COMPUTER MODEL DEVELOPMENT OF STRESS-STRAIN STATE OF GAS-PIPELINE SECTION IN SOIL SETTLEMENT CONDITIONS

Афанасьев К.Ю.

Аспирант, Национальный исследовательский Томский политехнический университет

РАЗРАБОТКА КОМПЬЮТЕРНОЙ МОДЕЛИ НАПРЯЖЕННО-ДЕФОРМИРОВАННОГО СОСТОЯНИЯ УЧАСТКА МАГИСТРАЛЬНОГО ГАЗОПРОВОДА В УСЛОВИЯХ ОСАДКИ СЛАБОСВЯЗАННЫХ ГРУНТОВ

Аннотация

В статье приводится исследование напряженно-деформированного состояния участка магистрального газопровода в условиях осадки слабосвязанных грунтов с использованием программного продукта ANSYS.

Ключевые слова: напряженно-деформированное состояние, газопровод, осадка грунта, предел текучести.

Afanasyev K.Y.

Postgraduate student, National Research Tomsk Polytechnic University

COMPUTER MODEL DEVELOPMENT OF STRESS-STRAIN STATE OF GAS-PIPELINE SECTION IN SOIL SETTLEMENT CONDITIONS

Abstract

The article presents research of the stress-strain state of main gas pipeline section in soil settlement conditions using software ANSYS.

Keywords: stress-strain state, gas pipeline, sediment soil, yield limit.

Objective features of a pipeline network of Russia are the difficult natural conditions of operation having negative influence on gas pipelines functioning that increases risk of ecological and technical safety. Deviations of deflected mode can appear while in service gas pipelines as a result of action of loadings which have been not provided by the project. The essential change of deflected mode can be result of gas pipeline spatial position change [1, 2]. Soil settlement promotes it which arises because of consolidation under influence of ground weight and gas pipeline vibration and reflux of excessive moisture. It reduces to the big curvature of pipes, their overstrain and as a result - to damage of a gas pipeline in the form of ruptures of welded joints and gaps in pipe walls for which elimination carrying out of repair work with stop of gas transportation. Therewith each of damage can reduce to considerable losses of gas [3]. Bugs for this reason are most frequent on the gas pipelines laid in sedimentary soil [4]. Definition of the deflected mode of underground main gas pipelines from operational loadings and influences will be inevitable to develop using the pipe calculation and the soil basis [5].

The purpose of this work is analysis gas pipeline behavior in loosely coupled soil and its research its deflected mode.

The underground rectilinear gas pipeline is considered. Let‘s admit that it is laid in dry soil which won’t be watered in rated term of operation. In this case vertical moving (collapse) results from ground consolidation under a pipe. The design data show that it is insignificant because soil pressure defined by the pipe weight doesn't exceed 0,5 Н/см² and not necessities to consider its influence on a gas pipeline position in comparison with its initial position.

Let's admit that the gas pipeline is laid in the water-deposited soil or periodically inundated territories. In the water-deposited soil collapse is defined by soil consolidation under a pipe. From soil mechanics it is known that completely water-deposited soil can be considered as the two-phase system and its consolidation is defined by filter water from interstices of soil skeleton under the influence of condensing loading [4].

Limiting gas pipeline collapse is S_пр. This gas pipeline collapse is called as stabilized. On fig. 1 S_пр is shown by dashed line on pipeline length. If the sedimentary soil distributed on all gas pipeline length then the collapse would be identical on all length. However in practice most typical alternation of sedimentary soil and pan soil is. On the last as it was already marked the soil sedimentary is almost equal to zero. In the middle of sedimentary soils site the gas pipeline collapse could reach limiting value S_пр. The gas pipeline is bent on a site l as is shown at fig. 1. As lengthening of pipes is realized only by their stretching on a site l and sites adjoining to it l₁ and l₂ in pipes there is stretching longitudinal force Р, and a site l starts to work as a rigid thread. Valid gas pipeline collapse S appears essentially less S_lim [4]. In pipes there are pressure from longitudinal force, bending force under the influence of pressure of an overlying soil, weight of the pipe with isolation and the pumped over product, operating internal pressure and temperature difference of  the pipe walls.

Fig. 1 - A gas pipeline collapse: a — a general view; б — a scheme

For definition of strains in the pipe wall exceeding admissible and defining change range of numerical characteristics of processes, influencing deformation the strains is made calculations (on durability), affected the gas pipeline collapse by means of software ANSYS.

Fig. 2 - Settlement scheme of the gas pipeline

The scheme of lowering of a site of the gas pipeline with following parameters is investigated: a external diameter is 530 mm, a thickness of the pipe wall is 11 mm, length is 12 m, working pressure is 9,0 МPа. The gas pipeline is made of steel 17Г1С with following mechanical characteristics: a limit of short-time strength σ_в is 490 МPа, a limit of yielding of metal σ_т is 350 МPа [6]. On the gas pipeline except working pressure following loadings operate: the distributed loading from weight of the pipe with the isolation, pumped over gas and pressure of an overlying ground is q_общ, pressure of a ground from below on the gas pipeline is q_г.

Fig. 3 - The stresses arising in a gas pipeline in soil settlement conditions

  The loadings operating on a gas pipeline are calculated according to СНиП 2.05.06-85*"Main pipelines» [7].

Fig. 4 - The stress distribution (σ) on gas pipeline length (l)

The assumptions corresponding  the most simple of possible variants to interaction of the pipe and the soil contacting are taken: in the beginning and the gas pipeline end there are no moving on axis Х, pressure of a ground is taken  20 % from operating loading from above, temperature difference of pipe walls isn't considered.

Fig. 5 - The deformations in a gas pipeline in soil settlement conditions

According to assumptions and the scheme represented at fig. 1 the settlement scheme is presented at fig. 2.

At fig. 3 and 5 the result of calculation of the gas pipeline site found by software ANSYS is shown, and at fig.  4 and 6 given result interpretation in a graphic kind is shown.

Fig. 6 - The deformations distribution (S) on gas pipeline length (l)

The received results allow to draw following conclusions:

• change of values of the stresses arising in a gas pipeline in soil settlement conditions, can reach the sizes close to a limit of yielding of metal that reduces level of reliability of a gas pipeline;
• stress values are variable on gas pipeline length. The gas pipeline sites located in a zone of alternation of sedimentary soil and pan soil are characterized by higher level of stress;
• the received results don't give a full formulation for acceptance of the design decision, it is necessary detailed researches of deflected mode deformed of a gas pipeline taking into account physical-mechanical properties of soil.