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Surface Well Completion Page 2
Basic training operator Oil & gas Advisafe Risk Management B.V.
© Copyright Advisafe Risk Management B.V.
Version 1.0
All rights reserved. No part of this publication may be duplicated, stored in a computerised
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means of photocopies, recordings or any other way whatsoever, without prior written
consent from Advisafe Risk Management B.V.
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Surface Well Completion Page 3
Content
1. Casinghead-housing and spools ............................................. 5
1.1. Summary of surface well completion .......................................................... 5 1.2. Casing head housing and spools ................................................................ 7 1.3. The Cameron CA casing hanger ................................................................. 9 1.4. The Cameron BRX type 2 casing hanger .................................................. 11 1.5. X-bushing with P-seal ................................................................................ 12
2. Tubingheads and metal seals ................................................ 14
2.1. Summary of tubing heads and metal seal .................................................. 14
3. Tubingheads and metal seal ................................................. 16
3.1. LDO tubing head ....................................................................................... 16 3.2. SRT tubing head ........................................................................................ 17 3.3. Metal seal.................................................................................................... 20 3.4. Boll-weevil tubing head ............................................................................. 21
4. Christmas-tree ....................................................................... 23
4.1. Summary of the Christmas-tree ................................................................. 23 4.2. The solid block Christmas tree .................................................................. 24 4.3. The components of the composite Xmas tree ........................................... 26 4.4. The Xmas tree for the production cross for gas lift oil wells ..................... 28 4.5. The Xmas tree setup on clusters ................................................................ 29
5. Gate valves ............................................................................. 31
5.1. Summary of Gate valves ............................................................................. 31 5.2. The principle structure of a gate valve ....................................................... 31 5.3. Requirements set for valves ....................................................................... 32 5.4. The method of sealing gate valves ............................................................. 33 5.5. Sealing compound method ........................................................................ 33 5.6. The McEvoy gate valve, model C .............................................................. 35 5.7. The Cameron gate valve, type F ................................................................ 36
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1. Casinghead-housing and spools
1.1. Summary of surface well completion
The functions of the surface well completion are:
• hanging the successive casings and sealing them off from one another and the
surrounding area;
• providing the option of shutting down well production;
• providing the option of making observations and having controlled access to the
well for various activities.
The completion consists of the following components:
• Casing head housing
• Casing head spools
• Tubing head
• Christmas tree
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Overview surface well completion
5 1/8” API 5000 type ‘F’ solid-block tree
6” double ‘P’ seal
Check port
Metal seal
SRT tubing head 10” x 6” API 5000
control head
Tubing hanger nippel
7 5/8” double ‘P’ seal
Plastic packing ports
Casinghead spool 16” API 3000 x
10” API 5000 Casinghead type ‘WF’
Check port
Casinghead housing 16” API 3000 x
Casinghead type ‘WF’
16” casing
10 3/4" ” casing
7 5/8” casing
5 1/2” casing
Plastic packing ports
Intermediate flange
Check port
CA slip and seal assembly 10” x 7 5/8”
x-busing 16” x 10 ¾” o.d. casing
CA slip and seal assembly 16” x 10 ¾”
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1.2. Casing head housing and spools
The casing head housing is screwed (and sometimes welded) on top of the conductor and
serves to:
• anchor the blowout preventer to the first casing stack during drilling;
• support the above-ground well completion;
• support the next, smaller casing stack via the casing head spool and the casing
hangers;
• seal the annular space between the two largest casing stacks;ensure access to the
annulus
Casing head housing and casing head spools
The casing head
housing is installed on
the casing (see figure).
There is a flange
connection on top, on
which the casing head
spool will be mounted
at a later stage.
The housing has two 3”
drain holes opposite
each other, with
flanges threaded on the
inside. The annulus
shut-off valves are
attached to this in
order to enable mud
circulation.
On the inside, the
casing head housing is
a bore that is partially
straight and partially
tapered.
Tubing head spool
Test channel
10” casing outlet
16”x10” casing head spool
BRX type 2 Casing hanger
x-bushing
16” casing outlet
Injection channel
Tubing 5 1/2"
16” casing outlet
Tubing 5 1/ 2“
Annulus 5 1/2 ” x 7 3/4”
Casing 7 3/4”
Annulus 5 1/2 ” x 10 3/4”
Casing 10 3/4"
Annulus 10 3/4” x 16”
Conductor 16”
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The first casing head spool is connected to the casing head housing by means of a
flange fitted with a metallic packing (ring joint). The number of casing head spools
depends on the number of casing stacks with which the well is equipped. In the case
of the figure above, only two casing stacks (the 10¾” and 7¾” casings) are taken in,
in addition to the 16” conductor, and only one casing head spool is used. The spool
connects the various flanges with reduced diameters, which correspond with various
consecutive casing stacks.
