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Horizontal and Vertical X-mas tree
A subsea Xmas tree is basically a stack of valves installed on a subsea wellhead to provide a controllable interface between the well and production facilities. It is composed of a variety of valves, which are used for testing, servicing, regulating, or choking the stream of produced oil, gas, and liquids coming up from the well below. Different types of subsea Xmas trees may be used for either production or water/gas injection. Configurations of subsea Xmas trees may differ based on the requirements of the projects and field developments. Functions of a subsea Xmas tree can be listed as the following:
- Enable flow of the produced fluid from the well or the injection of water or gas from surface facility into the formation (called injection tree), including protection fluids, such as inhibitors for corrosion or hydrate prevention.
- Stop the flow of fluid produced or injected by means of valves in a safe way.
- Control the fluid flow through a choke (not always mandatory).
- Monitor well parameters at the level of the tree, such as well pressure, annulus pressure, temperature, sand detection, etc.
There are two types of Xmas tree according to the configuration of valves, vertical Xmas tree and Horizontal Xmas tree. Horizontal trees are so called because the primary
valves are arranged in a horizontal configuration, and likewise vertical
trees have the primary valves arranged in a vertical configuration.Figure 1 shows the differences between two configurations.
Figure 1: Difference between horizontal Xmas tree configuration and vertical Xmas tree configuration
Vertical Xmas Tree
The master valves are located above the tubing hanger and swab valves together with master valves are stacked vertically. The production and annulus bore lays vertically on the body of the tree. The well completion is finished before installing the vertical Xmas tree. Since the tubing hanger rests on the wellhead, Xmas tree can be recovered without having to recover the downhole completion. This type is generally applied in subsea fields due to their flexibility of installation and operation.
Horizontal Xmas Tree
In contrast to vertical Xmas tree, the valves of horizontal Xmas tree are located on the lateral sides of the horizontal Xmas tree, allowing for easy well intervention and tubing recovery, thus this type of tree is very feasible for the wells that need many interventions. The tubing hanger is installed in the tree body instead of the wellhead. Consequently, the tree is installed onto the wellhead before completion of the well.
Horizontal vs Vertical trees
In the subsea industry much debate is generated comparing the
relative merits of horizontal and vertical sub-sea tree systems. This
article considers some of the primary drivers of this discussion and how
they may influence both system design and future operations.
A
key requirement of a subsea tree is that access is enabled to the “A”
annulus between the production tubing and casing. This is required for a
number of reasons, including pressure monitoring and gas lift. As an
example, any pressure buildup in the A annulus can be bled to the
production flowline via a crossover loop on the tree.
The original
designs of subsea vertical trees and tubing hangers were of a dual-bore
configuration. Prior to removal of the BOP, it is necessary to set
plugs in both the production and annulus bores. Access to both bores
requires the use of a dual-bore riser or landing string. The handling
and operation of dual-bore systems compared to monobore systems is more
complex, and time-consuming and, therefore, more costly.
On a
horizontal tree, access to the A annulus is incorporated into the tree
design and controlled by gate valves rather than plugs. This enables
operations with a mono-bore, less-complex riser or landing string, which
can deliver significant advantages, particularly in deep water. It is
exactly this logic that led to the introduction of tubing-head spools
for use with vertical trees, thereby offering many of the advantages of a
horizontal tree.
Hangoff location
Another key functional difference relates to the hangoff location of the completion. In a vertical tree system the tubing hanger is landed either within the subsea wellhead or within a tubing-head spool. The subsea tree is then installed on top of the wellhead or tubing-head spool. In the case of the horizontal tree, the tree is installed on top of the subsea wellhead, and the tubing hanger is landed within the tree body.
This key difference means that, in the case of the horizontal tree, recovery of the completion can be achieved without removal of the tree. In the case of the vertical tree, access to recover the completion first requires removal of the tree itself. This functional variation is another input into the choice between horizontal and vertical trees.
If the probability of a failure in the completion (requiring its recovery) is higher than a failure in the tree (requiring recovery of the tree), then there may be a strong case for favoring a horizontal tree. The converse also is true; if a failure in the tree is considered the highest risk, then application of vertical tree technology incorporating the most efficient opportunity for its recovery may be favored over horizontal tree technology, where recovery of the tree first requires recovery of the completion.
