Yu. P. Surkov

On the possibility of evaluating the stressed state of pipelines using a magnetic method

It is shown that the monitoring of the stressed state of pipelines with the use of a method based on the evaluation of magnetic properties of a pipe metal allows one to solve completely the problems of improving the reliability of a pipeline system of compressor shops, namely, evaluate the stressed state of pipelines, to detect stress concentration zones, monitor the change of stresses in a construction just during the repair of a pipeline and adjustment of pipeline supports, and optimize the character of the reduction in the stress level.

Feasibility of Diagnosing Corrosion Cracking in Main Pipelines

With a view to assessing various diagnostic techniques for corrosion and stress-corrosion aws, results of an in-pipe inspection of a pipeline section have been analyzed. The statistical data are presented in the form of plots of aw dimensions versus their positions over the length of the inspected part of the pipeline and also over the pipe cross section. In order to identify common characteristics of corrosion and stress-corrosion aws, results of the in-pipe inspection of the pipeline were analyzed, and it turned out that there are essential differences in most parameters of the corrosion and stress-corrosion aws. The analysis took into account geographical features of the pipeline and protecting potentials in the inspected parts of the pipeline. The most essential differences between the corrosion and stress-corrosion cracking are in the distributions of the aws over the inspected pipeline division. The pipeline sections with higher densities of stress-corrosion aws contained smaller numbers of corrosion aws, and vice versa. The ultimate aim of the reported research was to compare the opportunities offered by direct and indirect diagnostic techniques designed to detect corrosion cracking in main pipelines.

Estimating the probability of the propagation of stress corrosion cracks in compressor station pipelines

The conditions for the nucleation of stress corrosion cracks in linear and compressor station gas pipeline sections were considered. The potential for the use of pipes with a smaller diameter and a thicker wall in compressor station pipelines to create favorable stressed state conditions that promote the arrest, etching, and stabilization of stress corrosion cracks was shown. The fractographic inspection of pipe fragments with stress corrosion cracks allowed us to reveal an overload that was unrelated to the increase in the internal pressure of the given gas pipeline section. This overload was a major cause of such events as the reinitiation and propagation of stress corrosion cracks to a depth of nearly 50% of the pipe wall thickness. The overload itself was presumably caused by a construction factor connected with the position of a pipe on a pipeline route and its possible deformation (pinching) owing to soil and thermal instabilities.

Analysis of the development of stress corrosion cracking in pipelines of compressor stations

The problem of corrosion cracking in underground gas pipelines has been a focus of attention of gas-transporting enterprises for more than 15 years, but so far there is neither an unambiguous assessment of the main causes of this phenomenon nor acceptable methods for reducing it (except detection of stress corrosion cracking (SCC) flaws by means of in-tube flaw detection and the subsequent elimination of these flaws). One important feature of the manifestation of the problem of SCC in main gas pipelines is the selectivity of cracking (SCC is identified in a small fraction of pipes), the causes of which are unknown. It is still not clear whether this feature is due to the time delay of SCC or this effect has another origin and is associated with the resistance of the greater part of pipes to stress corrosion damageability. This issue is directly related to forecasting of the further development of stress corrosion damageability both for gas pipelines in which SCC has already been detected and for segments in which SCC problems have not yet arisen. An especially pressing task is assessing the possibility of the development of SCC in process pipelines of compressor stations situated in regions where the presence of the SCC process on linear segments has been identified, because equipment operation and the presence of maintenance staff on the station territory involve increased risk.

Assessing changes in stress-corrosion damageability on the basis of repeated in-tube nondestructive testing

Advances in the development of in-tube nondestructive testing (INT) have allowed obtainment of comprehensive quantitative information that concerns the state of an operational gas pipeline at the moment of inspection and characterizes the quantity, location, and dimensions of flaws. By means of repeated testing, it is possible to assess changes in the state of an operational gas pipeline during the time period between inspections, thus showing the dynamics of corrosive damageability on the time scale and under conditions of commercial operations (within the limits of sensitivity and resolution of the flaw detectors used). In this study, the INT results for a segment of a multistrand pipeline from the Krasnotur’ insk compressor station to the Lyaly a compressor station collected in 2000–2004 with ДMT-1400 and ДMTII-1400 magnetic-pig flaw detectors are considered. On the basis of the analysis of the INT results characterizing the development of corrosion and stress-corrosion damageability at the moment of inspection, the possibilities and conditions for the use of the obtained data for comparative assessment of the changes in corrosion-related damageability of gas pipelines are considered. According to the collected information, there is no evidence that stress-corrosion damageability is enhanced during the period between two inspections and that corrosion and stress-corrosion damageability are superimposed (observation of etched longitudinal grooves during second inspection).

