The casing head assembly is the first piece of pressure equipment on a well and the one nobody wants to revisit. It is welded or slipped onto surface casing, it carries everything above it, and it has to hold before the blowout preventer is nippled up, because after that the test becomes a stack test. One that is checked out properly at this stage saves the rig a day; one that is not becomes the reason a well is shut in during a drill.
Casing Head Assembly Flange and Pressure Rating
The top flange is what the BOP or the tree bolts to, so its rating is set by the well rather than by the casing. Assemblies run from 11 inches to 21-1/4 inches with working pressures of 2,000, 3,000 or 5,000 psi, and the casing sizes they accept run from 8-5/8 inches to 20 inches. A 13-3/8 inch surface string typically lands in a 13-5/8 inch body at 3,000 or 5,000 psi; a 20 inch conductor takes the 20-3/4 inch or 21-1/4 inch body. Side outlets are 2 inch line pipe or 2-1/16 inch studded, and their number and position decide how the choke line and the cementing lines are routed before the BOP arrives.
Bottom Connection: Slip, Thread or Weld
The bottom connection follows how the casing will be handled, not a preference.
Slip type grips the casing through slips, so the string can be cut and re-landed.
Threaded type gives a positive seal and comes apart for maintenance.
Welded type gives the most leak-proof joint where the casing is never recovered.
A welded head costs a weld that has to be inspected and stress relieved, and it is chosen when the surface string will stay in the ground for the life of the well. The mistake is deciding this in the office and discovering it on the rig floor, because a slip head cannot be turned into a welded head once the casing is cut.
Testing the Casing Head Assembly Before Nippling Up
The test uses a test plug or a cup packer held against the body and the bottom connection, and it happens before anything else goes on top. Two things make it fail for reasons unrelated to the steel: a hanger that has not landed on its shoulder, and a seal lubricated with the wrong compound. The test pressure is the rated working pressure of the assembly or the figure the well programme specifies, held long enough for the gauge to be read twice. If it drops, the leak is at the bottom connection far more often than at a flange, and finding that out before the BOP is on the well is the whole point of doing it now.
Outlets, Cementing and the Choke Line
The side outlets do more work than their size suggests. During cementing one outlet takes the returns while the other stays available for a pressure reading; after the plug bumps, the same outlet is where casing pressure is watched while the cement sets. Later it becomes the kill line connection or the point where a gauge is hung. A 2 inch line pipe outlet carries returns at normal rates, but a job that plans to reverse circulate through the casing head wants the larger studded outlet and valves to match. Crews that leave outlet valves until the last minute usually find one of them the wrong size.
Alignment Matters as Much as Pressure
An assembly can pass its pressure test and still cause trouble, because the flange face has to be square to the stack:
Level the flange face on two axes before the studs are tensioned.
Torque in the specified sequence rather than round from one side.
Fit a new ring gasket every time the flange is broken.
A casing head that is not plumb puts a bending load into the studs, and the leak shows up weeks later as a damp flange nobody can trace.
A casing head assembly is a small piece of equipment that decides whether the rest of the wellhead can be installed and tested. Get the rating, the bottom connection and the flange face right before the BOP lands, and the wellhead becomes ordinary work rather than a recurring problem.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
