4 Products
4.1 Separators
4.1.1 Liquid/liquid separation
In a liquid/liquid separator the produced water is separated from the oil. The water can then be injected into a reservoir for either disposal or pressure support. Pressure support means that the water is injected into the reservoir to partly counteract the pressure reduction that is caused by removing oil, water and gas. The higher the pressure in the reservoir the more oil and gas can be produced. If the water is injected for pressure support the quality requirements will be strict and the water will need further treatment to
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remove residual oil and sand. Removing water from the well stream decreases the pressure loss in the flowlines to topside so the production can be increased.
Five types of separators used for bulk water removal are presented in the following:
Conventional gravity separator (sand, oil, water and gas in the same tank)
Semi compact gravity separator (gas bypass line)
Pipe separator
Dewaterer
Decanter
Hydrocyclones
Conventional and semi compact gravity separator
The conventional gravity separator and the semi compact gravity separator have the same functional principle. A multiphase flow enters the separator and goes through a momentum breaker which separates oil and gas. Additionally, the momentum breaker’s function is to reduce incoming moment, prevent formation of drops from rough contact with the bottom of the separator and prevention of foam by controlling the shear forces by using smooth surfaces. There are different types of momentum breakers, i.e. cyclones and Evenflow. The gas rises and gather in the uppermost part of the separator (conventional separator) or is led through a pipe (semi compact separator).
Distribution baffles
Figure 4-1 Conventional gravity separator Figure 4-2 Semi-compact gravity separator
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Figure 4-3 Flow patterns in gravity separator
When the flow exits the momentum breaker there will be turbulence and gas, oil water is mixed. The flow is therefore lead through baffles plates (one or two), which are vertical plates with holes. In this way the flow is straightened making sure everything moves in the right direction so the separation will begin. Because of the different densities of the fluids they will be separated in layers: sand will gather in the bottom, then water on top of this, then the oil and on top the gas. Bubbles will rise and particles will sink. Sand is normally sedimented within the first two meters of the tank. The retention times in the tank are chosen after the quality of the separation required. The necessary retention time will vary by the composition of fluids and will determine the dimensions of the tank.
Advantages Disadvantages
Conventional Simple process
Low pressure loss or well fluids with small amounts of water or well fluids with small amounts of water
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Figure 4-4 Comparison of size for conventional and semi-compact gravity separator Pipe separator
The pipe separator is a long gravity separator. It is suitable for fluid compositions with heavy oil and small amounts of water. At the inlet, a gas harp separates the bulk free gas from the liquid. The gas is routed to an outlet section and recombined with the oil. The liquid and remaining gas enters the pipe separator where oil and water phases are separated. The layers of oil and gas have different velocities. Because of this the layers will rub out the drops in the emulsion layer and make water drops merge with water drops and oil drops merge with oil drops. The decomposition of emulsion is efficient.
Sediments will have sufficient time to sink to the bottom of the pipe because of the relatively small diameter. The pipe diameter is based on a design velocity of approximately 0.7 m/s. At the end of the pipe separator an outlet section splits the separated liquid phases.
Figure 4-5 Pipe separator
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Figure 4-6 Parts of Marlim SSAO separation module, showing gas harp and gas crossover (red), PipeSeparator™ (green) and outlet section (grey). White arrows indicate direction of flow in the PipeSeparator™.
Inline Dewaterer
Figure 4-7 Inline DeWaterer
The InLine DeWaterer is a compact cyclonic unit designed for efficient separation of bulk oil from water. The unit consists of one or more axial flow cyclones with fixed swirl elements. The technology has been developed and qualified in cooperation with Statoil.
The mixed oil-water flow enters the pipe spool (or for multiple liners a DeWaterer vessel) via the inlet nozzle and moves into the inlet compartment. Next, the mixed flow goes into the DeWaterer liner where it moves through the fixed swirl element generating a rotating flow. The centrifugal force makes the lighter phases, i.e. oil and gas, move towards the centre of the DeWaterer. The heavier phases, (i.e. water and sand) move to the outside of the liner. The lighter phases are extracted (counter current) through the reject, and the heavy phases are removed via the underflow.
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A major advantage of the system over the conventional gravity-based solutions is a potentially large reduction in the required vessel size. A reduced vessel size leads to a reduced overall station size and weight.
