As long as the drill pipe passes through the bend of the hole, even if the tension is eliminated, the accumulated fatigue damage still exists, so if the drill pipe passes through the bend of the hole several times, it is possible to fatigue damage the drill pipe. It is also important to note that a drill pipe in a given well may have been damaged even if there was no accident. Therefore. When it is used in the same well or another well, although it is attached to the top of the drill string, even if the bending stress acting on the drill pipe is within the rated value, it may become unbearable and cause accidents.

In addition, there are some other reasons that can cause bending stress of pure fatigue failure. For example, any bent drill pipe is often a potential factor in fatigue failure. A bent kelly will bend the first drill pipe below the turntable, causing fatigue failure once the stress on this drill pipe is high enough. A misalignment of the crown can also cause fatigue failure of the drill pipe because it causes bending stress on the kelly and drill pipe.

PH Low PH water-based mud will reduce the fatigue life of the drill tool. The PH value is the main factor in controlling corrosion fatigue, but how to accurately determine the low PH value sufficient to prevent fatigue damage is difficult. Many users believe that mud PH below 9.5 will reduce the fatigue life of the drill. Temperature Most corrosion rates will accelerate with the increase of temperature. The rate of corrosion will be accelerated as the mud flow rate increases. Local differences in steel composition or microstructure can increase the rate of corrosion. In general, the corrosion rate of high-stress parts is faster than that of low-stress parts.

This kind of failure is now the common cause of oil drill pipe failure. Corrosion may cause extremely serious damage in various or several combined forms of failure (erosion, wear out, fatigue). Sometimes, several forms of corrosion occur at the same time, but in general, one form of corrosion is the main cause of failure.

(1) Corrosives The main corrosives affecting drill steel are: oxygen, carbon dioxide, hydrogen sulfide, dissolved salts (chloride, carbonate and sodium sulfate, calcium, magnesium), various acids (formic acid, acetic acid, etc.).
(2) Factors affecting corrosion rate.