Stress state calculation and potential failure risk identification of LNG storage tank inlet pipeline

ObjectiveThe inlet pipeline, a critical auxiliary component of the LNG storage tank, experiences significant variations in pressure and temperature during operation, with a complex stress state. Identifying potential failure risks in the pipeline is essential for ensuring the safe operation of the LNG storage tank. MethodsTaking the inlet pipeline of a LNG terminal storage tank as an example, a numerical calculation model was developed for the temperature field of the pipeline. Real operating parameters were utilized as inputs to calculate temperature variations under precooling and liquid inlet conditions, resulting in the temperature distribution across the circumferential and wall thickness directions of the pipeline at the top of the LNG storage tank. 304/304L stainless steel was chosen for the LNG storage tank inlet pipeline, and mechanical property tests were conducted to assess the tensile and fatigue characteristics of the pipeline at both normal and ultra-low temperatures. Based on these test results, a stress calculation model for the inlet pipeline was established, using the temperature field simulation results as boundary load conditions to assess the stress state under various operating conditions. ResultsDuring precooling, the inlet pipeline exhibited a non-uniform circumferential temperature distribution, with a temperature difference of 40 ℃ between the top and bottom of the pipeline, and 23 ℃ between the inner and outer walls. At the onset of liquid inlet, the circumferential temperature became evenly distributed, while the inner wall temperature rapidly dropped to −160 ℃, resulting in a temperature difference of over 50 ℃ between the inner and outer walls. Under ultra-low temperatures, the pipeline’s yield strength and tensile strength reached 454 MPa and 1,463 MPa, respectively. At a strain of 0.8％, the number of cycles was fewer than 4,500 before failure, falling short of the expected cycles for liquid inlet within the LNG storage tank’s design life. Under normal service conditions, the structure of the inlet pipeline may change due to internal pressure and temperature, but the resulting stress is minimal. However, if internal leakage alongside high cold insulation temperatures occur in the tank top valve, combined with circumferential temperature unevenness caused by non-full flow operation, axial and circumferential stresses may exceed the material’s yield strength at the weld points with unequal wall thickness on the inner surface of the pipeline’s bottom, increasing the risk of crack failure. ConclusionThe significant temperature difference between the interior and exterior of the LNG storage tank inlet pipeline during operation is a critical factor influencing the pipeline’s stress. Future efforts should focus on optimizing operational processes to reduce this temperature difference, assess safety risks, and ensure stable and reliable pipeline operation.