深海天然气水合物钻探取心钻柱振动模态分析

Finite element analysis of modal vibrations of drill strings in deep-sea natural gas hydrate coring/drilling

作为深海天然气水合物钻探取心系统必不可少的关键部件,钻柱在深海复杂环境钻探取心过程中承受着非常复杂的外力,不可避免地会产生横向振动、纵向振动、扭转振动及其耦合振动。钻柱的共振会引起钻柱本身甚至整个钻探取心系统的严重失效,进而导致无法成功获取深海天然气水合物岩心。为此,根据国内外相关振动理论的研究成果,结合深海天然气水合物钻探取心的特点,利用ANSYS有限元方法对钻柱转动过程中的横向振动、纵向振动及扭转振动进行了模态分析,确定出钻柱的固有频率和临界转速,为避免钻柱的共振提供了理论依据,同时还得出以下结论:①当外力的激振频率和钻柱的固有频率相同时,钻柱内的交变应力和振幅相当大,此时会引起钻柱的共振,整个钻柱的振动加强;②横向振动引起共振的可能性比较小,纵向振动和扭转振动引起共振的可能性较大。

As an essential part in the equipment of deep-sea natural gas hydrate coring/drilling,drill strings have to bear such complicated external forces under deep sea environment that horizontal,vertical,torsional,and coupled axial vibrations occur inevitably.In addition,the resonate vibration of drill strings will lead to serious failures of drill strings or even of the whole coring/drilling system.In view of this,based on the research results of vibration theory at home and abroad,ANSYS finite element models,in combination with the characteristics of natural gas hydrate coring/drilling in deep sea,are adopted to conduct the modal analysis of the horizontal,vertical,and torsional vibrations of drill strings,thus the inherent frequency and the critical rotation speed of drill strings are determined,which provides theoretical basis for the prevention of resonate vibrations of drill strings.Meanwhile,conclusions are drawn as follows: a.when the vibration frequency of the external force equals the inherent frequency of drill strings,the alternative stress and vibration amplitude are getting significantly large enough to cause the resonate vibration of drill strings;b.comparatively speaking,the resonate vibration is easier caused by the vertical and torsional vibrations rather than by the horizontal vibrations.