this article presents the main operational and safety precautions to be taken when installing a xmas tree.

1. General Precautions

There are many methods of landing the tubing and leading control line(s) from the wellhead. Full instructions for this operation will be contained in the programme or in written standard procedures, however precautions to be taken include:

1. check all sealing or matting surfaces for damage before bringing them together;
2. check all sealing elements (metal ring gaskets, etc.) for damage and ensure freedom of movement in grooves or recesses;
3. check that the tubing hanger nipple is central and vertical;
4. use flange protectors (seal pads) where recommended;
5. before the BOP stack is removed, ensure that a two-way check valve or plug is installed in the hangar and pressure tested.

After the tubing has been landed, the control line(s) packed off and satisfactory pressure tests carried put, the Xmas tree is installed. Specific procedures will be governed by the type of Xmas tree and connections, however, the following general points apply:

• ·If an adapter flange is included in the assembly, it should be installed but only hand tight. The Xmas tree should be lowered into position and the adapter flange rotated as necessary to enable the Xmas tree to be orientated to correspond with the flowline hook-up. When the tree is aligned the bolts/clamps should be tightened.
• ·Plastic injection and pressure testing should be carried out carefully and methodically, adhering to the torque and test pressures stipulated in the programme or standard procedures. Injection of the plastic is considerably eased if it is slightly warmed using hot water, but it must not be melted. When the test pressures have been successfully completed, the injection screws should be backed out, the ports filled with plastic packing and the screws and plugs refitted in the injection ports. Following testing, the test pressure should be bled off from the test port, the test apparatus removed and the check valves and plugs refitted.
• Excessively high pressure can be generated when energising sealing elements: causing deformation of wellhead equipment and/or possible injury to personnel.
• ·Install the necessary control line, flow wing and kill wing valves and chokes, in accordance with the engineering drawings.
• ·The Xmas tree should now be API pressure tested against the two-way check valve or plug in the tubing hanger. The test should be carried out in two stages:

1.with flow and kill wing valves closed and choke open;

2.with valve downstream of choke and kill wing valve closed and flow wing valve and choke open.

• ·When the test has been completed without evidence of leakage past the seals, the two-way check valve/plug should be recovered from the tubing hanger.

2 Tubing back pressure valve

Safety critical activities occur during the completion between BOP stack removal and christmas tree installation, or at the beginning of repair work between christmas tree removal and BOP stack installation. Since, in these situations, the completion or workover fluid will usually have the minimum allowable density necessary to balance the formation pressure, this is not considered a sufficient safeguard against blowout. Normal practice is to install a downhole plug, preferably set in a packer tailpipe. A second barrier is provided by a tubing back pressure valve (BPV) fitted in the top of the tubing hanger (left handed square thread) or hanger nipple (landing nipple profile). The valve is provided either with a matching thread or with locking dogs, respectively, in addition to its seal. The valve is installed either by screwing it into the hanger using an extension rod or is set in the hanger by wireline methods. The BPV allows circulation down the tubing but automatically closes against well pressure.

2.1 Valve installation/removal

Installation or removal of the threaded valves through a Xmas tree is accomplished using a special lubricator. This consists of a rod which works through a yoke housing two stuffing boxes and to which the installation/removal tool is attached. By closing the vent valve and opening the equalising valve, well pressure acts on both the top and bottom of the rod. It can then be moved up or down, and/or rotated, by means of a friction wrench.

The wire line type BPV can be installed or retrieved by normal wire line methods.

To pressure test the tree and/or tubing hanger seals, a two way check valve must be landed in the hanger. In this instance, circulation down the tubing is impossible.

In new well applications it is not recommended to use threaded profiles for the BPV. This is a result of threaded profiles being more prone to damage/corrosion.