Published on 01 Mar 2019 by Tadd Pham
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 blowout preventer (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.
The SSTT should be composed of two major parts:
- The lower part (below the emergency disconnect latch) remains attached to THRT and contains a minimum of two barriers in the production fluid conduit capable of containing the anticipated well shut‑in pressure from below.
- The upper valve is strongly preferred to be a flapper type valve. A ball type valve in this position will be considered.
- The lower valve should be a ball valve capable of cutting wireline and coiled tubing.
- The SSTT valves should be fail safe close. In the failed closed condition the barriers and the cutting device should permit the passage of kill fluid at a suitable flow rate into the well, so that the well can be killed by bullheading in the event of a total failure of the SSTT or its control system. This feature is commonly known as ‘pump through’ capability.
- The upper part of the SSTT (above the emergency disconnect latch) should be capable of disconnection from the lower part of the SSTT and retrieval to the surface, and should be provided with hydraulic connections to suit the attachment of the hydraulic hoses surrounding the shear joint. The unlatch operation should be operable without altering the rig top tension. The unlatch function should be fail-as-is.
Ensure there is a retainer valve to prevent unlatching under tension that may cause the upper SSTT to jump, and may damage the interface when it lands on the lower half.
The Retainer Valve should have the following requirements:
- Fail-safe closed
- No pump-through capability
- Able to retain the contents of the landing string upon unlatch from the SSTT
The connections at the emergency disconnect latch interface between the upper and lower parts of the SSTT should be provided with check valves where appropriate to prevent the ingress of the contents of the marine riser into the control lines.
Some of the lower coupler halves may need to vent to allow the valves to be fail safe closed. Also, the coupler should be separable under full line pressure of the hydraulic functions.
Any electrical couplers communicating through the SSTT emergency disconnect latch interface should be wet mate-able and pressure sealing in both the coupled and uncoupled conditions.
The SSTT should incorporate an interlock to prevent release of the SSTT quick disconnect latch prior to closure of the upper (ball or flapper) valve.
The structural integrity of the SSTT should be adequate to withstand the combinations of internal and external loading possible under all conditions expected during handling, including both normal and contingency operations. The material from which the SSTT is made should be suitable for the duty required, and for the fluids with which it may come into contact (understanding that there will be limited exposure time to acids used in well stimulation, and to produced fluids).
The SSTT should have a primary mode of operation using hydraulic fluid and controls conveyed via an umbilical. In addition, the SSTT should be provided with a secondary operating means, in the event the primary mode becomes inoperable.
- Primary circuit functions:
- Open upper SSTT valve
- Close upper SSTT valve
- Open lower SSTT valve
- Close lower SSTT valve
- Open retainer valve
- Close retainer valve
- Lock emergency disconnect latch
- Unlock emergency disconnect latch
- Injection of hydrate inhibitor (methanol) between lower SSTT valves
- Bleed-off valve (for bleed-off below retainer valve)
- Secondary circuit functions:
- Close lower SSTT valve
- Unlock emergency disconnect latch
- Unlock Tubing Hanger Running Tool from Tubing Hanger