4. Modern problems of justifying the strength of pipeline systems

One of the main problems was a complex, interrelated deterministic, statistical, and probabilistic analysis of the determining parameters—safety margins , , and and mechanical properties and in Eqs. (1)–(4). According to Eqs. (2) and (3), the minimum allowable

Managing safety margins and for the purpose of their reduction should be carried out in accordance with ratio / , which is featuring, as shown on Figure 1, the hardening degree (or module) of tubular steels in the elastoplastic range beyond the yield point . For the majority of actually used pipe steels as they are improved with existing hardening methods, with the growth of and , the ratio / is increased due to preferential growth of

In the nomenclature and types of the previously used tube carbon steels (Figures 1 and 2) with reduced yield strength (less than 300 MPa) and a ratio / (less than 0.6), the traditional calculations of the yield strength with margins were of primary importance. With a further increase in the yield strength and decrease in the safety margin , the calculations for the ultimate strength with margins have become determinative, in accordance with

and an uncontrolled dangerous transition to large plastic deformations according to Eq. (2) remains, in fact, not explicitly reflected in Eq. (5), due to a reduction in the degree of hardening of steels with a simultaneous increase of and the ratio / . Such conclusion in the framework of modern concepts of strength calculations [1, 3–6] required a gradual transition from calculations in stresses to calculations in deformations е. This transition already received not only its scientific justification [6–8] but also its practical implementation in norms

However, in this case, the problem of increasing the danger of stability loss under

Figure 4. Coherence between strength margins and mechanical properties of pipe steels.

ð5Þ

stresses give the maximum quantitative coherence between these parameters:

(Figure 4).

88 Probabilistic Modeling in System Engineering

Eq. (5).

Four strategic tasks are being solved by methods of deterministic, statistical, and probabilistic modeling and calculation nowadays in Russia:


The solution of these tasks must meet the modern requirements of:


The tasks of justifying and ensuring industrial safety of pipeline systems in accordance with the criteria of strength, resource, and risks in compliance with the Federal Law No. 116-FZ "On Industrial Safety of Hazardous Production Facilities" are resolved with the coordinating and decisive role of Rostekhnadzor with the participation of the Russian Academy of Sciences, leading oil and gas companies as Transneft, Gazprom, Rosneft, the Russian Union of Oil and Gas Constructors and leading academic and industry institutes and universities.

The main directions of scientific research and applied developments in this direction are reflected in the proceedings of the I and II Forums on industrial safety [12].

The solution of problems of formation and development of industry norms and rules for substantiating the strength, durability, resource, and reliability of pipelines is concentrated in the research institutes of Transneft and Gazprom.

In normative documents [13] that are governing the industry, the following assumptions were made:

• Temporary technological heredity is not explicitly taken into account from the processes of obtaining the parent metal and the production of sheets and pipes in factories and enterprises. • Mechanical properties (including limits and ) of structural pipe steels in the process of pipeline transportation, construction, and operation of pipelines are assumed to be unchanged.

abroad, it has not yet been possible to obtain and justify this functional Fc with the appropriate statistical and probabilistic equations and parameters. The prerequisites for the formation of a

Currently, knowledge on the processes of aging and degradation in time τ of carbonaceous

• Natural aging (curve 1) of steels in the initial state (е = σ= 0) at room temperature to is characterized by a slow increase in the yield strength σy, reaching values of 1.1–1.25 in about 30–40 years τ; furthermore, the ratio of the yield strengths σyð Þτ to the tensile

• Thermal aging (curves 2I and 2II) of steels in the initial state (е = σ = 0) at elevated temperatures t<sup>1</sup> and t<sup>2</sup> (t<sup>1</sup> > t0; t<sup>2</sup> > t1) leads to an accelerated growth of the yield point

• Deformation aging (curve 3) of steels in the riveted state for е > 0 even at room tempera-

• Dynamic aging (curve 4) at elevated temperatures in the plastically deformed state (e > 0) under stress conditions (σ > 0) can be accompanied at first by an insignificant increase, while later there is a fall in yield strength σyð Þ τ; t;е; σ and strength σиð Þ τ; t;е; σ with a

In all cases of aging (curves 1–4), the ratio of the yield strengths σ<sup>y</sup> to the tensile strengths σ<sup>и</sup> increases (due to a smaller change in the tensile strength σ<sup>и</sup> as compared to the yield point σy). In the normative strength calculations [10], it is suggested not to take into account the areas of increase in the yield strength σyð Þ τ; t;е; σ; N due to aging, which goes to the margin of safety. In the refined basic and normative calculations of the strength of pipelines, one should take into

–10<sup>4</sup> h) with its subsequent reduction

http://dx.doi.org/10.5772/intechopen.75078

91

Probabilistic Analysis of Transportation Systems for Oil and Natural Gas

system of initial equations for the functional Fc are presented in [4, 8, 11, 13, 14].

and low-alloy steels is reduced to the following basic provisions (Figure 5):

<sup>σ</sup>тð Þ <sup>τ</sup>; <sup>t</sup> at the initial stages of exposure (up to 103

ture t<sup>0</sup> gives a smaller change of σyð Þ τ;e than the natural one.

decrease in the degree of hardening of tube steels in the plastic area.

strengths decreases.

(steel over ageing).

account [4–9, 14–16]:

Figure 5. Scheme of aging processes of pipe steels.


The normative approach has an important development element in comparison with [2, 13] in it, the strength and durability evaluation is carried out not only by nominal stresses but also by local deformations in the concentration zones created by structural, technological, and operational factors (welds, defects, corrosion). This makes the normative calculation of the strength of pipelines comply with both the modern deformation criteria [6, 7] and the norms in nuclear power engineering and rocket and space technology [9–11].
