After the blowout preventer comes off, the wellhead is capped by the assembly that will control it for the next twenty years. The x-mas tree assembly is that cap: master valves, cross, swab valve and tree cap, sized to a pressure class and trimmed for the fluid it will actually see. Most wellhead problems traced back to a tree are not valve failures but specification mismatches, where the class or the material was chosen for the drilling program rather than for the production stream.
What an X-mas Tree Assembly Seals and Holds
The tree suspends the tubing, seals the annulus between tubing and casing, and controls flow from the well into the surface line. It also provides access for well testing, wax removal, pressure recording and interventions such as washing, sand removal, acidising and fracturing. That combination of functions is why the assembly is built from a tubing head spool, master valves, hydraulic control valves, choke and kill lines and wellhead connectors rather than from a single body. The swab valve on top allows tools to be run without breaking the pressure barrier, and the tree cap closes the assembly when the well is shut in for a workover.
Pressure Class and Temperature Range
Vigor builds the assembly to API 6A and NACE MR0175 with rated working pressure from 2,000 to 20,000 psi (138 MPa). The standard temperature range is -46 to 121 degrees C in the LU class, which covers most production and injection wells at the wellsite; higher classes are available when the operator expects thermal cycling from steam or from a deep gas well. The class has to be chosen against the shut-in pressure the well can deliver, not against the pressure seen during drilling. In practice the common classes are 3,000 psi, 5,000 psi, 10,000 psi and 15,000 psi, and stepping up a class adds wall thickness and cost to every component in the stack. A 10,000 psi tree on a well that can only shut in at 6,000 psi is money spent on the wrong side of the barrier.
Material Class and PSL for Sour Wells
Material class runs AA through HH, product specification level PSL1 to PSL4, and performance requirement PR1 or PR2. In a sour well the class does the work: HH trim with the correct hardness limit is what keeps sulfide stress cracking out of the valve bodies and the ring gaskets. Hardness control is verified on the mill certificate, and a substitution at the supplier is the usual way an HH tree becomes a non-compliant one. PSL3 or PSL4 adds traceability and testing that a critical gas well is usually specified with, and PR2 adds the cyclic pressure and temperature qualification that a thermal well needs.
Connections and Make-Up at the Wellhead
The tree connects by clamp, thread, ring joint, flange or a clamp-flange combination. The ring joint is the usual choice for high pressure because the metal gasket is positively located, while a clamp connection speeds up a workover where the tree is lifted regularly. Make-up is a controlled operation: the flange bolts are torqued in sequence, the ring gasket is inspected for brinelling, and the assembly is pressure tested before the well is opened to the production line. A ring gasket that has been reused after a workover is a common source of a leak that only shows up when the well is put on production.
Confirm the ring gasket type and condition before make-up.
Torque flange bolts in a cross pattern to the specified value.
Pressure test the tree and the connections before opening the well.
X-mas Tree Assembly Checks Before Production
Before the well goes on line, the wellsite crew should walk the assembly from the tubing head up. Confirm the master valve and the swab valve operate through a full cycle, check the pressure gauges on the tubing and casing sides, and verify the choke responds through its range. On a workover, the same walk is done in reverse before the tree is lifted, so the job starts with a known valve position. Record the test pressures and the torque values in the well file, because the next workover will start by reading them. A tree that passes a function check at the wellhead is a barrier the operator can sign off on, and the record is what makes the sign-off defensible.
A tree is a barrier, and a barrier is only as good as its specification. Match the class to the shut-in pressure, the trim to the fluid, and the connections to the way the well will be worked over.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