The spool consists of a housing with a conical notch for the casing hanger at the top.
When the
Cameron CA casing hanger is used, there is space for the X-bushing with P-seal in
the lower flange. A filling and test port for the P-seal are also installed in the lower
flange. When the Cameron BRX casing hanger is used there are two filling ports
and two test ports in the lower flange. Two ports with thread and flanges in the side
wall of the housing provide access to the annular space.
Valves are installed on the side flanges of the casing head housing and casing head
spools. These valves provide access to the annular spaces to measure the pressure or,
if necessary, to vent the pressures accumulated.
The tubing head spool is installed on the top flange of the top casing head spool, in
which the tubing is hung.
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1.3. The Cameron CA casing hanger
After a casing stack is inserted at depth, the string with slips is hung from the rotary table.
The casing hanger is installed around the pipe. The casing, with the casing hanger around
it, is lifted with the hoisting facility and passes through the hole in the rotary table and the
BOP stack to the desirable depth, after which the casing hanger is secured.
The hanging system consists of two slips (components that enclose a flexible packing
component) and is called slip and seal assembly.
Cameron CA casing hanger with ‘slip and seal assembly’
16” casinghead housing
Slip A
Slip C
Seal
Slip D
Slip B
10 3/4" casing
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The whole packet is pressed together and kept together by long threaded bolts. By
hanging it the serrated slips are pushed against the outside of the casing. Slip D drags
against the conical inside wall of the casing head housing and ensures the hanging
process. The flexible packing component is pressed against the wall laterally and forms a
seal between the two successive casing stacks.
An advantage of the CA hanger system is that the hanger drops to the desirable level
around the casing and catches at the place with the serrated slips.
The disadvantages are:
• The casing hanger is not installed until the casing head housing or spool is
attached. In practice the hanger is placed through the BOP with the risk that the
hanger will get stuck or drag to a halt in the BOP.
• The 'slip and seal assembly' often leaks, while the additional seal formed by the X-
bushing with single P-seal is also inadequate.
To eliminate these disadvantages, a Cameron modified BPX hanger was developed, which,
working in collaboration with Cameron, resulted in the BRX type 2 casing hanger.
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1.4. The Cameron BRX type 2 casing hanger
This hanger is likewise used in a casing head housing as well as in a casing head spool.
Cameron BRX type 2 casing hanger
The conical casing hanger is screwed to the last casing pipe, which must be cut off at a
length accurately specified in advance and threaded. All this is lowered into the well from a
running tool which is screwed into the top of the casing hanger, till the conical section
drags against the conical support in the casing head spool. Dropping is a lot easier through
the BOP than with the slip and seal assembly.
The BRX hanger and the conical support of the casing head housing/spool both have a
smooth finish. The seal is obtained by two canvas seal rings in the BRX hanger. The BRX
type 2 hanger is tapped with a left-hand thread into which the running tool can be
screwed. After shut off, the running tool is unscrewed. At a later stage, the polished
surface right above the left-hand thread can be sealed with a double P-seal.
Injection port
P-seal
Casing head spool
Test port
Ring joing
16’ casing head housing
Ring joint
BRX hanger
10 3/4 “ casing
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The advantages of the hanger are:
• improved control of casing hanging through the BOP.
• better sealing systems.
1.5. X-bushing with P-seal
The purpose of the X-bushing with P-seal is to provide additional sealing in the casing
hanging (for CA casing hangers, see figure on the left).
Casing head spool
The X-bushing with P-seal
is enclosed in a chamber
bored at the bottom of
the casing head spool.
The seal by the P-seal can
be servo-assisted via an
injection port in the
flange and inspected via a
test port. The X-bushing
is a metal, ring-shaped
component that is slid
over the cut-off casing
end and is locked into the
chamber bored into the
casing head spool above.
Tubinghead spool
Test channel
Casing hanger
Casinghead spool
X-bushing
Casing hanger
Casinghead housing
Injection channel
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The sealed-off space between the X-bushing ring joint and the casing hanger is pressure
tested via a ½” test port and checked for leakage through the P-seal.
Only one P-seal is used for the X-bushing, which was found to be a shortcoming.
Nowadays a different type of casing hanging is used, with a double P-seal as a seal above
it. If one leaks, a good seal can still be obtained with the other P-seal.
The seal between the
housing of the casing head
spool and the outside
circumference of the X-
bushing is obtained by two l-
shaped seal rings in the
outside circumference of the
X-bushing.
