Damage Identification For Offshore Platform Underwater Members Based On Genetic Algorithm

Offshore platform are expensive and complex structures, structure damage probably cause loss of life and property and pollution. Identification of damage can ensure safely operation of them.Main methods of damage identification now are processes from responses to global modal parameters, then from change of modal parameters to structure damage. They are simple to operate, but not easy to get the correct damage position and degree since limitations such as noise of measurement, numbers and position of sensors , difficulties of exitation.This paper attempts to estimate damage's locations and degree through modal parameters changes of a single structure member, as an upplement to the global damage evaluation method described above.Weld craze is the damage of offshore platform underwater members discussed in this paper. Changes of the member are manifest as two forms: rigidity reduced at supports and water fulfilling into pipe. The main work has been done include:Firstly, Timoshenko theory is selected as the dynamics model of the beam. According to the differential equations of lateral bending free vibration of Timoshenko beam, the frequency equations of the beams are derived with elastic rotation constraints at supports. The natural frequencies of beams are obtained through the given boundary conditions. Comparison on frequency relationship between Euler and Timoshenko beam has been done. Numeric examples were given.Secondly, damage situation of underwater member has been analyzed, using Genetic Algorithm method, considering two aspects of debasement of rigidity at supports and water fulfilling into the pipe. Results from different damage situation and different modal test data are compared and summarized. Finally, based on a vibration signal acquisition and analysis system, assembling method of this paper as a module, a structure health monitoring system is developed.