In oil production, API standard drill collar（www.baowi-steel.com） threads are commonly used, which have the advantages of high sealing, strong load-bearing capacity, and fast loading and unloading speed. However, with the increasing number of H mined in directional wells, heavily corroded wells, deep wells, and ultra-deep wells, the reliability and sealing performance of API drill collar threads have been unable to meet the mining needs of oil and gas wells. Accidents caused by drill collar thread failure are common. occur. The loads that oil drill collars bear in the wellbore are very complex. The drill collar joints bear the effects of internal pressure, external pressure, axial load, bending load and torque at the same time. During the drilling process, the drill collars also bear the effects of dynamic loads. Under complex loads, drill collar failure is common in the oil drilling industry. Statistics on drill string damage abroad show that the average loss per drilling accident is approximately US$106,000, which is estimated to account for 14% of the total drilling costs. Among these damages, punctures are not included, fatigue cracks in which drilling fluid can still circulate within the drill pipe without actually breaking.
Through the analysis of many drilling accidents, it is not difficult to find that accidents caused by fatigue failure of drill collars account for a large proportion. Practice shows that most of the causes of drill collar thread failure are fatigue, and fatigue failure is mainly due to excessive alternating load on the drill collar. In the drill collar, the connection part between it and the thread is relatively fragile, so it is easier to generate higher alternating stress in this part and serious stress concentration occurs, so fatigue fracture is very easy to occur in this part. Therefore, dispersing the stress concentration at the threaded joint is the key to improving the working characteristics of the thread and reducing the drill collar failure rate.
A large number of practices at home and abroad have proven that when the average stress is far less than the service strength, tiny irregular cracks will gradually develop into large cracks and damage at the parts where the alternating stress acts, becoming a high-cycle low-stress fatigue fracture. Some alternating stress cycles do not have many cycles, but the average stress is quite high, which can also cause fatigue damage, causing low-cycle high-stress fatigue damage. Both forms exist in drill string fatigue failure.
At present, the main cause of drill string fatigue failure is due to the periodic alternating bending stress that occurs when the drill string rotates in a curved wellbore. When a curved pipe rotates, one side of it is first stretched, then compressed, and then stretched. If there is a transverse gap on the surface of the pipe, it will expand whenever the gap turns to the side that is under tension. open. When the stress at the bottom of the notch reaches a certain value, the notch will continue to expand at a certain rate, so that the remaining section is not enough to support the load surface fracture.
The fatigue damage of drill string threads generally has two forms: fatigue fracture and shear failure of the screw thread. Due to the bending of the wellbore or the dynamic load bending moment endured by the drill collar, it is subjected to alternating bending stress, which can easily cause fatigue damage. However, since the drill collar body is more rigid than the threaded joint, the stress is concentrated in the wellbore with a large inclination angle. When one or more drill collars are connected to the uppermost stabilizer, the drill collar above the stabilizer will droop sharply. , making the threaded connection on the upper part of the stabilizer prone to fatigue, causing excessive stress there and reducing the fatigue life of the drill collar. This problem will be more serious when the ratio of the size of the stabilizer to the size of the drill collar body increases, because the drill collar sitting on it will sag further, and the threaded connection of the bottom hole tool assembly will produce more in this case. Big fatigue damage.
Because the drill string is slender, in a vertical or nearly vertical wellbore, the bottom part of the drill string is subject to compressive load (the mechanical load acting on the drill bit and the liquid pressure acting on the surface of the exposed part of the drill string produce compression), When the compressive load is higher than the critical buckling load, buckling deformation of the drill string or joint will occur.
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