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Why Drill Pipe was Broken?
Oct 01, 2018

Causes of damage to the drill pipe during drilling

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First, 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 damage. In particular, it is most unfavorable when the drill pipe is subjected to two types of vibrations (wave joint vibration and spring pendulum vibration). The critical speed varies with the length of the drill, the size of the drill, the size of the drill collar, and the size of the wellbore.

Second, tensile damage

Tensile failure often occurs during the process of pulling the stuck drill pipe. When the upper pulling force exceeds the yield point, a "slim neck" deformation occurs at the weakest or smallest section of the drill pipe wall. If the upper pulling force exceeds the ultimate strength, the drill pipe will be pulled off. Tensile failure generally occurs in the upper part of the drill string. Because the upper drill is both under pressure and has to withstand the weight of the drill string.

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Third, fatigue damage

Fatigue damage is the most common in drill pipe failure. There are three basic types of fatigue damage:

1, pure fatigue damage

The drill pipe is subjected to periodic stresses of tension, compression, torsion and bending. Stretching and bending are the most dangerous stresses. At present, the main cause of fatigue failure of the drill pipe is the cyclic stress caused by the drill pipe rotating in the curved wellbore. In a curved wellbore, even if the drill collar has sufficient thickness, fatigue damage may occur and the location of the damage is not necessarily. When the drill pipe is bent, each time the drill pipe rotates, its corresponding position will be periodically stressed due to repeated tension and compression. The drill pipe near the drill collar is most likely to be bent because the drill collar has greater rigidity and resists 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 portion of the drill pipe, about 50 cm from the joint. As mentioned above, the joint is unlikely to bend, and the bending can only occur on the drill pipe body where the pipe wall is thin. In the position where the cross section changes, it acts like a vise fixing, making it a fulcrum of the bending force. If the drill pipe is uniformly bent over its entire length, the stress acting on the drill pipe becomes lower, and the number of stress cycles of fatigue failure can be increased.

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The values that determine fatigue failure are: the tensile load on the drill pipe at the bend of the drill pipe; the severity of the drill bend; the number of repeated stresses per drill pipe at the bend of the drill hole; the size of the drill pipe and the properties of the steel. Since the amount of tension exerted by the drill pipe is the key to fatigue failure, in deep wells, the upper hole bending tends to be a factor of fatigue damage. It should be noted that as long as the drill pipe passes through the bend of the drill hole, even if the tensile force is eliminated, the accumulated fatigue damage still exists, so if the bend is made through the drill hole several times, the drill pipe may be fatigue-damaged. It should also be noted that a drill pipe may have been damaged even if there is no accident in a well. therefore. When it is reused in the same well or other 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 be unacceptable, resulting in an accident. In addition, there are some other bending stresses that cause pure fatigue damage. For example, any curved drill pipe is often a potential factor for fatigue damage. A curved kelly bends the first drill pipe below the turntable. Once the stress on the drill pipe is large enough, it can cause fatigue damage. 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, the recess fatigue damage

The incompleteness of the drill pipe surface, whether formed by machining or formed during metallurgical processes, will greatly affect the fatigue limit of the drill pipe, the extent of which depends on the location, direction, shape and number of defects. If the notch is located on the drill pipe where it is not the main stress, the effect on fatigue damage is small. However, if the notch is located within 50 cm of the joint, this is the core of fatigue failure because this part is the maximum bending stress generated by the drill pipe. Longitudinal notches can diffuse stress without harm. However, even a subtle score with a sharp base angle increases the stress and causes damage. Several surface conditions of the drill pipe that can cause fatigue damage of the notch are listed below:

(1) Stamping pattern All lateral impressions can be stress concentration points. If the stamp is printed at an inappropriate position on the drill pipe, the lateral impression of the stamped font and the mark at any position will be the starting point for the fatigue of the drill pipe. Printing on the drill rod body is not allowed. The mark is placed on the thickened and disappeared part of the drill pipe with a large cross-sectional area, and the generated stress can be safely absorbed. In addition, the dot number can be used instead of the line number, and the Roman digital stamp can be arranged along the longitudinal direction of the drill pipe, and can also be used as a security mark.

(2) Arc burn If the drill pipe is used as the grounding wire for grounding, the arc between the iron bar and the rail will often cause the drill pipe to be burned on the circumference of the drill pipe. Although these small pits that are difficult to notice are extremely small, they are A wide burn band is formed on the drill pipe. With hard and brittle glass, it is very easy to accelerate fatigue damage.

(3) Rubber Guard Hoop The circumferential groove formed on the top of the rubber band on the drill pipe is another cause of frustal fatigue damage.

(4) Large tongs dents Among the various surface scars on the drill pipe, tongs are the most common and most serious. However, since it is longitudinal and does not have the same direction as the applied stress, it is not harmful to fatigue damage. If it deviates slightly from the vertical direction, it is easy to form a stress concentration point. It is necessary to hit the pliers on the joint without hitting the drill rod body as it may damage the drill rod.

(5) Kava scars Turntable slips generally do not leave damaging scars on the drill pipe. However, if the kava is poorly processed or poorly handled, the kava will bite the drill pipe.

(6) Cutting holes in the formation and the fractured metal in the well The surface of the drill rotates in the well, and the surface will rub against the hard well wall formation to leave a circumferential nick. Such scores tend to be triangular and are common on drill pipes and drill collars.

3. Corrosion fatigue damage

This damage is currently a common cause of early damage to the drill pipe. Corrosion can be extremely severely damaged by various or several types of integrated damage (erosion, wear, fatigue). Sometimes, several forms of corrosion occur simultaneously, but in general, there is always a form of corrosion that is the main cause of damage.

(1) Corrosive agents The main corrosive agents affecting the steel of drilling tools 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

PH value Low pH pH based mud will reduce the fatigue life of the drill. The pH value is a major factor in controlling corrosion fatigue, but how to accurately determine the minimum pH 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 increase with increasing temperature.

Flow rate The rate of corrosion increases as the mud flow rate increases.

Inhomogeneity Local differences in steel composition or microstructure increase the rate of corrosion.

High stress is generally faster in areas subjected to high stress than in low stress areas.

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