The conventional tree design that has been our subject comprises of a number of well proven parts that are assembled together to make one unit.
Most user companies have there own requirement over and above the API standards although they are mainly on overall configuration or a need for an extra valve or some additional chemical or control lines.
The main component is the valve block.
This is preferred to be made from one piece to minimize joints as they could be a potential leak point.
The preferred API 6A design for production of oil or gas is the gate valve. A number of these will be machined into the block body and arranged for physical operation by ROV and some others will be automated by hydraulic actuators with their operation integrated into the overall control system.
The arrangement shown below is fairly typical of this design in up to 300 meter depths.
The Master and Wing valves are red and the Swab valve is yellow.
The smaller valves on the right are for monitoring and control of the area below the tubing hanger called the annulus.
Xmas 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”. In this section we are referring to what is known as a conventional subsea tree. Note that a Xmas tree and wellhead are
separate pieces of equipment not to be mistaken as the same piece. The Xmas tree is installed on top of the wellhead. A wellhead is used without a Xmas tree during drilling operations when the drilling BOP is installed on the wellhead.
The primary function of a Xmas tree is to control the flow of well fluids and gas out of the well. A Xmas 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 flow through a flow line to the production facilities on the host platform or 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, valve and choke position feedback), and connection points for devices such as down hole pressure and temperature transducers (DHPT). On producing wells, chemicals may be injected to prevent production problems such as hydrates or scale.
The control system attached to the Xmas tree, controls the downhole safety valve (SCSSSV) while the tree acts as an attachment and conduit means of the hydraulic control line to the downhole safety valve.
A ‘Conventional Xmas Tree’ generally refers to a
dual bore tree which has two vertical bores running through the tree. The bores are typically 5” x 2” ID but can be built to specific customer requirements.
The larger 5” bore is the production bore through which the produced formation fluids flow. The smaller 2” bore is a conduit to the annulus and is used for access to the annulus with the likes of gas injection. A crossover line which is isolated with valves, connects both the production bore and annulus bore to facilitate circulation / communication between the bores.
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 Xmas trees no longer exists given the frame and support systems into which the main valve block is integrated.
Subsea Xmas trees have a large variety of valve configurations and combinations of hydraulic actuated valves. Examples are identified in API Specifications 6A and 17D.
The conventional tree has vertical bore valves. The two lower valves on each bore are called the master valves (upper and lower respectively). Master valves are normally in the fully open position and are never usually opened or closed when the well is flowing (except in an emergency) to prevent erosion of the valve sealing surfaces. The valves are hydraulically actuated, allowing a means of remotely shutting in the well in the event of emergency.
An actuated wing valve is located on the horizontal flow path and is normally used to shut in the well
when flowing, thus preserving the master valves for positive shut off for emergency or 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 valve at the top of the vertical bores is called the swab valve and lies in the path used for well interventions such as wireline and coiled tubing operations. 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. There is also an annulus swab valve to enable access to the annulus bore of the tubing hanger to facilitate plug setting in the tubing hanger.
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 floline 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 and flow measurement.
When a dual bore subsea Xmas tree is connected to a subsea wellhead it must interface with the tubing hanger previously installed in the wellhead.
The tubing hanger and tree must be correctly orientated so that the production and annulus bores are properly aligned with each other.
ASV - Annulus Swab Valve AMV - Annulus Master Valve AWV - Annulus Wing Valve PSV - Production Swab Valve
PUMV - Production Upper Master Valve PLMV - Production Lower Master Valve PWV - Production Wing Valve XOV - X-over Valve
TESTPORT
MECHANICAL OVERIDE
WELLHEAD CONNECTOR
TUBING HANGER SCSSV
LINE
AWV PWV XOV
CI CIV PT
ACTUATOR TREE CAP
TREE HUB
PRODUCTION CHOKE
HYDRAULIC STAB CONNECTOR
AMV PUMV
PLMV
PSV ASV
PT
API 6A Gate Valve
The API 6A Gate Valve is the industry standard and can be fitted with a variety of internal parts that suit different types of produced medium.
Crude oil as produced will have been analysed at the time the well was flow tested, this will have determined the proportions of water, oil and gas or Oil to Gas ratio.
