1. vibration, bending wear
When the drill string reaches the critical speed, the drill pipe will vibrate. This vibration will often cause the drill pipe to bend, excessive wear, rapid damage and fatigue failure. In particular, the drill pipe has two types of vibration (node vibration and spring pendulum vibration) when the most unfavorable. The critical speed varies with the length of the drilling tool, the size of the drilling tool, the size of the drill collar and the size of the borehole.
2. tensile failure
Tensile failure often occurs in the process of lifting the stuck drill pipe. When the lifting force exceeds the yield point, the "thin neck" deformation will occur at the weakest part or the smallest section of the drill pipe wall. If the pull-up force exceeds the limit strength just now, the drill pipe will be broken. Tensile failure generally occurs in the upper part of the drill string. Because the upper drilling tool has to bear both the pressure and the weight of the drill string.
3. fatigue failure
Fatigue failure is the most common failure of drill pipe. There are three basic types of fatigue failure:
1. Pure fatigue failure
The drill pipe is subjected to cyclic stresses of tension, compression, torsion and bending. Stretching and bending are the most dangerous stresses. At present, the main cause of fatigue failure of drill pipe is the periodic stress of drill pipe rotating in curved borehole. In a curved borehole, even if the drill collar has sufficient thickness, fatigue failure may still occur, and the location of the failure is not necessarily the same. When the drill pipe is bent, the corresponding position of the drill pipe will generate periodic stress due to repeated tension and compression for each rotation of the drill pipe. The drill pipe above the drill collar is most likely to be subjected to bending because the drill collar is more rigid and can resist bending, and bending will occur in the drill pipe above the drill collar. At the same time, the maximum stress on the drill pipe occurs at the end of the thickened part of the drill pipe, about 50cm from the joint. As mentioned above, the joint can not be bent, bending can only occur in the pipe wall thin drill pipe body, in this section change position, play a similar vise fixed role, make it become the fulcrum of bending force. If the drill pipe is bent uniformly over its entire length, the stress acting on the drill pipe becomes lower and the number of stress cycles for fatigue failure can be increased.
In addition, there are other causes of bending stress that can cause pure fatigue failure. For example, any bent drill pipe is often a potential factor for fatigue failure. The bent kelly will bend the first drill pipe below the rotary table. Once the stress on this drill pipe is large enough, it will cause fatigue damage. The misalignment of the crane will also cause fatigue damage to the drill pipe, because it will cause bending stress on the kelly and the drill pipe.
2. Fatigue damage of notch
The imperfect surface of the drill pipe, whether it is formed by mechanical processing or metallurgical process, will greatly affect the fatigue limit of the drill pipe, and the degree of influence depends on the location, direction, shape and number of defects. If the notch is located on the drill pipe where the main stress is not applied, the effect on fatigue failure is relatively small. However, if the notch is located within 50cm from the joint, it will become the core of fatigue failure because this part is the drill pipe where the maximum bending stress is generated. The longitudinal notch can diffuse stress without harm. However, a score that is even fine, but has a sharp base angle, increases the stress and leads to failure.
3. Corrosion fatigue damage
This damage is a common cause of early damage to drill pipe at present. Corrosion can cause extremely serious damage in various or several combined forms of damage (erosion, wear, fatigue). Sometimes, several forms of corrosion will occur at the same time, but in general, there is always one form of corrosion as the main cause of damage.
(1) Corrosive The main corrosive agents that affect the steel of drilling tools are: oxygen, carbon dioxide, hydrogen sulfide, dissolved salts (chloride, carbonate and sodium sulfate, calcium, magnesium), and various acids (formic acid, acetic acid, etc.).
(2) Factors affecting corrosion rate
Water-based mud with low PH value will reduce the fatigue life of drilling tools. The pH value is a major factor in controlling corrosion fatigue, but it is difficult to accurately determine the minimum pH value sufficient to prevent fatigue damage. Many users believe that the mud pH below 9.5 will reduce the fatigue life of the drilling tool.
Temperature, most of the corrosion rate will increase with the increase of temperature.
flow rate, as the mud flow rate increases will accelerate the rate of corrosion.
Inhomogeneity, local differences in steel composition or microstructure, can increase the corrosion rate.
High stress, generally in the high stress parts than the low stress parts of the corrosion rate faster.