1、 Vibration, bending and wear
When the drill string reaches the critical speed, it will vibrate when it reaches the critical speed, which will often cause the drill pipe to bend, excessively wear, damage quickly and fatigue. Especially, when two kinds of vibration (wave joint vibration and spring swing vibration) coincide with each other, the drill pipe is the most unfavorable. The critical speed varies with the length of the drilling tool, the size of the drill tool, the size of the drill collar and the size of the borehole.
2、 Tensile failure
Tensile failure often occurs in the process of pulling the stuck drill pipe. When the pull-up 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 lifting force exceeds the limit strength, the drill pipe will be broken. Tensile failure usually occurs in the upper part of the drill string. Because the upper drilling tool must 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:
- Pure fatigue failure
The drill pipe should bear the periodic stress of tension, compression, torsion and bending. Tension and bending are the most dangerous stresses. At present, the main reason of drill pipe fatigue failure is the periodic stress when the drill pipe rotates in the curved hole. Even if the drill collar has enough thickness, fatigue failure may still occur in a curved hole, and the location of failure is not certain.
When the drill pipe is bent, the corresponding position of the drill pipe will produce periodic stress due to repeated tension and compression. The drill pipe above the drill collar is most likely to be bent, because the drill collar is relatively rigid and can resist bending, and the bending will occur on 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 50 cm away from the joint. As mentioned above, it is impossible for the joint to bend, and the bending can only occur on the drill pipe body with thin pipe wall. At the position where the cross-section changes, it plays a similar role as a vise, making it the fulcrum of bending force.
If the drill pipe is bent evenly over its full length, the stress acting on the drill pipe will become lower and the number of stress cycles of fatigue failure can be increased.
The values that determine the fatigue failure are: the tensile load on the drill pipe at the bending position; The severity of borehole bending; The repeated stress times of each section of drill pipe at the bend of drilling hole; Drill pipe size and steel properties.
Because the amount of tension the drill pipe bears is the key to fatigue failure, so in deep wells, the bending of the upper hole often becomes the factor of fatigue failure.
It should be pointed out that as long as the drill pipe passes through the bending part of the borehole, even if the tension is eliminated, accumulated fatigue damage still exists. Therefore, if the drill pipe passes the bending part several times, it may cause fatigue damage of the drill pipe. It is also important to see that a drill pipe may have been damaged even if there is no accident in a well.
When it is used in the same well or other wells again, even if it is connected to the top of the drill string, even if the bending stress acting on the drill pipe is within the rated value, it may not be able to bear, resulting in accidents.
In addition, there are other reasons that can cause pure fatigue failure of bending stress. For example, any bent drill pipe is often a potential factor of fatigue failure.
The bent Kelly will bend the first drill pipe below the rotary table. Once the stress on the drill pipe is large enough, it will cause fatigue damage. If the crown block is misaligned, the drill pipe will be fatigued because it will cause bending stress on the Kelly and drill pipe.
- Notch fatigue damage
The imperfection of the surface of drill pipe, whether formed by mechanical processing or in metallurgical process, will greatly affect the fatigue limit of drill pipe, and the degree of influence depends on the position, direction, shape and quantity of the defects.
If the notch is located on the drill pipe where the main stress is not, the effect on the fatigue failure is slight. However, if the notch is within 50 cm from the joint, it will become the core of fatigue failure because it is the place where the drill pipe produces the maximum bending stress.
The longitudinal notch can diffuse stress without harm. However, even a slight notch with a sharp base angle can increase the stress and lead to failure. Several kinds of drill pipe surface conditions that can cause notch fatigue damage are listed as follows:
(1) All the transverse marks of the steel seal pattern can become the stress concentration points. If the steel seal is made at an inappropriate position on the drill pipe, the transverse mark of the steel seal and the mark at any position will become the starting point of drill pipe fatigue.
Printing on drill pipe body is not allowed. The mark is marked at the part where the drill pipe thickens and disappears with a large cross-sectional area, which can safely absorb the generated stress. In addition, the dot number can be used to replace the line number, and the Roman numeral steel seal can be arranged along the drill pipe longitudinally, which can also play the effect of safety marking.
(2) If the drill pipe is used as grounding wire for arc burn, the arc between the tie rod and the rail will be caused, which will cause the drill pipe to be burned on the circumference. Although these small pits which are not easy to notice are very small, they form a wide burn zone on the drill pipe. It has the advantages of hard and brittle glass, which is easy to accelerate fatigue failure.
(3) The circular groove formed at the top of the rubber hoop on the drill pipe is another reason for the fatigue failure of the notch.
(4) Tong tooth mark is the most common and serious of all kinds of surface marks on drill pipe. However, because it is longitudinal and not in the same direction as the applied stress, it does little harm to the fatigue failure. If it deviates slightly from the vertical direction, it is easy to form stress concentration points. Hit the pliers on the joint, not on the drill pipe body, because it may damage the drill pipe.
(5) Slip scar generally, the rotary slip will not leave any damage mark on the drill pipe. However, if the slip teeth are not processed or operated properly, the slip teeth will bite the drill pipe.
(6) The surface of the drilling tool is cut by the broken metal in the formation and in the well. When the drilling tool rotates in the well, the surface will rub with the hard wellbore formation and leave a circumferential notch. This kind of notch is often triangular and is common on drill pipe and drill collar.
- Corrosion fatigue failure
这This kind of failure is the common cause of early failure of drill pipe at present. Corrosion may cause very serious damage in various or several comprehensive damage forms (erosion, abrasion, fatigue). Sometimes, several forms of corrosion will occur at the same time, but generally speaking, there is always one form of corrosion as the main cause of damage.
(1) The main corrosive agents that affect drilling tool 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
Water based mud with low pH value will reduce the fatigue life of drilling tools. The pH value is the main factor to control the corrosion fatigue, but it is difficult to accurately determine the minimum pH value enough to prevent the fatigue damage. Many users believe that the mud pH value lower than 9.5 will reduce the fatigue life of drilling tools.
The corrosion rate of the vast majority of temperature will increase with the increase of temperature.
The velocity of the mud increases with the increase of the velocity of mud, which will accelerate the corrosion rate.
The local difference of the component or microstructure of the inhomogeneous steel will increase the corrosion rate.
The corrosion rate of high stress is faster than that of low stress parts.