DeWaterer is able to handle up to 30-50 % gas volume and both oil- and water continuous systems. Reject flow is normally around 15-25% of total incoming flow. For deoiling of a water stream, OiW outlet quality is typically <1000 ppm.
Inline Hydrocyclones
Figure 4-8 Inline Hydrocyclone
InLine Hydrocyclones are used to separate the residual oil from water by use of high centrifugal forces. The CDS Inline Hydrocyclone has a liner inlet that minimizes shear forces and thus oil droplet breakup. A major advantage of the Hydrocyclone over the conventional gravity-based solutions is a potentially large reduction in the required vessel size.
The mixed oil-water flow enters the Hydrocyclone vessel via the inlet nozzle and moves into the inlet compartment. Next, the mixed flow goes into the Hydrocyclone liner where it moves through the fixed swirl element where a rotating flow is generated. This rotation generates a high centrifugal force. The centrifugal force makes the lighter phases, i.e. oil (and gas), to move towards the separation chamber of the Hydrocyclone and the heavier phases, i.e. water (and sand), move to the outside of the liner. The light phase is extracted counter current through the reject, and the heavy phase is removed via the underflow.
Hydrocyclone liners have a low operational flow rate and it is necessary to group liners together inside a vessel.
Depending on inlet water quality and oil chemistry, OiW quality of <100 ppm after single stage Hydrocyclone may be reached. Two Hydrocyclone stages in series may be used to reach even better water quality but this setup will increase total reject flow.
50 4.1.2 Gas/liquid separation
A gas/liquid separator separates gas and liquid so that the fluids can be treated separately.
This is done in cyclones at the inlet of gravity separators, in vertical gravity separator or in VASPS (Vertical Annular Separation and Pumping System)
Figure 4-9 Left: CDS Gasunie inlet cyclone. Middle: Vertical gas/liquid separator with pre-separation pipe and inlet cyclone. Right: VASPS (Vertical Annular Separation and Pumping System)
Scrubber
A scrubber is a type of gas/liquid separator which main function is to prepare the gas for compression. It is used when there are small amounts of liquids. Liquid drops may lead to erosion in the compressor over time, and it is therefore important that the gas is as dry as possible. A vertical tank is the best solution for gas/liquid separation.
A typical scrubber consist of a distribution element, vane pack and spiralflow cyclones.
The inlet distribution element absorbs moment and coarsely split liquid and gas by use of centrifugal force (cyclone) or an Evenflow. The liquid and potential sand will accumulate in the bottom of the separator while gas will rise. Gas will hit vane packs which are mainly designed to ensure even distribution of gas and to remove bulk liquid and coalesce small liquid droplets into larger ones. After passing through the vane packs the gas will continue to a spiralflow cyclone which spins the gas. Because of centrifugal forces the liquid will gather at the walls while the gas rises in the middle. Water drops will stick to the plates and pour down to the bottom of the separator.
This principle is used on Ormen Lange where the amounts of liquid are small. Other benefits of this separator are that it has a small footprint and the vertical form makes it easier to remove sand.
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Figure 4-10 Scrubber with internals
Figure 4-11 Left: Inlet cyclone. Middle: Vane packs. Right: Spiralflow cyclone.
Decanter/caisson separator/dummy well
Decanter Process System technology is based on vertical separation units located mainly below seabed for separation of gas and liquid. Separated liquid is boosted to topside by ESP (Electrical Submersible Pump) or HSP (Hydraulic Submersible Pump) which are located within the separation units. The hole has to be about 120 meters deep because the pump is 75-90 meters long.
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Advantages: Known pump technology (the pumps are well known from applications in well stream). Can be installed and maintained with LWI equipment, procedure and technology is familiar to the client and the risk is low.
Disadvantage: more expensive than the alternative with separation tanks on the ocean floor.
Figure 4-12 Decanter separation system.
Inline DeLiquidiser
The InLine DeLiquidiser is an ultra compact separation solution developed by CDS/FMC in co-operation with Statoil. The DeLiquidiser separates liquid from a gas dominated stream within a pipe.
