If you’ve been around the oil & gas business, particularly in North America, you’re likely familiar with the LACT acronym. Lease Automatic Custody Transfer is the point where the custody of oil is transferred from the producer to the pipeline, or any other transportation means. Many interests can be involved: producers, carriers, royalty owners, state regulatory bodies, and taxing authorities. They all want to be assured of the exact quantity and quality of the transferred oil.
LACT units are currently the best method to transfer the ownership of crude oil from the production facilities to pipelines or trucks, as they have many advantages compared to other measurement methods, particularly tank gauging. These include increased measurement accuracy (capable of unattended measurement with maintained accuracy of ±0.25% or better), better use of labor, better scheduling of runs to pipelines, elimination of measurement errors due to tank bottom build-up or encrustation, and reduction in investment and maintenance costs.Design principles. The main components of a LACT unit are:
- Charge Pump
- Inline Strainer with Air Eliminator
- BS&W Probe and Monitor
- Automatic Sampling System
- Three Way Diverting Valve
- Custody Transfer Meter
- Prover Connections
- Back Pressure Control Valve
- Check Valve
- Pressure and Temperature transmitters
- Control Panel with Flow Computer
Operating principles. Oil from the production lease is typically stored in a tank. The tank normally has high and low level switches to start and stop the charge pump according to the oil level. An air eliminator can be present to discharge the gas present in the pipe, but not gas entrained in liquid; this entrained gas could otherwise be measured as part of the flow, which results in meter inaccuracy.
A probe monitors the flowing stream for basic sediment and water (BS&W) and communicates the result to the control panel. The monitor will control the 3-way valve: depending on the value of BS&W—usually set at 1%—the valve will send the ‘Merchantable’ oil to the meter, or send back the ‘bad’ oil to the tank for further treatment. The unit will also include a sampling system, used to determine oil quality and composite API gravity.
The meter will be suitable for custody transfer measurement—turbine, positive displacement, or Coriolis meters. It will accurately and precisely measure the fluid stream volume and accumulate the total throughput. In order to validate the measurement accuracy and reliability, a proving system will be needed. The LACT unit will include the necessary valves and tubing for the prover connection.
The backpressure valve is used to hold a minimum pressure against the LACT unit to ensure that the meter always operates against a pressure above the vapor pressure of the fluid being metered. A check valve is usually installed to prevent backflow of metered fluid from the pipeline back to the LACT unit.
The entire operation of the system is controlled by the LACT control panel, which provides capabilities for pump start/stop according to the oil level in the tank, BS&W diverting control, meter fail indication, and more.
Measurement challenges. In LACT units, the flow meters must measure and quantify the fluid with a high accuracy. They must be insensitive to high viscosity values and maintain the requested level of accuracy and reliability even with changing conditions and operating parameters. Meter technologies such as Coriolis and ultrasonic are designed to overcome these types of challenges, and provide stable measurement and proving results over extensive service periods.
Effective validation through proving operations (pipe provers, compact provers, Master Meters) will ensure the on-going meter accountability, while advanced in situ diagnostics, available for Micro Motion Coriolis meters, help identify abnormal events that can compromise the measurement between validations. Lifecycle services can help with reliable measurement through design, installation, commissioning, and ongoing maintenance and support.
Remote operations. LACT units are usually installed in remote locations and they are almost always unmanned. It’s important to have remote accessibility to all the data and operating conditions. Better insight into the process and early identification of uncertainties can help to contain costs of ownership and reduce fiscal risk. Solutions for remote operations include flow computers and RTU platforms with flexible software applications, and SCADA systems to monitor the process of fluid transportation.
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