Effect of operation on the stressed state of the piping of compressor stations located in different regions

Due to the increased operating time of operated compressor stations (CSs) and the current safety requirements, the engineering state of the pipelines of CSs must be surveyed to confirm the possibility of their further operation (extension of the operating life). The available factual material shows that the failure of an operated pipeline is either directly related to the effect of an unrated static or dynamic load that was the primary reason for the failure or is due to the presence of flaws that facilitate the appearance of an unrated load. The presence of an enhanced level of stresses may be a consequence of several factors, including errors in design; violation of the rules of assembly of structural elements; and operating conditions related to variation in the height positions of pipeline segments due to soil instability or subsidence and also due to buckling of supports. The latter factors are traditionally indicated as probable causes of overload for pipelines located in northern regions that are located on sandy soils and under conditions of deep freezing of soils. An evaluation of the actual level of stresses of production pipelines of compressor stations (CSs) and a comparison of the parameters of the stressed states of CSs located in different regions enables one to assess the effect of climatic factors on the operating conditions of pipelines. In this study, based on an analysis of the parameters of the stressed state obtained using magnetic methods for measuring stresses, the load level of pipelines is assessed on a scale from individual segments of a pipeline to a comprehensive assessment of plant piping.

Effect of operation on the stressed state of process pipelines in a pressure blower’s piping

The possibility of assessing parameters of a stressed state from changes in magnetic properties is considered. Such an option would allow determining the sign and magnitude of the effective stresses. Generalized indicators of the stress state of a pipe’s cross section may include data on maximum, minimum, and average “pointlike” stress in the investigated cross section, the amplitudes of extreme values, and the average stresses in the segments of the investigated cross section. It is shown that application of such generalized characteristics of a pipeline’s stress state allows one to sequentially change the scale (selection) of the evaluated segment and compare technological-equipment units through the use of the advantages of the local testing of stresses in combination with averaged parameters of a pipeline’s segment that has a respective scale.

Investigation of Behavior of Stress-Corrosion Cracks in a Pipes under Internal Pressure

Corrosion induced by ambient conditions causes generation of longitudinal cracks on outside surfaces of gas pipelines, and this can lead to breaks in main pipelines. The paper presents experimental data on behavior of cracks in sections of pipelines with aws under constant and cyclically varied (45 cycles) hydrostatic pressure, the pressure magnitude corresponding to operational parameters. Crack parameters (increasing depths and openings of cracks) were monitored by sensors of magnetic induction placed on pipes immediately over cracks. The data indicated that cracks with depths of up to 30–35% of the wall thickness did not show residual changes (in their depths and openings) after tests. Residual changes were detected in deeper cracks, which manifested as critical ows of metal near the crack edge and growth of cracks to their critical dimensions. Photographs of fractured surfaces showed that crack depths started to increase long before the ultimate fracture of a pipeline. Thus, tests conducted by applying high pressure to operating gas pipelines can lead to increases in crack dimensions without detecting them. This effect is actually detrimental to the tested pipelines safety. The absence of residual changes in cracks with depths of up to 30–35% of the wall thickness indicates that, possibly, new limits on aw dimensions should be established for decommissioning unsafe sections of pipelines, and these limits should take account of the hazard due to stress-corrosion cracks, depending on their dimensions and inside gas pressure.

Structural features of crack formation during corrosion cracking

Stress corrosion cracking (SCC) causes damage to main gas pipelines due to formation and development of cracks on the outer surface of a tube. Prevention of SCC crack development and fracture of a gas pipeline is an important problem of diagnostics that can be solved in various ways. The most effective approach involves systematic use of in-tube nondestructive testing and local testing in shafts, owing to which the position of a flaw and its linear size are determined. The hazard of the flaw (residual life of a tube with cracks) is found and a decision is made about the necessity and methods of flaw repair, such as cutting out the tube, grinding the flaw, and reinsulating the tube surface, and the date of repeated nondestructive testing is set. These measures ensure timely discovery of potentially hazardous flaws and make it possible to take the necessary steps for preventing development of SCC.

Corrugation of grain boundaries during high-temperature thermomechanical treatment

A new mechanism is proposed for the formation of wavy grain (crystallite) boundaries in a metal subjected to high-temperature thermomechanical treatment (HTMT). It is based on the corrugation of certain atomic metallic layers during deformation.