Manoeuvring Hydrodynamics And Stability Of Vessels Navigating In Proximity To The Bank

Under the background of trade globalization,modern ships are developed with large sizes,high speeds and to adapt to various waterway conditions.Such trend increases the frequencies of ship proceeding in ports,straits,estuaries,rivers,canals and other near-bank,shallow water areas,and the issue of ship navigation safety has been highlighted.The International Maritime Organization(IMO)has embarked on the initiation of manoeuvring criteria for ships in restricted waters,which will inevitably lead to higher requirements for the ship manoeuvrability.When a ship sails close to a bank,the bank effect and the shallow water effect produce a bank suction force and a yawing moment on the hull,thus causing a significant effect on the ship's motion.On the other hand,a ship is usually sailing near a bank at a lower speed than its design speed,and better directional stability is neeed in such water conditions.Therefore,in this study,a very large crude oil carrier(VLCC)and a submarine are adopted as the subjects.The hydrodynamics and motion stability in the manoeuvring motion near a bank are investigated through experimental approach and numerical calculation,and the ship path control problem is discussed in the near-bank environment condition.In this thesis,the study is conducted on the characteristics and variation mechanism of ship hydrodynamics when proceeding near a bank,and the method to evaluate the motion stability and the way to control motion stablility is proposed for guiding the navigation of real ships.Firstly,the ship manoeuvring model under the near-bank situation is established with the asymmetric hydrodynamic derivatives to account for the bank effect.According to the linear control theory,the response of the manoeuvring model to the external disturbance is divided into the steady component and the transient component,and the steady-state equation and transient equation of the motion response near a bank are obtained.Then,the creteria of straight line stability and directional stability are derived from the transient equations.To achieve the directional stability the proportional–derivative(PD)control is applied to the rudder angle,and thus the range of gain constants satisfying the directional stability criterion is calculated.The planar motion mechanism(PMM)tests of a VLCC model,KVLCC2,with different ship-bank distances and different water depths were carried out using the PMM test system in the circulating water channel(CWC).The uncertainty analysis in the hydrodynamic forces of bank effect test is firstly conducted in the domestic and overseas research work,with the assessment of systematic bias from errors of ship-bank distance and water depth proposed.The contribution of ship-bank distance and water depth error to the uncertainty of results of bank effect test is identified.The PMM test results show that the hydrodynamic derivatives do not change significantly with the ship-bank distance as the bank effect is separated from the hydrodynamic forces.With the water depth decreasing,most of the hydrodynamic derivatives except rudder derivatives grow substantially in shallow water.By introducing the hydrodynamic derivative into the steady-state equation,the critical value of ship-bank distance is obtained to meet the navigation safety requirements under different water depths.According to eigenvalue analysis of the transient equations with PD rudder control,the gain constants satisfying the directional stability greatly increase when KVLCC2 approaches to the bank and when it sails in shallow water.Therefore,the helmsmen should be concerned about high control sensitivity caused by water depth,ship-bank distance and other factors in order to take appropriate steering strategy.For the numerical prediction of hydrodynamic forces of near-bank vessels manoeuvring,Computational Fluid Dynamics(CFD)method based on viscous flow theory is adopted and a hybrid dynamic mesh technique is developed for the simulation of ship moving near the bank.The uncertainty analysis method in CFD is applied to the results of oblique towing tests of KVLCC2 and SUBOFF.The difference of the hydrodynamic characteristics between in a CWC and in a large towing tank is investigated.The applicability of the Tamura's velocity correction formula for the sidewall effect to the CWC oblique towing test is discussed and the correction formula of drift force is proposed for full form ships.The blockage effect on the bank effect test results due to the sidewalls of the CWC is analyzed by the numerical simulation.The numerical results of pure sway and pure yaw tests are compared with the experimental results,and the accuracy of the numerical method is validated.The dynamic PMM tests of KVLCC2 and SUBOFF under different ship-bank distances and water depths was numerically simulated.From the time history of the hydrodynamic forces,it is found that the amplitude of lateral force increases and moves toward the +Y ' axis when the ship-bank distance decreases,while the change of the yaw moment is not obvious.The amplitude of hydrodynamic forces of pure sway test in shallow water is greatly increased,and the hydrodynamic distribution of the pure yaw test changes greatly,leading to the large shift of the time history phases.The hydrodynamic derivatives of KVLCC2 near the bank shows that when the ship-bank distance decreases,the enhancing bank effect increases the added mass of sway and the added moment of inertia of yaw.And the suction force increases the viscous derivatives related to sway force,and the bow-out moment increases the viscous derivatives related to yaw moment.The shallow water effect increases the absolute value of all the accelerator derivatives and magnitudes of hydrodynamic derivative can be doubled or more.Using the numerical results of hydrodynamic derivative,KVLCC2 and SUBOFF is not stable in terms of the straight-line stability creterion.It is found that for the KVLCC2 needs “Control with mild sensitivity” to navigate stably near a bank in deep water when the ship-bank distance is greater than 2.8B(B means the ship breadth).When the ship-bank distance is smaller,“Control with high sensitivity'' is necessary for the gain cnstants of PD control.For the KVLCC2 proceedings near the bank in shallow water,“Control with mild sensitivity” is able to satisfy the directional stability creterion,and the directional stability mainly depends on the differential gain constant.For the motion stability of the SUBOFF in the vertical plane,with the submarine approaching to the seabed,the SUBOFF becomes inherently unstable.Finally,a multiple-input multiple-output(MIMO)manoeuvring motion response system is built using the rudder angle and speed variation as control input.The linear quadratic regulator(LQR)is used to control the rudder angle and speed variation in order to keep course near bank.The simulation results show that the overshoot of lateral distance and heading angle during the course-keeping process are reduced and the time to reach the equilibrium state is shorter with the help of MIMO system and LQR.For the near-bank path control,the model predictive control(MPC)algorithm is used to optimize the control inputs.The offset-free MPC scheme is designed to eliminate the steady state path error caused by bank effect effectively.Comparing the results of several controllers,the cross-track error of the path following system with MIMO system and offset-free MPC scheme is the smallest,in which the speed variation is more effective than that of the standard MPC.