Custody transfer
Encyclopedia
Custody Transfer in the oil and gas industry refers to the transactions involving transporting physical substance from one operator to another. This includes the transferring of raw and refined petroleum
between tanks and tankers; tankers and ships and other transactions. Meanwhile, Custody Transfer in fluid measurement
is defined as a metering point (location) where the fluid is being measured for sale from one party to another. During Custody Transfer, accuracy
is of great importance to both the company delivering the material and the eventual recipient, when transferring an material.
The term ‘fiscal metering’ is often interchanged with Custody transfer. ‘fiscal metering’ refers to metering that is a point of a commercial transaction such as when a change in ownership takes place, while Custody transfer takes place any time fluids are passed from the possession of one party to another.
Custody transfer generally involves:
Due to the high level of accuracy required during Custody transfer applications, the Flowmeters which are used to perform this are subject to approval by an organization such as the American Petroleum Institute
(API).
Custody transfer operations can occur at a number of points along the way; these may include operations, transactions or transferring of oil
from an oil production platform to a ship, barge, railcar, truck and also to the final destination point, such as a refinery
.
(Flow measurement
is primarily concerned with measuring the flow of ‘liquids’ and ‘gases’). Many flow measurement
technologies are used for custody transfer applications; these include differential pressure (DP) flowmeters
, turbine flowmeters
, positive displacement flowmeters
, Coriolis flowmeters and ultrasonic flowmeters
.
are used for the custody transfer of natural gas
to measure the flow of liquid, gas, and steam. The DP flowmeter consist of a differential pressure transmitter and a primary element. The primary element places a constriction in a flow stream, while the DP transmitter measures the difference in pressure upstream and downstream of the constriction.
In many cases, pressure transmitters and primary elements are bought by the end-users from different suppliers. However, several vendors have integrated the pressure transmitter with the primary element to form a complete flowmeter. The advantage of this is that they can be calibrated with the primary element and DP transmitter already in place.
The standards and criteria for the use of DP flowmeters for custody transfer applications are majorly lined out by the American Gas Association
(AGA) and the American Petroleum Institute
(API).
An advantage of using a DP flowmeters is that they are the most studied and best understood type of flowmeter. A disadvantage of using a DP flowmeters is that they introduce a pressure drop into the flowmeter line, this is a necessary result of the constriction in the line that is required to make the DP flow measurement.
One important development in the use of DP flowmeters for custody transfer applications has been the development of single and dual chamber orifice
fittings.
was invented by Reinhard Woltman , a German Engineer in 1790. Turbine flowmeters consist of a rotor with propeller-like blades that spins as water or some other fluid passes over it. The rotor spins in proportion to flow rate (see turbine meters
) . There are many types of turbine meters, but many of those used for gas flow are called axial meters.
The Turbine flowmeters is most useful when measuring clean, steady, high-speed flow of low-viscosity fluids
. In comparison to other Flowmeters, the Turbine flowmeter have a significant cost advantage over ultrasonic flowmeters
, especially in the larger line sizes, and it also has a favourable price compared to the prices of DP flowmeters, especially in cases where one turbine meter can replace several DP meters.
The disadvantage of turbine flowmeters is that they have moving parts that are subject to wear, a way to prevent this is by introducing more durable materials of construction, including ceramic ball bearings
.
are highly accurate
meters that are widely used for custody transfer of commercial
and industrial
water, as well as for custody transfer of natural gas and many other liquids to and from delivery trucks and for custody transfer of natural gas and most of other liquid
and gas
applications. PD flowmeters have the advantage that they have been approved by a number of regulatory bodies for this purpose, and they have not yet been displaced by other applications.
PD meters excel at measuring low flows, and also at measuring highly viscous flows
, because PD meters captures the flow in a container of known quantity. Speed
of flow doesn’t matter when using a PD meter.
and beverage. Coriolis technology offers accuracy and reliability
in measuring material flow, and is often hailed as among the best flow measurement technologies, however, there exists significant limitation in Conventional Coriolis meters for custody transfer. This is because Coriolis meters they have not performed well in measuring two-phase flow conditions, which involve a combination of gas and liquid mass
.
Flow is measured using Coriolis meters by analyzing the changes in the Coriolis force of a flowing substance. The force
is generated in a mass moving within a rotating frame of reference. An angular, outward acceleration
, which is factored with linear velocity
is produced due to the rotation. With a fluid mass, the Coriolis force is proportional to the mass flow rate of that fluid.
