Frequency shift in tracking the damage of fixed offshore structures

This study investigates a method of detecting and tracking damage in an offshore platform located in the North Sea under the excitation of a typical sea storm by performing a finite element simulation. A modal analysis and nodal displacement results from Finite Element (FE) simulation are adopted for analysing the dynamic characteristics of this platform. For this purpose, the JONSWAP spectrum and Morison equation are applied to simulate the typical wave force in the North Sea using Matlab and Simulink programmes. The associated mode shape vector obtained in modal analysis was correlated with the sea force vector which was the forcing function (force per unit length) to the structure. The determination of the dynamic response with realization to the real scenario was established by introducing some reduction of material stiffness in one of the diagonal member in stages. The results in time-displacement domain obtained from FE simulation were converted into time-acceleration domain before they were analysed in frequency spectrum using Power Spectrum Density in Matlab. The frequency analysis showed that there was a significant shift in peak frequency between the intact and damaged structure response with 30% stiffness reduction, provided that the sea force applied is in the direction of the damaged beam member orientation. This suggested that the shift in the natural frequencies of the dynamic response can provide a useful tool in monitoring the dynamic response and tracking the progression of fatigue failure of the structure. © 2011 Asian Network for Scientific Information.