The Estimation Of Natural Frequencies And Damping Ratios Of Offshore Structures

Abstract

This paper focuses on the estimation of natural frequencies and modal damping ratios from measured response spectra, with particular emphasis on the dynamic response of offshore structures to wind and wave excitation. At present, estimates of natural frequencies and damping ratios are computed from the location and half-power bandwidths of resonant peaks in a structure's ambient response power spectrum. While reliable natural frequency estimates are typically obtained in this manner, half-power bandwidth damping estimates are shown to be highly sensitive to the method employed in estimating the response spectrum. The lack of confidence bounds on natural frequency and damping estimates further restricts the utility of the estimates. An alternative method is developed based on a powerful method of spectral estimation known as the Maximum Entropy Method (MEM). The resulting technique yields estimates of natural frequencies and modal damping ratios as well as approximate statistics on the reliability of the estimates. Performance of this new method is explored through extensive Monte Carlo simulation of one and two degree-of-freedom systems. Conventional estimates are also simulated for comparison with the MEM parameter estimator. The use of the MEM parameter estimator is further illustrated with ambient response data from Shell Oil's South Pass 62C platform. The MEM parameter estimates show excellent agreement with natural frequency and damping estimates obtained during recent tests conducted using forced excitation.