To use Coriolis force for measurement, a Coriolis meter has two main components: an oscillating flow tube equipped with sensors and drivers, and an electronic transmitter
that controls the oscillations which analyzes the results and transmits the information. The Coriolis principle for flow measurement
requires the oscillating section of a rotating pipe to be exploited. Oscillation produces the Coriolis force, which can be sensed and analyzed to determine the rate of flow. Although, custody transfer operations do not typically feature rotating pipes.
published a standard called AGA-9. This standard lays out the criteria for the use of Ultrasonic flowmeters
for Custody Transfer of Natural Gas
”.
Ultrasonic flowmeters
were first introduced into industrial markets in 1963 by Tokyo Keiki (now Tokimec) in Japan. Custody transfer measurements have been around for a long time, and over the past ten years, Coriolis and ultrasonic meters have become the flowmeters of choice for custody transfer in the oil and gas industry.
Ultrasonic meters provide volumetric flow rate. They typically use the transit-time
method, where sounds waves transmitted in the direction of fluid flow travel faster than those travelling upstream. The transit time difference is proportional to fluid velocity. Ultrasonic flow meters have negligible pressure drop, have high turndown capability, and can handle a wide range of applications. Crude oil production, transportation, and processing are typical applications for this technology.
The use of ultrasonic flowmeters is continuing to grow for custody transfer. Unlike PD and turbine meters, ultrasonic flowmeters do not have moving parts. And pressure
drop is much reduced with an ultrasonic meter when compared to PD, turbine, and DP meters. Installation of ultrasonic meters
is relatively straightforward, and maintenance requirements are low.
A typical liquid custody transfer skid includes multiple flowmeters and meter provers. Provers are used to calibrate meters in-situ and are performed frequently; typically before, during, and after a batch transfer for metering assurance.
and the bias component. Each device has its manufacturer stated accuracy specification and its tested accuracy. Uncertainty
takes all the metering system factors that impact measurement accuracy into account. The accuracy of flowmeters could be used in two different metering systems that ultimately have different calculated uncertainties due to other factors in the system that affect flow calculations. Uncertainty even includes such factors as the flow computer’s A/D converter accuracy. The quest for accuracy in a custody transfer system requires meticulous attention to detail .
, API
, or ISO, and national metrology standards such as OIML (International), NIST (U.S.), PTB
(Germany), CMC (China), and GOST
(Russia), among others. These requirements can be of two types: Legal and Contract
.
(W&M) codes and regulations according to the country or jurisdiction in which the sale is conducted controls the wholesale and retail trade requirements to facilitate fair trade. The regulations and accuracy requirements vary widely between countries and commodities, but they all have one common characteristic - “traceability
”. There is always a procedure that defines the validation process where the duty meter is compared to a standard that is traceable to the legal metrology
agency of the respective region.
is a written agreement between buyers and sellers that defines the measurement requirements. These are large-volume sales between operating companies where refined products and crude oils
are transported by marine
, pipeline
or rail
. Custody transfer measurement must be at the highest level of accuracy possible because a small error in measurement can amount to a large financial difference. Due to these critical natures of measurements, petroleum companies
around the world have developed and adopted standards to meet the industry’s needs.
In Canada, for instance, all measurement of a custody transfer nature falls under the purview of Measurement Canada
. In the USA, the Federal Energy Regulatory Commission
(FERC) controls the standards which must be met for interstate trade.
of ±0.25% or better. The overall uncertainty is derived from an appropriate statistical combination of the component uncertainties in the measurement system.
Mass measurement and reporting are achieved by
The sampling system is a critical section during flow measurement. Any errors introduced through sampling error will generally have a direct, linear effect on the overall measurement.
and Pressure
Measurement are important factors to consider when taking flow measurements of liquids. Temperature and pressure measurement points should be situated as close to the meter as possible, in reference to their conditions at the meter inlet.
Temperature measurements that affect the accuracy of the metering system should have an overall loop accuracy of 0.5°C or better, and the corresponding readout should have a resolution of 0.2°C or better.
Temperature checks are performed by certified thermometers with the aid of Thermowells
Pressure measurements that affect the accuracy of the metering system should have an overall loop accuracy of 0.5 bar or better and the corresponding readout should have a resolution of 0.1 bar or better.
at the meter may be determined either by:
Most industries prefer to use the continuous measurement of gas density. However, both methods may be used simultaneously, and the comparison of their respective results may provide additional confidence in the accuracy of each method.
has an equal chance of favouring either party, the net impact
should be zero to both parties, and measurement accuracy and repeatability should not be valued. Measurement accuracy and repeatability are of high value to most seller because many users install check meters
The first step in designing any custody transfer system is to determine the mutual measurement performance expectations of the supplier and the user over the range of flow rates. This determination of mutual performance expectations should be made by individuals who have a clear understanding of all of the costs of measurement disputes caused by poor repeatability.
