A tubing head is a flanged device or short joint installed on top of the uppermost casing head before perforation. It supports the tubing string and seals the annular space between the tubing and production casing. It also provides access to the casing-tubing annulus through side outlets.
Function:
◆Suspends the tubing string in the well.
◆Seals the annular space between tubing and casing.
◆Provides a transitional connection between the casing head and the Christmas tree.
◆Two side outlets on the tubing head cross allow for water injection and well cleanout operations.
Structural Components
A tubing head consists of a cross, tubing hanger, sealing mechanism, lockdown screws, gate valve, pressure gauge, and flanges. The top of the tubing head connects to the Christmas tree via a special flange, using either a flanged or clamp-type connection.
Tubing Hanger
The tubing hanger sits in its tapered seat, suspending the tubing string and providing a seal between the suspended tubing and the tubing head seat. Offshore oil and gas completions typically include downhole safety valves, so tubing hangers must have passages for hydraulic control lines. For electric submersible pump wells, additional passages for downhole gas vent valves and electrical cable feed-throughs with seals are required. The tubing connections form a circular assembly that sits in the dual tubing hanger seat, aligning with the guide blocks on the tubing rack when installed.
Cross
The tubing head cross is the main body. Its bottom connects to the casing head, and its top connects to the Christmas tree, with steel ring seals at both ends. The cross provides a carrier and container for the tubing hanger.
The cross allows for forward and reverse circulation well cleanout, casing pressure monitoring, and various operations through the tubing-casing annulus.
Tubing Hanger Lockdown Screws
After the tubing hanger is installed, lockdown screws must be fitted. These screws lock the tubing hanger in place, preventing vertical movement and rotation.
Lockdown Flange Plate
The lockdown flange plate is installed on the casing cross. The tubing hanger sits on the seat of this flange, which compresses the packing to seal the tubing-casing annulus. It also secures the tubing, preventing it from being pushed out of the well due to high internal pressure.
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