Preventing, Dealing With Drill Pipe Fatigue.
Downhole pipe failures result in costly downtime as crews attempt to retrieve downhole pipe joints, bits, transmitters and backreamers, and the delays caused by interrupted production often affect other parts of projects.
Experienced drillers say that many pipe failures should never happen.
Problems often are the result of careless operational procedures, failure to properly maintain pipe, and placing damaged pipe in a drill string, even when it is evident that the joint should be replaced.
Tremendous forces are generated during directional drilling operations, and the stresses HDD pipe must withstand are not the same as those in vertical drilling or other applications in which pipe is used. Each length of pipe must be flexible enough to be steered effectively, yet have sufficient rigidity not to break or become permanently bent. Connections must be durable to resist wear from repeated use.
Drill operators and other crew members must understand how these forces affect HDD pipe during drilling and pullback and recognize the limits of pipe to withstand these stresses.
"Drill stem for HDD applications must be able to handle the torque, thrust, pull and bending forces it is subjected to while making pilot bores, backreaming and product pullback," says Mark Van Houwelingen, market analyst, trenchless product group, Vermeer Manufacturing Co. "At the same time, pipe must have the flexibility required to follow curves in the bore path."
When drill stem is not kept within the limits of its torque range, thrust and pullback limitations, and bending radius, metal fatigue may occur resulting in premature and costly failure.
"Bending and rotation of drill stem causes normal wear fatigue," says Van Houwelingen. "Over steering a drill stem beyond its allowable limits is probably the most common cause of premature metal fatigue and will usually result in premature failure of the drill stem. This is why it is recommended that bent drill stems be removed from the machine."
Makeup and breakout of drill stem joints also imposes stress on the drill stem. An improperly-torqued joint can result in pin (male connection) breakage or flaring of the box (female connection) and pin shoulders because the shoulders of the connection will not make proper contact and the pin will rock inside of the box, causing fatigue. Pipe joints must be properly lubricated with tool joint compound, not grease. Leaking joints mean the connection is bad or the pipe is damaged.
"Proper makeup and breakout techniques play a vital role in the life of pipe joints," Van Houwelingen says. "Fatigue in the tube body may or may not be the result of improper operational procedures. But if there is damage to threads and shoulders, there is most likely an operator problem that needs to be corrected. Forcing make-up and break-out can lead to galled, rolled or sharp threads. Improper alignment during make-up also can cause thread damage. These problems can be prevented by using proper make-up and break-out techniques."
"Handle drill pipe with care," says Mike Dvorak, Ditch Witch drill pipe and downhole tool manager, The Charles Machine Works Inc. "Never throw pipe around or bang ends against other pipe or objects. Make-up and break-out pipe slowly, and fully torque tool joints when adding pipe to the drill string. Plan every installation. Minimize entry angles when beginning a bore, and do not push pipe without rotation until half the pipe is in the ground. Steer in gentle, gradual bends, and don't stall pipe during drilling."
Rotating the sequence of pipe sections going in the drill string is always a good idea. For example, pipe lengths used at the beginning and end of the drill string on the last job should be somewhere in the middle of the string on the next job. Moreover, this practice is recommended by many drill rig and pipe manufacturers.
Of course, even when correct operational procedures are followed, wear affects drill pipe.
Metal becomes fatigued as it experiences repeated reverse stresses over a period of time, says Gary Haub, senior sales engineer at Drilltube International.
"With HDD drill rod," Haub continues, "fatigue begins when rod is pushed to make the bit cut, causing the pipe to bend. At each bend, metal particles on the outside of the bend stretch, while particles on the inside are squeezed. When pipe is rotated, stresses are reversed, allowing fatigue to accumulate."
Time and the severity of forces stressing metals affect fatigue. "Increasing a specific force shortens the time it takes metal to fail," says Haub, adding that a combination of three forces are responsible for most pipe failures:
* Torsion fatigue from operating rod beyond recommended rotational specifications;
* Compressive stress as rod is pushed to apply axial force to the drill bit; and
* Tension stress when drill tube is pulled.
"Corrosion, occurring when fluids with corrosive properties are in contact with drill rod for long periods, also is a factor in fatigue," says Haub. "To prevent premature corrosion damage, use corrosion inhibitors and flush with clean fluids after the bore."
In addition to these factors, Dvorak points out that pipe is subjected to increased wear when it comes into contact with rock or other objects in the soil that can score or cut pipe, creating weakened areas which may fail in the future.
Regular inspection of pipe can reveal various kinds of damage, although fatigue within the tube or body of the pipe may not be visible.
On ends of pipe, check for broken, cracked or flared joints and damage to threads. Keep joints clean using zinc or copper-based tool compound, not petroleum-based grease. Always use protective caps when pipe is not in use or is stored for a period of time.
On the tube, look for signs of cracks, cuts, gouges and excessive wear. Bent or bowed pipe should not be used. While pipe-straightening equipment is available and there are service companies who specialize in straightening pipe, many manufacturers recommend replacing bent pipe.
The pipe's connection point to the drill rig also must be inspected on a regular basis. "A worn or damaged drive chuck can damage every drill step connected to it," says Van Houwelingen. "When signs of wear are apparent, it should be replaced."
"Drill pipe is a wear item," says Dvorak. "Drill pipe will not last forever, and its cost must be factored into operating costs of each drill rig. When threads are worn out or connections leak drilling fluid or when the pipe tube is bent or worn past manufacturer's specifications, it is time to take it out of service. The amount of money saved by keeping damaged pipe in service or attempting to repair it is insignificant compared to the potential cost of pipe failure during a job."
Proper project planning, matching drill rig with pipe and downhole tools, understanding the functions and limitations of drill pipe, correct operational procedures, and following a regular pipe maintenance and inspection program help drill owners get the most from their drill pipe investment.
"The key to it all," says Van Houwelingen, "is training. The most crucial element to extend the life of drill stem is making sure that every operator for a particular machine is properly trained in the care and operation of the drill stem. The next is to make certain the operator cares enough to take care of the drill stem."
RELATED ARTICLE: The Many Functions Of Drill Pipe
Specialized HDD drill pipe performs several vital functions while drilling the pilot hole and during backreaming and product installation. The success or failure of every HDD project depends on drill pipe performance.
Pipe transmits engine horsepower into downhole horsepower in the form of thrust, pullback and rotational torque.
Downhole guidance system components are attached to the drill string behind the drill bit, and when a wireline quidance system is used, pipe is the conduit for the wire.
Drilling fluid is used to lubricate bore walls and downhole tools, suspend cuttings and cool the tracking system's downhole radio transmitter and is pumped downhole through the drill string.
To complete an installation, drill pipe pulls backreamer, pipe, conduit or cable into the ground through the pilot hole.