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.
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)
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.
Figure 1 – Conventional single-bore subsea tree. Courtesy of DrilQuip
