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
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5 Midstream facilities
Raw natural gas from the well consists of methane as well as many other
smaller fractions of heavier hydrocarbons, and various other components.
The gas has to be separated into marketable fractions and treated to trade
specifications and to protect equipment from contaminants.
5.1 Gathering
Many upstream facilities include the gathering system in the processing
plant. However, for distributed gas production systems with many (often
small) producers, there is little processing at each location and gas
production from thousands of wells over an area instead feed into a
distributed gathering system. This system in general is composed of:
• Flowlines: A line connecting the wellpad with a field gathering station
(FGS), in general equipped with a fixed or mobile type pig launcher.
• FGS is a system allowing gathering of several flowlines and permits
transmission of the combined stream to the central processing facility
(CPF) and measures the oil/water/gas ratio. Each FGS is composed of:
o Pig receiver (fixed/mobile)
o Production header where all flowlines are connected
o Test header where a single flow line is routed for analysis
purposes (GOR Gas to oil ratio, water cut)
o Test system (mainly test separator or multiphase flow meter)
o Pig trap launcher
• Trunk line – pipeline connecting the FGS with the CPF. Equipped with a
pig receiver at the end.
5.2 Gas plants
5.2.1 Gas composition
When gas is exported, many gas trains include additional equipment for
further gas processing to remove unwanted components such as hydrogen
sulfide and carbon dioxide. These gases are called acids and
sweetening/acid removal is the process of removing them.
Natural gas sweetening methods include absorption processes, cryogenic
processes, adsorption processes (PSA, TSA and iron sponge) and
membranes. Often hybrid combinations are used, such as cryogenic and
membranes.
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Gas treatment may also include calibration. If the delivery specification is for
a specific calorific value (BTU per scf or MJ per scm), gas with higher values
can be adjusted by adding an inert gas, such as nitrogen. This is often done
at a common point such as a pipeline gathering system or a pipeline onshore
terminal.
Raw natural gas from the well consists of methane as well, as many other
smaller fractions of heavier hydrocarbons and various other components.
