A SIMPLIFIED MATHEMATICAL MODEL OF THE EFFECT OF GAZ HYDRATES ON INTRA-TUBE CORROSION

Збільшення власного видобутку нафти і газу – один із важливих етапів забезпечення енергетичної безпеки держави. У короткотерміновій перспективі основним ресурсом збільшення видобутку є розробка малодебітних родовищ. Природний газ містить від 60 до 98 % метану з домішками, такими як вода (H2O), двоокис вуглецю (CO2), сірководень (H2S) та кисень (O2). Ці домішки здатні до серйозної корозії трубопроводу, а також ініціюють зародження газових гідратів. Сформовані гідрати можуть спричинити часткове або повне закупорювання внутрішньої частини газопроводу, і якщо швидко її не видалити, то це призведе до зростання тиску всередині труби і до можливої позаштатної ситуації. Розглянуто прогнозуючу модель швидкості корозії газопроводу під впливом газових гідратів. Модель базується на термодинамічних властивостях рідини та газу, що входять у сформований газогідрат під впливом різних умов експлуатації. Модель показує, що швидкість корозії збільшується з температурою, експлуатуючим тиском перекачування та кислотністю. Різке підвищення швидкості корозії негативно впливає на роботу трубопроводу, знижуючи експлуатаційний ресурс трубопроводу. Збільшення робочого тиску підвищує температуру гідратоутворення, але водночас призводить до збільшення швидкості ерозійної корозії. Дослідження та прогнозування морфології корозійних ушкоджень є надзвичайно важливим для досягнення та збереження цілісності матеріалів та конструкцій, зокрема газопроводів.Увеличение собственной добычи нефти и газа – один из важных этапов обеспечения энергетической безопасности государства. В краткосрочной перспективе основным ресурсом увеличения добычи является разработка малодебитных месторождений. Природный газ содержит от 60 до 98 % метана с примесями, такими как вода (H2O), двуокись углерода (CO2), сероводород (H2S) и кислород (O2). Эти примеси способны к серьезной коррозии трубопровода, а также инициируют зарождение газовых гидратов. Сформированные гидраты могут привести к частичному или полному закупориванию внутренней части газопровода, и если быстро ее не удалить, то это приведет к росту давления внутри трубы и к возможной нештатной ситуации. Рассмотрена прогнозирующая модель скорости коррозии газопровода под влиянием газовых гидратов. Модель базируется на термодинамических свойствах жидкости и газа, входящих в сформированный газогидрат под влиянием различных условий эксплуатации. Модель показывает, что скорость коррозии увеличивается с температурой, эксплуатирующим давлением перекачки и кислотностью. Резкое повышение скорости коррозии негативно влияет на работу трубопровода, снижая эксплуатационный ресурс трубопровода. Увеличение рабочего давления повышает температуру гидратообразования, но одновременно приводит к увеличению скорости эрозионной коррозии. Исследование и прогнозирование морфологии коррозионных повреждений является чрезвычайно важным для достижения и сохранения целостности материалов и конструкций, в частности газопроводов.Increasing domestic oil and gas production is one of the important stages in ensuring the country's energy security. In the short-term perspective, the main resource for increasing production is the development of low-yielding deposits. Natural gas contains from 60 to 98 % methane with impurities, such as water (H2O), carbon dioxide (CO2), hydrogen sulfide (H2S) and oxygen (O2). These impurities are capable of serious corrosion of the pipeline, and initiate the origin of gas hydrates. The formed hydrates can cause partial or complete closure of the inner part of the gas pipeline, and if not quickly removed, it will lead to increased pressure inside the pipe and to a possible extraordinary situation. In this paper, the prediction model of the corrosion rate of a gas pipeline under the influence of gas hydrates is considered. The model is based on the thermodynamic properties of liquid and gas, which are part of the formed gas hydrate under the influence of different operating conditions. The model shows that the rate of corrosion increases with the temperature, operating pressure of pumping and acidity. The sharp increase in the speed of corrosion negatively affects the operation of the pipeline, reducing the operational life of the pipeline. Increasing the working pressure raises the temperature of hydration, but at the same time leads to an increase in the rate of erosion corrosion. Research and forecasting of morphology of corrosion damage is extremely important for the achievement and preservation of the integrity of materials and structures, in particular gas pipelines. The proposed model enables prediction of corrosion processes on gas pipelines considering hydrate formation. The influence of pressure on the speed of corrosion processes is estimated and it is shown that under the most unfavorable conditions the rate of corrosion under the action of gas hydrates can reach 1,2 mm/yr (0,0033 mm/day. With such an indicator, the pipeline suffers significant damage in the absence of immediate repair measures. The sharp increase in the speed of corrosion negatively affects the operation of the pipeline, reducing the operational life of the pipeline. Increasing the working pressure raises the temperature of hydration, but at the same time leads to an increase in the rate of erosion corrosion. Also shown is the influence of the pH of the medium in which the gas hydrate is formed, according to the results of the prediction model, the rate of corrosion increases with the level of acidification of the medium