The gas initially flows through the flow conditioning element to equally distribute the liquid droplets across the cross sectional area of the pipe. The stationary swirl element then sets the gas dominated stream into rotation. As a result, gas migrates to the centre of the cyclone while the denser liquid phase forms a film on the outer wall of the DeLiquidiser. The gas exits the cyclone through the gas outlet pipe located in the centre of the main pipe. The gas outlet pipe is equipped with an anti rotation device which stops the gas swirl and recovers pressure. The liquid enters the annular space between the gas outlet pipe and main pipe and is drained to the liquid booth. The separated liquid contains some gas, which is recycled through the gas recycle line back to the tip of the swirl element. A liquid level in the booth is required to prevent gas carry under. The separated liquid is discharged through a liquid outlet nozzle in the bottom section of the booth.
Due to its compactness, the DeLiquidiser is a very effective solution for applications where a limited space is available or where space and weight reductions are key parameters. The DeLiquidiser can be used to de-bottleneck existing processes to increase
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production capacity and minimize footprint of new production systems. It is normally applied for inlet gas volume fractions of 90 - 99.5%, but is also applicable for higher inlet liquid fractions by taking into account special design considerations for the handling of liquid. The unit is capable of producing two single phase outlet streams from one multiphase inlet stream.
Figure 4-13 Inline gas from liquid separator. Figure 4-14 Inline liquid from gas separator Advantages: Is small and lightweight.
Disadvantages: Hard to control because of small volume, less robust, higher pressure losses than in a gravity separator, limited turndown, new technology, slugging and similar types of disharmonics in the system can lead to blow by or liquid carry over, less efficient than a big tank.
4.1.3 Sand handling
The main methods for removing sand in a process system are to use an Inline DeSander and/or removing the sand accumulated in the bottom of a separation vessel. The InLine DeSander separates solids from multiphase or liquid flow, based on density difference and centrifugal force.
The flow enters the desander (liner) and is set in spin via vanes. The spinning flow enters a constriction that intensifies the spin due to the conservation of angular momentum. It is this spin that separates the liquid and solids (based on density difference). The sand exits the desander liner as concentrated slurry. The cleaned fluid reverses flow direction and flows through the center core of the desander liner towards the vessel exit.
The DeSander can be used at various locations in a process plant. Typical application areas are inlet desanding (positioned at the inlet of a production system either upstream or downstream the production choke), and liquid desanding (positioned downstream a primary separation stage for example to protect an injection reservoir and water injection pump).
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Figure 4-15 Inline desander
Accumulated sand in the bottom of a separation is removed with Jetting nozzles (they fluidize the sand with water) and TORE (elements that suck up the fluidized sand). After the sand has entered the TORE unit it is flushed in to a sand tank which acts like a vertical solid/liquid separator. Here the sand will accumulate in the tank while water is removed near the top of the separator. When the sand tank is near full the sand is flushed with high pressure to the oil flowline (for removal topside), to the water injection line or to a separate water/sand injection line.
Figure 4-16 Desander tank
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nozzles
TORE®
Figure 4-17 Jet nozzles and TORE units in horizontal gravity separator
Sandhandling is used subsea but is still at an early stage subsea (under development).
Flushing of the separator and sand tank will be done in intervals or according to level measurements (a few times a week, even when there isn’t that much sand just to make sure it doesn’t stick). This is done while the rest of the separation process runs as normal, assuming a small amount of extra sand in the water. On the Tordis project it is expected that 50-500 kg of sand will pass through the separator per day.
Suction points
Ejector
Level transmitters
Re-combination point with outlet oil
/ gas stram Inlet cyclone
Distribution baffle
Figure 4-18 Sandhandling system in conventional gravity separator
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4.2 Pumps
A pump is a machine or device used for raising, compressing or transferring fluids. It increases the pressure or the velocity of the flow and helps bringing fluids topside when the pressure has decreased in the reservoir or re-injects water into the reservoir. We have subsea and submersible pumps. Subsea pumps are set on the seabed while submersible pumps are built in the wellhead or in a dummy well next to the wellhead. Subsea pumps can be installed with vessel instead of rig and give a possibly higher MTBF (Mean Time Between Failure). The main types of pumps are:
- Single phase pump - Multiphase pump - Hybrid pump
Single phase pumps have higher efficiency than multiphase pumps, so they will be preferred where possible. This means that the separation must give a GVF (Gas Volume Fraction) of less than 5% and that you do not have creeping. Creeping is a phenomenon that occurs when you have condensate in a pipe and friction between liquid and wall creates gas. The fluid composition/separation quality will decide which pump it is possible to use.
