فهرست مطالب نویسنده:
امین منشی زاده نایین
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چکیده- خطوط لوله مدفون به واسطه طول زیاد خود ناگزیر از عبور از گسل هایی هستند که حرکات بزرگ آن ها می تواند باعث بروز شکست و گسیختگی در لوله های مدفون شود. این حرکات بزرگ ممکن است در یک زمین لرزه رخ دهد و یا اینکه بر اثر حرکت خزنده گسل که در طول عمر بهره برداری خط لوله اتفاق می افتد، بوجود آید. بنابراین ضروری است رفتار لوله های مدفون در برابر حرکات گسل مورد مطالعه قرار گیرد. در ادبیات فنی موجود، تمرکز بیشتر بر مطالعه رفتار خطوط لوله در گذر از گسل های نرمال و امتداد لغز بوده است. در مطالعه حاضر، رفتار لوله های مدفون گذرنده از گسل معکوس با کمک نرم افزار آباکوس مطالعه شده است. ابتدا نحوه و روند شبیه سازی های انجام شده با نتایج آزمایشگاهی و عددی دیگر مقایسه شده است که نشان از صحت نتایج دارد. سپس مطالعه حساسیت بر روی تاثیر نوع خاک و پارامتر های ژئوتکنیکی آن و همچنین اثر نسبت عمق دفن به قطر لوله انجام گرفته است. مطالعات عددی انجام شده نشان می دهد که کرنش های فشاری علت اصلی وقوع گسیختگی لوله ها در گسلش معکوس هستند و استفاده از خاک های نرم و با تراکم کم و هم چنین کاهش عمق دفن لوله سبب کاهش مقادیر کرنش های فشاری و کششی ایجاد شده در لوله می شوند. این در حالی است که تغییر مدول الاستیسیته خاک تاثیر چندانی بر مقادیر کرنش های ایجاد شده در لوله ندارد. همچنین، افزایش زاویه اتساع خاک در جابه جایی های بزرگ گسل سبب افزایش مقادیر کرنش های ایجاد شده در لوله می شود.کلید واژگان: لوله مدفون, گسل معکوس, مدل عددی, نرم افزار ABAQUS, اندرکنش خاک و لولهPipelines are considered as lifelines, because they are used for transportation of different fluids such as natural gas, oil and water, which the human life depends on their existence. The damages to the pipelines are usually associated with human fatalities, financial losses and also environmental pollution. Earthquake wave propagation and permanent ground displacement (PGD) caused by surface faulting are potentially devastating natural events which threaten buried pipelines. Although small regions within the pipeline network are affected by faulting hazards, the rate of the damage is very high since fault movement imposes large deformation on pipelines. On the contrary, the whole of pipeline network is influenced by the wave propagation hazards, but the damage rates is lower which leads to lower pipe breaks and leaks per unit length of the pipe. On the other hand, buried pipelines due to their long length, have to pass through active faults which their large movements may lead to failure and rupture of the buried pipes. It is, therefore, essential to investigate the behavior of buried pipelines against fault displacements in order to mitigate the losses caused by these natural events and to try to keep them in service under various situations. Over the years, many researchers have attempted to analyze pipeline behavior via numerical, analytical an experimental modeling, but most of these works were designed to assess pipe response to strike-slip faulting and some were implemented to recognize the behavior of pipelines under normal faulting with right deformation angles. In the present study, In order to understand the behavior of the pipelines under reverse fault movements, the effects of different geotechnical and geometric conditions on the response of the pipes is examined. Numerical simulations have been conducted using the software ABAQUS based on finite element method. In most of the previous studies, a simplified beam-spring model was used to simulate the behavior of the pipes, but in this study a 3-D continuum model is employed to simulate the behavior of the buried pipes against reverse fault movements. In order to increase the accuracy of the analysis, it is tried to use the elements that best match with reality of the nature of soil and pipe behavior and the interaction between them. The results of the numerical study confirmed that the compressive strains in pipe caused by reverse faulting are larger than the tensile strains, thus compressive strains are considered as the main cause of the failure of the buried pipes in the reverse fault motions. Investigating the pipes behavior in different soil types demonstrated that the buried pipelines in loose and soft soils experience less amount of strain in comparison with those which are bureid in other types of the soils. This is due to the fact that the displacement of the pipeline in loose and soft soils is easier and there are less soil resistance forces against pipe displacement. The assessment of effect of soil dilatation angle illustrated that in large fault displacements, the amounts of pipe strain decline with the reduction of the dilation angle, while changing the modulus of elasticity of the soil has no impact on the response of the pipes. The results also showed that by reducing the burial depth, the level of strain induced in the buried pipes decreases.Keywords: Buried Pipe, Reverse Fault, Numerical Model, ABAQUS Software, Pipe, Soil, Interaction