The relative ease of recovery of tree or completion is then the key functional difference. The overall system choice is, in many ways, application-specific and dependent upon a large number of factors. However, there are additional primary factors that are important inputs to the selection process.
Influence of installation and intervention
Both horizontal and vertical tree systems use a landing string to run the completion through the BOP. In the case of the horizontal tree, the completion is normally run on a subsea test tree within the marine riser, and the tubing hanger is landed within the horizontal tree. The subsea test tree is an assembly of connectors and valves and is designed to carry out a number of critical functions.
Once the hanger is landed in the tree, correct orientation of the tubing hanger is critical to ensure communication of all hydraulic and electrical downhole functions. In the case of the horizontal tree, the tubing hanger is normally oriented passively using an orientation sleeve attached to the tree. This passive orientation does not rely on external input.
It is common practice that once a well is completed it will be flowed to the drilling rig to clean up the well or to carry out a well test. This test or cleanup is carried out with horizontal trees via the subsea test tree and a high-pressure riser within the marine riser. The primary function of the subsea test tree ensures that, should it be necessary to disconnect the rig from the BOP during the well test or cleanup, the valves within the test tree can be closed and an emergency disconnect carried out safely.
In the case of vertical trees, the completion is run on a landing string incorporating a tool that runs, locks, and orients the tubing hanger. This orientation function normally requires the tool to interface with a known reference, which commonly comprises a pin installed within the BOP. Once the tubing hanger is oriented correctly it can be landed in the wellhead, with the understanding that when the tree is landed and oriented, communication for all hydraulic and electrical downhole functions will be achieved.
Well cleanup or well testing on a vertical tree is typically only carried out after the well has been suspended and the BOP replaced by a dedicated test package and open-water riser. This test system comprises two main assemblies: the lower riser package (LRP) and the emergency disconnect package (EDP). In a similar fashion to the subsea test tree, this system enables the rig or vessel to safely disconnect in the event of an emergency.
Such LRP/EDP packages and open-water riser systems represent considerable capital investments, typically in the order of tens of millions of dollars. In comparison, subsea test trees can be rented on the open market on a per-day or per-well basis. As a result, they can have a much lower capital impact. This variance in the capital impact of installation and intervention equipment is often a key input into the choice of vertical or horizontal tree technology, particularly when the installed well count is relatively small.
Previously it was noted that an additional tubing-head spool can be run on top of the subsea wellhead. The tubing-head spool is simply an additional spool that is not unlike the body of a horizontal tree but without a production outlet. It broadly carries the same functionality as a horizontal tree body, including passive tubing hanger orientation and A annulus isolation. Using a tubing hanger spool in conjunction with a vertical tree can, in addition to enabling monobore landing string or riser operations, also allow the use of a subsea test tree with a vertical tree.
This potentially negates the significant capital cost of an LRP/EDP and open-water riser system. It is, however, noted that use of a tubing-head spool can require an additional BOP trip. In the case of a horizontal tree, the tubing hanger and completion are installed within the tree. This requires that the drilling program is closely coupled with the tree delivery schedule. The same is true for a tubing-head spool. The decoupling of tree delivery and the drilling program offers a degree of operational flexibility and again is a factor in tree selection.
Tree system weights
Tree system weight is an important operational parameter. The weight can influence lifting, handling, and installation operations and can have an impact on the required vessel capability. Many end users specify maximum weights for subsea tree systems. As a broad generalization it can be said that the functionally comparable vertical trees are lighter than horizontal trees, primarily driven by the fact that the horizontal tree is designed to interface with a BOP rather than a lighter and less demanding LRP/EDP.
It also is true to say that the functional demands being placed on subsea trees are growing. Valve sizes are increasing; required bending capacities are increasing; and more sensors and instrumentation are required, such as flowmeters.
Key takeaways
Application-specific parameters influencing the choice between either tree include operational risk, water depth, sidetracking, and more. There is no right or wrong choice – what delivers an advantage in one application may not in another. There are perhaps two key takeaways: First, the demands placed on subsea tree designs will continue to evolve, and application-specific parameters will influence the horizontal/vertical technology evaluation. Second, discussion on this subject will continue to exercise the minds of subsea engineers from around the world for some time to come.
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