High-order spectral analysis on the pitch motion of a semisubmersible

Abstract

The effects of wave conditions on the pitch motion were investigated on the basis of a series of wave basin tests under representative wave conditions to acquire the pitch motion characteristics of a semisubmersible. The high-order spectral analysis procedure is established and improved, deriving linear transfer functions and quadratic transfer functions (QTFs) via cross spectral and cross bi-spectral analyses, respectively. Results indicate that the former ones are unrelated to the encountered waves while the latter ones are highly dependent on the encountered waves. The distribution of significant QTFs is mainly concentrated on the off-diagonal line where the diff-frequency is equal to the natural frequency. Furthermore, with the increasingly rising wave conditions, the response gradually emerges on another off-diagonal line with diff-frequency equal to spectral peak frequency. Thus, the pitch motion in wave frequency would present certain nonlinearity in rough seas.