[Article Sharing] Horizontal and Vertical X-mast tree

Drivers influencing the evolution of horizontal and vertical trees | Exploration & Production Magazine

Source: http://www.epmag.com/drivers-influencing-evolution-horizontal-and-vertical-trees-698041#p=full

The capability and flexibility offered by modern subsea christmas trees and production systems is, by any measure, truly impressive. The industry has moved from simple production systems to more expansive ones incorporating complex controls and sensors and a range of monitoring and diagnostic systems.

This equipment operates in very challenging environments and stands as a testament to the skill and ingenuity of the teams that design, build, and install it.

The demands for the production of hydrocarbons from deep water at higher pressures and temperatures, coupled with a range of additional design constraints, ensure that subsea tree systems will continue to evolve to meet these challenges both now and in the future.

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.

Horizontal and vertical trees

Subsea production trees can be segmented into two main types: horizontal trees and vertical trees. 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.

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.