The purpose of the subsea test tree (SSTT) is to provide well barriers within the high pressure upper completion string, and to facilitate well control in the event of an emergency disconnection of the rig from the well, without requiring the BOP to shear the shear joint.
- The lower ball valve should be capable simultaneous cutting of coiled tubing (23/8 in. x 0.156 w.t.) and 0.438 in. monoconductor wireline.
- A hydraulic latch mechanism is incorporated above the SSTT valves to provide a remotely disconnectable and reconnectable interface within the marine riser.
- The SSTT interfaces with the slick joint at its lower end and with the shear joint at its upper end. The SSTT is spaced out by the slick joint to allow the blind rams to be closed above the latch interface and the shear joint to be positioned across the shear rams.
Externally, the SSTT should have a profile that allows it to be installed within the internal profile of the BOP stack (BOP Stack minimum ID = 18.75 in.). The SSTT should provide internal through porting to allow hydraulic communication to the various SSTT and THRT functions below the latch assembly.
The control umbilical should terminate above the packoff sub, and be through-ported to the top of the shear joint. Synthetic hose sections should be used for hydraulic control spanning the shear joint (metal tubes are not acceptable for this application). The synthetic hoses should terminate at the bottom flange of the shear joint, and hydraulic communication should be provided via internal porting from the shear joint lower flange to each lower control function as required.
Internally, the SSTT should have a bore suitable to enable the passage of wireline tools and tree equipment. All internal bore transitions are to be tapered so as not to interfere with or snag the wireline tools and equipment.
The SSTT should incorporate an anti-rotation feature to permit torque transfer during orientation of the Tube Hanger (TH) or actuation of downhole completion equipment functions (and to prevent application of torque to the hydraulic couplers in the landing string, etc.). A point of fixation in the tubing string 400 m below the TH, and the requirement to rotate the string a maximum of 180 degrees above that point, should be assumed for the purposes of determining the required torque capacity for the anti-rotation device.