Amin Ghafooripour - Assistant Professor, Islamic Azad University, Central Tehran Branch Iran, Science and Research Campus
Ali Akbar Aghakoochak - Associate Professor, Tarbiat Modarres University Iran

This paper discusses the dynamic behavior of an offshore structure. The ambient vibration tests were performed to identify the modal parameters. Spectral analysis seeks to describe the frequency content of a signal, and also the Partial stationary concept is described for the mathematically nonstationary signals that records from offshore structures.A method for using the Bessel-Kaiser window shape function to estimate damping value from spectral density function (PSD) is applied for gathered data. The Bessel-Kaiser window is an approximation to the prolate window and more efficient than the other window shape functions and ratio of the main lobe energy to the side lobe energy of spectrum is maximized. The results show the accuracy of method for further interpretations from ambient vibration tests of an offshore structure.