Christmas Tree
05.59
Christmas tree, or "tree" (not "wellhead" as
sometimes incorrectly referred to), is an assembly of valves, spools, and
fittings used for an oil well, gas well, water injection well, water disposal
well, gas injection well, condensate well and other types of wells. It was
named for its crude resemblance to a decorated tree.
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 a tree is to control the flow, usually
oil or gas, out of the well. (A tree may also be used to control the injection
of gas or water into a non-producing well in order to enhance production rates
of oil from other wells.) 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.
Tree complexity has increased over the last few decades. They
are frequently manufactured from blocks of steel containing multiple valves
rather than being assembled from individual flanged components. This is
especially true in subsea applications where the resemblance to Christmas trees
no longer exists given the frame and support systems into which the main valve
block is integrated.
Note that a tree and wellhead are separate pieces of equipment
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, and so on) frequently do not utilize any tree owing to no
pressure-containment requirement.
Valves
Subsea and surface trees have a large variety of valve
configurations and combinations of manual and/or actuated (hydraulic or
pneumatic) valves. Examples are identified in API Specifications 6A and 17D.
A basic surface tree consists of two or three manual valves
(usually gate valves because of their flow characteristics, i.e. 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 (hence the endurance
of the term "Christmas tree"). The two lower valves are called the
master valves (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 a having a Christmas tree with a
second swab valve.
Subsea trees are available in either vertical or
horizontal configurations with further speciality available such as dual bore,
monobore, concentric, drill-through, mudline, guidlineless or guideline. Subsea
trees may range in size and weight from a few tons to approximately 70 tons for
high pressure, deepwater (>3000 feet) guidelineless applications. Subsea
trees contain many additional valves and accessories compared to Surface trees.
Typically a subsea tree would have a choke (permits control of flow), a
flowline connection interface (hub, flange or other connection), subsea control
interface (direct hydraulic, electro hydraulic, or electric) and sensors for
gathering data such as pressure, temperature, sand flow, erosion, multiPhase
flow, single phase flow such as water or gas.
Sumber:
https://en.wikipedia.org/wiki/Christmas_tree_(oil_well)
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