·Table of Contents
·Computer Processing and Simulation
Numerical Method of Ranking of Defective Pipelines - Sections Based on In-line Inspection DataKlishin G.S., Seleznev V.E., Aleshin V.V.
(JSC «SPE VNIIEF-Volgogaz», Russia)
(OAO «GAZPROM», Russia)
For early prediction of failures at pipelines methods of non-destructive in-line inspection are widely used in the international practice. To carry out in-line inspection of pipelines inner pipe magnetic gauges (pigs) or ultrasonic pigs are implemented. They make it possible to determine geometry of a pipeline, detect geometrical parameters of wall defects of a pipe.
The extent of their hazard is determined on the basis of the results of defects' assessment according to the requirements of standards and normative documents [1-6]. Traditional conceptions of the theory of strength of materials are assumed as a basis in most of standards and normative documents. Periodical revision of these standards does not change their basis.
Repair or replacement of pipelines' sections is expensive procedure. So correct and qualitative assessment of hazard extent of pipelines' defects can result in high economical benefit at the expense of correct ranking of pipelines according to replacement and repair priority. However present - day standards to analyze hazard of detected defects of pipelines mainly give too high assessment on rejection of defective pipelines' sections. Many researchers noticed it while comparing the results of calculations performed based on present standards and normative documents with the data of burst tests [7,8].
Proposed method of ranking of defective pipelines' sections is based on 3D non-linear numerical analysis of strength of pipeline's sections being examined using in-line inspection data without simplifications in geometry of defects with consideration of all loads. This method of ranking of defective pipeline's sections is performed based on the results of strength numerical analysis.Strength analysis of pipelines' section is performed by finite element method and includes the following main stages : calculation of stress-strain state of pipelines' sections by beam finite element models with consideration of all factors effecting operating pipeline; exposing of the most stressed sections of a pipeline construction, determining of forces and moments at the sections' boundaries; more precise calculations of stress-strain state of hazardous sections by shell and solid finite element models; analysis of carrying capability of pipeline's sections on the basis of the theory of strength criteria and fracture simulation.
While performing strength calculation of a gas transmission pipeline and analysis of its fracture mechanism the following loads onto a defective pipeline are taken into consideration: excessive internal pressure, thermal deformation; distributive forces of ground influence, static and dynamic loads of ground-surface sources; initial strains of pipeline welded joints, residual strains of elastically-curved pipes, wind loads for above-ground pipelines; loads from external water pressure for underwater pipelines; loads arising while getting over karst gaps; loads concerning river or sea bottom relief at underwater passage etc.
To carry out non-linear 3D strength analysis specialists of JSC «SPE VNIIEF-Volgogaz» supported by the International Science and Technology Center (ISTC) developed proper computational techniques that implement commercial software such as «ANSYS», «LS-DYNA» and software of JSC «SPE VNIIEF-Volgogaz» [9-11].
Estimation of probability of pipeline's fracture is performed based on the following criteria: margin of safety (according to normative documents); carrying capability (according to normative documents); models of elastic-plastic fracture.
|Fig 1: Model of a corroded pipe section||Fig 2: Equivalent stresses in a corroded pipe section figure|
Fig 3: Equivalent stresses in a pipeline's section with wall lamination
Fig 4: Equivalent stresses in a pipeline's section with an erosive defect
Fig 5: Numerical simulation of fracture of a pipeline' section (initial stage)
Fig 6: Numerical simulation of fracture of above ground pipeline under pressure peak
While ranking defective sections of pipelines, calculation of stress-strain state is added to numerical simulation of a section's fracture being examined. The example of such modeling is presented in Fig.5, 6.
Experimental works were carried out at testing facility of gas transportation entity of «Volgotransgaz» according to test program developed by specialists of JSC «SPE VNIIEF-Volgogaz». The objective of hydraulic tests was determination of internal failure pressure of a pipeline defect section.
The key objectives of experimental works were obtaining the data for verification and improvement of numerical methods of defect pipelines' ranking being developed at JSC «SPE VNIIEF-Volgogaz» for gas and oil industry.
Upon experimental prove of numerical method of defect pipelines' ranking, the works were carried out by the following stages. Prior to hydraulic test, JSC «SPE VNIIEF-Volgogaz» performed series of analyses on mathematical simulation of stress-strain state and determination of ultimate pressure calculated value for a pipeline defect section to be subjected to hydraulic tests.
The example of computation model of a pipeline section with corroded cavern is presented in Fig.7. Distribution of equivalent stresses in a defect pipeline section under pressure close to failure pressure is presented in Fig. 8.
Fig 7: Computation model of a pipeline section with a cavern
|Fig 8: Equivalent stresses [MPa] at a pipeline section with a cavern|
|Fig 9: Simulation results of pipeline section fracture with a cavern (equivalent stresses [MPa])|
Fig.9 presents simulation results' examples of initiation and development of pipeline section fracture with a corrosive cavern obtained based on the criteria of elastic-plastic fracture.
Upon finalizing of numerical analysis of failure pressure, hydraulic tests were carried out. When carrying out hydraulic tests, pressure was being measured according to certified technique of JSC «SPE VNIIEF-Volgogaz» by pressure transducer MT 100 of 12235 model. The error of measurement made up at most 0,6%.
Registered according to measurement records fracture pressures were being compared with calculated values.
The results of numerical analysis and hydraulic tests showed that the discrepancy of calculated and experimental values of failure pressure obtained in the course of 7 tests makes up less 4%, that is within the limits of minimal, attainable for today error of determination of mechanical properties of pipelines' material. For example the accuracy of calculations of failure pressure for defect pipelines' sections used in present day technique for pipeline transport exceeds 20% [7,8].
Thus, carried out hydraulic tests proved the correctness of mathematical models, computation algorithms and criterion used in numerical methods of defect pipelines' ranking.
To implement up-to-date diagnostic technique and proper computational technologies for defective pipeline sections' ranking it demands considerable revision of present national and international standards and normative documents. Current standards and normative documents do not stimulate the development of new methods and tools to detect defects (sensitivity, resolution, precision) and methods of defective sections' ranking.
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