The seal between the casing
and the X-bushing on the
inside is obtained by a
canvas ring in the inside
circumference of the X-
bushing. This packing ring
can be activated via an
injection port with plastic
packing material, whereby
the P-seal comes in contact
with the casing.
X-bushing
P-seal
Non-return valve
Injection channel
Casinghead spool
1/2 “ test port
Non return valve
Shut-off cap
16”casinghead housing housing
10 3 /4” casing
X-bushing met P-seal
Pressure plug
Bleeder plug
Ring Joint Profile RX
L-seal
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2. Tubingheads and metal seals
2.1. Summary of tubing heads and metal seal
The tubing is connected in the bottom of the well to the packer, which seals the casing and
the tubing. The connection between the tubing and the annulus must also be sealed at the
top, meaning above-ground. The total weight of the tubing string must also be absorbed,
which is dealt with by the tubing head spool and the tubing hanger nipple connected to the
tubing.
Surface well completion
The tubing head spool is a
transition from the
narrowest casing to the
Xmas tree or to a
temporary adapter.
Internally, the tubing
head spool is processed in
such a way that the
tubing hangs in it with
tubing hanger nipple and
seals it. The method of
hanging depends to a
large extent on the
temperature of the
medium flowing through
during production. Tubing
is hung without
prestressing in cold
shallow wells.
In deep wells, large
differences in length can
be created between the
cold, enclosed well and
the warm producing well.
In such wells the tubing is
prestressed before
hanging, which means
installed in the cold well
after prestressing.
5 1/8 “ API 5000 type “F” Solid-block tree 6” double “P” seal
Check port
Metal seal
SRT tubing head 10” x 6” API 5000
conrol head
Tubing-hanger nippel
7 5/8” double “P” seal
Plastic packing ports
Casing spool 16” API 3000 x
10” 5000 Casinghead type “WF”
Check port
Casinghead housing 16” API 3000
Casinghead type “WF”
16” casing
10 3/4” casing
7 5/8” casing
5 1/2” casing
Plastic packing ports
Plastic packing ports
Check port
CA slip and seal assembly 10” x 7 5/8”
X-bushing 16” x 10 3/4 “ o.d. casing
Plastic packing ports
CA slip and seal assembly 16: x 10 3/4"
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As soon as the temperature of the well increases during production, this prestress
disappears for the most part due to expansion of the tubing material. However, enough
tensile strength will be left in the tubing for it to seal properly and not buckle, which would
cause production or wireline problems. The following tubing heads are mostly used:
• LDO tubing head
• SRT tubing head
• Boll-weevil tubing head
LDO and SRT tubing head is used in gas wells, in which the tubing is prestressed before
hanging in order to ensure that the tubing will always be stressed when hanging at
temperature differences. A boll-weevil tubing head is used for oil wells and injection, wells
in which the tubing hangs free in the centre of the casing.
A plug can be installed in the tubing hanger nipple for all three systems, thereby
protecting the well from fluid or gas flowing out during an exchange in completion above-
ground or when a blowout preventer (BOP) is installed. The above-ground completion can
also be pressure tested using this plug after installation or exchanges.
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3. Tubingheads and metal seal
3.1. LDO tubing head
This head is installed on the top casing head spool.
LDO head
The tubing is hung from the rams in the tubing head spool. The tubing hanger nipple,
which is attached to the tubing, is used for this purpose. A packing is installed in the rams
to ensure sealing on the polished surface of this nipple and the LDO head. The tubing is
hung by closing the rams around the polished section of the tubing hanger nipple so the
front of it rests on the rams.
The rams are opened and closed by screw spindles. One must pay attention that the
tubing hanger nipple is centred as accurately as possible. The rams are fitted with rubber
Production cross Tubing –hanger -nippel plastic or P-seal injection ports Soft metal seal ring (metal seal)
Control of test ports Intermediate flange
for metal seal
Ring joint
Body of LDO head
Body of LDO head
Conductor pin
V-packingset cover with
packed spindle (2x)
Seal check with bleeder
plugs
Bleeder plugs
Casinghead spool
plastic or P-seal injection ports with bleeder plugs
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seals which ensure that by tightening the spindles and the weight of the tubing a proper
seal between the tubing and the casing is obtained.
The tubing hanger nipple is threaded on both sides and has a landing nipple profile on the
inside so it can shut off a plug by means of wireline.
3.2. SRT tubing head
Nowadays so-called tension heads, type Cameron SRT, are installed in new gas wells. Also,
the LDO heads used before are now often replaced with SRT heads during a workover. This
tubing head is also installed on the top most casing head spool.
