What is a Horizontal Subsea Tree?

A subsea horizontal tree is designed so that all flow control valves are outside the central wellbore. A tubing hanger is located inside of a subsea horizontal tree body. A horizontal tree is well known as a workover friendly tree because it offers easy access for tubing retrieval because the valves are not located at the centre of the wellbore. Figure 1 shows an image of a horizontal subsea tree from GE oil and gas.

Figure 1 – Horizontal Subsea Tree Model VetcoGray M-Series MVXT and MHXT (Courtesy of GE Oil & Gas)

Figure 2 illustrates a simple diagram of a horizontal subsea tree.

Figure 2 - Horizontal Subsea Tree Diagram

Figure 2 – Horizontal Subsea Tree Diagram

A typical horizontal subsea tree consists of the following valves:

Production master valve – Its function is to open or close the main bore containing hydrocarbons.

Production wing valve – Its function is to control the flow of hydrocarbons into a subsea flow line.

Production isolation valve – Its function is to isolate production from a well and a flow line.

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 isolation valve – Its function is to isolate the annulus side.

Annulus workover valve – Its function is to provide monitoring and bleed off pressure between a tubing hanger and a tree cap

Important Features of Subsea Horizontal Trees

  • A tubing hanger is landed in the tree body.
  • All valves located externally provide a clear path for well bore access.
  •  An internal tree cap is run on top of the tubing hanger in which crown plugs can be set by wireline.
  • The large through-bore design of a horizontal tree allows installation and retrieval of a completion string, downhole equipment, artificial lift completions, etc through a tree body without having to nipple down the tree or disconnect flow lines.
  • Height of a horizontal tree is less than a vertical tree; therefore, risk of damage due to trawl boards is smaller than a vertical tree.

Brief Operational Sequences for Installing a Horizontal 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
  • Once a production casing/liner is cemented, a well must be temporarily suspended by installing temporary barriers
  • Nipple down BOP
  • Recover marine riser and BOP
  • Run a horizontal tree from the rig and test the tree
  • Recover a running tool to surface
  • Run BOP and marine riser on top of the tree
  • Remove temporary barriers from the well
  • Run completion and tubing hanger.
  • Set tubing hanger inside the tree and test
  • Retrieve the running tool to surface
  • Install plugs inside the tree to allow the safe removal of the BOP.
  • Nipple down BOP
  • Recover marine riser and BOP
  • Install a tree cap

This video from Expro group demonstrates how a horizontal subsea tree system is installed in deepwater operation. The video may be slightly different from the sequences stated above because each operator can perform operation differently. However, the overall concept is similar to the sequences.

References

2012. Subsea Engineering Handbook. 1 Edition. Gulf Professional Publishing.
Add to My References

William L. Leffler, 2011. Deepwater Petroleum Exploration & Production: A Nontechnical Guide, 2nd Edition. 2 Edition. PennWell Corp.

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.

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