Thread problems are the most common problem encountered with the use of drill pipe. The purpose of this section is to help locate potential causes and correct them. It is important to note that an exact cause may not be found but the problem can be made to go away.
One of the characteristics of thread problems is that it spreads throughout your drill string. This happens as one damaged thread connects with another. If the top sub becomes infected, then the problem propagates with every connection made with the top sub.
When a thread problem is discovered, it is important to remove the drill rod with the damaged connection and inspect the other drill pipe in the string.
Over torque is a common cause of thread problems. One way to prevent this is to torque the connections to the designated make-up torque. Make up torque is the amount of torque needed to properly tighten the connections so they act as a unit.
Most over torquing does not occur at the table. This most commonly occurs when the drill pipe is run down the hole loose i.e. when not enough torque is applied at the table. Under torqued connections tend to continue tightening down the hole and can over tighten.
Indications of over torque are swelled or mushroomed box end shoulders. Another indication is that the pin connection will stretch or break.
Galling can occur for a variety of reasons. Galling is caused by metal to metal contact between the pin and box threads. When this happen, the metal from the one thread is “picked up” by the other. Sometimes, this is minimal and the connections appear to work OK but create problems later.
A good grade of tool joint compound will prevent galling – if it is kept clean and applied at every connection. Tool joint compound is not an area to skimp on quality.
Sometimes galling is caused by stabbing. Stabbing is the term used to describe bumping the connections too hard when making connections. This can cause the thread crest to slightly “roll over”. If this “roll over” is significant enough, it will gall during make up or perhaps peal off creating an obstruction. This obstruction can be mashed during make up and create galling.
Cross threading can occur when the connections are not properly aligned when making up. Typically, when this happens, the connections will not shoulder and will be difficult to break.
Care during make up can prevent this type of damage.
What to look for
Drill pipe connections can be screwed together by hand. Sometimes there can be a slight gap – less than 1/8” that would require more torque than can be done by hand.
When making up the drill pipe, keep an eye on the holding wench. When the connections begin to be in a bind, it is typically noticed when the holding wrench or slips move to one side.
There are several factors involved:
One of the characteristics of thread problems is that it spreads throughout your drill string. This happens as one damaged thread connects with another. If the top sub becomes infected, then the problem propagates with every connection made with the top sub.
When a thread problem is discovered, it is important to remove the drill rod with the damaged connection and inspect the other drill pipe in the string.
Over torque is a common cause of thread problems. One way to prevent this is to torque the connections to the designated make-up torque. Make up torque is the amount of torque needed to properly tighten the connections so they act as a unit.
Most over torquing does not occur at the table. This most commonly occurs when the drill pipe is run down the hole loose i.e. when not enough torque is applied at the table. Under torqued connections tend to continue tightening down the hole and can over tighten.
Indications of over torque are swelled or mushroomed box end shoulders. Another indication is that the pin connection will stretch or break.
Galling can occur for a variety of reasons. Galling is caused by metal to metal contact between the pin and box threads. When this happen, the metal from the one thread is “picked up” by the other. Sometimes, this is minimal and the connections appear to work OK but create problems later.
A good grade of tool joint compound will prevent galling – if it is kept clean and applied at every connection. Tool joint compound is not an area to skimp on quality.
Sometimes galling is caused by stabbing. Stabbing is the term used to describe bumping the connections too hard when making connections. This can cause the thread crest to slightly “roll over”. If this “roll over” is significant enough, it will gall during make up or perhaps peal off creating an obstruction. This obstruction can be mashed during make up and create galling.
Cross threading can occur when the connections are not properly aligned when making up. Typically, when this happens, the connections will not shoulder and will be difficult to break.
Care during make up can prevent this type of damage.
What to look for
Drill pipe connections can be screwed together by hand. Sometimes there can be a slight gap – less than 1/8” that would require more torque than can be done by hand.
When making up the drill pipe, keep an eye on the holding wench. When the connections begin to be in a bind, it is typically noticed when the holding wrench or slips move to one side.
There are several factors involved:
- Manufacturer
- Operator
- Rig
- Drilling Conditions
Obviously, if the manufacturer does not manufacture the connections and drill rod per specifications, this can cause problems. When machining rotary shouldered connections, several characteristics are involved. There is the size, thread form, and taper that must meet specifications.
At one time, threading tools were basically made by the machinist. They ground their tool steel threading tool to match the thread form using a sight gage. Any more, just about every one utilizes pre-made threading inserts that automatically provide the correct thread form. Even though, care must be taken to make sure these tools are discarded when wear becomes significant. Worn threading inserts can lead to incorrectly machined connections.
