Assessment of the Structural Integrity of the Pipes with Anomalies Such as Local Elastic-plastic Deformations

Abstract

Pipelines are one of the most practical and economically efficient ways to transport dangerous and/or flammable substances, for which road or rail transport is often impossible. The evaluation of the processes that can negatively influence the performance of the pipelines is particularly important for assessing the risk associated with the operation of these technical systems and the potential for technical accidents. The anomalies that can be found on the pipes can be classified into two main categories. Imperfections that do not inadmissibly affect their load-bearing capacity and defects with significant negative influences on the correct operation and load-bearing capacity of the piping, which require supervision and maintenance measures. The influence of these anomalies and the processes that lead to the decrease of the pipeline-bearing capacity constitutes the main objectives of the analysis performed. The local elastic-plastic deformation anomalies are considered, for which the parameters of the geometric model of the defect profile, the conditions for generating these anomalies, and the evaluation methods, respectively, were analyzed, both analytically and experimentally.

1. Introduction

The probability of the appearance of anomalies on the pipelines is closely related to the mechanical resistance and tenacity characteristics of the material (steel) from which the pipelines are made. Anomalies that can be found on pipelines can be classified into material loss anomalies, which consist of the thinning of the pipeline wall through the loss of metal in the presence or absence of a corrosive process, and anomalies such as cracks and anomalies such as local elastic-plastic deformations.

The steels from which the pipelines are mainly made have the ferrito-pearlitic structure typical of carbon or low-alloy hypoeutectoid steels, and the increase in their mechanical resistance characteristics is achieved mainly by increasing the carbon concentration, which has the effect of increasing the percentage content of perlite in the structure. Modern steels for pipes have a structure with acicular ferrite (low carbon bainite), their high mechanical resistance characteristics being achieved mainly by obtaining a very fine grain and ensuring hardening effects by the precipitation of some intermetallic compounds. The weldability of these steels is satisfactory if the carbon concentration is not increased excessively and if the welding procedure and regime are chosen appropriately.

The carbon concentration of these steels does not exceed 0.30...0.31%, to obtain the higher degrees of resistance resorting to the use of manufacturing recipes with manganese concentrations higher than those typical of carbon steels. Flat strip semi-finished products intended for the manufacture of longitudinally or helically welded pipes are made from such steels by controlled rolling or thermomechanical rolling (rolling with high degrees of deformation in which working temperatures, heating, cooling, and deformation speeds are strictly controlled), which emphasize the presence in these steels of a wide range of microalloying elements (Nb, V, Ti, Mo, etc.).

Defining and classifying the factors that lead to the degradation and failure of pipelines is a problem for which specialized literature provides a multitude of solutions. To carry out the analysis of the factors and processes that lead to the progressive degradation and failure of pipelines, it was considered that the factors to be considered correspond to the potential hazards specified by the ASME B31.8S standard for risk assessment and development of pipeline integrity management plans intended for natural gas transport.

The classification of these factors (potential hazards) is presented synthetically in Table 1; as can be seen, the ASME B31.8S standard recommends the division into three classes of factors that can determine the failure of pipelines, and for each class, there are three categories of factors, each category having one or more factors.

Category	Type	Factor name	Category	Type	Factor name
A. Time-dependent			B. Stable
1	a	external corrosion	manufacturing related defects
2	a	internal corrosion	1	a	defective pipe seam
3	a	stress corrosion cracking	1	b	defective pipe
			welding/fabrication related
C. Time-independent			2	a	defective pipe girth weld
third party / mechanical damage			2	b	defective fabrication weld
1	a	damage inflicted by first, second, or third parties	2	c	wrinkle bend or buckle
1	b	previously damaged pipe	2	d	stripped threads / broken pipe
1	c	vandalism	equipment
2	a	incorrect operational procedure	3	a	gasket O-ring failure
weather-related and outside force			3	b	control equipment malfunction
3	a	cold weather	3	c	seal / pump packing failure
3	b	lightning	3	d	miscellaneous
3	c	heavy rains
3	d	earth movements

Table 1.

The categories of factors that can affect the integrity of natural gas pipelines.

