Research On The Time Domain Method Of Ship Motion For Quiescent Period Prediction

Many of the operations performed at sea are influenced by the wave induced motions.There would be some periods of calm in rough sea to allow ship to perform operations which is called Quiescent Period. If the Quiescent Period could be predicted exactly, the safety and efficiency of some operations such as helicopter landing and LNG connecting would be increased. We have done some work about the Quiescent Period Prediction in this paper.1,There is a summary about the developing progress of the Quiescent Period Prediction System and the several main technologies of this system. There are three key points of this system, the technology of wave measurement by radar, the technology of wave propagation and the technology of ship motion prediction. There is a conclusion that there would be enough Quiescent Periods during ship operation through the simulation of helicopter landing in rough seas.2, We have done some work about the technology of ship motion. There are two methods discussed in this paper,the one is spectrum method based on ship motion RAOs(RAO method),the other one is the time domain method based on impulse response function(IRF method). The main point of RAO method is the exactly calculation of RAOs. STF method is applied in this method to calculate RAOs. RAO method has been used in some typical quiescent period prediction systems. The main point of IRF method is the exactly calculation of impulse response function. In this paper, there are two methods be used to calculate the impulse response function,one is the frequency to time-domain translation method, the other is the direct time-domain method which used time domain strip theory. IRF method is the first time appeared in quiescent period prediction system.3, Time domain strip theory adopt three assumptions, the slender-body assumption, the high encounter frequency assumption and ignoring the free stream potential. Under these assumptions,we use two-dimensional boundary conditions on strip instead of three-dimensional boundary conditions on the whole ship. After the calculation of velocity potential on two-dimensional strip, we could solve the force and motion of ship in waves. This method could solve the problem about no-linear effects about ship motion and take less time than three-dimensional time domain method. Strip theory include a variety of hull forms: WigleyI, the S-175 containership, a 8000TEU containership, a 57000DWT bulk carrier and a KVLCC. This emphasizes the robust capabilities of the method and presents a variety of analyses for discussion.4, In order to check the suitability of the two methods, a series of experiments were carried out with a scaled model in seakeeping basin. It has an attached wave sensor that measures incident waves several meters before the bow. Through the experimental, we can have the conclusion that RAO method and IRF method could satisfy the demand of the quiescent period prediction system.