The tubing head assembly is the part of the wellhead that carries the tubing. It sits above the casing head, supports the string through a hanger, and seals the annulus between tubing and casing so the two pressures stay separate. It is also the component that has to be worked on most often, because every workover starts and ends with the tubing hanger. The rig crew lands and lifts that hanger more often than any other wellhead component, so the wear surfaces deserve a look on every trip. A tubing head that is specified on flange size alone will usually pass the pressure test and still give trouble at the load shoulder.
What a Tubing Head Assembly Supports
The assembly has three main parts: the tubing head, the tubing spool and the tubing hanger. The head provides the bowl and the load shoulder that carry the hanger, the spool spaces the head from the tree, and the hanger suspends the tubing and seals the annulus. Side outlets allow access for intervention, chemical injection and circulation, which is what makes the head useful beyond a pure support function. The outlets are also where a pressure gauge or a chemical line is connected, and a plugged outlet becomes a large problem during a test.
Flange Sizes and Working Pressure
Bottom flanges are 7-1/16 or 11 inches, with working pressure from 3,000 to 10,000 psi, and top flanges run 9, 11 or 13-5/8 inches at 2,000 to 10,000 psi. Outlets are 2 inch line pipe, 2-1/16 inch standard or 1-13/16 inch standard depending on the pressure class. The combination has to match the casing head below and the tree above, because a mismatch at either end becomes a crossover that nobody stocks on the rig. The flange class and the gasket type are ordered together, and the ring joint groove is inspected before make-up.
The 45 Degree Load Shoulder and Lock Screws
The hanger lands on a 45 degree load shoulder machined into the head, and the angle is what centres the hanger as it takes the string weight. A shoulder that is damaged or has debris on it will let the hanger sit high, and the tubing will be short by the amount the hanger is proud. Lock screws secure the hanger once it is landed, and they are run in sequence rather than all at once so the hanger is not pushed off centre. A hanger that lands high shows up as a tubing tally that does not match the plan, and the crew will spend an hour looking for the error in the wrong place.
Secondary Seal and Annulus Testing
Below the main seal there is a secondary seal that isolates the production casing and provides a way to test the wellhead. That test port is the reason the assembly can be pressure-tested without pulling the tubing: the annulus is pressurised between the two seals, and a stable reading proves both. A secondary seal that is never tested is an assumption, and the test is cheap at installation and expensive after a leak. On a workover the same port is used to confirm the seal before the tree is picked up, which is the last point where a repair is simple.
Land the hanger on a clean shoulder and check the height against the tally.
Run lock screws in sequence and torque them to the specified value.
Test the secondary seal before the tree goes on.
Tubing Head Assembly Installation Checks
Installation is a sequence of small checks. Confirm the flange gasket and the stud condition, make up the bottom connection to the specified torque, land the hanger and verify the seal, then test the annulus and record the pressure. On a workover, the same sequence runs in reverse, and the hanger is inspected for seal damage before it is reused. A head that is built to API 6A and tested at the works still depends on the rig crew landing the hanger properly. A wellsite check of the hanger height and the lock screw torque takes minutes and prevents a leak that would take a day to find.
A tubing head assembly is a support, a seal and a test point in one body. Match the flanges, protect the shoulder, and test the secondary seal, and the wellhead stays a barrier rather than a source of annulus pressure.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
