How Do Subsea Trees Work?
Source:  Rigzone October 21, 2013

Used on offshore oil and gas fields, a subsea tree monitors and controls the production of a subsea well. Fixed to the wellhead of a completed well, subsea trees can also manage fluids or gas injected into the well.

Since the 1950s, subsea trees have been topping underwater wellheads to control flow. A design taken from their above-ground cousins, subsea trees are sometimes called xmas trees because the devices can resemble a tree with decorations.

Subsea trees are used in offshore field developments worldwide, from shallow to ultra-deepwaters. The deepest subsea trees have been installed in the waters offshore Brazil and in the US Gulf of Mexico, and many are rated for waters measuring up to 10,000 feet deep.

Types of Subsea Trees

There are various kinds of subsea trees, many times rated for a certain water depth, temperatures, pressure and expected flow.

The Dual Bore Subsea Tree was the first tree to include an annulus bore for troubleshooting, well servicing and well conversion operations. Although popular, especially in the North Sea, dual bore subsea trees have been improved over the years.

These trees can now be specified with guideline or guideline-less position elements for production or injection well applications.

Standard Configurable Trees (SCTs) are specifically tailored for company’s various projects. A general SCT is normally used in shallower waters measuring up to 1,000 meters deep.

High Pressure High Temperature Trees (HPHT) are able to survive in rough environments, such as the North Sea. HPHT trees are designed for pressures up to 16,500 psi and temperatures ranging from -33 C to 175 C.

Other subsea trees include horizontal trees, mudline suspension trees, monobore trees and large bore trees. Companies that manufacture subsea trees are Aker Solutions, Cameron, FMC Technologies and Schlumberger.