Flare and atmospheric ventilation
Flare subsystems include flare, atmospheric ventilation and blowdown. The
purpose of the flare and vent systems is to provide safe discharge and
disposal of gases and liquids resulting from:
• Spill-off flaring from the product stabilization system. (oil,
condensate, etc.)
• Production testing
• Relief of excess pressure caused by process upset conditions and
thermal expansion
• Depressurization, either in response to an emergency situation or as
part of a normal procedure
• Planned depressurization of subsea production flowlines and export
pipelines
• Venting from equipment operating close to atmospheric pressure
(e.g., tanks)
The systems are typically divided into a
high pressure (HP) flare and a low pressure
(LP) flare system. The LP system is
operated slightly above atmospheric
pressure to prevent atmospheric gases
such as oxygen flowing back into the vent
and flare system and generating a
combustible mixture. With low gas flow,
inert gas is injected at the flare nozzle to
prevent air ingress.
Traditionally, considerable amounts of
hydrocarbons have been more or less
continuously flared. In these cases, a
continuously burning pilot is used to ensure
ignition of hydrocarbons in the flare.
Stronger environmental focus has eliminated continuous flaring and the pilot
in many areas. Vapors and flare gas are normally recovered, and only in
exceptional situations does flaring occur. To avoid the pilot flame, an ignition
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system is used to ensure safe ignition, even when large volumes are
discharged. One patented solution is a "ballistic ignition" system which fires
burning pellets into the flare gas flow.
8.5 Instrument air
A large volume of compressed air is required for control of pneumatic valves
and actuators, tools and purging of cabinets. It is produced by electrically
driven screw compressors and further treated to be free of particles, oil and
water.
8.6 HVAC
The heat, ventilation and air conditioning system (HVAC) feeds conditioned
air to the equipment and accommodation rooms, etc. Cooling and heating is
achieved by water-cooled or water/steam-heated heat exchangers. Heat
may also be taken from gas turbine exhaust. In tropical and sub-tropical
areas, cooling is achieved by compressor refrigeration units. In tropical
areas, gas turbine inlet air must be cooled to achieve sufficient efficiency and
performance. The HVAC system is usually delivered as one package, and
may also include air emissions cleaning. Some HVAC subsystems include:
• Cool: cooling medium, refrigeration system, freezing system
• Heat: heat medium system, hot oil system
One function is to provide air to equipment rooms that are secured by
positive pressure. This prevents potential influx of explosive gases in case of
a leak.
8.7 Water systems
8.7.1 Potable water
For smaller installations,
potable water can be
brought in by supply
vessels or tank trucks.
Photo: Lenntech Water
treatment and air purification
Holding B.V.
For larger facilities, it is
provided on site by
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desalination of seawater though distillation or reverse filtering. Onshore
potable water is provided by purification of water from above or below
ground reservoirs.
Reverse filtering or osmosis requires a membrane driving pressure of about
7000 kPa/1 PSI of pressure per 100 ppm of solids dissolved in the water. For
seawater with 3.5% salt, 2.5 MPa, 350 PSI is required.
8.7.2 Seawater
Seawater is used extensively for cooling purposes. Cold water is provided to
air compressor coolers, gas coolers, main generators and HVAC. In addition,
seawater is used for the production of hypochlorite (see chemicals) and for
fire water. Seawater is treated with hypochlorite to prevent microbiological
growth in process equipment and piping.
Seawater is sometimes used for reservoir water injection. In this case, a
deaerator is used to reduce oxygen in the water before injection. Oxygen
can cause microbiological growth in the reservoir. The deaerator is designed
to use strip gas and vacuum.
8.7.3 Ballast water
Ballast systems are found on drilling rigs, floating production ships, rigs and
tension leg platforms (TLP). The object is to keep the platform level at a
certain depth under varying conditions, such as mode of operation
(stationary drilling, movement), climatic conditions (elevation of rig during
storms), amount of product in storage tanks, and to adjust loading on TLP
tension members.
Ballasting is accomplished by means of ballast tanks, pumps and valves,
which are used in combination with position measuring instruments and
tension force meters (TLP) to achieve the desired ballasting.
If fresh water is produced, it can be used as ballast to avoid salt water.
Additionally, if ballast water has become contaminated from oil tanks, it must
be cleaned before discharge at sea.
8.8 Chemicals and additives
A wide range of chemical additives are used in the main process. Some of
these are marked in the process diagram. The cost of process chemical
additives is considerable. A typical example is antifoam, where a
concentration of about 150 ppm is used. With a production of 40,000 bpd,
about 2,000 liters (500 gallons) of antifoam can be used. At a cost of 2 € per
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liter, ($10 per gallon) in bulk, antifoam alone will cost some 4,000 € or
$5,000 per day.