When the oil ratio is high say 90% or more with the balance mainly water with a little gas and no presence of Hydrogen Sophie( (H2S)) this will be an OIL WELL. The parts or trim of this valve will be as listed in table 1.1 (page 14)
If the well is 95% gas with 2% water and 3% Carbon Dioxide (CO2) it will be a Gas well. The parts for this valve will be as listed in table 1.
If we have approximately equal parts of water, oil and gas this well will probable be left unfinished until later when it can be used as a Water Injection well so as to replace some of the oil that has been removed or to sweep oil towards another producer.
Production gate valves vary in bore size and pressure/temperature rating for subsea production.
They are usually 5” bore for oil and 6” bore for gas.
With a temperature rating of -20/+250 (API. P-U) Pressure ratings of 20 and 30,000 psi with temperatures of up to 450oC are available for other oilfield purposes.
Valve Actuators
Valve Actuators are very specialized single acting hydraulic cylinders that are fitted to valve bonnets so they can be remotely functioned.
Water based or oil fluid can be used as the power supply but where we are operating in an oceanic environment, water based is normally the preferred option.
This fluid is a mixture of 80% water, 19% glycol and 1% biocide and lubricating agents.
Certain companies prefer to use non foaming hydraulic oil and this undoubtedly offers considerably more lubricating qualities but since the units will rarely be functioned during there operational life lubrication is not an issue.
Operating pressure for an actuator will vary depending on two
things:-1) The operating pressure of the well.
2) The depth of the water. (1.3 lbs/mtr)
Inside the actuator are a piston and a large spring, because a spring is a highly stressed component we must make sure it is not allowed to become corroded. In order to do this, the spring housing will be filled with control fluid. When fluid is pumped in, the piston is displaced down, the spring gets depressed and its fluid is displaced into a holding chamber that is open to sea pressure at one end.
This is the holding position for an open valve.
Figure 21... API Gate Valve
Figure 23... Gate Valve
The API requirement for Xmas tree valves is fail safe closed (FSC). In the event of a loss of input fluid from a malfunction, feed line failure or a system command the large spring assisted by sea pressure acting on the Sea Chest will push the valve closed and fill the spring housing with fluid. This is called fail safe closed.
Before fluid can be filled into a system it should be taken from sealed drums that contain pre-filtered fluid that are in compliance with specification NAS.1638 CLASS 6.
When the system is completely charged it needs to be flushed. This flushing process will circulate the fluid aggressively around the system so as to dislodge any small microscopic particles from the actuators, the piping and any other part of the swept area.
Fluid cleanliness is the key to system reliability and at the manufacturing stage a fluid sampling system will be used to make checks by taking small amounts of fluid and checking them for contamination by comparing the sample with a known standard under a microscope.
When compliance with NAS.1638 CLASS 6 is achieved the fitter, QC inspector and often a third party representative will sign the system off as fit for purpose.
During the operating life of the system the fluid will slowly be circulated back to the control vessel or platform where it will be conditioned back to the NAS.1638 CLASS 6 standard.
System operating pressures will normally be from 3-5,000 psi in most areas of the world but can be as high as 20,000 psi so as to function the SCSSSV in a deep water high pressure well.
During the life of a field, possibly as long as 20 years, the only routine maintenance of the system will be fluid filtration and the examination of any particles of metal, rubber or plastics since they will be our first indicator of wear or erosion taking place somewhere in the equipment. By identifying the particles we will be aware of something happening and this could avert a costly shut-in of the well.
The actuator is attached to the valve body by the valve bonnet and is function tested as a complete unit.
These two parts are the interface and have to be supplied by the valve manufacturer and will allow any make of actuator to be fitted.
The actuator is subject to the same trim material requirements and falls under the same API classification as the valve.
The external surfaces will be finished in high tech high visibility epoxy paints so as to ease relocation in low or no light conditions.
All the actuators on a subsea tree are arranged to failsafe closed.
Three forces will combine to do this;-The internal spring force.
The ocean pressure on the sea chest.
The flowing pressure of the well acting on the unbalanced stem (one end in one end out of the valve).
The flowing pressure is by far the greatest and will automatically push the stem upwards and closes the valve.
During the Xmas tree installation the Xmas tree functions are controlled by the IWOCS system on the drilling rig. Once the tree is installed and operations on the drilling have been completed, the IWOCS system is recovered severing hydraulic control to the tree. The Tree Cap is then installed.
The tree cap is fitted with a number of hydraulic couplers which mate up with the hydraulic lines on the Xmas tree, providing a hydraulic link between the tree functions and the Xmas tree control pod.
