Your Ultimate Guide to Understanding Subsea Xmas Tree (2020)

In the oil and gas or rather petroleum industry, there are many and different terms that can really confuse you. If you are non-technical like me, am sure you have faced such problems at some point.

One such word is subsea xmas tree. Do you know what it means and how it works? I took time to research and put together this resource for you. Here is what I found out.

A subsea Xmas tree is basically a stack of vertical and horizontal valves installed on a subsea wellhead. It is installed provide a controllable interface between the well and production facilities. It is composed of a variety of valves, which are used for testing, servicing, regulating, or choking the stream of produced oil, gas, and liquids coming up from the well below. The buying cost or price varies according to spec and market conditions and is typically at US$1-3Million. 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.

Different types of subsea Xmas trees may be used for either production or water as well as gas injection. Configurations of subsea Xmas trees may differ based on the requirements of the projects and field developments.

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HISTORY AND OVERVIEW OF SUBSEA XMAS TREES

The first primitive Christmas Tree was used by the Hamill Brothers to bring Spindletop under control. It consisted of a T-valve, with a 6-inch and 8-inch valve on the vertical pipe, and a 6-inch valve on the horizontal pipe. The vertical valve was closed first, and then the valve to the horizontal pipe.

Christmas trees are used on both surface and subsea wells. It is common to identify the type of tree as either “subsea tree” or “surface tree”. Each of these classifications has a number of variations. Examples of subsea include conventional, dual bore, mono bore, TFL (through flow line), horizontal, mudline, mudline horizontal, side valve, and TBT (through-bore tree) trees.

The deepest installed subsea tree is in the Gulf of Mexico at approximately 9,000 feet (2,700 m). Current technical limits are up to around 3000 metres and working temperatures of -50 °F to 350 °F with a pressure of up to 15,000 psi.

The primary function of an oil and gas christmas tree is to control the flow of oil or gas out of the well.

When the well and facilities are ready to produce and receive oil or gas, tree valves are opened and the formation fluids are allowed to go through a flow line. This leads to a processing facility, storage depot and/or other pipeline eventually leading to a refinery or distribution center (for gas).

Flow lines on subsea wells usually lead to a fixed or floating production platform or to a storage ship or barge, known as a floating storage offloading vessel (FSO), or floating processing unit (FPU), or floating production, storage and offloading vessel (FPSO).

A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means, monitoring points (such as pressure, temperature, corrosion, erosion, sand detection, flow rate, flow composition, valve and choke position feedback), and connection points for devices such as down hole pressure and temperature transducers (DHPT). On producing wells, chemicals or alcohols or oil distillates may be injected to preclude production problems (such as blockages).

Functionality may be extended further by using the control system on a subsea tree to monitor, measure, and react to sensor outputs on the tree or even down the well bore.

The control system attached to the tree controls the downhole safety valve (SCSSV, DHSV, SSSV) while the tree acts as an attachment and conduit means of the control system to the downhole safety valve.

Differences Between Subsea Xmas Tree and a Wellhead

A subsea xmas tree and wellhead are separate pieces of equipment which should not to be mistaken as the same piece.

The Christmas tree is installed on top of the wellhead.

A wellhead is used without a Christmas tree during drilling operations, and also for riser tie-back situations that later would have a tree installed at riser top.

Wells being produced with rod pumps (pump jacks, nodding donkeys, grasshopper pumps, and so on) frequently do not utilize any tree owing the absence of a pressure-containment requirement.

How Subsea Trees Works

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.

A design taken from their above-ground cousins, subsea trees are sometimes called xmas trees because the devices can resemble a tree with decorations.

Below is an animated video by Aker Solutions demonstrating how a subsea xmas tree works or is intalled.

Types of Subsea Xmas 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.

Two main types 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.

Vertical Xmas Tree

Master valves are located above the tubing hanger and swab valves together with master valves are stacked vertically.

The production and annulus bore lays vertically on the body of the tree. The well completion is finished before installing the vertical Xmas tree.

Since the tubing hanger rests on the wellhead, Xmas tree can be recovered without having to recover the downhole completion. This type is generally applied in subsea fields due to their flexibility of installation and operation.

In this part, it emphasizes on vertical subsea Christmas trees or 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. 

Vertical trees can have dual bore configurations because this allows operators to monitor annulus pressure.

Horizontal Xmas Tree

In contrast to vertical Xmas tree, the valves of horizontal Xmas tree are located on the lateral sides of the horizontal Xmas tree.

This allows for easy well intervention and tubing recovery. Therefore, this type of subsea xmas tree is very feasible for the wells that need many interventions.

The tubing hanger is installed in the tree body instead of the wellhead. Consequently, the tree is installed onto the wellhead before completion of the well.

Subsea Xmas Tree Valves

Subsea xmas trees usually have a large variety of valves. These valves usually have configurations and combinations of manual or actuated valves which are either hydraulic or pneumatic.

You can find examples in API Specifications 6A and 17D.

A basic surface tree consists of two or three manual valves called gate valves because of their flow characteristics which is low restriction to the flow of fluid when fully open.

A typical sophisticated surface tree will have at least four or five valves, normally arranged in a crucifix type pattern and hence the use of the term “Christmas tree”.

