4 Oil and gas storage, metering and export
The final stage before the oil and gas leaves the platform consists of
storage, pumps and pipeline terminal equipment.
4.4.1 Fiscal metering
Partners, authorities and customers all calculate invoices, taxes and
payments based on the actual product shipped out. Often, custody transfer
also takes place at this point, which means transfer of responsibility or title
from the producer to a customer, shuttle tanker operator or pipeline operator.
Although some small installations are still operated with a dipstick and
manual records, larger installations have analysis and metering equipment.
To make sure readings are accurate, a fixed or movable prover loop for
calibration is also installed. The illustration shows a full liquid hydrocarbon
(oil and condensate) metering system. The analyzer instruments on the left
provide product data such as density, viscosity and water content. Pressure
and temperature compensation is also included.
Figure 9. Metering system
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For liquids, turbine meters with dual pulse outputs are most common.
Alternatives are positive displacement meters (pass a fixed volume per
rotation or stroke) and coriolis mass flow meters. These instruments cannot
cover the full range with sufficient accuracy.
Therefore, the metering is split
into several runs, and the number of runs depends on the flow. Each run
employs one meter and several instruments to provide temperature and
pressure correction. Open/close valves allow runs to be selected and control
valves can balance the flow between runs. The instruments and actuators
are monitored and controlled by a flow computer. If the interface is not
digital, dual pulse trains are used to allow direction sensing and fault finding.
To obtain the required accuracy, the meters are calibrated. The most
common method is a prover loop. A prover ball moves though the loop, and
a calibrated volume is provided between the two detectors (Z). When a
meter is to be calibrated, the four-way valve opens to allow oil to flow behind
the ball.
The number of pulses from it passes one detector Z to the other and
is counted. After one loop, the four-way valve turns to reverse flow direction
and the ball moves back, providing the same volume in reverse, again
counting the pulses. From the known reference volume, number of pulses,
pressure and temperature the flow computer can calculate the meter factor
and provide accurate flow measurements using formulas from industry
standard organizations such as API MPMS and ISO 5024. The accuracy is
typically ± 0.3% of standard volume.
Gas metering is similar, but instead,
analyzers will measure hydrocarbon
content and energy value (MJ/scm or
BTU, Kcal/scf) as well as pressure
and temperature. The meters are
normally orifice meters or ultrasonic
meters. Orifice plates with a diameter
less than the pipe are mounted in
cassettes. The pressure differential
over the orifice plate as well as
pressure and temperature, is used in
standard formulas (such as AGA 3
and ISO 5024/5167) to calculate normalized flow. Different ranges are
accommodated with different size restrictions.
Orifice plates are sensitive to a buildup of residue and affect the edges of the
hole. Larger new installations therefore prefer ultrasonic gas meters that
work by sending multiple ultrasonic beams across the path and measure the
Doppler effect.
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Gas metering is less accurate than liquid, typically ±1.0% of mass. There is
usually no prover loop, the instruments and orifice plates are calibrated in
separate equipment instead.
LNG is often metered
with mass flow meters
that can operate at the
required low temperature.
A three run LNG
metering skid is shown
above.
At various points in the
movement of oil and gas,
similar measurements
are taken, usually in a
more simplified way.
Examples of different gas
types are flare gas, fuel
gas and injected gas, where required accuracy is 2-5% percent.
4.4.2 Storage
On most production
sites, oil and gas are
piped directly to a
refinery or tanker
terminal. Gas is
difficult to store
locally, but
occasionally
underground mines,
caverns or salt
deposits can be used
to store gas.
On platforms without
a pipeline, oil is stored in onboard storage tanks to be transported by shuttle
tanker. The oil is stored in storage cells around the shafts on concrete
platforms, and in tanks on floating units. On some floaters, a separate
storage tanker is used. Ballast handling is very important in both cases to
balance the buoyancy when oil volume varies. For onshore, fixed roof tanks
are used for crude, floating roof for condensate. Rock caves are also used
for storage
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Special tank gauging systems such as level radars, pressure or float are
used to measure the level in storage tanks, cells and caves. The level
measurement is converted to volume via tank strapping tables (depending
on tank geometry) and compensated for temperature to provide standard
volume. Float gauges can also calculate density, and so mass can be
established.
A tank farm consists of 10-100 tanks of varying volume for a typical total
capacity in the area of 1-50 million barrels. Storage or shuttle tankers
normally store up to two weeks of production, one week for normal cycle and
one extra week for delays, e.g., bad weather. This can amount to several
million barrels.
Accurate records of volumes and history are kept to document what is
received and dispatched. For installations that serve multiple production
sites, different qualities and product blending must also be handled. Another
planning task is forecasting for future received and delivered products. This
is for stock control and warehousing requirements. A tank farm management
system keeps track of all stock movements and logs all transport operations
that take place.
4.4.3 Marine loading
Loading systems consist of one or
more loading arms/jetties, pumps,
valves and a metering system.
Tanker loading systems are complex,
both because of the volume involved,
and because several loading arms will
normally interact with the tanker's
ballast system to control the loading
operation. The tanks must be filled in
a certain sequence; otherwise the
tanker's structure might be damaged
due to uneven stresses. It is the
responsibility of the tanker's ballast
system to signal data to the loading
system and to operate the different
valves and monitor the tanks on
board the ship. Photo: Statoil