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در مطالعات مهندسی مرسوم، برای شبیه سازی رفتار لوله در برابر حرکات گسل از الگوی عددی ساده شده تیر- فنر استفاده می شود. از طرف دیگر به دلیل سهولت شبیه سازی ، بیشتر شبیه سازی ها متمرکز بر روی گسل های امتدادلغز بوده است. در مطالعه حاضر، از نمونه اجزای محدود سه بعدی و در قالب محیط پیوسته جهت شبیه سازی رفتار لوله های مدفون در برابر حرکات گسل معکوس استفاده شده است. جهت انطباق هر چه بهتر شبیه سازی با ویژگی های رفتاری لوله و خاک از عناصر پوسته ای1 و عناصر حجمی2 به ترتیب برای شبیه سازی لوله و خاک استفاده شده است. همچنین با در نظر گرفتن الگوی رفتاری کشسان – خمیری برای لوله و خاک، رفتار غیرخطی مصالح آن ها شبیه سازی شده است. در این مقاله، ضمن نقد و بررسی روش مرسوم تیر- فنر، اثر نسبت قطر به ضخامت لوله، زاویه شیب گسل و زاویه اتساع خاک بر پاسخ لوله مورد مطالعه قرار گرفته است. نتایج نشان می دهد که روش تیر- فنر مرسوم تنها در جابه جایی های کوچک گسل پاسخ های منطقی می دهد. افزایش نسبت قطر به ضخامت لوله، کاهش زاویه شیب گسل و افزایش زاویه اتساع خاک سبب افزایش مقادیر کرنش های فشاری ایجادشده در لوله می شود. همچنین، نتایج نشان داد که مقادیر کرنش های ایجاد شده در لوله با الگوی تغییر شکل لوله رابطه دارد.کلید واژگان: لوله مدفون, گسل معکوس, روش عددی, نرم افزار ABAQUS, اندرکنش خاک و لولهIn common practices, the simplified beam-spring model is applied for modeling the pipe behavior against fault displacement. On the other hand, due to the ease of modeling, most simulations have been focused on strike-slip faults and very rare studies have paid attention to the simulation of pipes crossing reverse faults. In the present study, the behavior of the buried pipes subjected to reverse fault motions have been investigated by using three-dimensional continuum finite element modelings. The ABAQUS software has been utilized in the simulations. By this software, the analyses have been performed by using the explicit method. To provide better adaptation between simulation and the behavioral properties of pipe and soil, shell elements and solid elements have been used for the modeling of pipe and soil, respectively. The material non-linearities associated with pipe-material and soil is modeled by considering elasto-plastic behavioral model for soil and pipe. In addition, interface elements have been considered between the soil and the pipe elements. As for the first stage of numerical modeling, the numerical simulation procedure was validated by simulating a large-scale physical model of a pipe crossing a reverse fault. Comparison of the results (in terms of axial compression strains of the pipe) obtained from the simulations with those of the physical model indicates a good match. In the next stage, the behavior of a pipe with a reverse fault motion is investigated from two different approaches. To this aim, the current approach, i.e. three-dimensional continuum modeling was compared with conventional beam-spring model, and the results of the simulations are compared. The results show that the beam-spring model gives logical answers only for small amount of fault displacements while for large fault motions, the model cannot consider correctly the justified behavior of the pipe. The reason is because of the governing local buckling of the pipe at large fault displacement, which cannot be well considered in the beam-spring model. In other words, the beam-spring model can only take the global buckling into consideration; however, this approach is not suitable to study the pipe behavior for large fault displacement, and thus, the problem should be studied by considering the continuum body of the soil as well as the pipe body. In this study, the effect of the diameter to pipe thickness ratio was investigated by using the 3D simulations. The results show that as the diameter to thickness ratio is varied, the failure mechanism of the pipe is changed too. As the diameter/thickness ratio increases, a local buckling is generated at small level of fault displacement, and hence, the resistance of the pipe against the local buckling decreases. In addition, the pipe deformation pattern is different. For the thicker pipe, the pipe deforms in a longer distance around the fault; however, the thinner pipe is crushed at the location of the differential fault displacement. As the other parameter that is effective on the pipe deformation pattern is the soil dilatancy. The numerical modeling indicates that as the soil dilatancy increases, the axial strains of the pipe augments too. The increase in dilatancy from zero to 30 degrees causes a double increase in the pipe strain level. The effect of fault dip angle on the pipe deformation is also investigated numerically. To do this, two faults with different dip angles of 40 and 70 degrees were considered in the modelings. It was found that as the dip angle of the fault is smaller, the level of the axial compression strains increases too. The rate of increase
in the axial strain to the fault displacement is higher too. The deformation pattern of the pipe is investigated, which released that the pipe is much more deformed and damaged for smaller fault dip angle (40 degree). As a conclusion, it can be briefly deduced that: 1) in order to study the deformation of pipe crossing reverse faults, 3D numerical modeling approach are more justified than the simplified beam-spring approach; 2) To reduce the pipe damage, the fill around the pipe should be filled with fines-grained soils, which have low values of dilatancy; 3) As the dip angle of the fault increases, the pipe should be selected as to be thicker in order to prevent local buckling of the pipe.Keywords: Buried Pipe, Reverse Fault, Numerical Model, ABAQUS Software, Pipe-Soil Interaction
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