The second step is to quantify the operating conditions which are not controllable. For a flow measurement, these can include:
The third and final step is to select hardware
, installation and maintenance procedures which will ensure that the measurement provides the required installed performance under the expected (uncontrollable) operating conditions. For example, the user can:
While the first and second steps involve gathering data, the third step may require calculations and/or testing.
, a CIF
sale or a DES
sale.
In the case of an FOB
(Free On Board) sale, the determination of the energy transferred and invoiced for will be made in the loading port.
In the case of a CIF
(Cost Insurance & Freight) or a DES
(Delivered Ex Ship) sale, the energy
transferred and invoiced for will be determined in the unloading port.
In FOB
contracts, the buyer is responsible to provide and maintain the custody transfer measurement systems on board the vessel for volume, temperature and pressure determination and the seller is responsible to provide and maintain the custody transfer measurement systems at the loading terminal such as the sampling and gas analysis. For CIF
and DES
contracts the responsibility is reversed.
Both buyer and seller have the right to verify the accuracy of each system that is provided, maintained and operated by the other party.
The determination of the transferred energy usually happens in the presence of one or more surveyors, the ship’s cargo officer and a representative of the LNG terminal operator. A representative of the buyer can also be present.
In all cases, the transferred energy can be calculated with the following formula:
E =(VLNG × DLNG × GVCLNG) - Egas displaced ± Egas to ER(if applicable))
Where:
E = the total net energy transferred from the loading facilities to the LNG carrier, or from the LNG carrier to the unloading facilities.
VLNG= the volume of LNG loaded or unloaded in m3.
DLNG = the density of LNG loaded or unloaded in kg/m3.
GCVLNG = the gross calorific value of the LNG loaded or unloaded in MMBTU/kg
E gas displaced = The net energy of the displaced gas, also in MMBTU
, which is either:
sent back onshore by the LNG carrier when loading (volume of gas in cargo tanks displaced by same volume of loaded LNG),
Or, gas
received by the LNG carrier in its cargo tanks when unloading in replacement of the volume of discharged LNG.
E(gas to ER) = If applicable, the energy
of the gas consumed in the LNG carrier’s engine room during the time between opening and closing custody transfer surveys, i.e. used by the vessel at the port, which is:
+ For an LNG loading transfer or
- For an LNG unloading transfer
flowmeters have displaced Positive displacement
and turbine
flowmeters. Meanwhile, for the custody transfer of natural gas, the ultrasonic flowmeters
has displaced the Differential pressure
and Turbine meters
. . Nevertheless, differential pressure, Positive displacement
and turbine flowmeters
remain viable choices for many types of liquid
and gas
applications. Positive-displacement
flowmeters are well suited to measure low flows, while turbine meters
are better suited for medium to high speed flows. Much of the future for traditional meters depends on how quickly standards bodies approve new technology flowmeters for applications.
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
between tanks and tankers; tankers and ships and other transactions. Meanwhile, Custody Transfer in fluid measurement
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
is defined as a metering point (location) where the fluid is being measured for sale from one party to another. During Custody Transfer, accuracy
ACCURATE
ACCURATE was established in 2005 by a group of computer scientists, psychologists and policy experts to address problems with electronic voting...
is of great importance to both the company delivering the material and the eventual recipient, when transferring an material.
The term ‘fiscal metering’ is often interchanged with Custody transfer. ‘fiscal metering’ refers to metering that is a point of a commercial transaction such as when a change in ownership takes place, while Custody transfer takes place any time fluids are passed from the possession of one party to another.
Custody transfer generally involves:
- Industry standards;
- National metrology standards;
- Contractual agreements between custody transfer parties; and
- Government regulation and taxation.
Due to the high level of accuracy required during Custody transfer applications, the Flowmeters which are used to perform this are subject to approval by an organization such as the American Petroleum Institute
American Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
(API).
Custody transfer operations can occur at a number of points along the way; these may include operations, transactions or transferring of oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
from an oil production platform to a ship, barge, railcar, truck and also to the final destination point, such as a refinery
Refinery
A refinery is a production facility composed of a group of chemical engineering unit processes and unit operations refining certain materials or converting raw material into products of value.-Types of refineries:Different types of refineries are as follows:...
