مدل ‌سازي آزمايشگاهي كمانش موضعي خطوط لوله‌ي دريايي داراي خوردگي تحت بارگذاري محوري متناوب

EXPERIMENTAL MODELING OF LOCAL BUCKLING OF CORRODED OFFSHORE PIPELINES UNDER AXIAL CYCLIC LOADINGS

در طول عمر خطوط انتقال نفت و گاز فراساحلي، به علل مختلف، امكان قطع و وصل سيستم انتقال نفت و گاز وجود دارد. اين قطع و وصل در خط باعث ايجاد كشش و فشار مي ‌شود، كه مي توانند به چين خوردگي، كمانش غيرخطي موضعي، يا خرابي خميري پيش‌رونده منجر شوند. از طرف ديگر، , مكانيزم ‌هاي ديگر خرابي مثلاً خوردگي رخ مي ‌دهد كه مي ‌تواند منجر به كاهش مقاومت و در نهايت خرابي در لوله شود. در اين تحقيق با استفاده از مدل ‌سازي آزمايشگاهي، پديده‌ي خرابي پيش ‌رونده در خطوط لوله‌ي خورده ‌شده‌ي دريايي تحت بارگذاري متناوب محوري مورد بررسي قرار گرفته است. نتايج به‌دست ‌آمده حاكي از آن است كه در نمونه ‌هاي داراي خوردگي امكان بروز خرابي پيش ‌رونده به ميزان زيادي تشديد مي شود.

Repetition of start-up/shutdown and temperature changes in an offshore pipeline will generate cycles of compression/relaxation in the line. This might lead to wrinkling and local or plastic buckling in the line. On the other hand, offshore pipelines are commonly exposed to corrosion attacks from sea water and chemically corrosive fluids inside the pipe. In the current study, the effect of corrosion defects on the tubes ratcheting response under cyclic axial loadings is investigated. The experimental modeling was aimed at providing comparisons between the behavior of intact and corroded tubes under small amplitude wrinkles and subsequent persistent cycles of axial stress ranges. The specimens were all first subjected to monotonic axial compressions to attain an initial pre-defined non-linear axial strain. The subsequent cyclic axial load was introduced to the specimen in a load control mode. The mean and amplitude values for the cyclic stress remained constant in each test but varied with different tests. The physical shape caused by the metal loss in a corroded tube is irregular in depth and in surface. Results obtained from specimens IC1(?) and IC2(?) indicated that the increase in the cyclic stress amplitude had a major effect on the ratcheting response. Ratcheting strain responses suggested that the shape of the initial non-linear path, the constant ratcheting rate (or the slope of the linear part of the response and the number of cycles to the failure) were substantially affected by variations in the amplitude of the cyclic stress. It was observed that surface imperfections had more detrimental effects on the axial ratcheting response of the defected tubes, as compared to the corresponding monotonic responses. It was noticed that the ratcheting problem, or the possibility of progressive plastic failure, was substantially exacerbated by the presence of corrosion defects.