BIM for the contractor and other designers to access the latest model. Two case studies are presented to illustrate the bidirectional conversion that allows an integrated coupled analysis of the building-piping system to account for their interactions.

Identifying the mechanism of the mixed-phase flow in the horizontal pipeline using computational fluid dynamic approach

In this study, computational analysis has been carried out using computational fluid dynamics (CFD). These calculations have been made to investigate the rheological behavior of the mixed-phase flow in horizontal pipelines. In order to study the shear stress in a vertical pipe, a new numerical model for oil-water dispersion in three dimensions has been developed. CFD software has been used to study the wall shear stress function and water droplet pressure. Using Reynolds numbers and the Navier-Stokes equations with k–turbulence factor to save energy, the flow range for the continuous process was explained. The results from a recent study on experimental methodology were simulated. In this study, the diameter of the tube is 40 mm and the length is 3.5 m and modeled and analyzed using Ansys software. Thus, the geometry has been imported and modeled using the CFD tool. The meshed model has been tested and converged accordingly. The primary data of the simulation have been verified with experimental results successfully. Oil droplet widths have previously been thought to be dependent on the flow Reynolds number, which was confirmed in this case study. Droplet diameter Dd was measured at 6 mm while the mixture moved at a speed of 1.9 m/s. It was found that the largest shear stress value was found at the top of the pipe, where the oil fraction (cut-off) was 0.3, in the simulation results for varied velocities (1.6, 2.5, 2.9 m/s) and oil fraction (cut-off) values. The results of the simulation analysis of the two-phase flow of crude oil for the horizontal pipe are wall shear stresses with different velocities for crude oil in the two-phase flow. As well as pressure drop at different velocities for the same fluids

Plastic behavior and fracture cross section of coiled tubing in the ram shearing process of subsea blowout preventer

For studying the plastic behavior and fracture cross section of sheared drill pipe are of important aspects for evaluating the rams' shear performance of subsea blowout preventer, the plastic deformation and fracture cross section of drill pipe are analyzed based on the plastic deformation theory and the hypothesis that the deformation process of drill pipe is a four-plastic hinge or an eight-plastic hinge mechanism, respectively, according to different ram structure configuration. The elastic-plastic deformation process of CT90 tube in the simulated shearing process is obtained, the final fracture cross section of CT90 tube is approximately a rhombus circle, which is agreeing with the analytical and experimental results. Furthermore, the structure parameters research shows that ram or pipe structures have limited effect on the peak/terminal value of pipe stress. The major length of rhombus circle increases first and then decreases with the increase of ram's blade mouth thickness, decreases with the increase of ram's V-shape angle and pipe wall thickness, while the minor length shows an opposite trend. The larger pipe outer diameter is, the greater major/minor length of rhombus circle is and the growth rate of major length is significantly faster than that of minor length.

Evaluating the Residual Stress and Its Effect on the Quasi-Static Stress in Polyethylene Pipes.

Residual stress is generated during the production process. It can significantly affect the mechanical performance of pressurized polymer pipes. In this paper, six polyethylene (PE) pipes, including three high-density PEs (HDPE) and three medium-density PEs (MDPE) provided by different suppliers, were tested using a one-slit-ring method to measure the residual stress distribution along the hoop direction. Finite element (FE) simulation and mechanical testing were also employed in an iteration process to obtain the mechanical parameters of the six PE pipes. For the same PE pipe code from different suppliers, the results show that the magnitude of the residual hoop stress can be very different, resulting in different mechanical behaviors. In addition, the results are proposed to explain the scenario that was reported previously, i.e., the different critical quasi-static stress (the time-independent stress) levels of the PE pipes with the same pipe code. Since the quasi-static stress is expected to dominate the long-term behavior of the PE pipes, it is of great importance to carefully consider the effect of the residual stress on the determination of the quasi-static stress.

A New Stress Monitoring Method for Mechanical State of Buried Steel Pipelines under Geological Hazards

Long-distance pipelines are threatened by a variety of natural geological hazards. A stress monitoring system driven by the strain-stress solution algorithm was proposed; it can achieve real-time maximum axial stress measurement by installing vibrating wire gauges (VWGs) on the surface of the pipe. To verify the effectiveness of the algorithm, a large-scale pipe mechanical loading experiment combined with a finite element model (FEM) was conducted. The results show that VWGs were reliable with a relative error of 1.19%∼7.98% compared with resistance strain gauges (SGs). The FEM was also reliable with a maximum relative error of 4.04% compared with theoretical analysis. When the reasonable combination modes of VWGs were chosen utilizing the least square method, the error of the pipe stress detection algorithm could be controlled within the range of −13.33∼16.66%. This pipeline stress monitoring technology can meet the requirement of 24-hour dynamic monitoring of the underground pipeline’s mechanical state, realizing the early warning of geohazards.

