Subsea tree (wet trees) is a system of valves, flow paths, piping, and connectors installed on a subsea wellhead to contain and control the flow of fluid from a reservoir or from the surface by injection. A control pod mounted on a subsea tree assembly provides a receptacle for an umbilical and contains the electronic and hydraulic components that control tree functions.
Two type of subsea Christmas tree are vertical Christmas trees and horizontal Christmas trees. They are different because of valves arrangement. The vertical trees have all valves arranged vertically; whereas, the valves in the horizontal tree are positioned horizontally.
In this part, it emphasizes on vertical subsea Christmas trees (conventional subsea trees). Vertical trees are manufactured in single bore and dual-bore configurations and pressure ratings are between 5,000 and 15,000 psi. The body of a Christmas tree can be made of carbon steel, low-alloy steel, or stainless steel depending on the operating environment. Figure 1 show the conventional single-bore subsea tree from DrillQuip.
Vertical trees can have dual bore configurations because this allows operators to monitor annulus pressure (Figure 2).
Figure 3 illustrates a simple diagram of a vertical dual bore subsea tree.
A typical vertical subsea tree consists of the following valves:
Production lower and upper master valve – Its function is to open or close the main bore containing hydrocarbons. Typically, a subsea tree has two master valves for a safety reason. The lower production master valve is designed for ROV operation.
Production wing valve – Its function is to control the flow of hydrocarbons into a subsea flow line.
Production swab valve – Its function is to provide access to the production bore during well intervention.
Crossover valve – Its function is to control flow between the production tubing and annulus.
Annulus master valve – Its function is to open or close the annulus bore.
Annulus wing valve – Its function is to control flow from the production flow line or control umbilical to the annulus.
Annulus swab valve – Its function is to provide access to the annulus bore during workover reentry.
Important Features of Dual Vertical Subsea Trees
- The completion tubing is landed in the high pressure wellhead housing.
- Two vertical bores allow access to the tubing and tubing/production casing annulus.
- Workover can be performed with a dual bore lower riser package, emergency disconnect package and riser.
- Each bore has a swab valve and a master valve; therefore, these provide two independent barriers to flow.\
- Most trees can be run and recovered with ROV intervention.
- It requires the BOP to be nippled down only one time after landing a completion string.
Brief Operational Sequences for Installing a Vertical Subsea Tree
- Spud the well riserless
- Run conductor with low pressure wellhead housing
- Cement conductor casing
- Drill and run surface casing with high pressure wellhead housing (wellhead)
- Cement surface casing
- Nipple up a subsea BOP on the high pressure wellhead housing
- Complete drilling and cementing program
- Run completion
- Set tubing hanger into the wellhead
- Set wireline plugs through tubing hanger running tool and test.
- Retrieve tubing hanger running tool.
- Nipple down BOP
- Pull BOP and marine riser to surface.
- Install vertical tree either from the rig or an alternative vessel and perform test on the tree
- Rig up intervention package
- Remove wireline plugs
- Recover intervention package
- Install a tree cap
This video from Expro group demonstrates how a subsea vertical sea system is installed in deep water operation. This can give you some ideas on how the tool works. The video may be slightly different from the sequences stated above because each operator can perform the operation differently. However, the overall concept is similar to the sequences.
2012. Subsea Engineering Handbook. 1 Edition. Gulf Professional Publishing.
Add to My References
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Jr. Adam T. Bourgoyne, 1986. Applied Drilling Engineering (Spe Textbook Series, Vol 2). Edition. Society of Petroleum Engineers.
J.J. Azar, 2007. Drilling Engineering. Edition. PennWell Corp.
The Australian Drilling, 1997. Drilling: The Manual of Methods, Applications, and Management. 4 Edition. CRC Press.
Steve Devereux, 1999. Drilling Technology in Nontechnical Language. Edition. Pennwell Pub.
OneSubsea, (2014), OneSubsea reaches subsea trees milestone [ONLINE]. Available at: http://www.offshoreenergytoday.com/wp-content/uploads/2014/03/OneSubsea-reaches-subsea-trees-milestone2.jpg [Accessed 6 August 2016].