Analysis On The Dynamic Of Drill String Of Offshore Riserless Drilling

Oil and gas resources are still the lifeblood of economic development in the world,and the consumption of which is still increasing.With the decreasing of oil and gas resources on land,the exploitation of oil and gas resources has gradually transferred from land to ocean and even deep water area.The riser was used to connect the wellhead and drilling rigs in conventional offshore deepwater drilling,the main function of which are insulating seawater,guiding drill string and casing into the wellhead equipment,constituting the circulation channels of drill string,and so on.However,the use of the riser will increase the load of platform and drilling costs,and the complex load on the riser also makes it easy to be damaged which will probably result in a catastrophic event.Riserless drilling technology is a new drilling technology which abandoned the use of riser directly,it can reduce the requirements of platform effectively,improve the safety of offshore deepwater drilling,and reduce the drilling costs.Riserless drilling technology in developed countries has been comparatively mature,it has been practiced in many wells,and the effect of which is significant.However,this technology in domestic is still in a fledging period and develop slowly,as the key equipment and control method of riserless drilling technology is limited to our country.Because of its high efficiency in deepwater drilling,riserless drilling technology has gradually become a national race of research.In this paper,the dynamics of drill string of riserless drilling would be studied rely on the National Natural Science Foundation of China "Study on dynamic response of whirling to drill string in deepwater riser drilling"(51405032).Firstly,this paper introduced the application prospect and research significance of riserless drilling technology,and analyzed the research situation of pipe string mechanics briefly.According to the dynamic problem of drill string of riserless drilling in seawater,dynamic theory of Rayleigh beam axis rotor,D'Alambert principle and Euler dynamic equation was proposed to establish the transverse vibration mechanics model when the rotational speed is 0 r/min,transient whirling dynamics model of drill string in rotating state.Then the finite element method(FEM)was used to solve the mathematical model.Secondly,this paper briefly introduced the main structure and basic principle of the key equipment of riserless drilling technology.According to the working condition of string in seawater,the load form and axial load of drill string unit were analyzed,and determined the calculation method of wave load of drill string.Furthermore,the transverse vibration mechanical model of drill string was established by analyzing the mechanical balance and torque balance of the drill string unit in seawater based on the D'Alambert principle.At the same time,the whirling dynamic model of drill string was established based on the Euler dynamic equation,and derived the differential equation of whirling drill string,and the corresponding boundary conditions and periodic conditions were given.Then,the central difference method with high precision was utilized to separate the higher order differential equations based on the mechanical model established in this paper.The dynamic response law of drill string in transverse vibration was obtained by solving the transverse vibration differential equation of drill string,and the influence of hook load,offset of platform,ocean current and wave on the transverse vibration of drill string was analyzed.At the same time,the expressions for calculating the critical rotation speed of drill string was derived based on the established whirl differential equation.The mechanical behavior of drill string was researched by solving the dynamic response of drill string under certain working conditions.Finally,according to the dynamic model established in this paper and differential equation solving method,the software named "Dynamic Analysis Software of Drill String in Riserless Drilling" was developed by using the C# and Matlab mixed programming technology.The software has friendly interface,reasonable structure and obvious hierarchy,which can improve the calculation efficiency of drill string dynamic in riserless drilling.This paper studied the dynamics of drill string under the condition of offshore drilling without riserless,and the motion law of drill string in seawater was explained objectively.The results of this study will provide an effective guide for the operation of riserless drilling,and also provide a theoretical reference for the further study of the dynamics of the drill string in riserless drilling.