Numerical Research On Mechanical Behavior Of Natural Gas-hydrate Drilling Coring Fidelity System

Fidelity sampling without disturbances of deep-sea natural gas hydrates is realized through rotary scraping drill bit driven by large size drill string which is special for rope sampling.In the process of drilling sampling,the safety reliability of drill string and the disturbances of gas hydrate stratum produced by drill bit are directly related to the success of obtaining the NGH physical samples.The paper digs into the mechanical behavior of natural gas hydrate drilling coring system by using the method of numerical simulation and theoretical calculation.Firstly,the paper does static analysis for the large size drill string which is special for rope sampling.On the premise of ignoring the dynamic factors,the paper analyses the stress and deformation,and finds out the stress concentration part of drill string,namely the dangerous section,and studies the influence of torque,drilling pressure and rotary speed, getting the changing regulation of the equivalent stress of drill string.On the premise of ignoring the geometry dimension of drill bit,the paper analyses the vibration modal of drill string,getting the top 10 inherent frequency and the resonate vibration critical rotation speed.Besides,the paper studies the influence of the thickness and the length of drill string and the length of drill collar for the inherent frequency,providing references for preventing the resonate vibration of drill string in the process of drilling sampling.Considering the influence of inertia force and damping effect,the paper simply analyses the axial displacement under the forces changing with time and rotate speed's influence to lateral vibration.Setting the rigid-flexible contact between the drill bit and NGH stratum,the paper analyses the disturbances of gas hydrate stratum produced by drill bit,calculates the disturbance rate and suggests measures to reduce the distuebance rate.At last, the paper conducts transient thermal stress analysis for the process of drilling sampling.In the process of drilling sampling for NGH,because of the cutting friction between the drill bit and NGH stratum,which can produce large amounts of heat,the in-situ state of NGH can be destroyed,even causing NGH decomposition. The transient thermal stress analysis can ensure the temperature changes,helps to understand the degree of the influence, and provides references for the fidelity sampling of NGH.