The main loads on the casing thread are as follows:
1. Make up torque. The casing joint must be tightened to a certain torque before it can be put into oil and gas wells. For API threaded joints, the size of the make-up torque is specified in API RP 5c2. For premium connections, the make-up torque is provided by each manufacturer. After the joint is screwed on, the internal and external threads are fitted with each other to produce a certain contact pressure, so that the casing joint can complete the sealing and connection functions. Research and application experience have proved that after API joint is buckled up according to the specified torque, the stress level of the joint is already very high and the part of the joint has entered the yield state without applying other loads. Premium connections have the same problem.
2. Tensile load. Generally, the tensile load is due to the self weight of the casing, and the joint closest to the wellhead bears the largest tensile load. In general, the maximum tensile load on the joint is the sum of all casing weights. However, if the casing is blocked during running in, the joint may be subject to tensile load greater than the self weight when the casing needs to be pulled.
3. Internal pressure load. In general, the casing string is filled with high-pressure oil or gas. In some wells, the pressure of oil or gas can reach tens or even hundreds of MPa.
In addition, casing joints sometimes bear external extrusion load, compression load, bending load, etc. in heavy oil thermal recovery wells, the thermal stress of the environment temperature can not be ignored. At the same time, the casing often bears not a single load, but several loads at the same time, such as torque, tension, internal pressure and external extrusion. Under the above loading conditions, API round thread and partial trapezoid thread are often difficult to ensure the structural integrity and sealing integrity of the casing string. According to API reports, 86% of the failure accidents of API casing occur at the screw connection, i.e. the joint.