Modeling And Simulation Of The Ship's Nonlinear Motions In Heavy Seas

Recently, there are often the occurrences of the wreckage and capsize of ship sailing in heavy seas, which cause much casualties and large loss of asserts. From the investigation and analysis, it is known that the main factors leading to the accidents lie in not enough consideration of the stability's deterioration while ship sails in waves. Using ship statics, dynamics, seakeeping and fluid mechanics theories, this thesis puts emphasis on the reaserch of the ship's stability problem in waves. It is a part of the work "Study on Safty Assessment of Stability and Strength for Bulk Ship in Heavy Waves" which is supported by Young Ivestigator Foundation under Grant No.95-05-05-31 of National Ministry of Communication of P. R. China. The contributions of this paper are in the following.Firstly, the heaving and pitching motions of ship in regular waves are discussed. For the convenience of calculations, the coefficients of the mathematical model proposed by other authors are reasonably simplified, and then the computer simulation researches are implemented. It not only has much significance in the prediction of ship's seakeeping and manoeuvring behavior in heavy seas, but also lays the foundations of the following caculations for the ship's large angle transverse stability. Secondly, a new method of calculation is presented to obtain the large angle transverse stability arm of ship in still water. The successive approximation is adopted to caculate the transverse section area with large heeling angle in still water. Moreover the influence of the trim is considered, caused by the unasymmetry of bow and stern during the transverse inclination. That is, we consider the balance of not only gravity and buoyancy, but also trim moment. Based on the arm of the large angle transverse stability in stilll water, a new method of calculating the big angle transverse stability in waves is also proposed. The computation of every transverse section's area is key to the new method. For the purpose of improving the precision, the influence on every transverse section's area is considered from the trim caused by rolling. All that must be known is the table ofoffsets of vessel and environment disturbances, and then the stability arm can be obtained efficiently for a certain heeling condition. Utilizing the results above, the transverse stability is thoroughly studied in the condition of not only wave crest and wave trough at midship but also wave crest at the any vessel's position. Finally, by use of the curves of the big angle transverse stability in waves obtained from the above-mentioned calculation method, and the related theories of ocean transportanion vehicle dynamics, the mathematical model of the large angle nonlinear rolling motion is established. The influence of the nonlinear damping moment is also discussed in the mathematical model, in order to make the simulation results are in a good agreement with practical situation. Then, all simulation researchs are carried out by use of Matlab's Simulink Toolbox in this thesis.