КОРРОЗИЯ ДЛИТЕЛЬНО ЭКПЛУАТИРУЕМЫХ ТРУБНИХ СТАЛЕЙ В СРЕДАХ ХЛОРИДНО ТИПА

Значна частина розгалуженої мережі магістральних трубопроводів України перебуває в експлуатації більше 30 років. Зі збільшенням термінів їх експлуатації все актуальнішою стає проблема ефективної та безперервної роботи трубопровідного транспорту, яка забезпечується організацією періодичної технічної діагностики стану елементів трубопроводів та ремонту в місцях виявлених недопустимих дефектів. На пізній стадії експлуатації нафтогазопроводів особливо актуальною науково-технічною проблемою стає належне забезпечення технічної надійності та безпечної експлуатації трубопроводів із застосуванням методів технічного діагностування, особливо, корозійних дефектів, а також розроблення ефективних методів оцінки працездатності експлуатованого матеріалу. Експериментальні дослідження виконували з використанням методів тензометрії та потенціометрії за розробленою методикою на базі раніше створеної в ІФНТУНГ автоматизованої випробувальної системи з ЕОМ, що дає змогу отримати високу точність та достовірність результатів експерименту. Досліджено вплив терміну напрацювання на деформаційну поведінку трубної сталі у ґрунтах хлоридного типу. Встановлено, що приріст деформації становить до 30 %. Визначено найнебезпечніші з корозійної точки зору хлоридні середовища – МС2 та МС3. Вивчено вплив величини механічних напружень та хімічного складу середовища на корозійну деградацію тривало експлуатованих трубопроводів у ґрунтах хлоридного типу.Most of oil and gas main pipelines in Ukraine has been in operation for more than 30 years. With the increase in the terms of their operation, the problem of efficient and continuous operation of pipeline transport becomes more urgent, which is ensured by the organization of periodical technical diagnostics of the state of pipeline elements and repair in places of detected inadmissible defects. At the later stage of exploitation of oil and gas pipelines the proper technical and technical problem is to ensure the technical reliability and safe operation of pipelines with the use of methods of technical diagnostics, especially corrosion defects, as well as the development of effective methods for assessing the working capacity of the exploited material. Experimental studies were carried out using strain gauge and potentiometry methods based on the previously developed computer-aided testing system with computers on the IFNTUNG, which allows obtaining high accuracy and reliability of the results of the experiment. The object of our researches were selected major gas pipelines of large diameter made of steel 17GS, which were in operation for 41 years. It is precisely this material of pipes that was widely used in the construction of main pipelines in the 70 s of the last century and is being exploited at present. The nature of the development of deformation processes in MS1-MS3 compared with air does not change significantly. We fix growth of deformation increments with increase of applied stresses. Unlike air, in the studied media there are no cyclical acceleration-decay deformations, which can be explained by the effect of the Rebinder effect.The angles of inclination of the final sections are increasing, which indicates a greater predicted duration of development of deformation processes under conditions of influence of the operating environment.If we compare the deformation behavior of the exploited and non-extruded material, it is easy to see the preservation of the air-creep tendency of the absolute magnitude of the creep deformation. At the same time, in all model environments, unlike the air, with the naked eye we fix the tendency to increase the absolute increase in the creep deformation and with the increase in the level of nominal stresses.The influence of the term of work on the deformation behavior of tubular steel in soils of chloride type is investigated. It has been established that the deformation gain is up to 30 %. The most dangerous from the corrosive point of view chloride environments – MS2 and MC3. Even for the smallest stresses studied, the indicators of the slope gain are in the range of 28-41 %, which indicates an increased sensitivity to the prolonged use of operating environments, and requires taking precautionary measures to ensure the operability of pipelines in such conditions.Значительная часть разветвленной сети магистральных трубопроводов Украины находится в эксплуатации более 30 лет. С увеличением срока их эксплуатации все актуальнее становится проблема эффективной и непрерывной работы трубопроводного транспорта, которая обеспечивается организацией периодической технической диагностики состояния элементов трубопроводов и ремонта в местах выявленных недопустимых дефектов. На поздней стадии эксплуатации нефтегазопроводов особенно актуальной научно-технической проблемой становится надлежащее обеспечение технической надежности и безопасной эксплуатации трубопроводов с применением методов технического диагностирования, особенно, коррозионных дефектов, а также разработка эффективных методов оценки работоспособности эксплуатируемого материала. Экспериментальные исследования выполняли с использованием методов тензометрии и потенциометрии по разработанной методике на созданной в ИФНТУНГ автоматизированной испытательной системы с ЭВМ, что позволяет получить высокую точность и достоверность результатов эксперимента. Исследовано влияние срока наработки на деформационную поведение трубной стали в почвах хлоридного типа. Установлено, что прирост деформации составляет до 30 %. Определены самые опасные с коррозионной точки зрения хлоридные среды – МС2 и МС3. Изучено влияние величины механических напряжений и химического состава среды на коррозионную деградацию длительно эксплуатируемых трубопроводов в ґрунтах хлоридного типа

