What is a subsea tree?

What is a subsea tree?

Subsea tree is a kind of wellhead equipment used to produce oil, such as flowing well and mechanical well. It is the main equipment for controlling and regulating oil and gas production in the top of oil and gas wells, mainly composed of casing head, tubing head and oil (gas) tree body.

The subsea tree is an assembly located at the opening to the top of the well, including valves for measurement and maintenance, safety systems, and a series of monitors. The tree connects the production pipeline and the oil outlet pipe from the well, and acts as an important barrier for the isolation of the top of the well from the external environment. It includes many valves that can be used to regulate or prevent the production of crude oil vapor, natural gas and liquid from flowing out of the well. The tree is connected to the production manifold system through the submarine pipeline.

20210601084419 33024 - What is a subsea tree?

Classification of subsea tree

There are many types of subsea trees, which are divided into water production trees (on platform deck) and underwater production trees (on the seabed) according to the application area; According to the installation position, it can be divided into vertical (or vertical) production tree, horizontal (or horizontal) production tree, plug-in (or caisson type) production tree; According to the structure, it can be divided into dry type, wet and dry / wet production trees; According to the layout of the well, it can be divided into satellite well tree and chassis well tree.

Single tube and double tube

1. Single tubing subsea tree
The single tubing subsea tree is installed on the wellhead equipment of single tubing completion. In addition to the split type and integral type, it can also be divided into single wing and double wing. According to the different types of wells or production methods of oil wells, different types of single tubing subsea tree can be formed by combining valves and nipples, and the connection methods can be flange type, thread type or clamp type.
2. Double tubing subsea tree
Dual tubing subsea tree is installed on the wellhead of dual tubing completion, which is used for production control of simultaneous and independent production of two oil layers. Double tubing completion is to run two parallel tubing strings (long tubing string and short tubing string) or two concentric tubing strings in the same production casing, and separate two oil (or gas) intervals by double tubing packer and single tubing packer.
3. Three tube subsea tree
With the development of technology and the demand of field application, three tube subsea tree has appeared.

Function of subsea tree

  • (1) Connect each layer of casing, seal the annular space of each layer of casing, and hang the weight of casing.
  • (2) Hang tubing and tools to support the weight of all tubing strings in the well and seal the annular space between tubing and casing.
  • (3) Control and regulate the production of oil well.
  • (4) Ensure the construction of various underground operations, facilitate well killing operation, tripping operation and other measures of construction, and carry out daily production management such as pressure measurement and paraffin removal.

Related components of subsea tree

1. tubing four-way and three-way
The oil pipe four-way and tee of oil production tree are connected with main valve, wax cleaning valve and wing valve. Common four-way and three-way are flange type and flange one stud type. The four-way oil tree is used as the double wing tree, and the three-way oil tree is the single wing one. Some platform tree tee and wax valve, main valve or wellhead safety valve, which is a whole type of subsea tree.
2. gate valve and plug valve
Gate valve is a valve that moves the parts (gate) perpendicular to the flow direction. A plug valve is a valve that closes the rotation of a component (plug), which can be cylindrical, conical or spherical. It plays a role in wellhead control.
3. oil nozzle
The nozzle is a component on the production tree to control the flow of the well, which is installed between the wing valve and the oil outlet pipeline. The nozzle can also be used to control the gas injection volume of gas lift well and the water injection volume of injection well. The nozzle is a throttle element. The reasonable production pressure difference of oil well is controlled by changing the oil nozzle with different hole diameter. The nozzle is made of high carbon alloy steel after heat treatment. The hole diameter is from 2-20mm, each difference of 0.5mm is one grade.
There are two types of oil nozzles, fixed type and adjustable type. The fixed nozzle is equipped with replaceable parts, which have fixed holes for throttling. The adjustable nozzle has a variable area hole controlled by the outside and an indication mechanism corresponding to the small hole area. The orifice of the nozzle is a small hole distributed on the tubular part. The change of the number of the holes in the cover along the axial direction of the outer sleeve of the tubular part is the change of the over-current area.
4. check valve
Check valve is a valve that only allows fluid to flow freely in one direction, so it is also called single flow valve. The check valve has a mechanism to automatically prevent the flow of fluid from other directions. Check valves on subsea trees are generally installed between wing and nozzle. Some oil well trees do not have check valves installed, but generally there are check valves on injection wells. Check valve has oil outlet, swing open, rising rod and butterfly type. The check valve can be full open or reduced. The end connection of the check valve is usually flange type connection and clamp type connection.
5. flange
Flange is the end connector with bolt hole and used to connect the sealing structure of pressure equipment. The flange conforming to APL specification is API flange. Blind flange has no central through hole, which is used to completely close flange end face or outlet connection. Blind flange and outlet flange to be closed shall have the same pressure class and can be equipped with the same sealing elements.
According to the different connection methods, the flange can be divided into threaded flange and welding neck flange. One side of the threaded flange is the sealing surface, the other side is the internal thread. One side of the welding neck flange is the sealing surface and the other side is the welding groove.
6. steel ring
The steel ring is a sealing element for flange connection. The commonly used API steel rings are R-type, RX type and BX type. R-rings and Rx rings can be interchanged in 6B flange. BX rings can only be used for 6bx type flange. RX and BX Steel rings have the properties of pressure self tight sealing, but they cannot be interchanged.
All 23 for R and Rx rings ° Surface roughness shall not be less than 63rms, all 23 for BX rings °
 Surface roughness shall not be less than 32rms. RX and BX rings have a pressure through hole that drill through the entire height of the rings.
7. bolt and nut
Bolts and nuts are components used to connect flange and clamp. The threads of bolts and nuts shall conform to API thread specification apistd5b.
8. back pressure valve (return valve)
Back pressure valve is installed in the oil pipe hanger. When removing the blowout preventer or installing or removing the oil tree, the back pressure valve is needed to seal the inner hole of the oil pipe; When the lower main valve is repaired and replaced, the back pressure valve is also needed to seal the inner hole of the oil pipe. There are two types of back pressure valves, one is fixed by thread, the other is fixed by expansion locking mechanism,
9. oil tree cap
The subsea tree cap is mounted on the wax removal valve on the top of the tree to provide an inlet for rapid longitudinal access to the tubing space. If the operation of wire rope, coiled tubing, production test, sliding sleeve switch, loading and unloading return valve, etc., the tree cap must be removed first.
10. subsea tree valve

  • Main valve, control oil and air flow into the main passage of the subsea tree.
  • Production gate, control oil and gas flow to oil outlet pipeline.
  • Wax cleaning gate, above which can be connected with wax cleaning nozzle, etc.
  • Throttle, control the production of the self injection well.
  • Small four-way, connecting parts.

Source: China Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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