The taper is typically within tolerance when machining on a CNC lathe. If threaded on a manually operated lathe, the taper is set by the machinist. With use, the taper attachments can wear causing the taper to be out of tolerance. Another possibility is that the taper attachment could be set incorrectly or the markings on the attachment are not accurate due to wear.
The size of the connection is very important. Typically, the blanked connection ( this is when the connection is ready for the actually treading) is measured to insure the size is correct. After the threads are machined, the connection is gauged utilizing an API-approved, hardened, ground thread gage. Since the connections are tapered, they are measured by the “stand-off” for the pin connection or “stand-in” for the box connection. The pin stand-off should be between 0.620” to 0.635”. The box stand-in should be 0.000” to 0.010”. This gage does wear and must be properly maintained.
Gauging the connections typically insures the connections are machined correctly as long as the taper & thread form are correct. That is a rather big “if” as a connection with thread form or taper problems can be machined to gage correctly even though it is not correct.
There are other manufacturing issues that can lead to thread problems. These include machining the connections to run true with the OD and the tool joints properly aligned with the tube. Connections can be made correctly but get damaged during the manufacturing procedures.
A good inspection procedure can catch most manufacturing problems.
Determining whether your problem is a manufacturing problem is not always obvious. You really need to depend on the manufacturer to inspect and make the determination that there was a manufacturing problem.
If you have some unused drill rods from the order, these can be closely inspected. If the problem was not just a one time error, these unused rods should have the same problem.
Once a connection has been used, it can no longer be accurately gauged. Also, if new connections have some anti-gall treatment such as phosphating, they can not be accurately gauged even if unused.
If the manufacturing problem is due to many of the factors listed above, the manufacturer may have experienced problems elsewhere and be aware of the potential for problems in the field.
From the manufacturer’s standpoint, if the thread problem is wide spread in your drill string but not elsewhere, they will tend to point towards other factors. This is logical and you should not be offended. With problems existing with only your drill string, I would typically consider manufacturing problems as a possibility but not likely.
Manufacturing defects should appear within the first few times the drill rods are used.
In any case, if you are not happy with your manufacturer’s results, you can hire an outside consultant.
Operator error is probably the most common cause. Many problems can arise from stabbing, under-torquing, cross threading, lack of use of a good grade tool joint compound, and un-clean connections. With carousel rigs, there is not just the connection at the table to consider but also the connection at the top of the mast – one you can not see.
Not noticing problem connections and removing damaged drill pipe from the rig can lead to complete destruction of the drill string. Not maintaining the top sub can also lead to problems.
One of the more critical times for an operator is when new drill pipe is added. A break in procedure should be followed to help these new threads survive their first few connections. New threads are not as forgiving as threads that have been in use and worn in.
Another critical time is when starting up a new drill rig with new drill pipe. Not only should the drill pipe be cared for as above, but the new rig will probably not run like your old rig.
The operator standpoint is typically that they have been operating their rigs for years without thread problems. If thread problems arise, especially with relatively new pipe, the operator may believe that the problem must be with the drill rods. However, this is not always the case.
Rig alignment is also an important, often overlooked, factor. Here we are looking at how the threads of the drill rod in the table align with the drill rod in the head. If this alignment is not correct, it will put undue pressure on the threads during make-up and break out. Even if this does not produce obvious thread problems, it will wear your connections prematurely.
Checking the rig alignment should be added to you periodic rig maintenance schedule. Check with the rig manufacturer to see how best to correct any problems.
For carousel rigs, alignment at the top is just as critical.
Drilling conditions can create problems. Difficult drilling conditions can add extra stress to the connections. Sometimes this can lead to pin or box failure.
Stress is accumulative. As the connections are stressed over time, the pin can snap off at the base. The box end can also beak off – typically at the back of the connections.
A variety of things can happen down hole that can be detrimental to thread life. Difficult conditions can lead to over torque. A sudden lead off by the bit can put excess stress on the connections.
When thread problems occur, probably the first thing to do is to inspect your drill string. See how far spread the problem has become and remove drill rods with damaged connections. Inspect the top sub and other tools.
If the drill pipe is relatively new or you think it could be a manufacturing problem, contact your supplier and get them involved. Digital cameras make for a relatively easy way to give the manufacturer a fast look at the problem.
Meanwhile, check all of the other factors mentioned. You want to be sure that when new pipe is put on the rig, that any problem issues have been resolved. If you think it could be a rig issue, get a serviceman out to check it out.
When everything is considered, we may still be scratching our heads as no one cause has been found. It could be that by looking everything over and making a few minor adjustments could have eliminated the problem. Maybe a one time event happened causing damage that was not discovered until it spread.
In any event, by checking out all of the possibilities, you increase the likelihood that the problem will disappear.
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