Imperfections are anomalies in configuration, dimensions, microstructure, etc. present in the pipelines, which do not inadmissibly affect their load-bearing capacity, and the defects are the imperfections with significant negative influences on the correct operation and the load-bearing capacity of the pipelines, which require maintenance measures. The classification of pipe imperfections and defects can be based on the configuration criterion that defines the following four categories of pipe imperfections and defects:

1.1 Imperfections and geometric defects produced by the local deformation of the pipes

Local deformations or dents are deviations from the circular shape of the cross-section of the piping, obtained by local deformation of the piping, inwards, without removing the material and, as a result, without reducing the wall thickness. Indentations influence the flow of gases in the pipelines and can cause major difficulties in performing cleaning or washing operations and in the internal inspection of the pipelines, by blocking the movement of working tools or PIG devices. Indentations of an elastic nature, produced, for example, by the interaction of pipes with pieces of rock can be eliminated by simply removing the cause (pieces of rock that produced the deformation); Gouges are areas of the pipes where the wall thickness has been locally reduced due to the removal of material through a mechanical action, and gouges are defects or imperfections of great severity because their presence can lead to the initiation of brittle cracking processes, representative types of such defects are shown in Figure 1.

Figure 1.

Imperfections and defects of local elastic-plastic deformation type.

1.2 Imperfections and defects such as material loss

These imperfections or defects consist in the general or local thinning of the wall of the pipeline through the loss of metal in the presence or absence of a corrosive process. The most common imperfections or defects in this category are as follows: areas of local thinning (areas on the surface of a pipe element, having the axial extension or length of the same order of magnitude as the circumferential extension or width, in which the material has been removed by corrosion and/or erosion), pinching or pitting (traces of local corrosion on the surface of a pipe element, in the form of cavities or holes, having the surface diameter of the ordinal size of the wall thickness of the respective pipe element), and representative types of such defects are shown in Figure 2.

Figure 2.

Imperfections and defects such as material loss.

1.3 Cracks

Cracks are the anomalies with the greatest harm, which produce strong mechanical stress concentration effects and significantly reduce the carrying capacity of the pipes. Their dimensions can change over time through stable growth, until they reach critical dimensions, at which unstable propagation and rupture of the tubing can occur. Depending on the toughness characteristics of the piping material, cracks can generate brittle fracture phenomena (which occur at high speeds and propagate over large distances, giving rise to damage with important consequences) or ductile fracture phenomena (which occur at small speeds and are preceded by plastic deformation processes, which consume an important part of the available energy and thus contribute to stopping the phenomenon and limiting its consequences), and representative types of such anomalies are presented in Figure 3.

Figure 3.

Cracks.

1.4 Other types of defects or imperfections

This category includes imperfections or defects that cannot be attached to one of the previously specified categories, for example: defects in the sealing systems of valves or fittings mounted on pipes, defects in threads, defects in the manufacture (lamination, welding) of pipes.

2. Characterization of the geometry of anomalies produced by local elastic-plastic deformation of pipes

The way of generating and modeling the geometry of the anomalies (imperfections and defects) produced by the local deformation of the pipes is a very delicate problem, considering both the complexity of the phenomena that take place in the deformed area and the need to obtain some methods for evaluating the bearing capacity of the pipes that present such anomalies.

In the specialized literature, you can find various geometric modeling (semi-elliptical, hemispherical, or hexagonal cavities) that take into account both the characteristics of the pipeline and the cause of these anomalies (third-party interventions). Figure 4 shows two of the most important geometric models that characterize the imperfections and defects produced by local plastic deformation of pipes [1, 2, 3].

Figure 4.

Two models for defining the geometric profile for an anomaly of the type of local elastic-plastic deformations.

Local plastic deformation represents a change in the circular section of a pipe or a distortion of the pipe in a circular section. In Figure 5, a geometric modeling model of the shape of these elastic-plastic deformation type anomalies is proposed. The depth of this type of anomaly is defined as the maximum reduction of the diameter in the defect area compared to the initial nominal diameter of the pipe. This definition of deformation depth includes both local deformation and any distortion in the circular section of the pipe (ovality).

Figure 5.

Scheme of the description of the configuration of a pipeline in the area of a local elastic-plastic deformation type anomaly.

To determine the stress state in the area of an imperfection and defects produced by the local deformation of a pipe, it is necessary to carry out an analytical description of the geometry of the anomaly. For the indentations with axial orientation on the pipelines, the analytical description of the shape was proposed by using a cylindrical coordinate system, of the type defined in the sketch in Figure 5 and a function with the following analytical form [4, 5]:

$$r\left(\phi ,z\right)=R-d_p\left(z\right)\left[1+\mathit{cos}\left(2\pi \frac{\phi }{{\phi }_0\left(z\right)}\right)\right]\left|\phi \right|\le {\phi }_0\left|\phi \right|>{\phi }_0$$E1

In Eq. (1) the circumferential anomalies profile are given as a deformation from the pipe radius R, the maximum of the deformation is given by the term d_p(z), and the circumferential deformation of the local elastic-plastic deformation defect is limited to the angle $2{\phi }_0\left(z\right)$, the measure of which is correlated with the defect width c_p(z),$2{\phi }_0\left(z\right)=\frac{c_p\left(z\right)}{R}$ [6, 7, 8].

