Drill pipe is expensive, as everyone involved with horizontal directional drilling (HDD) knows. One of the realities of working with drill pipe (more commonly known as “drill rod”) is the watch beetle of metal fatigue. The bending, flexing and over-torquing of the pipe on the rig and in the hole creates microcracks, reduces pipe wall thickness, or leaves the pin (male threads) and box (female threads) of the drill rod so badly damaged the drill rod cannot be properly made up into the drill string.
Over-steering the drill pipe and attempting bends too tight for the bend radius of the pipe do more damage, but these problems happen less frequently. These issues will always occur, but some commonsense precautions can help prolong the life of your drill pipe. (Choosing the correct drill rod is essential for project success. Learn more in Choosing Drill Rods for Trenchless Tunneling.)
Before You Drill
Shortening the life of your drill pipe begins on the rig floor. On the first day, as soon as the rig is in place, everyone wants to get downhole as quickly as possible. As the drill string is put together, the torque on the pin and box may be fudged – or worse, spun up to the maximum torque the rig can produce – while everybody is scrambling to assemble the drill string. Once the drill reaches depth, the operator turns the drill string sharply onto the planned path.
All of the foregoing activities represent a good recipe for early drill pipe failure.
When assembling the drill string on the rig, failure to tighten the pin and box to the correct torque stresses the threads of the male and female connections during drilling. Too much torque causes the box to try to force itself over the shoulder of the pin of the drill rod inserted into it. When this happens, the box flares and, as it shows itself over the shoulder the pin, it causes the head of the pin to mushroom.
The flared box is slightly shorter on one side and the face of the box on the exterior torque shoulder will not make up correctly because of the mushroomed pin. This situation leads to accelerated thread wear that and a shortened drill pipe life.
The same thing happens with a mushroomed pin. A mushroomed pin is shorter. When made up to a good box, it will not make up correctly and excessive thread wear will result, as will a shorter drill pipe life.
The solution is simple: Torque drill rods to the recommended makeup torque. Inspect the threads on the pin and in the box for wear. Use the correct lubricants.
Pipe Bend Radius
Another issue that can make the short-and-merry life of the drill pipe shorter still is the stress that accompanies over-steering the pipe – turning the pipe with a radius smaller than the bend radius of the pipe. This can result in a bent or broken drill rod, a problem that, if it occurs downhole, can require the presence of a wireline unit to fish the broken or damaged parts out of the hole. Even an experienced operator can have this problem.
Perhaps undisclosed or unknown subterranean conditions – for example, a granite shelf – impinges on the design pathway at a location unchecked by the geotechnical service may cause over-steering to avoid the obstacle. A loadout error at the shop may result in the wrong drill rod – one with the bend radius too large for the project – being sent to the job site with unintended long-term consequences.
Solutions are as varied and numerous as the potential problems here. Double-checking drill stem loads before they leave your shop will help ensure that the correct materials are job-site-bound. When your operators encounter unexpected subsurface issues, make sure they seek guidance: The long-term effects of over-steering a drill pipe may be far outweighed by the profits of the project.
Experienced operators maximize the life of their drill pipe by knowing the bend radius of the pipe for the job. Once in the hole, it’s up to the operator to monitor the drill pipe’s activity, including the turns and bends. One method of minding the bend radius is to divide the drill pipe’s stated bend radius – expressed as a percentage of slope per piece of drill rod – by the length of the drill rod. This quick-and-dirty formula gives a reasonably accurate bend radius that’s not to be exceeded.
Almost all drilling contractors have new pipe and old pipe, but they should never be run in the same drill string unless the old pipe is used at the very end of the drill string. If you integrate old pipe with new pipe in a drill string, there will be multiple connections between old pipe and new pipe; problems like mushroomed pins and flared boxes will be passed along from the old drill rod to the new drill rod.
When old pipe is used at the end of the drill string, the potential for damage to new drill rods is limited to the single juncture between the old and the new. Another way to lengthen the life of your drill pipe is to rotate the individual drill rods. It's important to keep your equipment in top shape. Learn more in Proper Maintenance for Drill Rig Equipment.)
The process of lengthening the life of your drill pipe should continue after the project’s completed and the drill rods are returned to the shop, but before the celebratory beers are cracked open. The threads in each pin and box on each drill rod can be visually inspected for damage.
Each drill rod should get a visual once-over to ensure there are no problems visible to the naked eye, and the threads should be greased in preparation for the next outing.