مطالعه عددی اثر گسلش معکوس بر پاسخ لوله‌های مدفون در خاک

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.

Numerical Investigation of Reverse Faulting Effect on the Response of Buried Pipes

Pipelines 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 strikeslip 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 beamspring model was used to simulate the behavior of the pipes, but in this study a 3D 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.