PSI - Issue 36

V. Zapukhlyak et al. / Procedia Structural Integrity 36 (2022) 378–385 V. Zapukhlyak, Yu. Melnychenko , І . Оkipnyi et al./ Structural Integrity Procedia 00 (2021) 000 – 000

384

7

   

) )        2   2 R S

(

t

  



− M M

.

(8)

( P U

 = − 0) exp

exp

−

R

S

(

j

j

2

2

+

2

2 R

2

+

  

S

S

where M R , M S – mathematical expectations R і S . It should be admitted, that the number of criteria of gas pipelines line section state assessment in function (4) can be larger or smaller depending on real conditions of gas pipelines installation and on the results obtained during diagnostics of pipeline wall state and insulation coating condition. As a study case, we will consider two supposed pipelines whose characteristics are given in Table 1, and we will make a comparative assessment according to the suggested method. It is known, that the above-mentioned pipelines are made of the same steel, buried under the same climate conditions and in the soil of the same type, and any erosion-type defect was not found. Having used the formula (4) and having made simple calculations, it was found, that for pipeline 1 defect severity is DB =0.48 , and for pipeline 2 it is equal to DB =0.57. Thus, according to the calculations made pipeline 2 will be more reliable under operation conditions. So, using the above-mentioned model, the defect severity can be calculated for any n sections, its value will be within the range DB =[0…1]. We can state that the closer to 1 the defect severity is, the better technical condition of the gas pipeline is considered to be and the environment will make a more favorable impact on it for its further use. Furthermore, one should solve the problem of maintenance optimization of the main gas pipelines objects and lines based on the complex system approach which would combine the methods of the theory of probability and system mathematical analysis to obtain the required analytical dependencies which have to determine the most suitable time period for preventive repairs and minimum average specific service costs.

Table 1. Characteristics of supposed pipelines.

Parameter

Pipeline 1 Gas pipeline

Pipeline 2 Gas pipeline

Usage

Diameter DN , mm Length L , km

1400

1400

40 1,0 35 18

50 1,0 40 17

Depth of pipeline burial, m Operation period, years

Number of failures

Length of sections with corrosion pits, km Length of sections with pitting corrosion, км Length of sections with insulation failure, km Length of sections with the inadequate depth of pipeline burial h ср.1 =0.6 m Length of sections with the inadequate depth of pipeline burial h ср.1 =0.8 m Weight coefficient of the defect – corrosion pit k кр.1 Weight coefficient of the defect – pitting corrosion k кр.2 Weight coefficient of the defect – insulation failure k із.

0.09 0.015 0.12

0.15 0.02

0.2 0.5

1,2

2,1

0.8

0.63

0.63

0.72

0.72

0.25

0.25

4. Conclusions To assess the actual state of the gas pipeline section a concept of defect severity (denoted by DB) is proposed to use to make the comparative analysis of some sections in the case when they are going to be taken out of service. DB is determined by calculations according to the results of the obtained information of any failures on the separate section and their previous reasons in the past, of any available defects of the pipe metal and insulation, of the soil influence on the pipeline material, of the mismatch in pipeline axis geometry and the regulatory documents and project, as well as by the results of the section failure-free operation forecast in the future. The choice of sections of the gas pipeline lines for the further transportation of natural gas-hydrogen mixture should be based on the principles of inconsiderable maintenance costs assurance.