Sub-Sea Trees and Controls
Authors: Hoes L.M. 
Published: December 2007

This paper presents a summary description of three types of sub-sea completions. Included in the considered systems are sub-sea trees of various degrees of sophistication which are landed on standard sub-sea wellhead systems. Running procedures, method of operation, and control systems used will be explained.
The first tree to be discussed has a single control line which operates the hydraulic wing valve only. This tree is set with diver assistance, with manually operated master, swab, and annulus valves.
The second tree has a single hydraulic control line that operates all the valves on the tree using pressure sensitive pilots. The system provides vertical rig control of valves and the hydraulic tree connector through a manifold spool and horizontal control from the platform after completion. TFL (through the flow line) loops and wireline set diverters are used for TFL work. A diver makes the flow line connections.
The third tree has the characteristics of the second tree discussed. in addition, it has a hydraulic flow line connector, an electro-hydraulic multiplex control system, and is set and operated completely without diver assistance.

Simple Tree
Fig. 1 illustrates a single control line, single function. diver assisted tree. The major components to be discussed are:Tubing hanger orienting bushing.Tubing hanger and running tool.Manually operated tree connector.Valves.Control system.

1. Tubing Hanger Orienting Bushing
Prior to running the tubing string and hanger, a tubing hanger Prior to running the tubing string and hanger, a tubing hanger orienting bushing is run. (Fig. 2).
The orienting bushing is landed in the connector below the blowout preventers. After it is landed, it is rotated until a spring loaded dog locks into a vertical slot in the connector and is oriented with respect to the guide posts. As the tubing hanger is lowered into the bushing, a lug on the hanger lands on a camming surface in the bushing. As the hanger is lowered through the bushing, the lug slides down and around the camming surface and the hanger is oriented with respect to the guide posts.

2. Tubing Hanger and Running Tool
The tubing hanger performs the same general functions as a land type hanger: suspending the tubing and isolating the tubing run or runs from each other and the annulus. There is usually one more vertical bore through the underwater hanger than there are tubing strings. This provides an access to the annulus with a vertical run through the hanger rather than horizontal access in the casinghead housing below the hanger as provided with land type equipment.
The underwater tubing hanger is lowered and landed with a running tool made up on drill pipe. The running tool is locked into the hanger at the surface with hydraulically actuated dogs on the running tool. These fit into a recess at the top of the hanger. (Fig. 3).
After landing the hanger, rotation of the drill pipe and running tool will mechanically wedge dogs out from the hanger into a machined recess in the casing seal assembly and lock the hanger securely into place.
The running toot is released from the hanger by dropping an unlock dart down the drill pipe and applying fluid pressure down the drill pipe onto the top of the dart. This pressure moves a sleeve from behind the dogs on the running tool which frees it from the tubing hanger.
In the top of the underwater hanger, just as with a land type hanger, there are seal subs or receptacles in each of the bores through the hanger. These receive the seals on the tree when it is landed.
In the production bore through the hanger, there is a sealing nipple preparation to receive ' a standard wireline plug. Normally, this plug is installed before the BOP's are removed and is retrieved with standard wireline retrieving tools after the tree has been tested.

3. Tree Connector
The connector (Fig. 4) used in this system is diver actuated. A series of locking dogs are run in by the diver to connect the tree to the wellhead.
The tree is run on the guidelines used for the preventer stack. Guide funnels attached to the tree connector give initial orientation to the tree and final orientation is achieved by the lug on the hanger neck engaging the vertical slot in the connector.