The Longuet-Higgins wave model is introduced to study the motion response of a moored JIP Spar interaction with freak waves using timedomain method. The wave characteristics of the freak waves are analyzed. The results show that the maximum of the sway motion appears at the back side of spar, while the maximum of the pitch motion appears at the front side. The maximum of the sway motion increases with the initial phase, spectral widths and spectral periods for the focus points located at the front side of the spar. The finding is helpful to the structural design of JIP Spars.
Ocean waves are irregular and nonlinear, the regular wave can not accurately reveal the real wave condition. Freak waves, also known as rogue waves, are large, unexpected and suddenly appearing surface waves that can be extremely dangerous to offshore platforms. In physical oceanography, the significant wave height (SWH or Hs) is defined as the average of the highest one-third (33%) of waves (measured from trough to crest) that occur in a given period. Nowadays, it is usually defined that waves with maximum height larger than 2.2 times of significant wave height value are called freak waves(Lavrenov and Porubov, 2006). A great deal of efforts have been made by experimentally and numerically study to find the generation mechanisms and the physical properties of freak waves (Baldock, 1994; Fochesato, 2007). Also, long time simulation is required to generate the suitable waves if random waves are used. However, focused waves generated by the superposition of component waves with different frequencies can well simulate the largest transient wave in a short time(Liu, 2010). One of the possible mechanisms of freak wave generation may be energy focusing, i.e., the wave energy is concentrated in a small spatial area during a short time, resulting in an abnormally large wave (Kharif, 2003; Yan, 2009; Mori, 2002; Clovanangeli, 2004; Kharif, 2008; Touboul, 2006; Baldock, 1994; Yan, 2010). Two kinds of numerical theories to get freak waves are linear and non-linear theory. Freak waves are usually simulated based on the Longuet-Hinggins model by phase modulation method or linear wave superposition(Liu, 2010; Huang, 2002; Kriebel, 2000; Pei, 2007; Li, 2007; Zhao, 2009). In non-linear theory, freak waves are usually generated by high order spectrum method. In general, the linear method can capture the main characteristics of freak waves to satisfy the basic engineering requirements, which can bypass the complex non-linear theory.