A better modeling of the profile geometry of local plastic deformation defects is obtained if an expression corresponding to a Gauss curve (in polar coordinates) is used in Eq. (2):

$$r\left(\phi ,z\right)=R_e-d_p\left(z\right)\mathit{exp}\left[-\frac{1}{2}{\left(\frac{z}{z_0\left(z\right)}\right)}^2\right]\left[-\frac{1}{2}{\left(\frac{\phi }{{\phi }_0\left(z\right)}\right)}^2\right]$$E2

where the magnitude of the deviation is given by the term d_p(z), the circumferential extension of the local deformation type defect is limited to the angle $2{\phi }_0\left(z\right)$, the measure of which is correlated with the width of the defect c_p(z), $2{\phi }_0\left(z\right)=\frac{c_p\left(z\right)}{R}$, and the axial extension variation is given by the term z limited to ± S_p/2, S_p represents the axial length of the anomaly.

To describe with this analytical expression, the shape of the cross sections of a pipeline in the area of an indentation, a computer application, PROFIL_DEF, was created using the Mathcad computing environment. Results obtained by using this computer product and which correctly and objectively describe, from an analytical point of view, the geometric profile of an imperfection or local deformation type defect, are presented in Figure 6.

Figure 6.

Results obtained when describing the geometric profile of an imperfection or local deformation type defect.

Figure 7 shows a semi-model of the geometric profile corresponding to an imperfection and defect of the local deformation type, made by considering several dimensions of the connection radius that the geometry of the anomaly presents in relation to the geometry of the pipelines that thus present anomalies. The geometric profile was divided into three contact zones corresponding to the three regions of the profile geometry (AB, BC, and CD).

Figure 7.

The general semi-profile of the geometry of an imperfection and local deformation type defect; a–AB zone, b–BC zone, c–CD zone. (a) a - semi-model of the geometric profile for elastic-plastic anomaly, r₁ - main deformation radius in the contact area, r₂- the secondary deformation radius in the contact area, α - main angle corresponding to radius r₁, β - the secondary angle corresponding to radius r₂, rm-the average radius of the pipeline. (b) case study.

Two examples of modeling the geometric profile of imperfections and local deformation type defects are also presented in Figure 7, considering the parameters and the method described above, with the observation that the method presented involves a rigorous and difficult determination in practice of the parameters that define defect geometry, modeling that was done using CAD software (AutoCAD).

The verification of the models designed to define the geometric profile of imperfections and local deformation type defects was carried out through experimental tests using ring samples taken from pipes for pipelines. Pipe rings with a diameter of De = 114.3 mm, wall thickness t = 4.6 mm; 5.4 mm and 8.0 mm and with a length of 50 mm were used for the experimental tests.

The tested pipe section was rested on a prism, and the application of force on the indenter was done gradually with the help of a testing machine, measuring the depth of the anomaly with the help of a dial gauge.

The graphic representations in Figure 8 summarize the comparisons of the results obtained, depending on the type of indenter used and the wall thickness of the pipe ring used to simulate the appearance of an imperfection and/or local deformation type defect.

Figure 8.

An example of the application of the proposed model for the analytical description of the configuration of local elastic-plastic deformation anomalies.

From the analysis of the experimental program, the proposed analytical modeling ensures good fidelity for anomalies with relatively small depth, of the type that must be evaluated to be accepted on pipelines.

3. Experimental determination of burst pressure of pipes with anomalies such as local elastic-plastic deformations

The experimental verification of the behavior of pipelines with different types of anomalies under mechanical stress [9, 10] is one of the methods by which their residual bearing capacity is established and leads directly to conclusions regarding the level of confidence that must be associated with the results of the assessment of the severity of the anomalies through analytical methods available. The research carried out concerned the pressure test until bursting of some pipes on which anomalies were made (imperfections and/or anomalies of the type of local elastic-plastic deformations; dents with gouges, considered to be the most dangerous of the anomalies). Two samples were made for the internal pressure test, and each sample consisting of a pipe and two ellipsoidal bottoms welded to its ends, on which three connections were mounted: one for the manometer, one for filling the sample with water and pressurizing to it, and one for venting the sample before pressurization.

The anomalies realized in the two samples subjected to the internal pressure test until breaking/smashing are suggestively presented in Figure 9, they are of the indentation type for the first sample, respectively of the indentation type with a gouge for the second sample, and their geometric characteristics are shown in Table 2.