Cameron SRT tension head
Production cross
Plastic injection port
Metal seal with controlport
Intermediate flange
Packing securing bolts
Packingset
Body
Ledge cover
Ledge cover
Stops of centring the tubing hanger
Controlof test port
Ring joint
Controlof test port
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Just as it is for the LDO head, the tubing is hung on the rams here by means of a polished
tubing hanger nipple provided with internal notches so that plugs can be shut off.
The advantages over LDO heads are:
• The rams only serve to hang the tubing in a central position, thus not to seal it.
• Because the rams drag to a halt on a stop, we know for a fact that the tubing
hanger is in the centre after the rams have been screwed in completely.
• A separate packing bushing (sandwich seal) ensures sealing. If there are any leaks,
plastic can still be injected to thereby rectify the leaks.
• A workover mast is no longer required should the packing need to be changed,
because the tubing hanger nipple continues to hang on the rams.
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Detail of SC-SSV hydraulic control line
though the wall of the SRT tubing hanger nipple
Access to the annulus
The tubing head housing has two openings on
the side through which a connection with the
annulus can be established. A set of double
valves are installed in either opening. Via one
set of valves, which are sealed in the open
position, the kill pump line is connected from
the kill pump manifold to the annulus so that
the well can be killed at any time.
A non-return valve is installed in the kill
pump to prevent the pipe and the manifold
from being subject to the full well pressure in
case of a leak between the tubing and the
annulus. The servo-condensate mixture that
serves to combat the corrosion of the tubing
by CO is also pumped into the annulus via
these valves. The annulus is filled entirely
with servo-condensate and can be injected
into the tubing via the injection valve in the
bottom most SPM. This is accomplished by
pumping the servo-condensate into the
annulus by means of a membrane pump.
The valves on the additional side openings to
the annulus are closed. An inspection port is
sometimes installed here.
SC SSV control line
Nowadays the ¼” control line to the SC SSV
usually runs via the metal seal through the
wall of the tubing hanger nipple and
thereafter to the safety valve via the annulus.
The purpose of the control line is to operate
the SC SSV hydraulically.
Plugged
Metal seal
Connection for hydraulic inspection
Control line to ball valve
Tubing hanger nippel
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3.3. Metal seal
The metal seal for the hanger nipple is the most important seal between the tubing and
the annulus. The seal consists of a soft metal ring, which is butted up against the tubing
hanger nipple between the Christmas tree and the flange beneath it.
Seal around the tubing hanger nipple
An inspection or test port of the
space between the tubing
hanger nipple and the soft
metal ring provides indications
on possible leakage through
the inside. Leakage passing
the soft metal ring on the
outside must be detected by
listening for air passing or
using a gas detector. This test
port is also used as a passage
for the hydraulic control
pressure for the underground
safety valve (SC SSV).
However, not one leak has
been detected during 10 years
of experience with metal seals.
Leakage along the plastic seals
(P-seals) in the Christmas tree
can be rectified by injecting
more plastic. This seal only
serves to prevent corrosive
fluid from the tubing from
corroding the metal seal.
Intermediate flange, specially installed for
the soft metal ring seal.
Flange of composite production cross. Tubing-hanger-nippel
Control of testport
Plastic injection port P-seals
Metal seal
Ring joint
Packing securing bolts
Intermediate flange specially installed for
the soft metal ring seal.
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3.4. Boll-weevil tubing head
The boll-weevil hanging method is used for oil wells since the requirements imposed on
the sealing are not as strict as those for gas wells, as the pressures are lower. A polished
hanger nipple is also used here, with notches for installation of plugs by the Wireline
department.
Boll-weevil hanging
The short hanger nipple is tube-threaded on both sides: on one hand to establish the
connection between the hanger nipple and the tubing, and on the other hand so that the
boll-weevil hanger assembly can be put into position in the boll-weevil spool through the
BOP by means of auxiliary tubing screwed into the top.
The boll-weevil hanging is a combination of a boll-weevil hanger assembly and a boll-
weevil spool, which is installed on the top most casing head spool. The boll-weevil hanging
consists of a spherical cone fitted with a packing ring. The purpose of the pressure bolts is
to compress the packing component in order to obtain a good seal between the tubing and
the casing head. A plug can be screwed into the hanger nipple to allow for pressure testing
and for temporary protection when the above-ground well completion is exchanged.
Sometimes a Cameron type H two-way check valve (a non-return valve that works in two
directions) is installed so that the well can be temporarily shut down when the above-
ground well completion or adaptor are exchanged for a BOP, or vice versa. This valve has
Hanger nipple
Packing ring
Spherical cone with packing component
Packed pressure bolt
Guide pin
Wireline plug rabbet
Lip
Chamber
Boll weevil
Polished for sesaling the wireline plug
Tubing
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Surface Well Completion Page 22
two seats and seals both above and below the valve (function of an inside BOP) and is
thus also suitable for pressuring test the BOP.