.
About Custody Transfer
Custody transfer is one of the most important applications for flow measurementFlow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
(Flow measurement
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
is primarily concerned with measuring the flow of ‘liquids’ and ‘gases’). Many flow measurement
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
technologies are used for custody transfer applications; these include differential pressure (DP) flowmeters
Pressure sensor
A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the...
, turbine flowmeters
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
, positive displacement flowmeters
Positive displacement meter
A positive displacement meter is a type of flow meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. Positive displacement flowmeters make volumetric flow measurements taking finite increments or volumes of the fluid...
, Coriolis flowmeters and ultrasonic flowmeters
Ultrasonic flow meter
An ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in...
.
Differential Pressure Flowmeters
Differential pressure (DP) flowmetersPressure sensor
A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the...
are used for the custody transfer of natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
to measure the flow of liquid, gas, and steam. The DP flowmeter consist of a differential pressure transmitter and a primary element. The primary element places a constriction in a flow stream, while the DP transmitter measures the difference in pressure upstream and downstream of the constriction.
In many cases, pressure transmitters and primary elements are bought by the end-users from different suppliers. However, several vendors have integrated the pressure transmitter with the primary element to form a complete flowmeter. The advantage of this is that they can be calibrated with the primary element and DP transmitter already in place.
The standards and criteria for the use of DP flowmeters for custody transfer applications are majorly lined out by the American Gas Association
American Gas Association
The American Gas Association , founded in 1918, is an American trade organization representing natural gas supply companies and others with an interest in the manufacturing of gas appliances as well as the production of gas...
(AGA) and the American Petroleum Institute
American Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
(API).
An advantage of using a DP flowmeters is that they are the most studied and best understood type of flowmeter. A disadvantage of using a DP flowmeters is that they introduce a pressure drop into the flowmeter line, this is a necessary result of the constriction in the line that is required to make the DP flow measurement.
One important development in the use of DP flowmeters for custody transfer applications has been the development of single and dual chamber orifice
Orifice plate
An orifice plate is a device used for measuring the volumetric flow rate. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which states that there is a relationship between the pressure of the fluid and the velocity of the fluid...
fittings.
Turbine flowmeters
The first turbine flowmeterFlow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
was invented by Reinhard Woltman , a German Engineer in 1790. Turbine flowmeters consist of a rotor with propeller-like blades that spins as water or some other fluid passes over it. The rotor spins in proportion to flow rate (see turbine meters
Gas meter
A gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly...
) . There are many types of turbine meters, but many of those used for gas flow are called axial meters.
The Turbine flowmeters is most useful when measuring clean, steady, high-speed flow of low-viscosity fluids
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
. In comparison to other Flowmeters, the Turbine flowmeter have a significant cost advantage over ultrasonic flowmeters
Ultrasonic flow meter
An ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in...
, especially in the larger line sizes, and it also has a favourable price compared to the prices of DP flowmeters, especially in cases where one turbine meter can replace several DP meters.
The disadvantage of turbine flowmeters is that they have moving parts that are subject to wear, a way to prevent this is by introducing more durable materials of construction, including ceramic ball bearings
Ball bearing
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit...
.
Positive Displacement Flowmeters
Positive displacement (PD) flowmetersPositive displacement meter
A positive displacement meter is a type of flow meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. Positive displacement flowmeters make volumetric flow measurements taking finite increments or volumes of the fluid...
are highly accurate
ACCURATE
ACCURATE was established in 2005 by a group of computer scientists, psychologists and policy experts to address problems with electronic voting...
meters that are widely used for custody transfer of commercial
Commerce
While business refers to the value-creating activities of an organization for profit, commerce means the whole system of an economy that constitutes an environment for business. The system includes legal, economic, political, social, cultural, and technological systems that are in operation in any...
and industrial
Industry
Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...
water, as well as for custody transfer of natural gas and many other liquids to and from delivery trucks and for custody transfer of natural gas and most of other liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
and gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
applications. PD flowmeters have the advantage that they have been approved by a number of regulatory bodies for this purpose, and they have not yet been displaced by other applications.
PD meters excel at measuring low flows, and also at measuring highly viscous flows
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, because PD meters captures the flow in a container of known quantity. Speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
of flow doesn’t matter when using a PD meter.
