This section will compare the pros and cons of both vertical subsea trees and horizontal subsea trees and list the criteria to select a subsea tree.
Vertical Subsea Christmas Trees
Advantages
- Vertical trees require only one time BOP nipple down.
- No wireline plug to be removed from a tree for a well intervention program.
- Better for fields that don’t expect to do workover or have small chance of doing the workover
- Simpler and cheaper to change a vertical tree when compared to a horizontal tree
Disadvantages
- If the workover operations such as recompletion, changing tubing, installing extra downhole tools, etc. are required, a vertical tree must be removed in order to install BOP on top of the well.
Horizontal Subsea Christmas Trees
Advantages
- Better for fields that expect to do workover quite often because a tree does not need to be removed.
- Can have a larger bore tubing for a horizontal tree than a vertical tree
- Lower total height of a tree
Disadvantages
- Less flexibility for operation if the delivery of a tree is delayed.
- Two runs for subsea BOP and riser are required
- Completion string must be removed if replacement of a tree is needed.
- Two wireline plugs (crown plugs) must be removed before starting any well intervention program. There have been several cases when people have faced a lot of difficulty for removing the plugs. This can lead to extra time and cost for the operation.
Subsea Tree Considerations (Vertical vs Horizontal)
- The cost of a vertical tree is similar to a horizontal tree based on similar specification.
- Changing out horizontal trees is more cost and time consuming than replacing a vertical tree because whole completion string must be removed. Vertical subsea trees can be replaced by using rig or specially equipped light well intervention vessels. However, horizontal subsea trees are required only rig to replace the trees. This results in big expenditure. Therefore, horizontal trees should be used when there is very low possibility that a tree must be changed out. It is very imperative to do extensive probabilistic study prior to selecting the horizontal trees.
- Two BOP and riser runs are required for a horizontal tree, whereas only one time of BOP and riser run is needed for a vertical tree.
- For wells requiring several completion changes during the life cycle of a well, a horizontal subsea tree will save time and cost since there is no need to remove a tree or flow lines. Modern vertical trees are connected to a flow base which allows the flowlines to remain connected when removing the tree.
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.