بررسي مسيله‌ي فرايند آبشستگي در اطراف خطوط لوله‌ي دريايي با در نظر گرفتن ورق زيرسري نفوذناپذير در زير لوله

Investigation of Local Scour Process around Marine

- در رابطه با بررسي ميزان آبشستگي در اطراف خطوط لوله‌ي دريايي تحقيقات زيادي صورت گرفته اما در نظر گرفتن صفحات زيرسري در زير لوله‌ي تفاوت عمده‌ي اين تحقيق با موارد مشابه قبلي است. نصب صفحات نفوذناپذير باعث مي‌شود تا خطوط جريان زيرزميني شكل گرفته در زير بستر، واقع در زير خطوط لوله، طولاني شده، در نتيجه گراديان فشار در عرض لوله كاهش يابد. كاهش گراديان فشار، يكي از عمده‌ترين و اساسي‌ترين عوامل شكل‌گيري و توسعه‌ي آبشستگي در زير خطوط لوله‌ي دريايي و شكل‌گيري پديده‌ي رگاب در بستر زير خطوط لوله است. اين پديده به‌عنوان يكي از نتايج حاصل از غلبه‌ي گراديان فشار بر وزن مستغرق ذرات رسوب بستر مطرح است. در پژوهش حاضر ابتدا اثر خطوط لوله با قطرهاي مختلف و اثر آرايش سايد باي سايد آن‌ها روي فرايند آبشستگي بررسي مي‌شود؛ سپس با نصب ورق زيرسري نفوذناپذير زير خطوط لوله، براي هر يك از آرايش‌هاي لوله‌هاي موازي، اثر اين صفحه روي پديده‌ي آبشستگي مطالعه مي‌گردد. نتايج آزمايشگاهي حاكي از آن است كه استفاده از ورق‌هاي زيرسري نفوذناپذير در زير خطوط لوله‌ي دريايي تا حدودي مانع شكل‌گيري تونل آبشستگي در زير خطوط لوله شده و آن‌ها را از خطرهاي ناشي از آبشستگي حفظ مي‌نمايد.

The main aspect of this research study is investigation on scour phenomenon around marine pipelines with respect to submerged impermeable plates beneath the pipe. So far, numerous investigations have been done but the main difference of this study with them is installation an impermeable base plate under the pipelines. Installing these impermeable plates mainly causes that the developed streamlines under the bed, located under the pipeline, are lengthened and accordingly the pressure gradient reduces across the pipe. Reduction of the pressure gradient may be considered an important issue since one of the most important criteria of establishing and progressing of scour under pipelines is the formation of piping phenomenon under the pipelines. This phenomenon is due to dominance of the pressure gradient on floating weight of the bed materials. For this research study, firstly scour phenomena under the pipelines were investigated both for unprotected pipelines and protected pipelines with submerged impermeable plates, with the piping phenomenon under the pipelines were being considered for both cases. The experiments were carried out in a channel with 10m length, 0.25m width and 0.5m depth. P.V.C. pipes with four diameters (i.e., 2cm, 3cm, 4cm and 5cm) and 0.5cm thickness were investigated. The bed materials were consisted of sediment particles with median size of 0.50 mm and geometric standard deviation of 1.43. The specific gravity of bed materials was 2.65. All experiments were run at clear water conditions. For each test the steady approach flow was adjusted so that the ratio of velocity to critical velocity was equal to about 0.85 on the centerline of the flume. A number of rectangular galvanized iron plates with 25cm in length, 0.7mm thickness and various dimensions of breadths were selected as countermeasure tools for scouring phenomenon. Then, the scour depth beneath two parallel pipelines with side by side arrangements and their interaction on scouring process were investigated. Finally, with installing impermeable base plates under the pipelines, the effect of these plates on scour phenomenon were studied for any arrangements of the parallel pipelines. The experimental results showed that installation a plate under the pipeline could prevent the formation of scour hole to some degrees and protect them from scour hazards. For all test cases of unprotected pipelines, the maximum dimensionless scour depth beneath the pipelines decreased when the pipe diameter increased. For side by side arrangement of two parallel pipelines, the formed scour depth decreased when the distance of pipelines increased. Then, for practical purposes, in order to reduce the maximum scour depth it is recommended that the distance of two parallel pipelines increases as far as possible or a submerged base plate may be used beneath the under pipeline. For all test cases of pipelines with impermeable base plates, the scour process will not further occur unless due to wake vortices.