The Research Of Joint Strength Of API Round Thread Connection

As one of the most popular oil goods, the usage of API round thread connection is accounted for about50%of all casing. But the joint is the weakest place in service environment, which makes us recognize the significance of studying on it.1. Physical TestingBy utilizing the advanced experimental equipment of Tubular Goods Research Institute of CNPC, unconventional physical testing under galling、combined load and compression are conducted, which shows us that:(1) Thread slippage is main failure form of round thread connection;(2) The joint strength will reduce greatly(23.7%) under severe galling, slight galling has little impact on it. But slight galling can develop as severe galling by careless operation;(3) The joint strength will decrease about10%under combined load testing, and the optimum make-up torque of this specification casing in Sulige region is6240-6500N·m;(4) The joint strength will decrease under compression load, the relation between joint strenth and compression load is:F=-0.002t3+0.2871t2-9.6119t+1896.2.2. Finite Element Analysis(FEA)The paper presents the simulation of contact pressure distribution on each teeth by using FEA, the results show that:(1) The contact pressure on each engaged teeth is very unevenly. The big end is most prone to stress destruction because there undertakes most of the axial load;(2) The contact pressure on the small end drops fastest when inner pressure exists, which makes the small end will develop as the leak path most easily.Therefore, in order to increase the joint strength and sealing ability, the contact pressure distribution and the bear load of each teeth should be improved, and the contact pressure on small end should be enhanced.