Damage detection and localization method based on a frequency spectrum change in a scaled tripod model with strain rosettes

• For the correct shape of frequency spectrum at a point 3 FBG sensors in rosette configuration should be used instead of 1 sensor.
• The successful detection of the damage by all strain rosettes regardless of their location.
• The correct localization of damage based on RMSD estimator applied to selected new peaks which appeared after the damage.

A method for detection and localization of a damage in a model of an offshore support structure is addressed in this paper. The damage has been simulated as a dismantled flange located in one of the upper braces of a tripod model. The strain measurements have been obtained from a network of Fibre Bragg Grating (FBG) sensors bonded to the model. The root mean square deviation (RMSD) estimator applied to a chosen part of averaged and smoothed frequency spectrum of the strain data has been considered here as a damage indicator. A capability of the method has been checked for the freely hanging model as well as for the tripod fixed to a table. Experimental results have been supported by numerical modal analysis using Abaqus software. The results show that additional peaks have appeared in some frequency regions revealing vibration modes which are associated with damage in the structure. Applying the RMSD estimator to the regions where new peak has emerged it is possible to detect the damage by comparing the same part of an averaged frequency spectrum of the same rosette before and after the damage or by comparing the spectra of two rosettes for model with the damage without referencing to the healthy state. The good localization performance of the RMSD estimator for some new frequency peaks can be justified by analysing the corresponding localized numerical maximal in-plane principal strain modes. The determination of damaged structural member is understood as the correct damage localization.