Dynamic Analysis And Damage Detection Research Of Undersea Pipeline

Dynamic analysis of the undersea pipeline is studied theoretically and numerically. The damage types of the pipeline are summarized and various damage detection methods are studied. And magnetic flux leakage of the pipelines is simulated by using finite element method.Firstly, take the pipeline in Chengdao Area of Shengli Oil Field as example, the forces of the pipeline and the dynamic response of the pipeline is analyzed. Firstly, the vibration differential equation of the pipeline is written when the load is applied. The finite element method model of the suspended span of the pipeline is built. Then, the modal analysis of the model is studied. Next, the vibration of the pipeline due to the waves and the currents are analyzed, respectively. It is found that the stress at the winding pipe is the maximum for the vibration of the suspended span caused by the wave load. Forced by the cycle stress, the pipeline will have a lower fatigue lifetime. Considering the corrosion and the welding defect, the welding line at the end of the riser is the weakest position of fatigue damage and brittle fracture.Secondly, the damage types of the pipeline and the reasons of the pipeline damage is analyzed. The outer detection methods of the pipeline, the inner detection method of the pipeline and the corrosion detection methods are discussed. Regulations about the damage detection in the related codes are introduced. According to the reason, damages can be sorted into three categories, that is corrosion damage, mechanical damage and welding damage. Among all the detection methods, magnetism detection caterpillar can be used in various kinds of pipelines, for it does not need coupling medium between the sensor and the wall of the pipe. But it can only precisely detect a corrosion of over 20% of the wall thickness. Ultrasonic detection can have a higher sensitivity than the magnetism detection, it can detect a corrosion of over 10% of the wall thickness and an increment of 1% to 2% of the metal wastage, but the ultrasonic detection is more expensive. It is used only in the pipes that transfer liquids. Also, the regulation and detection method in the codes such as ASME, API and DNV is discussed, so as to give references to establish the damage detection regulation in the pipeline.Also, detection of magnetic flux leakage is analyzed numerically. The finite element method model was built using ANS YS software. The signal of magnetic flux leakage is calculated when damage occurs at the wall of the pipeline. And also the signals are analyzed and compared when damage occurs at different sizes. It is found that the peak value is approximately in proportion with the depth of the damage. The higher the depth, the signal increases. The peak value slightly decreases when the length of the damage enhance. The peak value of magnetic flux leakage evidently decreases when the width of the damage increases. And finally, the signal of magnetic flux leakage is analyzed when noises exist.