Christmas_tree is essentially the heart of the offshore
hydrocarbon production system. It is the primary means of well control and
plays a key role in the emergency shit down system. The Christmas tree sits on
the top of the wellhead casing system and represents the interface between the
well and the production and process facility.
The Christmas tree consists of an assembly of gate valve
which control the floe of hydrocarbons. It may consist of individual valves
bolted together from which the name Christmas tree was originally derived or it
may feature a cast or forged steel solid block into the valves chests are
machined. Occasionally it is a combination of two. In all cases the valve seats
and gates are removable for replacements or repair.
A wellhead skid controls the operation of the Christmas tree
and mudline safety valves. The skid permits valves to be operated locally,
remotely or via ESD system and timing mechanisms provide a means of controlling
the speed and sequence of valve operation. This sequence would normally be
close of wing valve, master valve and mudline safety valve.
During an ESD operation, complete closure of the Christmas
tree valves should be effected within approximately 45 seconds according to API
recommendations, the only organisation to provide guidance on this particular
aspect .
Wellheads can be Dry or Sub sea completion. Dry Completion
means that the well is onshore on the topside structure on an offshore
installation. Sub-sea wellheads are located under water on a special sea bed
template. The wellhead consists of the pieces of equipment mounted at the
opening of the well to regulate and monitor the extraction of hydrocarbons from
the underground formation. It also prevents leaking of oil or natural gas out
of the well, and prevents blowouts due to high pressure formations. Formations
that are under high pressure typically require wellheads that can withstand a
great deal of upward pressure from the escaping gases and liquids.
These wellheads must be able to withstand pressures of up to
140 MPa (1400 Bar). The wellhead consists of three components: the casing head,
the tubing head, and the 'Christmas tree'.
A typical Christmas tree composed of a master gate valve, a
pressure gauge, a wing valve, a swab valve and a choke is shown here. The
Christmas tree may also have a number of check valves.
At the bottom we find the Casing Head and casing Hangers.
The casing will be screwed, bolted or welded to the hanger. Several valves and
plugs will normally be fitted to give access to the casing. This will permit
the casing to be opened, closed, bled down, and, in some cases, allow the
flowing well to be produced through the casing as well as the tubing. The valve
can be used to determine leaks in casing, tubing or the packer, and will also
be used for lift gas injection into the casing
The tubing hanger (also called donut) is used to position
the tubing correctly in the well. Sealing also allows Christmas tree removal
with pressure in the casing.
There are wells drilled into the reservoir, and the central
conductor along with the surrounding jackets / annulus rises up to the
Production Deck / Cellar deck of the platform. On top of the well head, an
assembly of valves is placed, which has the form of a cross. This assembly of
valves together with the flanges is called a Well Head Christmas tree.
The Christmas tree has many Manual valves, and a number of
Actuated valves. The actuated valves usually found on the Christmas tree are as
follows:
1. Sub-Surface Safety valve:- Sub-Surface Safety valve is a
hydraulic operated valve, the location of which is below sea-level, above
sea-bed. The actuator of this valve need to be very small, as it gets enclosed
within the Annulus of the conductor. The actuator is usually hydraulically
operated. The control line for the hydraulic supply for the SSSV runs within
the conductor, and terminates at a connection on the Christmas tree.
2. Surface Safety Valve or Master Valve:- This isolates the
tree from the productions tubing. Christmas tree have two master valves
referred to as the upper and lower master valves. The lower master valves is
opened first and closed last. This ensures minimal flow of hydrocarbon over the
valve seat, thus protecting if it from abrasive particles and ensuring a good
seal is maintained.
In most cases the lower master valve is manually operated
and the upper master valve is operated via hydraulic or pneumatic actuator and
is connected into the emergency shutdown system. The actuators are fail safe in
operation. The valve is held open by oil or pressure against a compressed coil
spring.
The master gate valve is a high quality valve. It will
provide full opening, which means that it opens to the same inside diameter as
the tubing so that specialized tools may be run through it. It must be capable
of holding the full pressure of the well safely for all anticipated purposes.
This valve is usually left fully open and is not used to control flow.
Master valve is the first actuated valve on the Christmas
tree, located above the Mezzanine deck of the platform. The Actuator is bigger,
and can be pneumatic or Hydraulic, based on the Christmas tree requirement
The pressure gauge. The minimum instrumentation is a
pressure gauge placed above the master gate valve before the wing valve. In addition
other instruments such as temperature will normally be fitted.
3. Wing Valve:- Wing valve comes on the arm of the Christmas
tree, on the line where the flow line starts. The actuator is again hydraulic
or pneumatic based on requirement.The wing valve can be a gate valve, or ball
valve. When shutting in the well, the wing gate or valve is normally used so
that the tubing pressure can be easily read.
Christmas tree may be manufactured with one or two wing
valves. One valve is permanently connected to the hydrocarbon process system
and is fitted with hydraulic or pneumatic actuator. The other valve is manual
in operation and permits injection of chemical or gases into the well without
disturbing production pipework.
Both valves are offset from the vertical lines so that a
clear entry into the well s maintained through the swab valve for wire line
work. The flow of gas from the well regulated by wing valve operation or by
choke fitted above the wing valve.
4. Well Service Valve:- The Well Service Valve may be
present on some Christmas tree, where Diesel pumping is required for initial
start-up. It is on the other arm of the Christmas tree, and usually the size is
lesser than that of the wing valve.
The Wellhead valves are all controlled by a Well Head Control
panel, which gives the hydraulic & pneumatic supply for opening / closing
these valves. There is logic built in the WHCP for allowing the safe closure of
all these well head valves, in case of an emergency, either due to process
upset or due to emergency / fire. In addition to these valves, the other
instrumentation which is associated with the Christmas tree are the Pressure
gauges and Transmitters for monitoring the Annulus pressures, the Flowing Tube
Head Pressure, etc.
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