The two lower valves are called the master valves consists of upper and lower respectively. Master valves are normally in the fully open position and are never opened or closed when the well is flowing, except in an emergency to prevent erosion of the valve sealing surfaces.

The lower master valve will normally be manually operated, while the upper master valve is often hydraulically actuated, allowing it to be used as a means of remotely shutting in the well in the event of emergency.

An actuated wing valve is normally used to shut in the well when flowing, thus preserving the master valves for positive shut off for maintenance purposes. Hydraulic operated wing valves are usually built to be fail safe closed, meaning they require active hydraulic pressure to stay open. This feature means that if control fluid fails the well will automatically shut itself in without operator action.

The right hand valve is often called the flow wing valve or the production wing valve, because it is in the flowpath the hydrocarbons take to production facilities or the path water or gas will take from production to the well in the case of injection wells.

The left hand valve is often called the kill wing valve (KWV). It is primarily used for injection of fluids such as corrosion inhibitors or methanol to prevent hydrate formation. In the North Sea, it is called the non-active side arm (NASA). It is typically manually operated.

The valve at the top is called the swab valve and lies in the path used for well interventions like wireline and coiled tubing. For such operations, a lubricator is rigged up onto the top of the tree and the wire or coil is lowered through the lubricator, past the swab valve and into the well. This valve is typically manually operated.

Some trees have a second swab valve, the two arranged one on top of the other. The intention is to allow rigging down equipment from the top of the tree with the well flowing while still preserving the two-barrier rule. With only a single swab valve, the upper master valve is usually closed to act as the second barrier, forcing the well to be shut in for a day during rig down operations.

However, avoiding delaying production for a day is usually too small a gain to be worth the extra expense of having a Christmas tree with a second swab valve.

Components of a Subsea Xmas Tree

Typical components of a typical subsea Xmas tree include:

  • Tubing hanger system.
  • A tree connector to attach the tree to the subsea wellhead.
  • The tree body, a heavy forging with production flow paths, designed for pressure containment, annulus flow paths may also be included in the tree body.
  • Tree valves for the production bore, the annulus, and ancillary functions. The tree valves may be integral with the tree body or bolted on.
  • Valve actuators for remotely opening and closing the valves. Some valves may be manual and will include ROV interfaces for deepwater.
  • Control junction plates for umbilical control hookup.
  • Control system. This includes the valve actuator command system and includes pressure and temperature transducers. The valve actuator command system can be simple tubing or a complex system, including a computer and electrical solenoids depending on the application.
  • Choke (optional) for regulating the production flow rate.
  • Tree piping for conducting production fluids, crossover between the production bore and the annulus, chemical injection, hydraulic controls, etc.
  • Tree guide frame for supporting the tree piping and ancillary equipment. In addition, it provides and guidance for installation and intervention.
  • External tree cap for protecting the upper tree connector and the Xmas tree itself. Tree cap often incorporates dropped object protection or fishing trawl protection.

Functions of a Subsea Xmas Tree

  • Enable flow of the produced fluid from the well or the injection of water or gas from surface facility into the formation (called injection tree), including protection fluids, such as inhibitors for corrosion or hydrate prevention.
  • Stop the flow of fluid produced or injected by means of valves in a safe way.
  • Control the fluid flow through a choke (not always mandatory).
  • Monitor well parameters at the level of the tree, such as well pressure, annulus pressure, temperature, sand detection, etc.

Subsea Well Construction Sequence

Vertical xmas tree: The following would be the sequence for constructing a vertical subsea xams tree.

  • Spud weel. Drill top hole
  • Run BOP stack on marine riser
  • Drill to TD. Run & cement liner
  • Run downhole completion & tbg hanger
  • Install temporary barriers
  • Recover BOP stack
  • Deploy and test xmas tree
  • Remove temporary barriers
  • Connect flowline jumpers and flying leads
  • Flow test well
  • Recover intervention package
  • Run xmas tree cap
  • Commission well from platform
  • Install protective cover

Horizontal xmas tree: This below would be how you would construct or install a horizontal Christmas tree in oil and gas sector.

  • Spud well. drill top hole
  • Deploy BOP stack on marine riser
  • Drill to TD. run and cement liner
  • Install temporary barriers
  • Recover BOP stack
  • Deploy and test xmas tree
  • Redeploy BOP stack on marine riser
  • Remove temporary barriers
  • Run downhole completion and tbg hanger
  • Test completion and interfaces
  • Flow test well
  • Install TH plug and internal tree cap
  • Connect flowline jumpers and flying leads
  • Recover intervention string
  • Recover BOP stack
  • Commission well from platform
  • Install protective cover

Conclusion

In conclusion, subsea Xmas trees have been topping underwater wellhead to control flows since 1950s.

Subsea trees are used in offshore field developments globally. This is from shallow to ultra deepwaters. For example, the deepest subsea trees have been installed in the waters offshore Brazil and in the Gulf of Mexico.

Aker Solutions, General Electric, Cameron, FMC Technologies and Schlumberger are some of the companies that manufacture subsea Xmas trees.

It is my sincere hope that you have found this information and resource helpful in your oil and gas industry’s journey.

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