Numerical Research On Mechanical Behavior Of Large-size Sampling Drill String For Deep-sea

Considering the particularity structure and process and complexity loads of the sampling drill for natural gas hydrate drilling in deep-sea water,the Natural Gas Hydrate Coring engineering prototype and supporting technology research group developed the undisturbed packing fidelity sampling drill system to drill the samples.Combined with the research status,the paper use theoretical analysis and numerical methods to research and analyze the mechanical behavior of sampling drill string.Firstly,the paper researches the static characteristics of the sampling drill string.Ignoring the dynamics of sampling process and according to the mechanism of external loads which sampling drill string suffer,the paper researches the force and deformation characteristics of the sampling drill string by applying loads to the structure in the static mode.The result of the most dangerous conditions shows that the maximum equivalent stress of the sampling drill string is 266 MPa,occurs in the drill pipe at the joint of the drill pipe and drill collar;investigate the statics characteristics law of the sampling drill string by exerting different drilling pressure,torque,waves and water flow.The result shows that the drilling pressure and torque having greater impact on equivalent stress of the sampling drill string,the waves and water flow having greater impact on deformation of the sampling drill string.Secondly,the paper researches and analyzes the vibration modes of the sampling drill string,obtains the horizontal,vertical,and torsional vibration of the first 20 natural frequencies and the corresponding vibration modes of the sampling drill string.The paper also researches and analyzes the influencing law of the natural frequency of vibration of different lengths of drill collar,obtains the first 20 natural frequencies of vibration of the sampling drill string of different lengths drill collars.In order to prevent the occurrence of resonance of the sampling drill string in the process of drilling sampling string,the paper calculations the critical speed of the sampling drill string when it occurs resonance.Then,the paper researches and analyzes the transient dynamics of the sampling drill string in consideration of the basis of the inertial force and the damping force,obtains the changes law of the nodes' lateral and vertical displacement and elements' equivalent stress which in the dangerous locations under the combined action of the waves,water flows and waves,water flows,drilling pressures and torques.The results show that in the start time,the sampling drill string presence large laterally and longitudinal vibrations.Finally,the paper researches the stability of the sampling drill string,conducts eigenvalue buckling analysis and nonlinear buckling analysis,the results show that the nonlinear buckling analysis is more suitable for sampling drill string' stability analysis.Through computational analysis,obtains the 10 order critical buckling load of the sampling drill string,shows that the maximum applied drilling pressure shall not exceed 60 KN.