Statistical characteristics of dent defect sizes in gas pipelines

STATISTICAL CHARACTERISTICS OF DENT DEFECT SIZES IN GAS PIPELINES

Zorin A.A.

Ural Federal University, Ekaterinburg andreyzorin1989[AT]gmail.com

Timashev S.A.

Ural Federal University, Ekaterinburg

Science and Engineering Center «Reliability and Safety of Large Systems and Machines» of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg, timashevs[AT]gmail.com Ensuring reliability and uninterrupted gas supply to the consumer are priorities for Russian gas transportation system and currently are not fully solved. At the design stage the pipeline reliability is ensured by right choices of its geometric parameters, steel brand and comprehensive corrosion protection.

Due to the inevitable presence of defects during operation (originated during construction and installation, or formed during operation or repair, and identified by inspection of the pipeline) measures have to be taken to ensure and maintain the desired level of pipeline reliability. For these purposes it is generally necessary to carry out calculations of the static and dynamic strength of pipeline sections with defects, followed by reliability analysis. The vast majority of existing methods use deterministic approach for assessing gas pipelines strength and reliability. The main reason for this is lack of sufficient, representative volumes of initial data.

In this paper a statistical analysis of the size of dents (one of the most common defects in pipelines) revealed by in-line inspection (ILI) of gas pipelines 5720 km long is described. The pipelines diameters are from 219 mm to 1420 mm. The number of identified dents in pipes of various diameters is shown in Figure 1. According to the figure, the highest number of dents found in gas pipelines with diameters 530 mm and 1020 mm. This is due, presumably, to the fact that these diameters are used most frequently and, consequently, the volume of the ILI for the same diameters of the pipelines is maximal (Table 1).

50 00 tLi 377

530 720

S20

1020 1220 1420 Pipe diameter, mm Figure 1. Number of detected dents in pipelines of different diameters

To determine the statistical characteristics of dents identified during the ILI on the same type of gas pipelines define the number of dents on each a combination "diameter-wall thickness" (Table 2). It shows that a number of combinations "diameter-wall thickness" con- tains very small number of dents. This is primarily due to the low frequency of usage of this type of pipes, mainly where the pipeline intersect with roads and railways. In these areas pipes with increased wall thickness are used to enhance their reliability. Because of this specificity combinations of "diameter - wall thickness", which revealed less than 10 dents, further were not considered.

Table 1

ILI volume and number of detected dents on pipelines of different diameters

Table 2

Distribution of the number of detected dents on pipelines with different combination "diameter - wall thickness"

Histograms of the basic geometrical parameters of dents with a combination of "diameter - wall thickness" 530x7 mm are shown in Figures 2-4. Smoothing of the histogram by

36 the x --method shows that with a confidence level of 0.99, they obey the two-parameter Weibull law. It also turned out that the size of dents on all other gas pipelines from Table 2 also obey this law.

SOO 1000

Dent length, mm

Figure 2. Histogram of dents lengths in pipes whose "length - wall thickness" is 530x7 mm

600 SOO

Dent width, mm

Figure 3. Histogram of dents widths in pipes whose "length - wall thickness" is 530x7 mm 400r

40 30

Dent depth, mm

Figure 4. Histogram of dents depths in pipes whose "length - wall thickness" is 530x7 mm

According to the results of statistical analysis of dents sizes, revealed during ILI of pipelines with various diameters, their main parameters are described by the Weibull distribution. The analysis was performed using a software algorithm designed by the first author. The software algorithm selects out of the total data set information about found dents, separately for each combination of "diameter-wall thickness". It calculates basic statistical parameters of dents samples, determines whether the sample is well described by the Weibull distribution and stores the results in the analysis file (See Table 3).

This data is recommended for practical use when evaluating residual life, probability of failure, time of next diagnostics, time and the amount of preventive maintenance. Statistical characteristics of the basic geometrical parameters of dents for different combinations of gas pipelines "diameter - wall thickness"

W(k, X) - The Weibull distribution notation

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