Bearing capacity of elements of a gas transportation system

. The issues of improving the methods of calculation and design of the gas transmission system (GTS) are considered, taking into account the analysis of loading modes and peculiarities of its individual nodes. The analysis of design schemes for evaluating the performance of elements of the gas transmission system according to the strength criterion is given, and a probabilistic method for assessing the reliability of butt welds of pipelines is selected. The features of the operating conditions of the system are considered and the method for calculating equivalent strength indicators and loading modes is presented. A unified approach is applied for determining the probability level of failure-free operation of parts and components of a structure. A comparative analysis of traditional and new methods for assessing the performance of the considered system is given. The interval values of the parameters included in the dependence to determine the probability level of a failure-free operation of a welded joint are clarified. A computational method is proposed, which allows to increase the carrying capacity of the GTS in the probabilistic aspect.


Introduction
The efficiency and reliability of elements of building structures are the main conditions for the efficient operation of the gas transmission system (GTS). At the same time, strength is the main criterion of working capacity, and ensuring the strength reliability of structural elements is the main task of designing these systems [1][2][3][4]. Traditional methods of strength calculations are usually performed according to three calculation schemes [3.5]: conditional, not reflecting loading conditions and not taking into account numerous factors affecting the strength of the structure [6 ]. Under these conditions, some accuracy is introduced by the calculation of the permissible safety factor [s] ], [ / max s s     R (2) where max  is the maximum effective voltage.
To ensure the required accuracy of calculations, a differential method is used to determine

Methods
In the expressions (1), (2), all parameters are treated as deterministic and their median values are used in calculations. But all of them are random variables, basically obeying the normal or logarithmically normal distribution law. Similarly, the real modes of loading of structures in most are random, caused by the stochastic nature of the flow of production and operational processes [8][9][10]. As shown by computational practice [3], the use of expressions (1), (2) with median values of their parameters may lead to an increase in the material intensity of structures by 15 ... 20%, which is unacceptable in up-to-date conditions, characteristic of the development of large engineering structures and main GTS, as well as in mass output of products and equipment for various purposes. This is also dictated by the requirements of expanding construction projects and infrastructure, as well as increasing the capacity, productivity and operating speeds of manufactured products and equipment [11,12]. In order to fully utilize the entire life of the bearing capacity of a GTS structure with minimum material consumption, it is advisable to switch to probability models for reliability by the strength criterion instead of the design schemes (1), (2), which ensure the condition [ 6,13] pass to the probabilistic methods of calculating the reliability of strength criterion allowing for the specified design service life for the required level of probability of trouble-free operation to ensure the condition of [ 6,13]

Entering in (3) the coefficients of variation
, after transformations, we obtain an expression that combines the design schemes (1) -(3) and allows us to make an assessment of the reliability of the main factors acting on the structural strength of the GTS [6,14]: (4) For the reasonable application of the relation (4) . 1c): 2 -gas pores and non-metallic inclusions, that break the uniformity and become microvolumes for the occurrence and development of fatigue cracks; 3 -lack of penetration in the lower zone of the seam, weakening the bond between the deposited and base metal [5]. To the specified factors of a constructive and technological nature, an operational one is also added, which, due to the non-stationary mode of operation of the GTS, considers the specification of its operating parameters. Strength calculation of pipelines has distinctive features -a complex stress state arises in the pipe walls, and the network loading mode is mostly non-stationary and often stochastic. This implies in the calculations to apply a twostage transition to equivalent parameters: a -from a complex to a linear stress state; bfrom non-stationary loading mode to equivalently stationary.

(Method of determining equivalent parameters)
For pipelines, in the first approximation, the design case of loading a thin-walled vessel with internal pressure p occurs, in which walls a flat stress In a welded seam it can be written that, In the calculations of the strength mode loading is represented by the maximum pressure max p in the pipeline, which is then at the presence of non-stationary loading is replaced by the equivalent value pmaxэ. As shown in [15], the pressure p in the gas pipeline may vary within Calculations on the bearing capacity of structures under non-stationary loading conditions are based on the principle of linear summation of cyclic damage [3,6] where i N is the number of loading cycles at the given i p ; ip N -cyclic durability at the onset of failure with the same i p , determined from the equation of the fatigue curve of the weld; a -the sum of relative damages, which in most cases is assumed: a = 1. Correlation (6) allows the variable loading mode to be replaced by an equivalent stationary mode using the equivalent maximum load ( э max 2  stress) and э N cyclic durability in the calculations.
Using the equation of the fatigue curve of the seam, according to (6) we can write that [16][17][18] where 0 R  is the endurance limit of the laboratory sample (

(Probabilistic estimation of parameters)
To determine the coefficients of variation According to (7) and (8), the value э max 2  is a product of three independent random variables and therefore it can be written down [6,14] that where coefficients are introduced that take into account variations:

Conclusions
which, in turn, depend on the parameters of various factors affecting the performance of the gas transportation system and included in the functional relationships with other parameters in the form of two-and three-parameter regression equations. This involves identifying and building a system of interrelated equations, and, on this basis -corresponding nomogram which allows taking into account the complex effects of all the factors and the actual operating conditions of GTS determine optimum values of the parameters and, first of all, the maximum level of probability of system uptime [20][21][22]. Ensuring the reliable operation of complex technical systems involves the implementation of a wide range of studies related to the design, production, testing, maintenance, diagnostics and repair of these systems. Each of these problems includes the stages of their implementation. This paper addresses the design problem of GTS, which includes the following steps: 1. The number of system elements that were previously used and tested in operation should be maximum. 2. If possible, it is necessary to apply the modular design principle more, using ready-made and tested components, mechanisms and subsystems with standard elements. 3. The system should be equipped with modern monitoring and measuring and safety devices to monitor the state of system operation, promptly identify problems and eliminate emergencies. 4. The system should be designed to ensure maximum maintainability, which allows in the shortest possible time and without interruption of the GTS to perform repair work.
This work has been carried out in the frame of "Development ant preservation of Scientificresearch sector" programme, financed by State Committee of Science of Republic of Armenia.