*** Seminar by Yuming Liu, Wednesday, 4/11/07, noon-1pm in Room 3-343 ***

Title: Nonlinear Resonant Response of a Floating Platform in Surface Waves

Speaker: Dr. Yuming Liu

Speaker Affiliation: MIT Department of Mechanical Engineering

Date: 11 April 2007

time: noon - 1pm

Location: 3-343

Abstract:

To avoid violent motions, floating marine structures are often designed with natural frequencies away from the spectrum of ocean waves. Even so, large resonant responses may still be generated by excitation from nonlinear interactions of waves with body motions. Recent experiments indicate that a spar platform experiences large heave and pitch resonant motions when the incident wave frequency is much larger than the heave and pitch natural frequencies. Such resonant motions are not predicted by classical theories without considering nonlinear wave effects. This nonlinear mechanism has received little attention because of the complex nonlinear wave-body dynamics involved. In this work, we investigate nonlinear wave-body interaction effects on dynamic instability of marine structures.

We derive a second-order solution of nonlinear wave effects on the motion of a deep truncated vertical circular cylinder in the frequency domain based on the long-wave assumption. Based on this, a time-domain analytic model is developed for predicting the resonant response of a spar platform in both regular and irregular waves. In this model, both nonlinear restoring force and nonlinear wave excitation are considered up to second order. The prediction of spar motions in regular waves compares well with the experimental measurements. It is found that the observed resonant heave- pitch motion results from the second-order difference-frequency interaction between waves and body motion. The implication of this nonlinear mechanism to the study of dynamic instability of ships and freely dropping objects will also be discussed.

Biography:

Yuming Liu is currently a Principal Research Scientist in the Department of Mechanical Engineering of Massachusetts Institute of Technology (MIT). He received his Ph.D degree in Hydrodynamics in 1994 and afterwards assumed postdoctoral associate and research engineer positions at the Department of Ocean Engineering of MIT. He received the S.B. degree in Civil Engineering from Hohai University, Nanjing, China, in 1985 and the S.M. degree in Coastal Engineering from the University of Florida in 1988. His main research interest is in marine hydrodynamics with applications to the design and analysis of ships and offshore platforms as well as the prediction of ocean waves.