Coriolis flowmeters
Coriolis flowmeters have been around for more than 20 years and are nowadays preferred in major process industries such as chemical and foodFood
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...
and beverage. Coriolis technology offers accuracy and reliability
Reliability
In general, reliability is the ability of a person or system to perform and maintain its functions in routine circumstances, as well as hostile or unexpected circumstances.Reliability may refer to:...
in measuring material flow, and is often hailed as among the best flow measurement technologies, however, there exists significant limitation in Conventional Coriolis meters for custody transfer. This is because Coriolis meters they have not performed well in measuring two-phase flow conditions, which involve a combination of gas and liquid mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
.
Flow is measured using Coriolis meters by analyzing the changes in the Coriolis force of a flowing substance. The force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
is generated in a mass moving within a rotating frame of reference. An angular, outward acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
, which is factored with linear velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
is produced due to the rotation. With a fluid mass, the Coriolis force is proportional to the mass flow rate of that fluid.
To use Coriolis force for measurement, a Coriolis meter has two main components: an oscillating flow tube equipped with sensors and drivers, and an electronic transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
that controls the oscillations which analyzes the results and transmits the information. The Coriolis principle for flow measurement
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
requires the oscillating section of a rotating pipe to be exploited. Oscillation produces the Coriolis force, which can be sensed and analyzed to determine the rate of flow. Although, custody transfer operations do not typically feature rotating pipes.
Ultrasonic flowmeters
“In June 1998, The American Gas AssociationAmerican Gas Association
The American Gas Association , founded in 1918, is an American trade organization representing natural gas supply companies and others with an interest in the manufacturing of gas appliances as well as the production of gas...
published a standard called AGA-9. This standard lays out the criteria for the use of Ultrasonic flowmeters
Ultrasonic flow meter
An ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in...
for Custody Transfer of Natural Gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
”.
Ultrasonic flowmeters
Ultrasonic flow meter
An ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in...
were first introduced into industrial markets in 1963 by Tokyo Keiki (now Tokimec) in Japan. Custody transfer measurements have been around for a long time, and over the past ten years, Coriolis and ultrasonic meters have become the flowmeters of choice for custody transfer in the oil and gas industry.
Ultrasonic meters provide volumetric flow rate. They typically use the transit-time
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
method, where sounds waves transmitted in the direction of fluid flow travel faster than those travelling upstream. The transit time difference is proportional to fluid velocity. Ultrasonic flow meters have negligible pressure drop, have high turndown capability, and can handle a wide range of applications. Crude oil production, transportation, and processing are typical applications for this technology.
The use of ultrasonic flowmeters is continuing to grow for custody transfer. Unlike PD and turbine meters, ultrasonic flowmeters do not have moving parts. And pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
drop is much reduced with an ultrasonic meter when compared to PD, turbine, and DP meters. Installation of ultrasonic meters
Ultrasonic flow meter
An ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in...
is relatively straightforward, and maintenance requirements are low.
Components
Custody transfer requires an entire metering system that is designed and engineered for the application, not just flowmeters. Components of a custody transfer system typically include:- Multiple meters/meter runs;
- Flow computersFlow computerA flow computer is an electronic computational device which implements the required algorithms to turn the raw data received from flow meters to which it is connected into volumes at base conditions....
; - Quality systems (gas chromatographs to measure energy content of natural gas and sampling systems for liquid);
- CalibrationCalibrationCalibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
using in-place or mobile provers for liquid, or master-meter for liquid or gas; and - Supporting automationAutomationAutomation is the use of control systems and information technologies to reduce the need for human work in the production of goods and services. In the scope of industrialization, automation is a step beyond mechanization...
.
A typical liquid custody transfer skid includes multiple flowmeters and meter provers. Provers are used to calibrate meters in-situ and are performed frequently; typically before, during, and after a batch transfer for metering assurance.
Accuracy
In the ISO 5725-1 standard “accuracy” for measuring instruments is defined as “the closeness of agreement between a test result and the accepted reference value”. This term “accuracy” includes both the systematic errorError
The word error entails different meanings and usages relative to how it is conceptually applied. The concrete meaning of the Latin word "error" is "wandering" or "straying". Unlike an illusion, an error or a mistake can sometimes be dispelled through knowledge...
and the bias component. Each device has its manufacturer stated accuracy specification and its tested accuracy. Uncertainty
Uncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...
takes all the metering system factors that impact measurement accuracy into account. The accuracy of flowmeters could be used in two different metering systems that ultimately have different calculated uncertainties due to other factors in the system that affect flow calculations. Uncertainty even includes such factors as the flow computer’s A/D converter accuracy. The quest for accuracy in a custody transfer system requires meticulous attention to detail .