Research on reduction of residual stress in girth welds of large-diameter natural gas pipes by ultrasonic

ABSTRACT Excessive residual stress of the girth weld in pipes will directly damage the pipe strength during its operation. Accurate measurement and effective reduction of the residual stress can ensure the safety of natural gas pipes during their service. In this paper, an X80-type large-diameter natural gas pipe section with girth welds is taken as the research object. The coercive force method is used to characterize the residual stress of the girth weld and its surroundings, and then the distribution law of the residual stress of the girth weld and its surroundings is obtained based on the measurement results of the Blind-hole method; the ultrasonic impact method was used to reduce the girth weld residual stress, and the effect of the ultrasonic impact method on the residual stress reduction was studied by comparing changes of the distribution characteristics of the coercive force in the girth weld and its surroundings before and after the residual stress reduction in the natural gas pipes. Lastly, the in-situ test specimen is made to carry out the uniaxial tensile test so as to study the effect of the ultrasonic impact method on the tensile mechanical properties of pipe welds. This study clarifies the influence law of the ultrasonic impact on the mechanical properties of weld toe materials, and provided technical theoretical guidance for the reduction of field residual stress in natural gas pipes.

Stress and Pressure Pulsation Analysis of Low Temperature Compressor Piping System in LNG Vaporizing Station

LNG (Liquefied Natural Gas) vaporizing stations are usually built in the cities and towns, and the BOG (Boiled Off Gas) pressurizing system is a very important element. In the pressurizing system, the severe vibration of the low-temperature reciprocating compressor may lead to a failure of the pipeline system and the equipment. Therefore, this paper analyzes the stress and pressure pulsation of the BOG compressor piping system in the LNG vaporizing station. The beam model was used to establish the pipe model. The static, harmonic and modal analysis were carried out based on the plane-wave theory and the pressure-fluctuation theory, and the influence factors of support spacing, the settlement of the fulcrum foundation, pipe pressure and elbow angle were analyzed. The main conclusions are as follows: (1) the unbalanced excited force caused by pressure pulsation greatly affects the stress of the exhaust pipe and compressor outlet pipe, and has less influence on the stress of the suction pipe and compressor inlet pipe; (2) although unbalanced excited force is generated in the elbow, it also has an impact on the straight pipe stress; (3) adding an expansion joint to the pipe of the BOG compressor can effectively reduce the stress of the pipe its the displacement, and can increase the flexibility of the pipe.

Allowable Cracks Related to Penetration for Part-Through Cracks in Pipes Subjected to Bending Stresses

Abstract Fully plastic collapse stresses for circumferentially part-through cracked pipes subjected to bending stresses are estimated by limit load criteria provided by the ASME Code Section XI. Allowable crack depths were determined by using the limit load criteria and that are tabulated in the ASME Code Section XI for different plant service-level conditions. On the other hand, crack penetration bending stresses for part-through cracked pipes were estimated by using the local approach of limit load criteria. By using these criteria, the study presented in this paper obtained allowable crack depths at penetration for circumferentially part-through cracked pipes. Comparing the allowable crack depths obtained by both methods for each service level, it is evident that the allowable crack depths at penetration calculated by the local approach of limit load criteria are almost always smaller than those at fully plastic collapse stresses calculated by the limit load criteria. It was found that the allowable crack depths provided by the ASME Code Section XI are less conservative for crack penetrations.

Anchoring and suspending pipe string mechanical behavior of slip ram for subsea blowout preventer

For exploring the anchoring and suspending pipe string mechanical behavior of slip ram for subsea blowout preventer is of critical importance for reliable sealing wellhead in an emergency, the wedge stress theory and thick-walled cylinder theory are applied to analysis anchoring and suspending mechanical behavior of slip ram, respectively. The stress of slip inserts is very tiny in the approaching pipe process and stress concentration occurs at the slip inserts edge when contacting pipe, while the stress of slip inserts is pre-volatile and then gradually stabilized in the suspending process; the mean stress is approximately uniform in each slip inserts at the final moment. The numerical results indicated the wedge stress theory and the thick-walled cylinder theory are applicable to analyze the anchoring stress and suspending stress (mean stress) of slip inserts respectively. The visible damage and plastic deformation occur in the slip or CT90 tube after completing the anchoring and suspending operation whether numerical or experimental results. Furthermore, the sensitive parameters research shows that the slips are damaged in the anchoring stage depends on their structural strength, while in the suspending stage depends on their structural shear strength.