This enables the Xmas tree to be functioned by the host platform.
Conventional layout trees are made to be used in a variety of locations and water depths of up to 2,000 meters.
Figure 24... Actuator
Beyond this depth a different design will be used that we will see later.
The guideline-less system for water up to 3500 meters uses the same parts but the are guided together by moving the drilling rig with the equipment hanging in the water until a funnel–up captures a sleeve down then orientation guides present the parts correctly to be locked together.
When this activity is going on, our eyes are the cameras and lights onboard the ROV supporting equipment.
With the exception of the cap, the tree will probably remain on the seabed for all its production life of twenty years or more.
Within the frame work of the tree, we also have two other parts that play a major role during that period.
These are the:
Choke and the Control Pod
The difference with them is we have the ability to remotely, with the ROV remove parts that have worn beyond an acceptable condition of have suffered some component failures.
Choke
A Choke is bolted into the flow system downstream of the wing valve and as its name suggests it is a device that will regulate flow from the well.
This device is a made very heavily with thick sections so as to withstand high pressure and erosion from flowing well fluids.
They are also fitted with an aperture that will be variable so we can make adjustments up or down on the flow rate from the Xmas tree the choke rating is quoted in controlled velocity (C.V.)
There is a high rate of erosion of the choke insert, it is made from a very wear resistant man made alloy called Tungsten Carbide. The wear is usually higher at the start up and early years of high flow rate where particles are still breaking loose from the reservoir.
The flow through the choke is supersonic and therefore it has a high decibel rating but because we are in some depth of water this is not an issue.
When the insert is worn to where the necessary adjustment cannot be achieved it can be removed and replaced on the seabed remotely using an ROV
deployed by a support vessel thereby avoiding the need for a drilling rig.
Control Pod
The Control Pod is other item on the Xmas tree this is the unit that will receive electronic signals and translate them into hydraulic commands then send that to the requisite function. The unit is completely self contained and will function in all water depths simply by adjusting the operating pressure of the hydraulic section for valve actuators and the nitrogen atmosphere inside the casing is increased
Figure 25... Adjustable ChokeFigure 26... Control Module Internals
to prevent collapse from sea pressure.
Hermetically sealed inside the outer brightly coloured can is the electronic package this is normally a solid state system that is designed to be long life and reliable. It also contains fluid accumulators that hold 210% for re total swept volume of all the valve actuators and the SCSSV. This volume is sufficient to give two complete cycles of the entire system.
As high pressure fluid is used it will be replenished from the platform down the umbilical line.
On the left is the removable orange upper pod sitting inside its angled catchment funnel with the
“V” orientation slot at the front. On the right shows the electronics section and the orange actuator operating fluid storage accumulators.
The umbilical line has a finite pressure rating, should we need a higher pressure fluid it may be impractical because of flow rate or physical size just make things bigger.
We must then use an Intensifier to boost the pressure the intensifier works by applying umbilical pressure to a piston of a given diameter which will in turn boost the pressure at it’s outlet.
All the Xmas Tree pipework terminates in the section under the control pod and will be subjected to the same cleaning and flushing procedures.
At this point a large and very strong plate will be used and this is populated with one half of the fluid couplers with the other half being on the underside of the control pod.
The lower half will be fitted with a floating guide sleeve to reduce the positional criticality of the couplers. The two large nuts that will hold the upper and lower halves together and prevent separation of the couplings can be seen on the upper right and lower left of the plate.
The area of the tree where this unit is located is well protected by bumper bars to ensure its safety.
Cathodic Protection is part of the Xmas tree design and 99% pure zinc anodes will be welded to the framework to ensure a perfect connection.
The amount of zinc needed is calculated using the temperature and salinity of the water and the estimated life of the immersion period.
Tree Cap
Figure 27... Subsea Control Module
Figure 28... Tree Assembly with cap
Remember...
The function of the Flowbase is to provide the export line orientation and isolation vale to shut off the flow.
(a) Where is the master valve in a conventional tree, in a horizontal or vertical bore?
(b) What is the function of the tree cap?
(c) What is the function of the choke?
(d) When is a hydraulic intensfier used in the control system?
You will find the answer to Test Yourself 5 on page 51
Figure 29... Hydraulic Intensifier Figure 30... Control Module Mounting Base