Dynamic Response Of Marine Stern Ramp Under Moving Mass

In this paper,the safety control strategy of vehicle lightering on the sea has been researched.The transfer system on the sea is consist of Roll-On-Roll-Off ship(RORO)and semi-submerged ship.Vehicles are transported by stern ramp of RORO ship.The stern ramp is the research object.The stern ramp's dynamic response under moving mass at the three-level sea state is the core content of the paper.The technical route is adopted to analyze the vibration characteristics of the ramp both from theory and numerical analysis.The key parameters affecting the lightering on the sea are discussed to provide a certain reference for the design of transfer system and the stern ramp.(1)Based on the transverse vibration theory of thin plates and Kirchhoff's hypothesis,the differential equations of free vibration and forced vibration of ramp are established.The mechanical coupling model and geometric coupling model between the moving loads and rectangular plate are built.Combining with the finite element method,the method of solving the governing equation of the stern ramp under moving mass is obtained.(2)The transfer system adopts the horizontal and side-by-side layout with three DOF,which is including heave,pitch and roll.The stern ramp is regarded as a thin plate with four-node contact with 15m long.Ramp's material is CCS-AH36 steel.First,the principle of modal analysis are described.The ramp's natural frequency is between 6.4 and 29.8Hz obtained by modal analysis.Then,the second-order response system of the ramp is established based on structural dynamics.In addition,the results of the hydrodynamic response of the two ships are transformed into the constrained displacement spectrum of the ramp.The maximum stress response of the structure is 217MPa,which satisfies the allowable stress.It occurs at the junction of the first and second panels of ramp,which is the weak position of the ramp.For higher sea conditions,when the strength of the stern ramp at sea is insufficient,the structural reinforcement of this position may be given priority.(3)Based on the transient dynamics theory,the dynamic response of the ramp under moving mass is studied.First,the principle of contact dynamics is described and the coupling vibration numerical simulation model is established in ANSYS.The rationality of the model is verified by an example.Futhermore,taking five groups of parameters as examples,the simulation calculation is carried out.The changing parameters include the direction of excitation spectrum,the form of loads,the different speed of vehicle,the different boundary conditions and the different form of vehicle wheel.In the end,the time history curve of dynamic response of the ramp is obtained.The factors affecting the safety control of the lightering are discussed.Suggestions for the transfer of vehicle at sea are to use low speed,lay a spring compensation device and preferentially transfer the same type of tracked vehicle.(4)Taking the design of a RORO ship's stern ramp as an example,the dimensions of each component of ramp is described in detail.Detailed design of the dimensions is the first and second panels,deck beams,deck longitudinal,hinges,bolts,etc.Then,calculation verification is carried out by rules for classification of sea-going steel ships.Lastly,the strength of ramp under the moving mass is checked,including the comparison of the maximum stress response of the stern ramp with the allowable stress,the comparison of the torsion angle with the standard heel angle.Some suggestions are provided for the design of similar lightering structures.