Influence of loads in the process of laying on the resource of sea pipelines

Purpose: In the process of laying on the bottom of the sea material of the pipeline undergoes single-cycle alternating load. The purpose of the work is to determine the effect of pre-operational loads on the resource of marine pipelines. Design/methodology/approach: The influence of the method of construction of pipelines on their stress-strain state is analysed. According to the real modes of packing of sea pipelines, the loading regime is programmed and the laboratory modelling of the pipelaying process by the S-method has been programmed. Findings: According to the results of one-cycle shift load were obtained characteristics of the hysteresis loop. It is proposed to simplify the mathematical description of the hysteresis loop of the pipeline laying cycle in the given form. It was shown that the preload during the construction process negatively affects the durability of the pipeline material due to the exhaustion of its plasticity resource, reducing it to 70%. Research limitations/implications: In the future, investigations into the effect of overloading and overloading during the repair of pipeline sections on their durability and on the safe exploitation of resources should be continued. Practical implications: The developed method of estimation of influence of preoperational loads in the process of pipeline laying on its safe exploitation resource is used in gas-extraction enterprises. Originality/value: To forecast the deformation behaviour of the pipeline material in the laying cycle, it is efficient to use diagrams of a sign-changing single-cycle bend, which were built considering the creep. The fatigue life capability of a steel pipeline depends on the history of the pipeline load in the laying cycle. Ratio σ*0.2c / σ* 0.2t and εyc / ε yt can use as power and deformation criteria for evaluating Bauschinger effect. It is suggested that fatigue damage is determined by the width of the hysteresis loop

Corrosion-mechanical behavior of gas main steel in saline soils

The study of the deformation behavior of the pipeline material in conditions of loads and influences simulating the operation allows better estimation of the residual life of the pipelines and more accurate forecasting of the operating costs. The kinetics of deformation in 6 model environments simulating soil electrolytes was studied. Corrosion-mechanical tests were performed and the impact of the soil electrolyte chemical composition and the applied mechanical stresses on the corrosion behavior of the pipeline steel was studied. Significant increase in the corrosion activity of the soil electrolyte due to the synergistic effect of the corrosion-active components was found in the specified ratios of the ionic molar concentrations of sulphate and chloride ions (0.05 M: 0.1 M)

Estimation of temporary decommissioning impact on reliability and durability of oil pipelines

Purpose: Due to changes in the volume of oil transportation, part of the pipelines must be temporarily decommissioned. The purpose of the work is theoretical and experimental study of pressure fluctuations during operation of oil pipelines and their impact on the durability of the pipeline material in work state and after temporary decommissioning (conservation). Design/methodology/approach: The results of oil pressure fluctuation research have given a chance to choose the terms of experimental research providing of the pipe metal mechanical features changes research during oil pipeline exploitation. Findings: Fatigue test modes are selected based on the calculations of the mathematical model developed. Experimental studies of the dependence of the fatigue strength of the pipe material on the conditions of operation have been carried out, which made it possible to determine the parameters of the fatigue curve of the samples. Has been defined that fatigue strength for the new metal pipe samples is more for 20-25% than for the metal samples which had the contacts with lime milk and for 30-40% more than for metal samples which were under exploitation. Research limitations/implications: In the future, more combination of "pipe material - preservation medium" should be explored to establish pain of general regularities. Practical implications: The probabilistic curves of the pipeline non-destruction are constructed, which will be used for practical calculations of the reliability and durability of the 13G1S-U steel pipelines. Originality/value: Mathematical model is made that describes non-stationary oil moves in oil pipeline, that has been caused by jump like changes of oil supply in oil pipeline, on this basis was defined, that oil pressure in oil pipeline provides within non stationary process in a range of 0.4-0.6 Hz frequency and amplitude fluctuation of pressure is 0.1-0.5 MPa