The valve is loosened and pulled with a special pulling tool, whereby any pressure under
the valve can be equalised, either with or without the use of a lubricator. The tubing stack
is screwed into the bottom most threaded box. Its tensile stress is transferred to the
casing head via a chamber with lip and the cone.
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4. Christmas-tree
4.1. Summary of the Christmas-tree
The Christmas-tree is the part of the above-ground well completion between the tubing
head and the flow pipe. The purpose of the Christmas tree is to provide the option of
closing the gas and/or oil flow off from the formation by means of a manually operated
valve or by means of an automatically operated valve: the Otis valve. Furthermore, the
flow pipe can be opened or closed by means of the flow-arm valve.
Above-ground, gas wells are completed to as great an extent possible with a production
cross forged from one piece (solid–block Xmas tree), which consists of one body or
housing in which the various valves are installed. Older gas wells are still equipped with a
composite production cross (composite Xmas tree). The various cut-off valves are
assembled together in a so-called Y-form block.
In comparison to the composite cross, the forged cross has the following advantages:
• Noticeably fewer flange connections, thus less chance of leakage, which above all
provides more safety in case of a blowout or a fire in an adjacent well.
• Less high, thus more easily accessible to operating personnel; moreover, a lower
well cage will suffice.
A disadvantage of the forged production cross in comparison to the composite production
cross is apparent when damaged valves are changed. It may happen that the body is
seriously damaged near one of the wing valves (e.g. the flow arm valve) or near the top
valve. In that case, the entire Xmas tree must be exchanged and thus the entire well must
be killed. In case of a composite production cross, the entire Y piece with wing valves and
top valve can be changed, for example, just by closing the two master valves.
For various reasons there are two main valves, the master valve and the Otis (also
referred to as the first and second master valves):
• First of all, the gas flow can be shut off by means of the manually operable valve if
the Otis valve is not closing for one reason or another, while this is desired.
• Second, statutory safety requirements stipulate that installation components may
only be worked on if two closed valves are installed in series, between the
pressurised part of the installation and the pressure-free part. This implies that if
the flow pipe valve must be repaired, for example, this can take place in safe
conditions if both master valves are closed and the pressure-free part, in which the
flow pipe valve is installed, has an open connection to the atmosphere.
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Surface Well Completion Page 24
4.2. The solid block Christmas tree
The solid –block Christmas tree is used for most gas wells. These consist of a forged steel
block, which includes the valves.
The solid block Christmas tree
Top + cap E-top valve
C-tubing- casing- connection valve
bypass
Scheme Bril flange
Tubing-casing connection
Flow-arm valve
Otis operated master valve
Flow line Sandfilter
Master valve
Intermediate flange
Tubing-head spool (type LDO)
Kill pumpline
Alarm switch
Non return valve L zero point for wireline depth gauge
Casinghead spool
Casinghead housing
5” VAM casing as return stack
7” casing
10 3/4” casing
20” stovepipe
16” casing
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The Christmas tree consists of the following components:
• the manually operable master valve
• the Otis–operated master valve
• the block in which the valves are installed
• a flow arm valve
• a top valve
• an additional valve on the other side of the flow arm valve
• a top connection with cap (access for well service activities).
The Otis valve is similar to the bottom master valve, with the difference that an automatic
operating mechanism is installed instead of a hand wheel. This operating mechanism,
usually called the Otis actuator, is activated when the pressure in the Christmas tree
drops too low. This may be the case, for example, if a pipe ruptures or if the NTS or
flapper valve in the tubing closes.
The Otis actuator also closes the valve if the control air required to operate Otis stops
flowing or if a failure occurs in the installation. There is also the option of closing the Otis
during regular production from the control room by means of an electrical signal. The Otis
valve is of great importance for safety.
The gas supply to the production installation can be closed off by means of the flow arm
valve.
The top valve is a vertical access required for a large number of well service activities,
such as wireline, perforation and stimulation.