Figure 9.

Realization of the anomalies on the two samples subjected to the test at internal pressure until bursting.

No.	Sample 1	Sample 2
	Dent	Gouge		Dent
	Depth [mm]	Depth [mm]	The length on the direction longitudinal [mm]	Depth [mm]
1	25	0,5	40	25
2	30	1,0	40	30

Table 2.

The geometric characteristics of the anomalies made on the samples.

The samples that were subjected to the internal pressure test and were made from pipe sections with a diameter of De = 163 mm, the wall thickness being t = 8 mm. The stand on which the samples were tested is reproduced schematically in Figure 10, in which the constructive elements of the high-pressure pump and the work platform from the composition of this stand are presented.

Figure 10.

Scheme and main components of the stand used for internal pressure testing of pipe samples with local surface anomalies.

When carrying out the experimental research on the samples subjected to the tests, four strain-resistive transducers were applied around the anomaly, two in the circumferential direction (TER 1 and TER 3) and two in the axial direction (TER 2 and TER4) and two strain-resistive transducers, one in the circumferential direction (TER 5) and one in the axial direction (TER 6). During test, the computer controls the acquisition of data with the help of the SPIDER 8 device through the dedicated software CATMAN.

The method of carrying out the tests and the results obtained (quantitative and qualitative), regarding the behavior of the samples during the tests, are presented below:

1. The sample from Pipe 1 resisted up to the pressure 120 bar when failure occurred through cracking of pipe near the joint between the caps and the pipe, outside the defect—see Figure 11;

2. The sample from Pipe 2 withstood up to a pressure of 225 bar, when the sample failed by breaking next to the cavity indentation anomaly—see Figure 12.

Figure 11.

Images regarding the behavior of sample 1 in the internal pressure test.

Figure 12.

Images regarding the area where the burst occurred for sample 2.

The processing of the experimental results (see Figure 13) was carried out by determining the mechanical stresses in the circumferential direction and in the axial direction, using the known formulas:

Figure 13.

The results of the experimental analysis by the resistive tensometry method. a. results for sample, b. results for sample 2.

$${\sigma }_{\mathit{\theta ij}}=\frac{E}{1-{\mu }^2}\left[{\epsilon }_{\mathit{\theta i}}+\mu {\epsilon }_{\mathit{zj}}\right];{\sigma }_{\mathit{zij}}=\frac{E}{1-{\mu }^2}\left[{\epsilon }_{\mathit{zj}}+\mu {\epsilon }_{\mathit{\theta i}}\right],$$E3

where E represents the longitudinal modulus of elasticity, and μ—Poisson’s coefficient for the sample steel; the specific deformations in the circumferential direction ε_θi or in the axial direction ε_zj (i and j being the identification numbers of the transducers).

For the determinations, tensor-resistive transducers were used, with a base of 10 mm and resistance 120 Ω. With each of these transducers, at different pressures, during the test of the sample, the specific deformations in the circumferential direction and in the axial direction were determined.

4. Conclusions

The main causes that determine the degradation of pipelines, usually installed underground, are corrosion, third-party interventions and manufacturing, and construction defects; in the last period, there was a significant increase in concessions produced by third-party interventions/interferences.

The principles of codification of imperfections and defects of different types and categories ensure the synthetic and, at the same time, comprehensive specification in the technical documentation regarding the operation, exploitation, maintenance of pipelines of any anomalies found, and their causes; also, the coding of imperfections and defects is particularly useful for the development of maintenance procedures for pipelines.

Anomalies of the elastic-plastic deformation type may have the character of dents (deviations from the circular shape of the cross section of the pipe tubing, obtained by local deformation of the pipe, inward, without removing the material and, as a result, without reducing the wall thickness), gouges (areas of pipe where the wall thickness has been locally reduced due to material removal by mechanical action) or pitted indentations (indentations that have gouges at the bottom of the deformed zone).

Modeling the geometry of anomalies produced by local elastic-plastic deformation is particularly important because it serves to develop evaluation procedures and characterize the severity of these anomalies.

Experimental research into the operational behavior of pipelines with anomalies is usually carried out on stands that allow pressure testing of pipe or pipe sections, with or without anomalies. Usually, these stands can ensure the determination of the stress states of the tested samples and the pressure at which the respective pipe samples fail.

The stand designed and built allows the investigation of the behavior of pipes with or without anomalies, being able to provide both results obtained with the help of electro-tensometry transducers applied to the sample and the bursting pressure of the sample.