What is a Horizontal Subsea Tree?

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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)

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What is a Vertical Subsea Christmas Tree (Conventional Subsea Tree)?

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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

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Successful Topside Installation of the Ivar Aasen in the North Sea (Video)

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The heavy lift crane vessel ‘Saipem 7000’ has completed the installation of all ‘Ivar Aasen’ topside modules at the Ivar Aasen oil field in the Norwegian sector of the North Sea.For the lift, the crane vessel used its dual 7,000t cranes to lift the combined 15,000t modules from a Cosco heavy lift vessel onto a steel jacket attached to the seafloor.

The main part of the topside was built in Singapore and transported through the Suez Canal to the North Sea aboard the MV Xiang Rui Kou. The living quarters along with the flare boom and other modules were picked up along the way. Ivar Aasen field operator Detnorske says first oil is expected in December 2016.

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Surface Christmas Tree (Dry Tree) Basic Knowlege

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In the oil and gas industry, a Christmas tree is referred to as a series of valve & spool assembly fitted on top of the well.  A Christmas tree is installed on top of the last casing spool on a surface well or the high pressure wellhead housing for a subsea well. Figure 1 demonstrates the diagram of a Christmas tree and wellhead of a surface wellhead. The Christmas part is located at the top part (a blue box) and the wellhead part is the lower section (a red box). Many people get confused about what a Christmas tree and a wellhead are and many times they think of them as the same thing.

Its functions are as follows;

  • Allow reservoir fluid to flow from the well to the surface safely in a controlled manner.
  • Allow safe access to the wellbore in order to perform well intervention procedures.
  • Allow injections as water or gas injection.
  • Provide access to hydraulic line for a surface control sub surface safety valve (SCSSSV)
  • Provide electrical interface for instrumentation and electrical equipment for electrical submersible pump (ESP)

In this section, it will describe about a surface Christmas tree (Dry Tree) which is referred to as any Christmas tree used above water level. A Christmas Tree consists of a series of valves and the components are shown and described below; Continue reading →