There are generally two types of pumps used subsea;
Dynamic pumps like centrifugal pumps, helicoaxial multiphase pumps and hybrid pumps that are a combination of helicoaxial and centrifugal.
Positive displacement pumps like twin screw pumps
The dynamic pump generates differential pressure by adding kinetic energy to the fluid and converting that to pressure. The impeller accelerates the fluid to a certain velocity and the diffuser covert the kinetic energy (velocity) to pressure. Dynamic pump will by design have a maximum differential pressure set by the speed, impeller diameter and number of stages.
Positive displacements pump moves a fixed volume from the low pressure side to the high pressure side. In theory this pump has an infinite differential pressure but the differential pressure is limited by mechanical design (clearances, bearing selection, design pressure of housing, etc) and the available shaft power.
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Figure 4-19 Pumps in a processing system
Min. flow loop secures minimum flow to the pipe by opening on low flow to recycle the fluids, Figure 4-19. In this figure we see two examplleess of pumps used in the separation system, at the top a multiphase booster pump and at the bottom a water re-injection pump.
4.2.1 Single Phase centrifugal
Centrifugal or single phase pumps are applicable when having only one type of fluid composition (i.e. boosting of liquid and water injection) with less than 10 % GVF (Gas Volume Fraction). Single phase pumps can give a higher differential pressure and has higher efficiency than other types of pumps.
Figure 4-20 Single phase pumps Mode of operation:
Increase speed of the fluid with a round impeller. The fluid enters the pump in the center and is flung to the exit with high speed.
58 Advantages
Good size performance ratio
Can be designed for high capacity and Δp
Simple mechanical design
Field proven topside and subsea
Several suppliers
Can be made with tungsten Disadvantages
Low tolerance to entrained gas
High churning and forming of foam and emulsion
Poor performance at high Δp and low flow
High NPSH (Net Positive Suction Head)
4.2.2 Multiphase helico axial
The multiphase helico-axial pump uses special axial pump impellers suitable for pumping both liquid and multiphase fluids. The design is optimized to handle multiphase, hence the design is not optimized on efficiency as a centrifugal pump would be. This is the dominating technology for subsea boosting as it has a wide operating range with respect to GVF. It is applicable for more types of fluid compositions and run with up to 100 % GVF but the differential pressure created above 95% is very limited.
Figure 4-21 Left: Sulzer Helicoaxial MPP. Right: sketch of impeller and diffuser Mode of operation
Hydrodynamic. Flow enters and is distributed around a cylinder in the middle, the flow enters the cylinder from the bottom passes through impellers which adds velocity and diffusers which add pressure.
59 Advantages
High flow rate capability
Field proven topside and subsea Disadvantages
Differential pressure reduced at high GVF
Runs at high RPM (Revolutions Per Minute)
Sensitive to liquid slugs
High thrust loads
Impellers are shapes to avoid separation of gas and liquid through the pump so that the fluid is homogenic and everything moves with the same velocity. Used for medium to light oil.
Figure 4-22 Sulzer Pump 4.2.3 Hybrid pump
The hybrid pump is a combination of helico-axial multiphase and radial single phase hydraulics. The first few stages comprise helico-axial impellers that enable compression and dissolution of gas into the production fluid. The subsequent stages comprise radial single phase impellers for conveying the production fluid with less than 5 percent by volume of free gas, thus delivering higher head per stage and higher efficiency as compared to the helico-axial impellers. The inlet GVF for selecting a hybrid pump is generally limited to 20 percent.
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Figure 4-23 Hybrid pump (From Sulzer) 4.2.4 Multiphase twin screw
The multiphase twin screw pump uses the principle of positive displacement and operates from very low to very high gas fraction. This is more predominant topside but is also field proven subsea. The twin screw pump can generate higher differential pressure at higher GVF than a helicoaxial pump but cannot operate at 100 % gas for a long period.
Figure 4-24 Twinscrew multiphase pump. Top: Close-up of screws. Bottom: Principle overview Mode of operation:
Volumetric
61 Advantages
Field proven topside
Widely used (Most multiphase pumps used topside are twin screw pumps)
Differential pressure independent of GVF(Gas Volume Fraction)
Differential pressure independent of GVF(Gas Volume Fraction)