The most common chemicals and
their uses are:
Scale inhibitor The well flow
contains
several
different
contaminants,
such as salts,
chalk and
traces of
radioactive
materials. As pressure and temperature change, these
may precipitate and deposit in pipes, heat
exchangers, valves and tanks. As a result, they may
clog up or become stuck. The scale inhibitor prevents
the contaminants from separating out. Scale or
sediment inhibitor is applied to wellheads and
production equipment.
Emulsion breaker Water and oil cannot mix to form a true solution.
However, small drops of oil can disperse in water and
small water drops can disperse in oil. Such systems
are called emulsions; oil-in-water (o/w) and water-inoil (w/o), respectively. The drops are held suspended
by electrostatic repulsion, and will form a distinct layer
between the oil and water. Sand and particles are
normally carried out by the water extracted in water
treatment. However, the emulsion can trap these
particles and sink to the bottom as a sticky sludge that
is difficult to remove during operation. Although the
emulsion layer will eventually break down naturally, it
takes time, too much time. An emulsion breaker is
added to prevent formation and promote breakdown of
the emulsion layer by causing the droplets to merge
and grow.
Antifoam The sloshing motion inside a separator causes
foaming. This foam covers the fluid surface and
prevents gas from escaping. Foam also reduces the
gas space inside the separator, and can pass the
demister and escape to the gas outlet in the form of
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mist and liquid drops. An antifoam agent is introduced
upstream of the separator to prevent or break down
foam formation by reducing liquid surface tension.
Polyelectrolyte Polyelectrolyte is added before the hydrocyclones and
causes oil droplets to merge. This works by reducing
surface tension and water polarity. This is also called
flocculation, and polyelectrolyte flocculants allow
emissions to reach 40 ppm or less.
Methanol (MEG) Methanol or monoethylene glycol (MEG) is injected in
flowlines to prevent hydrate formation and prevent
corrosion. Hydrates are crystalline compounds that
form in water crystalline structures as a function of
composition, temperature and pressure. Hydrates
appear and freeze to hydrate ice that may damage
equipment and pipelines.
For normal risers, hydrates form only when production
stops and the temperature starts to drop. Hydrate
formation can be prevented by depressurization which
adds to startup time, or by methanol injection.
On longer flowlines in cold seawater or Arctic
climates, hydrates may form under normal operating
conditions and require continuous methanol injection.
In this case, the methanol can be separated and
recycled.
Hydrate prediction model software can be used to
determine when there is a risk of hydrate formation
and to reduce methanol injection or delay
depressurization.
TEG Triethyleneglycol (TEG) is used to dry gas (see the
chapter on scrubbers and reboilers).
Hypochlorite Hypochlorite is added to seawater to prevent growth
of algae and bacteria, e.g., in seawater heat
exchangers. Hypochlorite is produced by electrolysis
of seawater to chlorine. In one variant, copper
electrodes are used, which adds copper salts to the
solution that improves effectiveness.
Biocides Biocides are also preventive chemicals that are added
to prevent microbiological activity in oil production
systems, such as bacteria, fungus or algae growth.
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Particular problems arise from the growth of sulfate
reducing bacteria that produces hydrogen sulfide and
clogs filters. Typical uses include diesel tanks,
produced water (after hydrocyclones), and slop and
ballast tanks.
Corrosion inhibitor Corrosion inhibitor is injected in export pipelines and
storage tanks. Exported oil can be highly corrosive,
leading to corrosion of the inside of the pipeline or
tank. The corrosion inhibitor protects by forming a thin
film on metal surfaces.
Drag reducers Drag reducers improve the flow in pipelines. Fluid
near the pipe tries to stay stationary while fluid in the
center region of the pipe is moving quickly. This large
difference in fluid causes turbulent bursts to occur in
the buffer region. Turbulent bursts propagate and form
turbulent eddies, which cause drag.
Drag-reducing polymers are long-chain, ultra-high
molecular weight polymers from 1 to 10 million u), with
higher molecular weight polymers giving better drag
reduction performance. With only parts-per-million
levels in the pipeline fluid, drag-reducing polymers
suppress the formation of turbulent bursts in the buffer
region. The net result of using a drag-reducing
polymer in turbulent flow is a decrease in the frictional
pressure drop in the pipeline by as much as 70%. This
can be used to lower pressure or improve throughput.