Custody Transfer Requirements
Custody transfer metering systems must meet requirements set by industry bodies such as AGAAGA
-Medicine:* Adequate, Appropriate or Average for Gestational Age, referring to prenatal growth rate* Anti-gliadin antibodies* Androgenetic alopecia* Aspartylglucosaminidase-Business:* AGA , Autogen Gasaccumulator AG, 1920's German car company...
, API
American Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
, or ISO, and national metrology standards such as OIML (International), NIST (U.S.), PTB
Physikalisch-Technische Bundesanstalt
The Physikalisch-Technische Bundesanstalt is based in Braunschweig and Berlin. It is the national institute for natural and engineering sciences and the highest technical authority for metrology and physical safety engineering in Germany....
(Germany), CMC (China), and GOST
GOST
GOST refers to a set of technical standards maintained by the Euro-Asian Council for Standardization, Metrology and Certification , a regional standards organization operating under the auspices of the Commonwealth of Independent States .All sorts of regulated standards are included, with examples...
(Russia), among others. These requirements can be of two types: Legal and Contract
Contract
A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...
.
Legal
The Weights & MeasuresWeights and Measures Act
A Weights and Measures Act is an Act of Parliament determining trade law where the weight or size of the goods being traded are important. For example, if a bottle of milk is for sale and has a label saying it contains one pint, then the law states that it must contain that amount.-United...
(W&M) codes and regulations according to the country or jurisdiction in which the sale is conducted controls the wholesale and retail trade requirements to facilitate fair trade. The regulations and accuracy requirements vary widely between countries and commodities, but they all have one common characteristic - “traceability
Traceability
Traceability refers to the completeness of the information about every step in a process chain.The formal definition: Traceability is the ability to chronologically interrelate uniquely identifiable entities in a way that is verifiable....
”. There is always a procedure that defines the validation process where the duty meter is compared to a standard that is traceable to the legal metrology
Metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
agency of the respective region.
Contract
A contractContract
A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...
is a written agreement between buyers and sellers that defines the measurement requirements. These are large-volume sales between operating companies where refined products and crude oils
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
are transported by marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
, pipeline
Pipeline transport
Pipeline transport is the transportation of goods through a pipe. Most commonly, liquids and gases are sent, but pneumatic tubes that transport solid capsules using compressed air are also used....
or rail
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...
. Custody transfer measurement must be at the highest level of accuracy possible because a small error in measurement can amount to a large financial difference. Due to these critical natures of measurements, petroleum companies
Petroleum industry
The petroleum industry includes the global processes of exploration, extraction, refining, transporting , and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline...
around the world have developed and adopted standards to meet the industry’s needs.
In Canada, for instance, all measurement of a custody transfer nature falls under the purview of Measurement Canada
Measurement Canada
Measurement Canada is a Canadian government agency of Industry Canada whose mission is "to ensure equity and accuracy where goods and services are bought and sold on the basis of measurement, in order to contribute to a fair and competitive marketplace for Canadians." The agency is headed by a...
. In the USA, the Federal Energy Regulatory Commission
Federal Energy Regulatory Commission
The Federal Energy Regulatory Commission is the United States federal agency with jurisdiction over interstate electricity sales, wholesale electric rates, hydroelectric licensing, natural gas pricing, and oil pipeline rates...
(FERC) controls the standards which must be met for interstate trade.
Liquid Custody transfer
Custody transfer of liquid flow measurement follow guidelines set by the international bodies. By industrial consensus, liquid flow measurement is defined as having an overall uncertaintyUncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...
of ±0.25% or better. The overall uncertainty is derived from an appropriate statistical combination of the component uncertainties in the measurement system.
Volume or Mass Measurement
Liquid flow measurements are usually in volumetric or mass unit. The choice of measurement is set by DTI. Volume is normally used for stand-alone field tanker loading operations, while mass is used for multi-field pipeline or offshore pipeline with an allocation requirement.Mass measurement and reporting are achieved by
- Measurement of volume flow rate (for example, by turbine or ultrasonic meter) and fluid density
- Direct mass measurement by Coriolis meter
Sampling System
An automatic flow-proportional sampling system is used in flow measurement to determine the average water content, average density and for analysis purposes. Sampling systems should be broadly in accordance with ISO 3171.The sampling system is a critical section during flow measurement. Any errors introduced through sampling error will generally have a direct, linear effect on the overall measurement.
Temperature and Pressure Measurement
TemperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and Pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
Measurement are important factors to consider when taking flow measurements of liquids. Temperature and pressure measurement points should be situated as close to the meter as possible, in reference to their conditions at the meter inlet.
