Aleshin V. Numerical structural analysis of industrial pipelines for enhancing their safety.

NUMERICAL STRUCTURAL ANALYSIS OF

INDUSTRIAL PIPELINES FOR ENHANCING

THEIR SAFETY

Aleshin V.

Computation Mechanics Technology Center , Russia

Abstract: This paper presents computation technology for structural analysis of the industrial

pipeline systems subjected to multifactor loads. Numerical structural analysis of pipelines under nominal and failure loads is executed by the finite element method taking into account all the data of technical inspections. Industrial applications of the computational technology are illustrated with the examples for gas transmission pipeline segments.

1. Introduction

Service lives of industrial pipelines are tens of years. It is well known that one of the main causes of failure of industrial pipeline systems is a decrease in the carrying capacity of pipeline structures due to the occurrence of pipe wall local defects at both the construction and operation stages. In particular, according to GAZPROM data, within the last 30 years up to 60% of natural gas transmission pipeline ruptures were caused by the pipe wall corrosion. Besides, accidents at the industrial pipeline networks are caused by abnormal loads on the pipelines during their operation. For instance, for buried pipeline segments, failures can be caused by such abnormal loads as uncontrollable soil displacements (dolines, soil heaving, landslips, etc.) and the mechanical influence of earth-moving machines on the pipeline itself, as well as on the surrounding soil. To increase safety level of operating pipelines, to reduce failures entailing harmful consequences it is necessary to reveal, to evaluate the remaining strength and to repair timely all critical pipeline segments.

Over the last few years, the continuous progress of methods and tools for technical inspection brought them to a level which allows for objective data to be obtained both on the actual pipelines spatial position and on the geometry and location of existing pipe wall defects. For example, utilization of “smart” (or intelligent) magnetic flux leakage (MFL) and ultrasonic high resolution pigs for in-line inspection (ILI) of pipelines allows to

measure accurately geometric shape (Fig.1) of the corroded regions and establish their locations on the pipeline.

Fig. 1. Morphology of corrosion defect determined using ILI-data

Nevertheless, conventional methods for the evaluation of the stress state of pipeline structures based on simplified methods of strength of materials theory do not allow for an adequate analysis of industrial pipeline systems strength with the accuracy required by up-to-date conditions. It is mainly concerned with the analysis of operating pipeline systems which contain sections with local pipe wall defects and are subject to various, including abnormal loads, during their operation. In particular, to evaluate remaining strength of the corroded pipeline segments it is used traditionally semi-empirical technique [1] and its numerous modifications all over the world. The common disadvantage of these methods application is the excessive conservatism. In most cases their usage gives essentially low estimates of remaining strength of the corroded pipeline segments resulting in unnecessary cutouts. In some cases application of the methods like [1] can entail underestimation of really critical corrosion defect (cluster of defects). This underestimation followed by failure as a rule. It is due to the conservatism and functional narrowness of the standard methods currently used that all great efforts and expenditures of both vendors and consumers of modern technical inspection tools would not yield any significant increase of safety level or reduction of the accident rate at industrial pipelines. On the other hand, the present-day numerical methods of continuum mechanics and computer performance allow for new approaches towards the structural analysis of the complex industrial pipeline systems to be developed.

2. Computation technology for structural analysis of the industrial

pipelines

Computation technology for numerical simulation of multiaxial nonlinear stress state of the industrial pipelines taking into account all technical inspection data was developed at Computation Mechanics Technology Center (CMTC). This technology is meant for high accuracy evaluating actual strength of the operating pipeline segments. In particular, the developed technology allows to reveal the most critical corroded pipeline segments, to

calculate burst pressure and maximum allowable operating pressure (MAOP) for each segment, to plan economically effective repair or replacement of critical segments. Numerical structural analysis of nonlinear stress state of the pipeline segment is executed by finite element method (FEM) considering its multifactor loading and all technical inspection data including ILI-data, external inspection (displacement of the pipeline axis from the design position), geophysical research (soil shear and soil heaving, settlements), and etc.

Fig. 2. Solid model of the corroded pipeline segment

The main points which are the base for the algorithm of structural analysis and evaluation of remaining strength of the defective pipelines can be outlined as follows [2,3]:

 analysis of nonlinear stress state of the pipelines is executed with minimal

simplifications of their structure (including 3D geometry of the corroded defects),

 numerical simulation of the pipelines is executed step-by-step with successive usage of beam, shell and solid FE-models and using the results of the previous step for set of the boundary conditions at the following step,

 two simulation methods of nonlinear interaction between the pipeline and

surrounding soil are used at different steps: semi-empirical dependences of soil resistance to axial and lateral pipe displacements (at the first step); simulation of soil surrounding the underground pipeline segments as elastic-plastic continuum (at the next steps),

 burst pressure and MAOP (accounting safety margin) for each defective pipeline

segment are determined basing on analysis results of multiaxial nonlinear stress state in the most stressed zones and as the result of numerical simulation of pipeline rupture also.

