The connection between coiled tubing and the bottomhole assembly is the weakest point in any milling or reaming job, and it is the one place where a failure cannot be recovered by pulling harder. An internal slip connector addresses that point directly: the slips bite on the inside of the pipe, the outside diameter stays flush with the coil, and torque goes through drive teeth rather than through grub screws. On a job where the motor is straining and the coil is starting to wind up, that difference is what keeps the string together. The same joint has to hold while the coil is snubbed into a live well, so the tension path is tested before the torque path ever is.
What an Internal Slip Connector Holds Together
The connector joins the coil to the tool string and carries both tension and torque. It is chosen when the outside diameter of the assembly has to match the coil, which is the normal case in a completion with a small clearance, a liner top or a nipple profile that the BHA must pass. Sand washing and gas lift strings use the same connector because the flush profile leaves nothing to hang up on a ledge. In a well with a liner top, a flush connector is often the only way to get the BHA past the transition without a wiper trip.
Slip Engagement and Torque Path
The slips sit inside the pipe and are locked by rotating the connector body. Drive teeth on the slip engage the bottom sub, so rotational torque from the tool string passes into the pipe through a positive mechanical engagement instead of friction. The alternative, a grub screw arrangement, depends on point loading and can back out under vibration. A non-rotational slip pattern also gives bidirectional torque resistance, which matters when a motor stalls and the string is worked free in both directions. The slip contact area is spread along the pipe wall rather than concentrated at a few screws, so the coil is less likely to be crushed at the connection.
Sizes Matched to Coiled Tubing Strings
The connector is specified by coil size rather than by tool size. Common combinations include 1-1/2 inches coil with a 1.5 inches maximum outside diameter, 1-3/4 inches coil with 1.75 inches, 2 inches coil with 2 inches, and 2-3/8 inches coil with 2.375 inches, with bores from 0.5 to 1 inch depending on wall. Top threads are 1 inch AMMT on the smaller sizes and 1.5 inches AMMT on the larger ones. A connector that is one size off will either not enter the pipe or will not seal against it. Wall thickness matters as much as diameter: a 0.125 inch wall and a 0.175 inch wall of the same outside diameter need different slip sets to develop the same grip.
Installation Without Make-Up Torque
There is no torque specification to chase at the wellhead. The connector is fitted to the pipe, an overpull is taken to set the slips and prove the connection, and the job proceeds. The one preparation step that is easy to skip is the pipe end: the inner circle of the coil has to be ground smooth so the seal ring can seat, and a burr left from the cutting operation will leak no matter how much overpull is applied. A short pressure test on the connection before the BHA is run is a cheap way to find that out at surface instead of at depth.
Grind the coil inner surface before fitting the connector.
Record the overpull used to set the slips in the job file.
Keep replacement slips on location; they are a wear item.
Internal Slip Connector Checks After a Milling Job
Milling and reaming put the highest torque through this joint, and the inspection after the job is where the next failure is prevented. Look for slip tooth marks that have flattened, check the seal ring for cuts, and confirm the drive teeth are not rounded over. A connector that has been worked hard can still pass a visual check and fail on the next run, so the slip set is replaced on a cycle count rather than on appearance.
A flush connector is a small piece of steel that decides whether the coil and the BHA stay married under load. Set the slips properly, protect the seal surface, and the torque path stays inside the design.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
