A Time Domain Method For Predicting Wave-induced Heave Motion Of Buoy-type Wave Energy Converters

The prediction of buoy motion is very important in designing, optimizing and controllingthe buoy-type wave energy converters. There are three possible situations, including “floating”on waves,“suspending” above waves and “submerging” in sea water, which the buoys mayexperience. However, the current potential flow methods are only able to analyze the situationwhen buoys are “floating” on waves theoretically. Therefore, the paper derives a time domainmethod for heave motion according to the motion characteristics of the buoys. The maincontents and conclusions are as below:Divide the fluid force into instantaneous buoyance due to undisturbed incident wavesand scattering force, and derive the detail expressions based on potential flow theory.In the derivation, the wetted surface is treated as time-varying but not a mean andinvariable one, and the memory effect of scattering waves is assumed being ignored.Analyze the reason of ignoring the memory effect in theoretically, and point out it isthe result of power take-off force.Establish the heave motion equation and solve it in time domain. Commercialsoftware SESAM is used to calculate hydrodynamic coefficients.Compare the simulation motion with the real sea test data from a wave powergenerating platform “Haiyuan1” and analyze the influence of memory effect on thesimulated result. The comparison reveals that the derived method matches the realsea test data well. Besides, the assumption of ignoring the memory effect has beenproved to be reasonable in the analysis.Compare the derived method with the linear time domain method by using the testdata. The comparison reveals that the derived method is more accurate and fasterthan the linear time domain method.Methods and computational examples based on the derived method for short-termand long-term prediction of buoy motion and delivery power have been given.In conclusion, the derived method has taken the influence of time varying wetted surfaceand the power take-off force into concern, reflecting the motion characteristics of the buoysand possessing the advantages of less time-consuming and high accuracy. Besides, the research lays a theoretical foundation for the further study on the mechanism of wave-inducedmotion of wave energy converters and an application foundation for developing new deviceswith higher energy harvest efficiency.