Study On Ship Manoeuvrability And Navigation Safety Limits In Actual Seas Based On Unified Theory

The traditional study and related norm establishment on ship manoeuvrability were mainly for the case of calm water.However,the environmental disturbing forces such as wind and wave force,can bring considerable effects to manoeuvring motions when ship sails in actual seaway.In 2013,the International Maritime Organization(IMO)released Interim Guidelines for Determining Minimum Propulsion Power to Maintain the Manoeuvrability of Ship in Adverse Conditions,and the basic requirements of navigation safety have been put forward.Moreover,due to the complexity of manoeuvrability in waves,the International Towing Tank Conference(ITTC)established an expert committee of ship manoeuvrability in waves in recent years.In this dissertation,the sub-models including manoeuvring hydrodynamic force,propeller force,rudder force,wind force,radiation hydrodynamic force,wave force and so on,were established based on the Manoeuvrability-Seakeeping Unified Theory.The effect of propeller loads on rotating speed and power of main engine under the operational limits was particularly taken into account.Besides,the PID algorithm was used to realize the course control.Finally,the four-degree-of-freedom(surge-sway-roll-yaw)manoeuvring motions were modeled in irregular waves,and it was programmed based on the MATLAB platform.In the manoeuvring motion mathematical model,the sub-models of wave force and radiation hydrodynamic force were focused on.Firstly,the database of the first and second order wave force in several working conditions was set up in advance to interpolate and call the wave force values according to real-time navigation speed and relative wave direction.Secondly,Newman’s Approximation Method was used to calculate the real-time coefficient matrix of complete drift force including difference-frequency components.Thirdly,the fluid memory effect induced by radiation motions was considered through retardation function integral calculated by semi-analysis method,and the high-frequency part of needed radiation damping was corrected at the same time.In addition,some verification and validation works were carried out for the program reliability.Based on the mathematical model above,the simulations and analyses of ship manoeuvring motion in actual seas were conducted,including turning,zig-zag and straight navigation tests.On one hand,the abilities of turning,course changing and course keeping were analyzed through the navigation tracks,characteristic parameters and time history of manoeuvring motions.Moreover,the detailed effects of different sea states,navigation speeds and wave directions on the manoeuvring motions were explored physically.On the other hand,the study on minimum propulsion power and navigation safety limits in adverse conditions were carried out.Based on the related requirements released by IMO,the minimum propulsion power to maintain the manoeuvrability of ship in adverse conditions was calculated rapidly and effectively through direct simulation method.Furthermore,it was expanded to the limits of main engine power and safe sea state in multiple conditions based on the navigation safety criterion rules proposed in this dissertation.The manoeuvring motions in actual seas can be simulated relatively completely and accurately with the mathematical model established in this dissertation,and series of cases and analyses can be performed dispensing with substantial time,manpower or other resources by CFD simulations or model tests.Meanwhile,the research method in this dissertation can provide reference and be applied to the main engine power decision or navigation advice,respectively for the ships under design or in service.