Test results are given for specimens of steel 20 cut from industrial pipelines that had been operated for 16 or 24 years at a deposit in West Siberia subject to the action of a medium containing hydrogen sulfide. It is shown that there is a change in the degradation coefficients characterizing the cracking resistance distribution in the tube wall thickness.During prolonged use of pipelines, their mechanical characteristics may alter considerably because of fatigue in the metal caused by hydrogen sulfide attack. It is therefore important to evaluate the residual working life of industrial welded pipelines on the basis of the degradation in the tube steel on prolonged use (over 20 years), for which special experiments have been performed.We used specimens cut from industrial oil pipelines that had been operated for 16 or 24 years at the Samotlor deposit in West Siberia. We used specimens cut from the lower part of the pipelines, which are most damaged by corrosion and erosion, after the pipelines had been dismantled for industrial reasons.The tubes were made of steel 20 having the following % composition: 0.2 C; 0.3 Si; 0.55 Mn; 0.04 S; 0.04 P; 0.15 Cr; 0.1 Ni; 0.1 Cu.Flat specimens with working parts of size 35 × 4 × 1.5 mm were made from various layers in the pipe wall. To determine the degree of degradation, the specimens were tested in tension with a low strain rate of ε = 10 -5 sec -1 with a UMÉ-10 apparatus in order to determine the mechanical characteristics σ 0.2 and ψ, which are most sensitive to the embrittlement. The cracking resistance was also determined from the metal construction parameter R mc used in the mechanics of failure, namely resistance to microcleaveage, which is dependent on the structural state, in particular the grain size and thicknesses of the plates of cementite inclusions, and is not dependent on external factors, in contrast to other integral criteria for the mechanical properties.The metallography was performed with a scanning electron microscope (SEM) type JSM-35CF (Japan) fitted with an Ortec (USA) microprobe unit. The nonmetallic inclusions and the finely divided particles of secondary phase were examined with a Quantimet-720 TV microscope (Britain) and the JSM-35CF SEM. The residual contents and distributions of hydrogen and sulfur were determined by local mass spectral analysis with a laser microprobe, while the microhardness was determined with a PMT-3 tester in accordance with GOST 9450-76.
The main reason for pipeline weld failure is the formation of cold cracks in the heat-affected zone on account of the increased tendency to brittle failure. The cooling time after welding affects the tendency to cold cracking, and the critical cooling time is related to a parameter characterizing the cracking resistance for various steels. Basic principles are presented for welding low-alloy tube steels at low temperatures.The exploitation of new oil and gas deposits makes it necessary to construct industrial pipelines in northern regions of West Siberia, where building work can be done preferentially in the winter at low air temperatures (down to -50°C). A basic step is installation by welding, which largely determines the reliability, and in actual pipelines, cracking is most often due to welded joints.A survey of the reasons for welded joint failure in pipelines indicates that the main one is the formation of cold cracks in the heat-affected zone (HAZ) because of the elevated tendency to brittle failure.The cracking probability increases on welding at low air temperatures, which is due to the presence of quenched structures and extensive hydrogenation of the welded joint.Working reliability is largely determined by the proper choice of materials in combination with the best conditions for installation welding.There are no scientifically based criteria for choosing welding modes for negative temperatures, which hinders the definition of the best technology and often leads to unjustified complication and considerable expense on account of additional measures (heating before and after welding, thermal insulation of the installed components, and so on).Published data [1-5] and our studies show that one should use the parameter σ pmin characterizing the cracking resistance of steel in order to evaluate modes of welding and to define sound conditions for preventing coal cracks. For industrial pipelines of diameter 114-512 mm with wall thickness δ up to 16 mm, the critical value of σ pmin at low temperatures (down to -60°C) is 360-400 MPa [5,6].We have found that one should choose the conditions for heating installation joints in pipelines to eliminate cracking in the weld in the winter on the basis of the cooling time for the weld metal from 300 to 100°C, namely t 100 300 (Fig. 1), since as the cooling time for the metal in that temperature range increases, so does the release of hydrogen into the sur-
Analysis of paraffin-waxing of oil field equipment at West Siberian oil fields has shown that the number of wells troubled by wax deposits has been going up every year. Wax deposition causes fall in oil recovery and entails additional cost for deposit removing.Many researchers have studied the distinctive features of the process of wax deposit formation from model solutions [1][2][3]. Analysis of the obtained data has revealed that they are highly inconsistent even for such simplified systems as artificial mixtures of wax and solvent. That is why, to prevent resin-wax deposits (RWD), it is essential to study their formation process in real oils. Until now, this process has been poorly studied because of wide diversity of properties and compositions of oils.In view of development of new oil fields characterized by heightened bed temperature and substantially increased wax content as well as of oil fields located in regions having permafrost soils, there is a pressing need for investigating the composition and properties of deposits formed in a wide range of temperature gradients.The reagents under development generally act selectively on the oil or a group of oils distinguished by a definite ratio of waxes, resins, and asphaltenes. It was therefore necessary to conduct further research into the composition and properties of the formed resin-wax deposits as a function of the difference in the temperatures of the oil and the metal rod being cooled ∆t that reached 60°C. It covers the temperature variation range in which wax deposition on well equipment occurs.In the experiments, we used in-situ oils with diverse component compositions, which were collected from four clusters of wells located in various corrosion-active areas of the Samotlor field (Table 1). An aliquot of the oil was heated to 70°C and held at this temperature for 30 min. Next, the vessel containing the oil was placed in a water container and a cylindric steel rod filled with ice was dipped into the oil. The oil temperature was monitored by a thermocouple. The RWD formation process continued for 1.5-2 h, after which the deposits formed were extracted and submitted to thorough analysis.The deposit composition was studied by using well-known procedures [4], which made possible identification of waxes, resins, and asphaltenes. The composition of the identified waxes was determined by gas-liquid chromatography, the surface activity of the resins separated from the deposits was determined from the acid number (GOST 11362-76), and the carbon dioxide content in their pyrolysis products was determined by a procedure described in [1]. The properties of the RWD were determined by using conventional procedures described in [5,6].As evident from Fig. 1a-c, the mass fraction of the deposits from the well clusters III and IV is 2.0-6.2 mass %. The variation in the mass fraction of the deposits with temperature gradient has a maximum in the temperature region where the oil is saturated with wax. This maximum is assignable to crystallization of the wax at a temperatu...
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