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Surface Well Completion Page 26
4.3. The components of the composite Xmas tree
The composite Xmas tree
Top + cap E-top valve
scheme
Bypass
C-tubing-casing connection valve
Y-shape block
Flow-arm valve
Otis operated master valve
pilot solenoid
3 bar air Orifice union
Flowline
Sand filter
Master valve Tubing-head spool (type LDO)
Kill pump line
Non return valve L
Zero point for wireline dept gauge
Tubing-casing connection
Brill flange
Alarm switch
Casing head spool
Casinghead housing
7” casing
5” VAM casing as return stack
10 3/4” casing
16” casing
20” stovepipe
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Surface Well Completion Page 27
The composite tree is put together from separate components. This Xmas tree consists of
the following components, from top to bottom:
• the manually operable master valve
• the Otis valve (secondary valve)
• the 3-way piece
• the valve for the tubing casing connection
• a flow valve
• a top valve
• a top connection with cap
Various valves are installed on the 3-way piece:
• the Otis valve at the bottom
• three valves from left to right at the top:
o the valve in the tubing casing connection
o the top valve with the top connection on it
o the flow pipe valve
The tubing casing connection serves to release excessive pressure in the annulus to the
flow pipe via the tubing casing connection valve. The top valve with connection serves to
enable wireline work. The flow line valve can shut off the gas supply to the installation.
A choke is installed in some cases past the flow pipe valve. One can make a well produce
at different speeds by means of various chokes. In most cases, however, one is liable to
find the choke in the installation.
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Surface Well Completion Page 28
4.4. The Xmas tree for the production cross for gas lift oil wells
The Xmas tree is built out of the following components:
• casing hangers and nipples
• flow line
• gas lift supply pipe
• the usual valves.
Example of a Xms tree for gas lift oil wells
The flow line is equipped with a
hand choke, which is used when
the well is started. Remote
operable valves (ROVs) are
installed in the gas lift pipe as
well as in the flow line. The ROVs
are connected to a number of
shut-down actions; the latter
depends on the situation (hot
well, H2S well, etc.)
Examples of shut-down actions
are:
• On ROV in flow line
o HPSD – high pressure
shutdown
o LPSD - low–pressure
shutdown
o HTSD – high temperature
shutdown
• On ROV in flow line and gas
lift pipe
o Leak alarm
o H2S alarm
• Emergency stop from the
measuring station
If a corrosion inhibitor pump is
installed, it will also stop at a
shut-down action.
T-piece
bullplug
Production valve
Xmastree cap
Adapter flange
Top valve or lubricator valve
Flow wing of Injection valve
Tubing-casing connector
Liftgas
Casing valve
Conductor valve
4 1/2", 3 1/2” or 2 7/8”
7”
10 3/4” 24”
T piece
Master valve or main valve
Adapter flange
Tubinghead spool (boll weevil)
Casingheadspool
Casinghead housing
16”
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4.5. The Xmas tree setup on clusters
As indicated in the figures in the preceding two chapters, all onshore wells have a cellar.
Above-ground well structure of an offshore gas well
The purpose of the cellar is to
keep the most frequently used
master and operating valves
of the Xmas tree at a workable
height and to catch any small
oil or condensate/servo leaks.
In principle, cellars are never
made any bigger than strictly
necessary to accommodate
the part of the above-ground
completion, such casing head
housing, casing head spools,
tubing head spool and annulus
connections.
The annulus valves,
connection and tapping
facilities are not operated on a
daily basis. One must descend
into the cellar for this, unless
extension tools are used. The
accumulated rain water is
suctioned from the cellar from
time to time. The depth of the
cellar can be approximately
2.5 metres or more,
depending on the number of
casings to be sealed off from
one another and the type of
the well. The immediate
environment is also taken into
consideration by keeping the
tree as low as possible. Some
completions are recessed in a
cellar for the greater part.
Top cap
Top valve (open)
Flow line (injectionline)
Flow-arm valve (open)
Otis-actuated master valve (main valve)
Hand operated master valve
Sealed in open position
Tpiece
1/2 “ kerotest valve
Intermediate platform
Intermediate deck
cellardeck
Controlline outlet
Tie-line valve
Closed and blinded off
No tubing-annulus connector pipe
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The Xmas tree setup on platforms
The space available on offshore production platforms is restricted as a result of which the
freedom and options during design are also reduced. Platforms have no cellar, but a cellar
deck is still referred to. Stairs with an intermediate platform are installed from the cellar
deck to operate the valves.
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5. Gate valves
5.1. Summary of Gate valves
The gas and oil industry makes frequent use of gate valves.
Gate valves are the only valves installed in the Christmas tree. These valves must meet
various requirements. We will discuss two sealing principles for this type of valves at this
point.
The gas the Christmas tree and the valves are in contact with is at high pressure, can
contain H2S and can contain fluid or sand particles. The high speeds of the gas that are
generated when the valve is opened and closed require special sealing provisions to limit
wear from erosion.
5.2. The principle structure of a gate valve
In principle, a gate valve consists of a body with exchangeable, parallel seats, in which the
gate can move.