Corrosion-fatigue failure of tractor trailers metal materials in aggressive environments

The processes of corrosion-fatigue failure of materials in contact with mineral fertilizers are insufficiently studied. As a result of joint influence of atmospheric corrosion and mechanical loads, about 70 to 80 % of machine parts get out of order, 20 to 25 % of which are failures caused by operating overload due to the strength loss because of atmospheric corrosion. A large part of metal structures of agricultural vehicles used to transport mineral fertilizers is under the direct influence of aggressive environments and dynamic loads that occur during the motion by field roads. Saturated solutions of the most aggressive working environments used in agricultural production, in particular ammonium sulphate and nitrophosphate are investigated to reduce fatigue resistance of ordinary steels groups – St3 and St5 and quality steels – 10 Steel, 15 Steel, 20 Steel, 25 Steel when loaded at all levels. The fatigue endurance limit decreases in comparison with air up to 2.02 times in a solution of ammonium sulphate, and to 2.32 times in a solution of nitrophosphate. In organic fertilizer environments, compared to distilled water, the conditional fatigue endurance limit increased to 9 %. The properties of the given materials as an inhibitor of corrosion-fatigue failure were discovered and proved

Numerical simulation of the stress state of an erosion-worn tee of the main gas pipeline

Purpose: To investigate the strength of tees with regard to their erosion wear, it is necessary to consider the complex three-dimensional geometric shape of the erosion worn inner surface of the tee. In addition, the study of the strength of the erosion worn tees of the main gas pipelines is complicated by the occurrence of additional stresses caused by changes in the direction of movement of the gas stream, resulting in an uneven pressure distribution in the inner cavity of the tee, and the temperature difference in its walls. Design/methodology/approach: Methodology for complex numerical three-dimensional simulation of the stressed state of tees of the main gas pipelines, taking into account the gas-dynamic processes that occur in the places of these defects, erosion wear of the tee wall, temperature difference in the tee walls. Findings: The acceptable parameters of erosion defects of tees of gas pipelines, and residual life of tees with erosion defects of the wall should be determined. Research limitations/implications: The developed model does not take into account internal corrosion and corrosion products as an additional erosion factor. Further studies plan to develop a model of corrosion-erosion wear of pipeline elements. Practical implications: The developed technique allows determining the location of erosion defects, estimating the strength and determining the residual life of tees with erosion wear of the wall in order to ensure their reliability, to rank such defects according to the degree of danger, to determine which of them are critical and need an immediate repair. Originality/value: Based on the gas-dynamic processes occurring in the internal cavity of the main gas pipelines’ tees, the complex three-dimensional geometric form of wall erosion defects, and temperature difference, the technique of three-dimensional simulation of stress state of the main gas pipelines’ tees is develope

Simulation of thermomechanical processes in disc brakes of wheeled vehicles

Purpose: Ensuring the required operational reliability of disc brakes by forecasting their technical condition taking into account thermomechanical processes. Design/methodology/approach: Differential equations of rotation of a rigid body around a fixed axis are solved, it is established that the equations of motion and the equations of thermal conductivity are indirectly related. The use of these analytical dependences provides a better understanding of thermomechanical transients. Findings: The solution is obtained on the basis of the differential equation of thermal conductivity of the hyperbolic type, which does not allow an infinite velocity of propagation of temperature perturbations in contrast to the differential equation of thermal conductivity of the parabolic Fourier type. The obtained analytical dependences provide a better understanding of thermomechanical transients and develop a theoretical basis for determining stresses and heat fluxes in solving problems of reliability and durability of disc brakes. Research limitations/implications: The work uses generally accepted assumptions and limitations for thermomechanical calculations. Practical implications: It is shown, that transients in a mechanical system - a brake disk at impulse loadings cause emergence of thermal effects which arise under the influence of external loadings. Originality/value: The application of these analytical dependences provides a better understanding of thermomechanical transients and develops a theoretical basis for solving problems of reliability and durability of disc brakes