Conventional trees, commonly known as vertical trees, are the original subsea xmas tree design. The horizontal tree was an evolution of the vertical tree whereby the production master valve was relocated to the horizontal bore allowing the tubing hanger to locate in the body of the tree.
Vertical Tree
The most notable difference between the two tree types is the location of the tubing hanger. The vertical tree tubing hanger orientates, lands and locks in the wellhead prior to landing the tree. This has the advantage of being able to perform all drill rig operations (installation, well clean up and test) prior to running the tree. Once the tubing hanger is installed, the drill rig can move to the next well while the remainder of the completion is installed by a more cost-effective installation vessel.
Completing a well in this manner can be advantageous if the schedule is critical, as the tubing hanger system typically has a shorter lead time than the tree system. However, this also comes with risk as the interface between the tree and tubing hanger will not have been proven onshore at SIT if the tubing hanger is installed prior to the mechanical completion of the tree.
If opting for a tubing head spool solution, the risk further increases as there will be an additional unproven interface between the XT and THS flowspools.
Vertical completions only require one BOP trip to drill and complete the well. This becomes more financially attractive for deeper water developments as one BOP trip (to sea bed and back to moonpool) can take up to 4 days. Considering rig costs estimates can be up to $1M per day, removing one BOP trip can be particularly cost effective.
Production fluids passing through a subsea xmas tree (vertical or horizontal), are directed vertically then redirected by means of a barrier, through a horizontal bore. The design of the vertical barrier differs between a vertical and horizontal tree. In a vertical system, flow is diverted via a gate valve, designated as the production swab valve (PSV). To fulfil regulatory requirements, a tree cap is installed providing as a second vertical barrier to the environment. Gate valves are considered more reliable than plugs. Furthermore, they are not as susceptible to debris and misalignment when comparing against other barrier designs. For this reason, many operators favour this type of isolation.
Vertical systems enable recovery of the tree without having to recover the completion string. This may be advantageous if the tree is complex and the completion string is simple. Recovering the tree in this scenario, is more cost effective than the horizontal equivalent.
Vertical system also allows for the option to drill, complete and suspend the wells with drilling rig, and install the XT at a later date using a light well intervention vessel. This removes the delivery of the XT from the critical path of drilling and completion.
Disadvantages of a vertical tree system include the well isolation method. Typical vertical tree systems achieve well isolation post-BOP recovery / prior to tree installation, via a wireline plug. Recovery of the installed plugs post tree installation, require the use of a LRP / EDP stack. Few SPS suppliers can provide 7” completion and workover riser (CWOR) system for rental thus operators need to consider purchasing the system. The requirement for a large capital investment coupled with the limited SPS supplier rental pool, can discourage operators from selecting vertical tree systems.
The purchase cost of a multi-bore CWOR is relatively small for shallow water depths but for deeper water its cost becomes a dominant factor. The cost can be justified if absorbed over a multi-well development, but can be prohibitive in a one- or two-well development.
That said, recent improvements in downhole component technology have enabled the use of downhole ball valves to provide well isolation rather than wireline plugs, eliminating the need for EDP / LRP. The valves are either operated via the SCM, ROV hot stab or via pressure cycling.
While this type of system has clear advantages in terms of simplifying the required installation equipment, tubing installed ball valves can inadvertently close during production due to increase in control line temperature. There is also a risk of the valve failing to open when the well is ready to start-up. If this risk materialises, the operator would have to mobilise a drilling rig back over the well and perform a milling operation.
Horizontal Tree
The full vertical bore aspect of the HXT design obviously does not allow vertical bore valves in the XT, so HXTs are configured with the valve bores located horizontally within the tree body. This allows the XT to be equipped with a production bore larger than that normally allowed in a VXT. i.e. 7” production bore with a 5” nominal bore XT.
Horizontal trees have the advantage of allowing recovery of the completion string without having to recover the tree and jumper. This can be advantageous with complex completion strings with high risk of recovery.
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