Temperature measurements that affect the accuracy of the metering system should have an overall loop accuracy of 0.5°C or better, and the corresponding readout should have a resolution of 0.2°C or better.
Temperature checks are performed by certified thermometers with the aid of Thermowells
Thermowell
Thermowells are used to protect temperature sensors used to monitor industrial processes while permitting accurate measurement. They are typically installed in piping systems and subject to both hydrostatic and aerodynamic forces...
Pressure measurements that affect the accuracy of the metering system should have an overall loop accuracy of 0.5 bar or better and the corresponding readout should have a resolution of 0.1 bar or better.
Gaseous Custody transfer
Custody transfer of gaseous flow measurement follow guidelines set by the international bodies. By industrial consensus, gaseous flow measurement is defined as mass flow measurement with an overall uncertainty of ±1.0% or better. The overall uncertainty is derived from an appropriate statistical combination of the component uncertainties in the measurement system.Volume or Mass Unit
All gaseous flow measurement must be made on single-phase gas streams, having measurements in either volumetric or mass units. The choice of measurement must conform to those set by DTI.Sampling
Sampling is an important aspect, as they help to ascertain accuracy. Apt facilities should be provided for the purpose of obtaining representative samples. The type of instrumentation and the measuring system may influence this requirement.Gas Density
Gas densityDensity
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
at the meter may be determined either by:
- Continuous direct measurement, by on-line densitometerDensitometerA densitometer is a device that measures the degree of darkness of a photographic or semitransparent material or of a reflecting surface. The densitometer is basically a light source aimed at a photoelectric cell. It determines the density of a sample placed between the light source and the...
- Calculation, using a recognised equation of state together with measurements of the gas temperature, pressure and composition.
Most industries prefer to use the continuous measurement of gas density. However, both methods may be used simultaneously, and the comparison of their respective results may provide additional confidence in the accuracy of each method.
Best Practices
In any custody transfer application, a true random uncertaintyUncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...
has an equal chance of favouring either party, the net impact
Impact
Impact may refer to:In science :* Impact , a high force or shock over a short time period* Impact crater, a meteor crater caused by an impact event* Impact event, the collision of a meteoroid, asteroid or comet with Earth...
should be zero to both parties, and measurement accuracy and repeatability should not be valued. Measurement accuracy and repeatability are of high value to most seller because many users install check meters
The first step in designing any custody transfer system is to determine the mutual measurement performance expectations of the supplier and the user over the range of flow rates. This determination of mutual performance expectations should be made by individuals who have a clear understanding of all of the costs of measurement disputes caused by poor repeatability.
The second step is to quantify the operating conditions which are not controllable. For a flow measurement, these can include:
- Expected ambient temperatureTemperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
variation; - Maximum static line pressurePressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
; - Static line pressure and temperature variation;
- Maximum allowable permanent pressure loss;
- Flow turndown; and
- Expected frequencyFrequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of flow variation and/or pulsation.
The third and final step is to select hardware
Hardware
Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....
, installation and maintenance procedures which will ensure that the measurement provides the required installed performance under the expected (uncontrollable) operating conditions. For example, the user can:
- Select a static and/or differential pressure transmitter which has better or worse performance under the given real-world operating conditions.
- Calibrate the transmitter(s) frequently or infrequently.
- In the case of a DP flowmeter, size the primary element for a higher or lower differential pressure (higher DP's provide higher accuracy, at the expense of higher pressure loss).
- Select a flowmeter and pressure transmitter with faster or slower response.
- Use long or short impulseImpulseIn classical mechanics, an impulse is defined as the integral of a force with respect to time. When a force is applied to a rigid body it changes the momentum of that body...
lines, or direct connect for fastest response.
While the first and second steps involve gathering data, the third step may require calculations and/or testing.
General Formula For Calculating Energy Transferred (LNG)
The formula for calculating the LNG transferred depends on the contractual sales conditions. These can relate to three types of sale contract as defined by Incoterms 2000: an FOB saleFOB (shipping)
FOB is an initialism which pertains to the shipping of goods. Depending on specific usage, it may stand for Free On Board or Freight On Board. FOB specifies which party pays for which shipment and loading costs, and/or where responsibility for the goods is transferred...
, a CIF
Incoterm
The Incoterms rules or International Commercial terms are a series of pre-defined commercial terms published by the International Chamber of Commerce widely used in international commercial transactions...
sale or a DES
Incoterm
The Incoterms rules or International Commercial terms are a series of pre-defined commercial terms published by the International Chamber of Commerce widely used in international commercial transactions...
sale.
