Custody transfer
Custody Transfer in the oil and gas industry refers to the transactions involving transporting physical substance from one operator to another.
To complete standards and/or agreements and achieve maximum accuracy all parties included in fuel distribution processes (sellers and buyers, transport & storage services, fiscal depts) must follow the custody transfer procedures, appropriate measurements and related documenting operations must be fully implemented.
Turbine flowmeters consist of a rotor with propeller-like blades that spins as water or some other fluid passes over it.
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.
Coriolis meters do not have any moving parts and provide long term stability, repeatability, and reliability.
Because they are direct mass flow measurement devices, Coriolis meters can handle the widest range of fluids from gases to heavy liquids and are not impacted by viscosity or density changes that often effect velocity based technologies (PD, Turbine, Ultrasonic).
Yet modern principles based on the Coriolis effect or ultrasonic flow measurement will inform the user by means of diagnostic functions.
The Coriolis principle for flow measurement requires the oscillating section of a rotating pipe to be exploited.
Oscillation produces the Coriolis force, which traditionally is sensed and analyzed to determine the rate of flow.
Modern coriolis meters utilize the phase difference measured at each end of the oscillating pipe.
Ultrasonic flow meters have negligible pressure drop if recommended installation is followed, have high turndown capability, and can handle a wide range of applications.
A good example of this is a Lease Automatic Custody Transfer (LACT) unit in a crude oil production facility.
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”.
Custody transfer metering systems must meet requirements set by industry bodies such as AGA, API, or ISO, and national metrology standards such as OIML (International), NIST (U.S.), PTB (Germany), CMC (China), and GOST (Russia), DSTU (Ukraine) among others.
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.
These are large-volume sales between operating companies where refined products and crude oils are transported by marine, pipeline or rail.
Due to these critical natures of measurements, petroleum companies around the world have developed and adopted standards to meet the industry's needs.
In the US, the Federal Energy Regulatory Commission (FERC) controls the standards which must be met for interstate trade.
Custody transfer of gaseous flow measurement follow guidelines set by the international bodies.
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.
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.
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.
[16] 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.
