طول فعال خطوط لوله ي انتقال گاز در برابر زمين‌ لغزش

Active Length of Natural Gas Pipeline Subjected to Landslide

در صورت رخداد زمين‌لغزش در محل عبور خط لوله‌ي انتقال گاز، تغييرشكل لوله و خاك اطراف آن اجتناب‌ناپذير است. طولي كه از خط لوله كه در اثر تغييرشكل‌هاي زمين جابه‌جا مي‌شود، طول فعال نام دارد؛ كه مجموع طول‌هاي مهاري به علاوه‌ي عرض زمين‌لغزش است. در پژوهش حاضر، طول مهاري خط لوله تحت اثر زمين‌لغزش و مقايسه‌ي آن با روابط آيين‌نامه‌يي مطالعه شده است. با استفاده از روش اجزاء محدود و نرم‌افزار A‌N‌S‌Y‌S و درنظر گرفتن زاويه‌ي قرارگيري خط لوله‌يي به طول 1500 متر عمود بر جهت رانش، طول مهاري براي 4 قطر مختلف لوله تحت فشارهاي داخلي متفاوت در دو نوع خاك بررسي شده است. نتايج نشان مي‌دهند طول مهاري براساس آيين‌نامه‌هاي طراحي، به دليل تخمين كم نيروي محوري ايجاد شده در مقطع لوله، 35٪ كمتر از طول مهاري حاصل از تحليل بوده و رابطه‌ي جايگزيني براي آن پيشنهاد شده است كه ميانگين خطاي آن نسبت به نتايج تحليل، 4٪ است.

Natural gas transmission pipelines have a vital role in human life and thus, they are classified as lifelines. Long-term disruption in the pipeline operation due to landslides would have substantial direct and indirect consequences. Landslides are generally accompanied by large movements of the ground and result in the deformation of the pipeline and the surrounding soil. In buried pipelines, a significant length of the pipeline moves relative to the soil during a landslide. This movement activates the friction between the soil and the pipeline. The location where the relative movement and the friction diminish to zero is called the virtual anchor point. The clear distance between this point and the landslide where the relative movement occurs is known as the anchor length. Active length of the pipeline is the distance between two virtual anchor points on either side of the landslide and it is the sum of the two anchor lengths and the width of the landslide. During a landslide, the pipe within this length would be highly stressed and it is prone to failure. The active length is considered as the protected length and to reduce the risk of pipe rupture, construction of bents, joints, and equipment should be avoided within this length. Thus, an accurate estimate of this length is an important consideration in the design process. In this study, finite element analyses are carried out to evaluate the anchor lengths of common gas transmission pipelines in Iran. The analyses are performed using Ansys software platform. The pipe is modeled by nonlinear pipe elements and the pipe-soil interaction is modeled by Winkler springs. Buried pipes with diameters of 12, 20, 30, and 48 inches under internal pressure of zero, 500, 1000 psi in two different soils are analyzed. The results of the analyses indicate that the anchor length increases with increasing pipe diameter. The soil properties also significantly affect the anchor length, but the internal pressure has only marginal effect on the anchor length. The relevant equation of the anchor length provided by the PRCI guidelines is about 35% less than the anchor length obtained from the analyses. A more accurate equation for the anchor length is proposed in this study.