Fig. 3. Pattern of equivalent von Mises stresses [MPa] in the corroded pipeline segment

Fig. 4. Simulation of tee pipe rupture

This technique allows to evaluate actual remaining strength of the critical pipeline segment with the accuracy essentially exceeding the results of the methods [1] or similar ones. Fig. 2-4 present the examples of analysis results for multiaxial nonlinear stress state of the defective gas pipeline segments. These results were obtained using the developed technology. Fig. 2 presents computation solid model of the corroded pipeline segment generated according to ILI-data. Fig. 3 presents pattern of equivalent von Mises stresses in this segment under operating loads (internal pressure and temperature rise). Fig. 4 presents the results of numerical simulation of tee pipe rupture under failure pressure.

The developed computation technology was validated by the results of full-scale hydrostatic testing for rupture of the pipes having artificial and natural pipe wall defects. Deviation of the calculated and experimental values of burst pressure did not exceed 5% in all cases. It should be noticed that calculation of burst pressure for the corroded pipe using standard methods [1] usually gives the error from 30% to 70%. Over the last few years, the technology has been widely used for safety analysis, state evaluation and simulation of failures for numerous industrial pipeline systems (both in Russia and abroad) with the accuracy of the manifested results being up to present-day standards.

3. Automation of corroded pipeline numerical analysis

Effective application of the developed computation technology can be proved by successful solutions of many practical problems concerned with analysis of remaining strength of corroded segments and maintaining the integrity of pipelines. These problems were solved by CMTC experts mutually with the experts from gas and oil companies. Solution of these problems has shown that it is necessary to develop program procedures allowing maximum automation of computation model generation, application of loads and assignment of boundary conditions, control for numerical analysis and assessment of the results. On the one hand, usage of theses procedures reduces essentially time which is necessary for analysis of the particular defective pipeline segment and excludes possible user’s errors. On the other hand, it gives the opportunity to the users to become quickly familiar with the technology and apply it in their practice.

General-purpose commercial FEM-software were used as the tool for numerical structural analysis of pipeline networks while development of the technology. At present the program modules for automated numerical simulation of stress state and evaluation of remaining strength of the corroded pipelines were realized in the environments of ANSYS (Fig. 5) and ABAQUS.

The user has to form the input data file to work with the program modules meant for automated numerical structural analysis the selected pipeline segment. This file is the spreadsheet of specified format which contains spatial coordinates of the key points of the pipeline axis line, physical and mechanical properties of materials (pipes and soils), outside diameter (OD) and wall thickness of the pipes. Also the user has to have the specified format file containing information about geometric shape of corroded region surface (“matrix of remaining thickness” [3]). The methods for this file formation can be different depending on the type and capabilities of the user’s technical inspection tools. For example, while using the smart pigs this information is in output data of ILI. This information has to be converted into the specified format only.

After preparation of the input data file the user starts the command file in FEM-software environment and inputs the required parameters in the dialog mode at different steps of analysis: the name of job, the names of input data file and file of remaining thicknesses matrix, the values of operating loads and etc. All procedures of FE-models generation, numerical analysis and presentation of the results are executed automatically at three steps of technology and don’t require from the user any additional actions. Special program module included into the technology allows the user to predict accurately values of burst pressure and MAOP for each defective segment. It is possible as a result of automated execution of the corresponding iterative procedure.

Fig. 5. Command menu of the computation technology in ANSYS GUI

Features of the computation technology are continuously enhanced. It gives to the user the most comprehensive tool for analysis of the defective pipelines. For example, the current version includes the procedures for automated numerical analysis of cold-bent corroded pipe elbows (taking into account the residual stresses and strains) and excavated and backfilled underground pipeline segments.

4. Concluding remarks

Computation technology developed on the base of modern achievements in numerical structural analysis allows to evaluate actual carrying capacity of the operating pipelines with high accuracy and to provide maximal economical effectiveness of technical inspection and repair of the defective pipeline segments. Thus application of this technology helps to reduce accidents rate and to increase safety level of industrial pipeline networks.

References

1. ANSI/ASME B31G: Manual for determining the remaining strength of corroded pipelines. ASME, New York, 1984.

2. Ed. by Aleshin V.V. and Seleznev V.E.: Numerical structural analysis of underground pipelines. Moscow: Editorial URSS, p. 320, 2003

3. Seleznev V.E., Aleshin V.V., et al.: Numerical simulation of gas pipeline networks: theory, computational implementation, and industrial applications. Ed. by V.E. Seleznev, Moscow: KomKniga, p. 720, 2005.