Active Damage Detection Of LNG Tank Under Static And Dynamic Loads Using Piezoelectric Sensors

Liquefied natural gas(LNG)is very pressurized and cryogenic gas used in whole over the world for different purposes.Due to easy transportation,the natural gas is compressed at-163 0 C /-265 F.Among the tank types Independent tank is used mostly for transporting LNG via both ship and road.Type C independent tank is very popular to carry LNG due because it is very simple to construct and low cost.However,due to high pressure and having no secondary barrier,damage or leakage can occur on the tank body.If it happens,the environment and livings will be in danger as there are many accidental histories.So the tank should be monitored to escape from this accidents.In this thesis work,a horizontal pressure vessel has been assumed as LNG type C independent tank.Firstly,the mechanical behavior of the LNG type C tank was analyzed under different static pressure stress by ANSIS software in order to identify the damage location.Considering eight internal stress conditions,the stress cloud map and deformation distribution under different conditions were analyzed.The results show that the tensile stress and displacement of the tank gradually increase with the increases of internal static stress.The maximum stress and displacement occurred in the middle of the tank.Under a certain pressure,the middle part of the tank will be damaged or even failed.Secondly,the dynamic performance of the LNG type C tank was analyzed under different earthquake motions considering different site conditions because of the different frequency properties of different site soils.The result shows that site IV gives more deformations comparing to site I,while shows large acceleration than that under site I.Finally,the active monitoring technology based on PZT piezoelectric sensor was used to monitor and analyze the damage status of LNG tank.Based on PZT sensor and actuator,the active monitoring system was established by using the graphic software LABVIEW.Based on the wavelet transform,the energy of the signals under different damage was analyzed,and the damage index used to characterize the damage condition of the structures.The results show that when the tank has no crack,the signal energy is large and the damage index is close to zero,which shows the structure is in healthy condition.With the increase of cracks in the tank,the signal energy is weaker and weaker,and the damage index increases gradually,indicating that the structural damage is getting larger and larger.However,when the tank crack is very large,the signal energy is weak,the damage index nearly approaches one,and the tank is in a state of serious damage.The test results show it is effective to detect the damage of LNG tank using the PZT-based active healthy monitoring.