Experimental study of strain prediction on wave induced structures using modal decomposition and quasi static Ritz vectors

Abstract

Offshore structures are continuously subjected to dynamic loading from wind and waves which makes fatigue an important parameter for the structures expected lifetime. Monitoring the vibrations of the structure using real time operating data enables an assessment of the general health state of the structure.This paper proposes a method for an accurate full-field prediction of the strain history. Experimental mode shapes are found by the use of operational modal analysis and expanded to strain modes using a well correlated finite element model. The measured response from the structure is divided into two parts using complementary filters: Low frequency response caused by the quasi-static effect of the waves acting on the structure, and the high frequency response given by the modal properties of the structure. The high frequency response is then decomposed into modal coordinates using the experimental mode shapes. Strain histories are predicted by multiplying the modal coordinates with the expanded strain mode shapes. The low frequency response is decomposed using Ritz-vectors corresponding to the shapes that the structure vibrates with due to the wave loading. Strain Ritz-vectors are then extracted from the finite element model by applying a load corresponding to a representative wave and the strain history for the low frequency response is found by multiplying the decomposed signal with the strain Ritz-vectors. Finally the combined strain history is found by adding the strain histories from the low and high frequency responses.To validate the theory tests were performed on a scaled model of an offshore structure where the strain history was predicted using only the response from the accelerometers.