Principle of a direct acting gate valve
A packed screw stem with
pressure bearing is installed in the
bonnet or cap of the valve. The
gate can be opened or closed by
turning the hand wheel of the
stem, since the gate is moved
upward and downward along the
thread of the stem. If the gate is
in the closed position in the
bottom of the body, one uses the
term 'direct acting valve'. There
are also valves whereby the gate
rather seizes against the cap or
bonnet in closed position. This
type is called 'reverse acting
valves' and they are used in
Christmas trees in combination
with an Otis U (pneumatic and
hydraulic) and UX actuator.
Clockwise rotation of the hand
wheel closes a direct acting valve.
handwheel
Pressure bearing
Packing
Seal
Cap or bonnet
House or body
Screw spindle
Gate
Exchangeble seat
Internal thread (left)
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The two gate valves we will discuss are both direct-acting valves. They have a nonrising
stem. One cannot see from the outside whether the valve is open or closed. Gate valves
are open/close valves and not control valves. The valve may never be left in the middle
position, thus half open and half closed, for example.
If the hand wheel of an open gate valve (at the top gate position) is turned counter-
clockwise, it will seize up because the left-hand thread of the stem comes to the end of the
thread in the gate (Cameron type F) or in the lift nut (McEvoy model C). If the gate
valve is closed, the hand wheel also seizes against a shut-off valve if turned clockwise any
further. For these valves tightening the hand wheel has no effect on sealing, but it does
close the sealing valve installed for changing the stem packing.
After the valve has been opened or closed, one must turn the hand wheel back at least a
quarter turn to prevent the thread between the stem and the gate as well as the gate itself
from seizing. Such seizing up can be aggravated by temperature variations during
production and production stops and by the ambient temperature. Turning back a quarter
turn also assists with detecting the closed/open position of the valve, namely by first
turning the hand wheel clockwise. If the valve is closed, the hand wheel blocks after a
quarter turn or half a turn, and in open position one can rotate further (direct acting
valves).
5.3. Requirements set for valves
Valves must above all be safe. This leads to a number of technical requirements, such as:
• Valves must be bubble tight, even though there is dirt in the gas or the valve is
slightly damaged. It often happens that valves start leaking anyway after some
time, so that pressure is built up past the closed valve. The speed at which the
pressure builds up depends on the volume of the space past the valve and the
magnitude of the leak.
• The valve must be operable under all conditions, even if the valve separates a large
pressure difference, i.e. operable by one person (opening/closing).
• The number of turns required to open and close the valve must be limited.
• If the valve is open, passage must be straight, thereby preventing or limiting
turbulence (erosion).
• The valve must be resistant to corrosive and/or erosive components in the oil or
gas as well as to weather and wind.
• The valve must be resistant to very high well pressures.
The following requirements are set for maintenance work on a valve:
• valves must be of such structure that the packings and the bearings of the gate
spindle can be inspected and/or replaced during production without removing the
valve or releasing the pressure on it
• valves must make it possible to replace the gate or the seats at the location;
• valves must require little maintenance.
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5.4. The method of sealing gate valves
Gate valves can be sealed in various ways. Different methods are used for the gate valves
in use in the gas and oil industry. We will succinctly discuss the two most important
methods:
• Metal–to–metal seal method
• Sealing–compound method.
Metal–to–metal seal method
If a valve is in the closed position, the valve will be under full pressure, while there will be
lower pressure past the valve. This pressure difference results in a force.
Principle of a direct acting gate valve
5.5. Sealing compound method
The force due to the pressure difference on the valve is often insufficient to guarantee a
100% seal.
Moreover, bigger leaks can be caused by irregularities or scratches.
For these reasons, an additional sealing compound is applied automatically between the
gate and the seat to ensure complete sealing. The surfaces of the gate and the seat are
then finished normally smooth and parallel.
This sealing method is used in the McEvoy valves.
A packed screw stem with
pressure bearing is installed in
the bonnet or cap of the valve.
The gate can be opened or
closed by turning the hand
wheel of the stem, since the
gate is moved upward and
downward along the thread of
the stem. If the gate is in the
closed position in the bottom of
the body, one uses the term
'direct acting valve'. There
are also valves whereby the
gate rather seizes against the
cap or bonnet in closed position.
This type is called 'reverse
acting valves' and they are
used in Christmas trees in
combination with an Otis U
(pneumatic and hydraulic) and
UX actuator.
