A field layout to produce a small reservoir with an economical production life of 30 years.
FIELD CHARACTERISTICS
• Gas Production: 20MSm3 / sd (223 kg/s)
• Reservoir Pressure: 270 bar, 2280m
• Reservoir Temperature: 91ºC
• Expected life: 30 years
• CO2 Re-injection @210 bar:
2600 Tun/sd, 0.03 kg/s
Subsea System
• 3 x 4 Slot Production Template/Manifold
• 1 CO2 Injection Well
• Subsea Control System
• Workover System
Field Location
• Norwegian Sea
• 330m Water Depth
• Transportation to Melkoya
• Pipeline 160km
• Electro Hyd Fibre Optic Umbilical
• 9 Production Subsea Xmas Trees
• 1 Injection Subsea Xmas Trees
• 1 Workover Riser System Figure 40... Field Layout
Templates
As its name suggests, this is a component that has a number of well centres accurately incorporated into its structure.
It is quite normal for 6-8 wells to be clustered together in a closely confined space and this means we can possible operate all of them from one control pod and umbilical line making a saving on the capital budget of a substantial amount of money. Because this is a one piece unit, the flow-line, umbilical lines and piping are all built-in. Many connections will automatically make-up together when the Xmas tree is landed in its allocated position.
The large cylinders are the suction feet these units weight will sink it into the sand, water will be allowed to escape from the top of each cylinder and it will sink-in. we will achieve it level to half of one degree using this method.
Guide posts can be seen protruding from the four corners of each drilling guide base and a depth indicator can be seen marked on the cylinder on the right.
When landed, all posts marked No1 will face due North.
Drilling and completion work is now complete and you can see the four Xmas trees are in place.
With the manifold and control pod connected the protective structure is fitted with angled corner posts to comply with industry standards.
This type of template will be fitted with Xmas trees especially designed for re-injecting carbon dioxide that is a bi-product of some other process like exhaust gasses from a gas burning power station.
When mixed with water this gas becomes very corrosive so all wetted parts will be to API HH Trim.
This technology is new and is likely to become
Figure 41... Snohvit field SchematicFigure 42... Manifold Well Slots
widespread as reservoirs become depleted of oil we can use them for the long term shortage of all types of greenhouse gasses. A lot of development will have to take place before a system like this and the associated pumping systems will see it in common use.
This is an example of a CO2 Xmas tree the format is horizontal or Spool design and will fit into and connect up to the facilities provided on the template.
Because of the nature of this trees service conditions and the restrictive access to two sides all the intervention or adjustment points are carefully worked out.
Top access will be used for choke and control pod removal or replacement.
Here the tree is on the rig finger deck being checked to ensure all the systems are operating before deployment.
The ROV intervention docking buckets are see with there function marked on tape used to cover the luminescent signs because they we be impaired if exposed to direct sunlight.
The tape is removed just before deployment.
What do the signs mean?
PMV ...Production master valve.
MIV.2 ... Methanol injection valve No2 XOV ...Cross over valve This illustration shows the IWOCS system as it is on the rig floor with its riser and other tooling and control umbilical reels. Below it is the well control package, Xmas tree and support structures.
Above the template are the remove/replace tools for
the choke, control pod and the panels that make up the over-trawl protective structure.
On the left of the template are two flow lines the top one is a 16” oil export lin going to the shore terminal for onward shipment by tanker and the lower smaller one is bringing CO2 from shore for injection back into a zone that does not connect to the oil reservoir.
To the right are tooling packages that will be required from time to time is a pigs unit (Propelled, Information, Gathering System) this will be pushed along the 16” line by the oil flow and check the line for corrosion, cracks and other defects storing the information as it goes.
Figure 40 shows the gas injection Xmas tree in the test pit with the safety equipment stacked and locked to its top.
The pit will be filled with water and pressure tested with Nitrogen gas to its maximum working pressure and held for 60 minutes.
No bubbles are allowed over that test period.
The size of the stacked-up system is evident as seen here in the test pit prior to it being filled with water in preparation for nitrogen gas testing.
At this stage the assembly would weigh approx 120 tons.
Figure 43... Production Equipment Installed
Figure 44... Field Layout
Remember...
ROV Intervention Docking Bucket label meanings:
PMV ... Production Master Valve MIV.2 ..Methanol Injection Valve No. 2 XOV ...Cross Over Valve
Figure 45... Subsea Equipment Components
Figure 46... Gas test in water
Test Yourself (a) How is the structure held in position? 9
(b) What is a template?
(c) How are the produced fluids transported to the terminal?
