Wellhead components explained: casing head to christmas tree, and the surface test tree

Every string of casing run into a well has to be hung off at the surface and sealed, and the final string of tubing has to be held and controlled. The wellhead is the stack of forged bodies that does that, one component per string, with the christmas tree on top controlling what comes out. Read from the bottom up, the stack follows the order in which the well was drilled and completed.

/01The stack, from the bottom up.

/02Hangers and annulus seals.

A hanger transfers the weight of a string into the wellhead body and seals the annulus around it. A slip-type casing hanger grips the casing with toothed slips and packs off the annulus — ours is listed for 7" casing in an 11" casing-head bowl, with hardened slip teeth. A mandrel hanger is threaded onto the string and landed in the bowl, which gives a positive seal and a defined bore; our mandrel tubing hanger is rated 10,000 psi. Outlets on the heads and spools give access to each annulus for monitoring and pumping.

/03The christmas tree.

Our conventional trees are five-valve assemblies: two master valves on the vertical bore, a swab valve above them for access to the well, and two wing valves on the side outlets leading to the flowline and choke. The trees are listed in 2-9/16" at 5,000 psi and 3-1/16" at 10,000 psi, with a 10,000 psi sour tree in material class EE. The frac tree is a different animal: a 7-1/16" bore at 15,000 psi, PSL 3 and PR2, sized for the rates and pressures of a fracturing job rather than for production.

/04Surface test trees.

A surface test tree, or flowhead, is a temporary tree used during well testing and intervention. Our listings cover five configurations: conventional frac trees with two master, one swab and two wing valves at 10,000 psi sour and 15,000 psi; a wellhead-mounted tree at 15,000 psi; a coiled-tubing tree with a pack-off and an integral shear/seal BOP; a snubbing tree with ram-type strippers; and a 7-1/16" subsea tree with retainer valves, listed to API 17D. The surface trees have a 5-1/8" vertical bore and hydraulic gate valves on a 3,000 psi pilot; on the frac and wellhead-mounted trees the swab valve is manual.

/05Classes and documents.

Wellhead equipment carries the same API 6A markings as valves: our listings are temperature class P, material class AA for standard service and EE for sour, PSL 2 or PSL 3, and PR1 or PR2. What those codes mean is in reading an API 6A nameplate. Flanged connections between components each need the correct ring and bolting — see ring joint gaskets — and every component should travel with its material and test certificates.