A casing string never lands exactly where the plan says it will. The shoe is a metre high or low, the hanger sits deeper than expected, and the space-out has to be corrected with something. That something is a pup joint: a short, non-standard length of API pipe that closes the gap between the string as designed and the string as it actually goes in the hole. Getting the length right is a measurement exercise; getting the grade and connection right is what keeps the correction from becoming the weak point.
What a Pup Joint Does in a String
Standard tubulars come in lengths around 31 feet (9.45 meters), and a string assembled from them will not hit an exact depth. The rig crew measures every joint into the tally for that reason. A pup joint adjusts the height of the string so the hanger lands in the right place and the tools below sit at the intended depth. The same joint is used in tubing strings for space-out above a packer, in drill strings for a short correction, and in casing strings where the shoe has to be set at a specific point in the formation.
Lengths and Grade Coverage
Vigor supplies pup joints from 0.75 to 20 inches nominal outside diameter in lengths of 2, 3, 4, 6, 8, 10 and 12 feet, with custom lengths on request. Grades cover H40, J55, K55, N80, L80, C90, T95, P110, C110 and Q125, plus 13 percent chromium for corrosive service. The grade has to match the string it joins, because a pup joint in a P110 casing string that is made from J55 becomes the limiting component in the load path.
Connections and Make-Up
Connections include EU, NU, SC, LC, BC and premium threads, in pin by pin, box by pin or box by box configurations. The joint is made up with the same torque procedure as the rest of the string, and the tally has to be updated with the actual length rather than the nominal one. A pup joint that is measured with a tape at the rack and made up at a different temperature will still be close enough for space-out, but the number that goes on the tally should be the measured one.
Pup Joint Ordering for a Casing String
The order needs four things: outside diameter, wall thickness or weight, grade and connection. Add the end finish and the required length, and state whether a mill certificate and a hydraulic test are required. For a casing string that will be cemented, the pup joint also has to pass the same drift as the rest of the string, or the cementing plug will hang up at the one joint that was ordered in a hurry.
Measure the joint at the rack and record the actual length.
Match grade and wall to the string, not to the nearest stock item.
Confirm drift before the joint goes into the elevator.
Handling and Inspection at the Wellsite
Pup joints are handled more than most pipe because they are short and easy to move, and that is how threads get damaged. Keep the thread protectors on until make-up, lift with a sling rather than a chain, and inspect the seal area for dents before the joint goes in the string. On a workover, keep the pup joints in a rack with the sizes marked, because the wrong length at the wrong moment costs a trip. A pup joint that has been in the rack for a year also needs its threads re-inspected, since storage damage is more common than handling damage.
A pup joint is a small correction with a large consequence if it is specified loosely. Order it against the string, measure it before it goes in, and the space-out lands where the completion design expects it. On the next trip, the tally is the only record of what was actually installed.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