You will find the answer to Test Yourself 9 on page 51
Section 10 - Case Study
This is a case study of a modern subsea development where the water depth is 1200m and beyond the capacity of guidelines. The production is oil that has high paraffin wax content. The oil temperature at the seabed and through the Xmas tree is moderate so it is necessary to insulate the tree from the cold seabed temperature of +2 degrees Centigrade.
Hydrates seen as gas bubbles are also present in the immediate vicinity.
The initial installation and any field life interventions need to be provided for and incorporated into the tree design in such a way that work can be done using specialized vessels instead of a full blown drilling rig.
The functioning of a production subsea tree systems and valves is considerably less than a platform surface tree this will save on normal wear and tear so that should a problem develop below the tree.
If it is of horizontal or spool configuration it can be reworked without the removal of the tree.
In this study, after a tree shut down the wax content is of some concern, it is necessary to slow down the cooling rate and keep the tree warm this will keep the oil from thickening and ultimately solidifying and making the tree non-functional.
Tree insulation is crush resistant epoxy foam that is applied to all the areas that normally carry the production fluids areas like the tree block, the choke and the flow line connection.
This field will produce about 120,000 barrels per day, and will be connected to a ship shape vessel called a Floating Production, Storage and Off-loading Unit.
The (FPSO) is anchored in position and will process the produced fluids and transfer them to an export tanker.
A total of 16 trees will be installed and the field life is estimated to be 22 to 25 years.
At some point during that period some maintenance or repair work will be needed for
example:-Corrosion or erosion in the production tubing, Damage or wear in the SCSSV, Wear of the choke insert or its adjusting mechanism.
Production Xmas tree valves will have been specified to have a 25 year operating life without any maintenance hence the need for strict manufacturing controls on the assemble of valves and actuators the long term lubricants used and their ability to function in ever increasing hostile conditions.
Because of the number of trees in the field it is more economical for the operator to buy their own tools and have them maintained and tested prior to each time they want to use them.
Field conditions are:
Field: ... West Africa Water Depth: ... 1200 metres
Tree Type: ... 5” x 2” Conventional Pressure: ... 5,000 psi
Temp: ... API Class U (-18C to 121C) Weight (Tree): ... 31 Tons
Weight(TH Spool): ... 25 Tons
Trim: ... Prod H-H, Annulus E-E Valve: ... API 6a Gate valves Actuator Type: ... Hydraulic
Overrides: ... ROV Rotary W’Head Connector: ... MD-H4
Choke: ... KI Stepping Type Control Pod: ... OS Multiplex Type
Wellhead System: ... Heavy duty High capacity Flowline Connector: ... 5-1/8”
The equipment and the running tool stack-up are shown here.
Lower most is the Tubing Spool, it provides the connector to lock on to the wellhead and a mandrel that mimics the wellhead with the internal bore to land, seal and lock the Tubing hanger in place. The export line also is attached here.
Above that is the Xmas Tree and that is locked on to the tubing spool (the white vertical pipe between the two is the insulated flow line downstream of the choke and where the export flow line will be connected.
The two components above the tree are running tools The Lower Riser Package (LRP) and the Emergency Disconnect Package (EDP) these two are normally kept together however if an emergency arises when the tree is locked to the seabed and the drilling rig is still attached on the surface we can close the lower and release the upper to allow the rig to deal with the surface emergency.
Hydrates are gas molecules encased in an ice shell and where the seabed is soft they will rise to the seabed. The ice melts and we then have frozen free gas, as this gas rises towards the surface it can get trapped by our Xmas tree or other parts. When this happens it can cause a malfunction by freezing moving parts as it forms into ice blocks.
In this case we are interested in preserving production heat rather than allowing the tree to freeze so as long as we have the well flowing everything will be OK.
When hydrates are know to be present the drilling rig will set a Mud-mat at seabed level this is a steel plate that is larger than any equipment the will be used or fitted above it.
Any hydrates will then have to migrate out towards the plate edges then continue upwards outside the equipment in a harmless fashion as the picture below shows. This is about 3 meters above the seabed.
Figure 47... Equipment Stack-up
Figure 48... Hydrate build up
Production Heat must be conserved at all costs and this is why the areas that contain vital valves and other parts have to be protected from the cold ocean by a high strength and high efficiency epoxy foam.
The seabed conditions in this case at 1200 meters will be 2 degrees Centigrade and 1580. phi. The window of time available to us to remedy any problem or take action using chemicals to prevent the solidification of the wax content of the oil is small only a few hours.
The temperature slices shown here illustrate how the core temperature drops in a time frame of 12 hours.
