Dynamic characteristics analysis and optimization design of drill string composed of titanium/steel drill pipes
Continuous increase of drilling depth makes the load condition of drill string more and more severe.Titanium alloy has the characteristics of small density,high yield strength and low elastic modulus.In this study,according to the design requirements of tensile allowance for wellhead drill pipes,three types of composed drill strings with titanium/steel drill pipes were designed,namely composite drill string 1(use titanium drill pipes throughout the entire Φ101.6 mm drill pipe sec-tion),composite drill string 2(use titanium drill pipes on the upper part of the Φ101.6 mm drill pipe section)and compos-ite drill string 3(use titanium drill pipes on the lower part of the Φ101.6 mm drill pipe section).In order to explore the dy-namic characteristics of the composite drill strings,a dynamic model of the composite drill string based on the Hamiton prin-ciple was proposed,using the node iterative method and Newmark-β method to solve the model.The whirl characteristics,dynamic stress and vibration characteristics of the three composite drill strings were analyzed,and compared with convention-al steel drill string.The results show that the use of the titanium alloy drill pipe can effectively slow down the whirl velocity and dynamic stress of the drill string,among which the whirl velocity and dynamic stress of composite drill string 3 are the smallest.Based on the vibration characteristic strength,the drilling operation parameters can be optimized,and a recommended drilling operation parameters chart of composite drill string 3 was formed,and a reasonable range of operation parameters was given.