Clockwise rotation of the hand
wheel closes a direct acting
valve.
handwheel
Pressure bearing
Packing
Seal
Cap or bonnet
House or body
Screw spindle
Gate
Exchangeble seat
Internal thread (left)
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Surface Well Completion Page 34
Mc-Evoy gate valve - model C (closed position)
Screwcap Hand wheel
Grease nippel Seal valve and slanted
seat for exchanging
packing and bearing
lift nut to move
the gate along
the spindle
Slide block
Space for water-repellent grease
Exchangeble seats
Pressure bearing
Spindle packing
Bonnet
Reservoir for sealing compound
Nipple with venting plug
Lefthand threaded screw spindle
Piston
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5.6. The McEvoy gate valve, model C
All McEvoy solid block Christmas trees are equipped with this type.
Mc-Evoy gate valve - model C (closed position)
Screwcap
Seal valve and slanted
seat for exchanging
packing and bearing
lift nut to move
the gate along
the spindle
Slide block
Exchangeble seats
Space for water-repellent grease
Bonnet
Grease nippel
Pressure bearing
Spindle packing
Bonnet
Nipple with venting plug
Lefthand threaded screw spindle
Piston
Reservoir for sealing compound
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Surface Well Completion Page 36
The gate of the McEvoy gate valve consists of two parallel halves kept apart against the
seats by pressure springs. Because the gate is parallel, it cannot seize between the seat
rings, and additional tightening of the hand wheel has no effect on the seal. The valves in
gas flows can be subject to more serious erosion at the time the gate practically closes the
passage. The two so-called sealing surfaces must be very smoothly polished to guarantee
complete sealing.
The eventual seal between the gate and the seats in the McEvoy valve is established by a
special sealing compound.
This compound is stored in two reservoirs on either side of the gate. The reservoirs can be
topped up via external grease nipples, which is very important for correct performance of
the valve. The advantage of this type of valve is that if any leakage is detected, it can
easily be remedied in a short time by injecting a sealing compound.
The space underneath the gate can be filled up with water-repellent grease to prevent
accumulation of fluids, hydrates and dirt.
5.7. The Cameron gate valve, type F
The Cameron gate valve, type F (see figure). A great deal of research has been done to
limit wear of the gate and the seat and to find a good solution for such wear. Gas flows
through a very narrow space at high speeds especially when a valve is just being opened
and nearly closed. The erosion that occurs at that time is extensive and the wear on the
gate and the seat as well as leakage appearing at a later stage will be the result.
The Cameron gate valve is fitted with a seat that can rotate (rotating seat). The intention
of this is that a new piece of the seat is exposed to wear by the gas flow each time. This
way, the wear of the seat is distributed evenly across the entire circumference.
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Cameron gate valve – type F (closed position)
Hand wheel connection piece
Shear pin
Pressure piece
Grease nipple, also pressure release with non-terun valve
Gate
Seat
Teeth aong the circumference Palm mechanism
Hand wheel
Pressure bearings
Screw spindle with left hand thread in gate
Seal valve and slanted seat for sealing when the packing set is exchanged
Screw cap
Bonnet flange
Packingcap
Center pin
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Surface Well Completion Page 38
The valves are mostly not fitted with a rotary seat and a catch mechanism. Problems
caused by teeth breaking off were more serious than the advantages.
The valve gate is not shared and moves back and forth between two seats through a
threaded stem. The seal is formed by a metal to metal seal. The gate and the seat require
a very accurate and smooth finish for this reason. In order to protect the gate, the stem
and other internal parts from overload, the force applied to the hand wheel is transferred
to the stem via a shear pin. This shear pin is not part of the McEvoy gate valve, model C.
The forces on the stem during operation are absorbed by two pressure bearings. One
packing set, enclosed by a screwed pressure piece, ensures that no leakage can occur
along the stem. Since the packing pressure piece is attached by means of thread, the
pressure bearing at the Cameron valve can be exchanged under pressure without other
precautions being taken. If the packing set must be changed during production, the space
for this above the packing can be closed off from the pipe pressure, just as is the case for
the McEvoy valve.
A slanted disc (front) is installed on the stem, which moves up after the valve is closed by
turning it further clockwise (due to the left-had thread in the gate) and will seal against
the slanted seat in the bonnet flange. The pressure that prevails above the slanted seat
can be released via the lubrication nipple with the non-return valve. To do so the ball is
pushed from its seat by means of a special tool, whereby the pressure can be released.
The screw cap can be safely turned loose and the packing set inspected or replaced only
after no more gas is escaping.
Tightening the hand wheel has no effect on the seal, just as is the case for the McEvoy
valve. Maintenance of the valve is simple thanks to its structure. The seats and the gate
can easily be replaced. Depending on the use, the valve must be lubricated from time to
time. One disadvantage of the Cameron valve type F is that, if it turns out during wireline
work that the valve leaks, it must first be repaired, while the McEvoy valve, model C, can
usually be sealed again by refilling up the sealing compound reservoir.