It can be seen how all the heat has been lost and a serious situation would exist. Although this has never happened it illustrates the planning and equipment design development and testing that has
to be done to let planning a recovery take place. The Epoxy Foam Insulation as applied to the Xmas tree block is 75 mm thick. This makes the assembly of actuators and other components very difficult as can be seen here.
The vulnerable part of the tree is the valve cavity area that contains the sliding gates.
Solidified wax filling this will prevent the reopening of the cooled valve that was closed in the hot condition the actuators are not powerful enough to overcome this condition.
The insulation will delay the point where it is impossible for the valves to operate to somewhere in the region of 57 hours but this may not be sufficient to save a well and that would be a multi-million loss in any currency.
Before the Xmas tree can be approved for service.
To prove the strength of the casing it must be proof tested using water at twice the operating pressure for 5,000psi (10,000 test) and 1.5 times for higher rated Xmas trees this is necessary since the casing has no second barrier between the reservoir and the ocean. A second functional test for valves, actuators and choke is done with nitrogen gas at the full rated pressure, and is held for one hour with no bubbles permissible.
Remember...
We use a floating production system with off-take facilities to take the oil to the customer
Figure 49... Tree thermal profile (producing)
Figure 50... Tree thermal profile (cooling)
Figure 51... Tree Thermal Insulation
Here the Xmas tree is seen being lowered in to the water by the drilling rig by its running tool this will be hydraulically released after the tree is locked in place and pressure tested.
All that remains now is to connect the flow-line and the control umbilical. In this case these will run from a collecting, processing and exporting vessel called a Floating Production Ship with Off-take facilities (FPSO).
An FPSO is a complex vessel that accepts the crude oil directly from the wells and separates it out into oil gas and water. The oil will be stored within a double skinned hull the gas will be used to power all the on board systems and the water spun clean in hydro-cyclone until it contains less than 40 parts per million (PPM) in this condition it is cleaner than the ocean so it can be put in there.
When the FPSO’s tanks are full a large tanker will moor behind it and using a hose connection will
off-take the contents and deliver them to a customer in any part of the world.
All FPSO’s are custom built and fitted for their principal use but can be rebuilt for a second life at a later date. This one has a capacity of 400,000 barrels ( x 42 = 1.68 million gallons.)
Rising vertically up the ships side we can see the risers that bring the high pressure oil from the seabed. They are rigid at this stage but because we are in deep water the vessel is held on station by a series of thruster that are controlled by computer that are taking constant information from a number of satellites.
Below the hull, the risers will be flexible to allow for the slight movement of the vessel and in the event of a runaway they will drop and seal automatically to prevent damage or spillage.
(a) What is the water depth?
(b) What is the problem we are addressing?
(c) How do we overcome this problem?
(d) Are there any other seabed hazards?
(e) How do we take the oil to the customer?
You will find the answer to Test Yourself 10 on page 51
Figure 52... Tree deployment Figure 53... Bonga FPSOTest Yourself 10
Check Yourself 6
(a) 1) Since the tubing hanger is run after the tree is in place, a workover requiring the tubing to
(b) One plug fits in the tubing hanger above the horizontal passage. The second plug fits in the Internal Tree Cap.
(c) Using a Cam Actuated Running Tool (CART) run on a landing string.
Check Yourself 7
(a) The landing string provides well control functions and disconnection capabilities during well installation, workover, intervention and well test operations on wells with horizontal subsea Xmas trees.
Check Yourself 8
(a) 50 tons snag load
(b) To protect subsea production equipment and fishing nets and gear
Check Yourself 9
(a) Suction legs
(b) A component that has a number of well centres accurately incorporated into its structure.
(c) By pipeline
(e) Floating production system
Check Yourself 1
(a) Fixed Platform (b) Tension Leg Platform (c) Subsea Development
Check Yourself 2
(a) The requirements for material, testing and certifications for subsea equipment in general
Check Yourself 3
(a) Mandrel or Hub (b) 7 Million pounds (c) 3 Casing hangers
(d) Fits onto top of Blowout Preventer (BOP)
Check Yourself 4
(a) Flowline and Umbilical (b) Hydraulic pressure
(c) It suspends and seals the removable tubing string inside the wellhead
(d) The IWOCS controls and monitors the
deployment, the operation and the retrieval of subsea production equipment such as Tubing Hangers, Landing Strings and Xmas Trees
Check Yourself 5
(a) A verticle bore
(b) It provides 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.
(c) It is a device that regulates the flow from the well.
(d) To boost the pressure from the umbilical.