In the mining of petroleum gas and oil, a Christmas tree refers to an intricate assembly consisting of a series of valves, fittings and spools that sit on top of a gas or oil well to regulate outward and inward flow of fluid and gas. The terms wellhead and Christmas tree are often interchangeably used to refer to this apparatus. However, the wellhead is an entirely different sub-system sitting on the very mouth of the well. The Christmas tree sits on top of a wellhead.
A wellhead is only used without a tree during drilling, or in a well drilled from rod pumping, where a pressure control system is not necessary.
The contraption hardly looks like a Christmas tree. Nothing about its rigid metal structure evokes the notion of Christmas, but it was named Christmas tree due to its resemblance to the series of lights at the start of a drag racing strip, which is also called a Christmas tree.
Wet Christmas trees, also known as subsea trees, are installed on submerged (subsea) wells. They rest on the high-pressure wellhead-housing surface of the well. Dry trees or surface trees fit on the wellhead of surface wells.
Subsea Christmas Trees
To collect gas buried beneath the depths of the ocean, where several rich gas wells have been discovered over recent years, you need a subsea Christmas tree.
The subsea functionality has drastically improved with complexity in design. While the structure still goes by Christmas tree, the resemblance in appearance is completely lost due to additional support and control structures to facilitate submerged operation. Subsea trees are designed for either vertical or horizontal orientation. They vary widely in size and weight depending on the application. Wet trees have more valves and control instruments compared to surface trees. Most notably, the subsea Christmas tree has a choke, subsea control interface and an array of sensors for gathering environmental data such as pressure, sand flow, erosion and temperature.
There are various kinds of wet trees rated for specific depths, pressure, temperature and well output. Subsea Christmas trees operate from shallow depths of a few metres to deep-sea mining enterprises at thousands of feet. The three most popular types used in the petroleum industry are:
Conventional and Dual Bore Subsea Trees
Vertical Christmas trees can have a conventional single bore or dual bore configuration that allows operations to monitor annulus pressure.
Standard Configuration Trees
SCTs are designed for shallow water drilling at depths not exceeding 1,000 metres.
High-Pressure High-Temperature Trees (HPHT)
These are trees designed to withstand a rough environment with pressures of up to 16,500 psi and temperatures as high as 175 degrees Celsius.
The Role of Christmas Trees in Gas Mining
Christmas trees are vital components in the mining of gas. Industrial gas regulators and valves fitted to the tree enable miners to control the flow of high-pressure gas from the well’s outlet. Besides outlet flow control, the tree is also responsible for these functions:
• Allowing the injection of gas and liquid into the well
• Providing access through a hydraulic line for the surface-controlled subsurface safety valve
• Allowing safe access to the well buoy for well intervention procedures
• Providing an electrical interface for control instruments and submersible pumps
• Injection of flow control elements in the flow line
• Serving as a safety barrier
• Bleeding excess pressure from the annulus
• Relaying crucial changes in well characteristics to the control centre
Fluid injection into the well may serve various purposes in the mining process. Injection provides pressure and temperature control, erosion and corrosion control, alteration of flow rate, as well as composition and blockage control. Injection may also be used to control the output of linked wells sourcing from a common reservoir.
How Mined Gas Is Collected
Gas reservoirs under the sea experience tremendous pressure that pushes the gas upwards through a drilled well. The immense pressure is essential for the mining process since little energy is required in bringing the gas to the surface. However, the pressure presents a problem to the safety and control of the flow rate of the well. To deal with this problem, massive Christmas trees with multiple robust valves are used to guide the gas to the surface. Industrial gas regulators then control the gas flow throughout the refining and storage plant.
Subsea Christmas trees have four functional building blocks: the valves, the choke, the tree connector, and the tubing hanger. The connector serves as the interface between the tree and the wellhead. The tubing hanger supports the gas uptake pipes to the surface or floating rig. The choke controls the well’s production rate and underlying pressure. Most units are fitted with a double master valve system for fault tolerance, in case one should fail. Wing valves on either side of the main valves control flow in the injector channels.
Hydraulic mechanisms control the movement of the choke and the opening and closing of the valves. These processes use automated sensor controls with manual overrides.
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