Experimental evaluation of the modal parameters changes of a telescopic bulk dock structure

An experimental and analytical estimation of changing modals parameters of a 132 meters long telescopic 3D frame ship loading structure is presented in this article. The structure consists of an external 110 meters length 3D steel truss than can move in the horizontal plane and an internal 3D trust that works as an extending arm. The total combined length of the structure after full extension of the telescopic arm is 132 meters. The extension length of the arm changes continuously according to the ship position and loading conditions. Due to the redistribution of mass and rigidity, the modal parameters of the structure changes continuously. In some occasions due to the load rate and load weight, important vibrations occur in the system. In order to understand the reason for the apparent vibration amplification and to validate an analytical model for risk assessment of the structure, several ambient and forced vibration measurements were performed on the structure. To capture the space - frequency characteristic, time - frequency system identification techniques were evaluated and are presented in this paper. The time-frequency methods presented in this article are the Short Time Frequency Spectrum, the Hilbert-Huang Transform (HHT) and the Wigner-Ville distribution (WVD). Some of the advantages and difficulties for each method are indicated. It is concluded that it is possible to monitor and to reproduce analytically, the complex response of this structure.