Study On Vibration Diagnosis Of Crack In The Jacket Platform

Offshore platform structures serve in the vile ocean environment in a long term, and they undergo the interaction of various loads, such as wind loads, wave loads, currents, and ice loads; Sometimes they also suffer from earthquakes, typhoons, tsunamis, ship collisions and other accidents; The structure itself also suffers from environment corrosion, halobios accreting, seabed eroding and so on. Sometimes, the offshore platform structure made up of pipe may crack under dynamic loads conditions. If the structural cracks can not be found out ahead, it will expand and cause structural damage to the entire platform, resulting in fatal marine accidents. Therefore, it is important to discover the existence of the microcrack in time before the marine accidents occur.This paper adopt the vibration diagnosis method to diagnose the platform cracks. Major tasks include three aspects:(l)Taking a all-around platform which has four legs as example, founding the finite element model, discussing the influence of different positions and different sizes of cracks on platform's vibration mode. The numerical calculation shows that the natural frequency do not change significantly when the cracks are small. In this case, it is difficult to diagnose the damnification of platform components by the changes of the frequency before or after the damnification. However, the model shape changes obviously before or after the damnification.(2) In order to find a better index for platform crack diagnosis, using beam as model, discussing the sensitivity of various indexs to cracks. Based on the study of various structural diagnostic indexs, we bring forward a new index for diagnosis-vibrational mode margin curvature. Study has shown that this index is more sensitive than other diagnostic index.(3) Adopting the index put forward in this paper to diagnose components of the platform. Taking the leg pile and horizontal bar of the platform as example to verify the index can also be applied to crack diagnosis of the pipe on platform.