Force Analysis And Calculation For Righting A Capsized Ship

In sea transportation,a ship may capsize sometimes during its operation.Marine salvage first involves the uprighting of the capsized ship,essentially bringing the deck to point up.The traditional marine salvage scheme for a capsized ship lacks the necessary design theory and calculation method because the uprighting process is performed with past experiences.Therefore,this dissertation reports a study on the mechanical property of the uprighting process of a capsized ship by combining theoretical calculations and the use of a simulation software.The related calculation formulas were derived.The effects of factors such as floatation,stability,ground force,soil resistance,the velocity of the uprighting process,and hydrodynamic force on the uprighting process were evaluated.Based on the hydrostatic theory for ships,first the righting force model and righting force moment model were established using Euler angle parameters,and then the Newton-iterative method was used to solve the righting force.The calculation formulas of 2D/3D righting force moment were derived from the variation quantity of trim angle.Four types of damaged compartments during the uprighting process are proposed by analyzing the flooding water,free surface,and air pocket in the damaged compartments.The transmission conditions of all the forms of damaged compartments are also given.When the ship run aground on homogeneous soil,the penetration can be calculated by setting up rectangular nodes on the ground area parallel to the horizontal and vertical axes of the ship.Then,the distributions of ground force and soil resistance moment were deduced.When the ship run aground on heterogeneous soil,the effect of soil on hull can be divided into three parts:the shear resistance moment between the hull and soil,the soil-blocking resistance moment,and the compaction resistance moment.The ground force formula was deduced using the relationship between the pressure of soil unit area and sinkage proposed by M.H.Letoshnev,and the shear resistance moment between the hull and soil was obtained.The soil-blocking resistance moment was computed using the Rankine theory for lateral passive earth pressure.The compaction resistance moment was obtained from the compression work done by the ship to the soil.The simulation results of the uprighting process of a capsized ship on water show that the stability of the capsized ship can be decreased by properly adjusting the quantity and position of free water in the earlier phase of the uprighting process.The free water is also one of the decisive factors for the inclining moment in the later phase of the uprighting process.The stability of a capsized ship can be changed by the displacement of damaged and capsized ship depending on the change in the flooding quantity.The damaged side tank decreases the stability of the capsized ship,thus favoring the uprighting process in the earlier phase,and increases the difficulty of the uprighting process in the later phase.The simulation results of the uprighting process of a grounded and capsized ship show that the initial stability range and righting moment are small.In the later phase of the uprighting process,a significantly large righting moment is needed to stabilize the ship because of the high positive/negative moment.A grounded and capsized ship in deep water has a wide initial stability range,but a relatively small righting force moment is needed to balance the ship in the later phase.When a ship run aground on multiple points or soft soil areas,too many ground points decrease the computational efficiency and increase the risk of miscalculation.Therefore,the numbers and positions of ground points should be selected based on the requirement of hull strength and operating conditions.Using the rigid-body dynamics theory,the motion equation of the capsized ship was established.The calculation equations of inertial hydrodynamics force and added mass were derived from the function principle of fluid mechanics.The formula of viscous hydrodynamics force was obtained from Taylor series expansion,and the computation simplification method of viscous hydrodynamics force was determined using the motion characteristics of the capsized ship.The hydrodynamic pressure was derived from the potential flow theory,and the calculation formulas of irregular and regular wave forces were obtained using the Gauss formula.Based on the stress characteristics of a capsized ship,the wind force was solved using the direct calculation method.Then,the calculation method for current force was also obtained from the formula of wind force deduced by Aage C.