Time-domain Analysis Of Motion Responses Of Floating Structures In Irregular Waves

Ever since the dawn of human civilization,mankind has been sharing the sun and the moon with ocean.Since the beginning of the new century,the exploitation and utilization of the ocean are drawing more and more attention across the world.From offshore drilling platform to detecting AUV,from private yacht touring around lovely reef to tough cruiser marching across the ocean,various floating structures can be spotted across the oceans.To have a better interation with the ocean,not a single minute has been wasted to enhance the the construction technique and research development,of which the simulation of ocean situation is a key part with vital importance.Regular waves has been adopted in both engineering and research as a simulation method for the sea.However,the real waves in ocean is rather complicate physical phenomena affected by numerous factors.Irregualr wave theories should be induced to conduct a more precise simulation of sea waves.This paper aims at introduce a method to analyse the time-domain motion responses of floating structures in irregular waves,doing its bit for a more reasonable and economical design.A method of solving the boundary integral equation is developed by adopting the higher order boundary element method to discretize by the boundary integral equation based on the Rankine source method.An analysis method of time-domain problem is achieved by solving the boundary integral equation an d the equation of motion in each time step with the time-marching method.In this paper,a program is developed to study the constant problem along with the time-domain diffraction,radiation and free motion problems,and the reliablity of the method is ve rified.The developed time-domain analysis method is applied to numerical simulation of motions of a FPSO in regular waves.Then,an ITTC spectrum and a white noise spectrum is employed to generate the irregular waves,which the ship is advancing in.Finally,motions of the FPSO in irregular waves are simulated by the above-mentioned developed numerical model.Computed results of the ship motion responses are validated by compared with the data derived in regular wave condition,and the main factors that have deep influence on ship motions are analyzed.Irregular waves consist of components with various frequencies,of which the low-frequency part asks for larger calculating area to conduct wave-absorbing process better,and the high-frequency part requires denser grids to obtain more accurate result.Besides,the paper introduces a method which can correct the wave spectrum efficiently.The validity of the method is confirmed by comparing the corrected two-parameter P-M spectrum along with the white noise spectrum to the target spectrum.The limitation of the method is also discovered,which is that only the analysis results using the certain frequencies which was used to simulate the spectrum at the beginning that can achieve relatively accurate results.To those frequencies that were not chosen originally,the calculation error would be magnified.At last,the effect of corrected spretrum to the motion responses of floating bodies are analyzed.