Research On Wave Motion Compensation Of Two Side-by-Side Positioned Ships

When two vessels float parallel alongside at sea to do the operation of replenishment, thesix-degree-of-freedom motions of them are much larger than themselves alone, due to theimpact of wave and hydrodynamic interaction. In certain cases, the relative motions are toolarge, even leading to the collision of their superstructure. In order to avoid the abovementioned danger and to improve the sea state of side-by-side ship operation, a variabledamping wave compensation method is presented, which restrict the roll and sway of the twoside-by-side ships.The research for the variable damping wave compensation was carried out by theoryanalysis, computer simulation and model test in this dissertation. The key problems whichwill be solved in this paper are as follows: the characteristics of relative motion of twoside-by-side ships, the short time prediction of relative motion, the variable damping wavecompensation and its control strategy, the signal acquiring and processing for model test, theacceleration signal processing, and model test for relative motion and variable damping wavecompensation. The main contents of this paper are:1. Based on the three dimensional potential flow theory, the characteristics of relativemotion between two side-by-side ships are studied in the regular and irregular waves. Anda new method for analyzing relative motion characteristics of two side-by-side ships inirregular waves was presented which based on AQWA and spectrum analysis.2. The prediction of relative motion between two side-by-side ships are studied. The relativemotion of low seas is predicted by AR model. The relative motion of high seas ispredicted by chaotic Elman neural network. Chaotic Elman neural network fits for thenonlinearity, nonstationarity and chaotic characteristics of two side-by-side ships in waves.The optimal embedding dimension acquired by phase space reconstruction is used for thenumber of input nodes, and a chaotic back-propagation algorithm was applied, in whichchaotic noise is added into weight update process. It can obtain the the characteristics ofthe relative motion between two side-by-side ships under the prevailing conditions andenvironments. The simulation result shows that the proposed method can predict therelative motion of two side-by-side ships effectively.3. The variable damping wave compensation based on magneto-rheological technology andits control strategy are studied, which presents a new idea for the research of wavecompensation. By consideration of the relative motion characteristics, variable damping wave compensator is designed and manufactured, which is based on themagnetorheological technology. Its mechanical model was established by the analysis ofits performance. By taking full advantage of its controllable damping force characteristics,temporal state-space equations of variable damping wave compensation of twoside-by-side ships is built. The clipped optimal control strategy is designed and realized torestrict the roll motion. By taking the two side-by-side ship models in waves as example,numerical simulation and model test are executed to prove the efficiency of variabledamping wave compensation. The results shows that variable damping wave conpensationcan reduce the roll motion responses of two side-by-side ships.4. Signal acquiring and processing for model test is studied, especially about the processingof acceleration signal. Acceleration signal processing is key to the side-by-side shipsmodel test. In order to improve the accuracy of the phase and amplitude of the results ofacceleration integration, a method based upon EMD adaptive filtering and frequencydomain integration is presented. The acceleration signal is decomposed by EMD into nIMF, and then based upon appropriate rules, the number of IMF pertaining highfrequency(h) is determined, and they are adaptively filtered to cancel noise. Finally, FFTtransformation is applied, frequency domain integration is done, and then IFFT is used toacquire displacements. Simulation results show that this acceleration signal processingmethod is better than the pure frequency domain ingegral. The data processing result ofside-by-side ship model test also show the validity of this method.5. Model test similarity principle is determined, and model test for side-by-side ship wavecompensation is established. Model test is performed for relative motion of twoside-by-side ships and variable damping wave compensation, which further validate theeffective reduction of roll motion responses by variable damping wave compensation.