In the case of an FOB
FOB (shipping)
FOB is an initialism which pertains to the shipping of goods. Depending on specific usage, it may stand for Free On Board or Freight On Board. FOB specifies which party pays for which shipment and loading costs, and/or where responsibility for the goods is transferred...
(Free On Board) sale, the determination of the energy transferred and invoiced for will be made in the loading port.
In the case of a CIF
Incoterm
The Incoterms rules or International Commercial terms are a series of pre-defined commercial terms published by the International Chamber of Commerce widely used in international commercial transactions...
(Cost Insurance & Freight) or a DES
Incoterm
The Incoterms rules or International Commercial terms are a series of pre-defined commercial terms published by the International Chamber of Commerce widely used in international commercial transactions...
(Delivered Ex Ship) sale, the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
transferred and invoiced for will be determined in the unloading port.
In FOB
FOB (shipping)
FOB is an initialism which pertains to the shipping of goods. Depending on specific usage, it may stand for Free On Board or Freight On Board. FOB specifies which party pays for which shipment and loading costs, and/or where responsibility for the goods is transferred...
contracts, the buyer is responsible to provide and maintain the custody transfer measurement systems on board the vessel for volume, temperature and pressure determination and the seller is responsible to provide and maintain the custody transfer measurement systems at the loading terminal such as the sampling and gas analysis. For CIF
Incoterm
The Incoterms rules or International Commercial terms are a series of pre-defined commercial terms published by the International Chamber of Commerce widely used in international commercial transactions...
and DES
Incoterm
The Incoterms rules or International Commercial terms are a series of pre-defined commercial terms published by the International Chamber of Commerce widely used in international commercial transactions...
contracts the responsibility is reversed.
Both buyer and seller have the right to verify the accuracy of each system that is provided, maintained and operated by the other party.
The determination of the transferred energy usually happens in the presence of one or more surveyors, the ship’s cargo officer and a representative of the LNG terminal operator. A representative of the buyer can also be present.
In all cases, the transferred energy can be calculated with the following formula:
E =(VLNG × DLNG × GVCLNG) - Egas displaced ± Egas to ER(if applicable))
Where:
E = the total net energy transferred from the loading facilities to the LNG carrier, or from the LNG carrier to the unloading facilities.
VLNG= the volume of LNG loaded or unloaded in m3.
DLNG = the density of LNG loaded or unloaded in kg/m3.
GCVLNG = the gross calorific value of the LNG loaded or unloaded in MMBTU/kg
British thermal unit
The British thermal unit is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F...
E gas displaced = The net energy of the displaced gas, also in MMBTU
British thermal unit
The British thermal unit is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F...
, which is either:
sent back onshore by the LNG carrier when loading (volume of gas in cargo tanks displaced by same volume of loaded LNG),
Or, gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
received by the LNG carrier in its cargo tanks when unloading in replacement of the volume of discharged LNG.
E(gas to ER) = If applicable, the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
of the gas consumed in the LNG carrier’s engine room during the time between opening and closing custody transfer surveys, i.e. used by the vessel at the port, which is:
+ For an LNG loading transfer or
- For an LNG unloading transfer
The Future
New technology meters such as Coriolis and ultrasonicUltrasonic flow meter
An ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in...
flowmeters have displaced Positive displacement
Positive displacement meter
A positive displacement meter is a type of flow meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. Positive displacement flowmeters make volumetric flow measurements taking finite increments or volumes of the fluid...
and turbine
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
flowmeters. Meanwhile, for the custody transfer of natural gas, the ultrasonic flowmeters
Ultrasonic flow meter
An ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in...
has displaced the Differential pressure
Pressure sensor
A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the...
and Turbine meters
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
. . Nevertheless, differential pressure, Positive displacement
Positive displacement meter
A positive displacement meter is a type of flow meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. Positive displacement flowmeters make volumetric flow measurements taking finite increments or volumes of the fluid...
and turbine flowmeters
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
remain viable choices for many types of liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
and gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
applications. Positive-displacement
Positive displacement meter
A positive displacement meter is a type of flow meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. Positive displacement flowmeters make volumetric flow measurements taking finite increments or volumes of the fluid...
flowmeters are well suited to measure low flows, while turbine meters
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
are better suited for medium to high speed flows. Much of the future for traditional meters depends on how quickly standards bodies approve new technology flowmeters for applications.