For some applications it is worthwhile to consider second-order wave effects. Especially the influence of low frequency second-order forces on systems moored in waves may cause resonant behaviour, resulting in large oscillating motions at the resonant frequency of the moored system. To describe this phenomenon we first derive the second-order waves in the case that we have a simple spectrum of waves consisting of two monochromatic plane waves with frequencies close to each other. Next a derivation of the second-order low frequency drift forces by means of a local expansion and far-field expansions is given. As a classroom example we consider the forces on a vertical wall caused by the reflection of such a wave system. This serves as an introduction to formulation of the low frequency motion of a moored system.
KeywordsMoor System Free Surface Condition Drift Force Hull Surface Monochromatic Plane Wave
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