OIL & GAS

Custody Transfer Operations

Metrology

Recomentations

1. Introduction

Metrological support of custody transfer operations in the oil and gas industry is not just a technical procedure — it is the foundation of trust between contract parties. In cases where oil, petroleum products, or gas are transferred, financial settlements worth millions of dollars are at stake. Even a minor measurement error can lead to serious legal, economic, and reputational consequences.

Modern standards and requirements applied in custody transfer regulate every detail — from the selection of measurement instruments to calibration procedures and documentation. Engineering solutions are complemented by regulatory frameworks such as GOST, OIML, API, and ISO. Together, they form an integrated system aimed at ensuring the accuracy and traceability of measurement results.

This document presents the Pro meter approach to metrological support of projects: systematic, practice-based, aligned with international standards and focused on achieving minimal uncertainty. It is addressed to engineers, designers, metrologists, and decision-makers in the field of custody transfer.

The introduction serves as a starting point for any discussion about the quality of measurements. It defines the goals, explains key terminology, and highlights the importance of unified approaches as the basis of contractual relationships.

Our conviction is clear: metrology is not just about the instrument — it is a tool for ensuring fairness, transparency, and efficiency in transactions. This is why Pro meter views it as a strategic element in the entire supply chain.

2. Principles of Custody Transfer

Custody transfer of hydrocarbons is a legally significant process that underpins settlements, deliveries, and contractual obligations. It is based on strictly defined principles aimed at ensuring objectivity, repeatability, and transparency of measurement results. Any deviation or uncertainty in ownership transfer can lead to financial disputes, which is why strict requirements apply to custody transfer systems.

Key principles include: use of verified and certified measuring instruments, application of approved methodologies, agreement on measurement units between parties (barrels, tons, cubic meters), and mandatory documentation of all data with legal validity.

A critical aspect is dual control: both the seller and buyer conduct independent measurements followed by result comparison. This reduces risks and builds trust. Regulators, metrological laboratories, and legal experts participate at all stages — from project planning to arbitration.

Another vital condition for successful custody transfer is traceability — the ability to validate the accuracy of each step, from sensor calibration to final delivery. Only such a system ensures protection of the interests of all stakeholders, including government agencies, traders, and end users.

Pro meter adheres to these principles in all project implementations, helping clients not just to measure — but to build a custody transfer system that is protected, transparent, and reliable.

3. Measurement Methods

A wide range of measurement methods is used in the metrological support of custody transfer operations. Each method has its own specifics, application areas, and accuracy requirements. The correct choice of method is a key factor when designing a custody transfer system.

In practice, two main groups of methods are used: dynamic and static.

Static methods are applied when measuring volume in tanks, most often at terminals or storage parks.
These methods are based on measuring the liquid level (using level gauges), followed by volume calculation using calibration tables or hydrostatic pressure data. Despite their seeming simplicity, these methods require high accuracy in tank calibration and temperature factor accounting.

In addition, indirect methods are used — when mass or volume is calculated based on measured values of density, temperature, and pressure.
Combined schemes (e.g., flowmeter + tank) increase reliability and help verify the readings.

Pro meter applies proven combinations of methods tailored to the product specifications and operating conditions. If needed, backup lines and automatic samplers are implemented to ensure quality control and independent verification.

4. Oil and Gas Flow Measurement Systems (Custody Transfer Units)

Custody Transfer Units (CTUs) are integrated engineering and metrological systems that ensure accurate measurement of mass, volume, and quality indicators of oil, petroleum products, and gas. These systems serve as the technological foundation for custody transfer operations, combining measurement instruments, computing units, data archiving, and reporting.

A typical CTU structure includes: metering lines with flowmeters, quality control modules with densitometers and temperature sensors, provers (for calibration), automatic samplers, flow computers (e.g., OMNI-6000), SCADA interfaces, and data archive servers. All components operate as a single system, ensuring data reliability at every stage.

CTUs are classified by purpose: commercial, control, pipeline, terminal, loading, and receiving systems. Depending on the operating environment, CTUs can be installed above ground, underground, in arctic or tropical climates. They may be manual, semi-automatic, or fully automated.

The lifecycle of a CTU includes design, approval, modernization, and decommissioning stages. Each step requires close coordination between engineers and metrology services, taking into account site-specific features: temperature conditions, aggressive media, and maintenance access.

Pro meter develops CTUs based on customized technical specifications, ensuring compliance not only with standards but also with real-world engineering and metrological conditions. This approach allows for the creation of reliable systems with minimized uncertainty, tailored to each project’s needs.

5. Metrological Support

Metrological support is a set of organizational, technical, and methodological measures aimed at ensuring the accuracy and reliability of measurements. It lies at the core of the entire custody transfer cycle and guarantees compliance with established standards and legal requirements.

Key elements of metrological support include: verification and calibration of measuring instruments, development of procedures, determination of mass and volume calculation algorithms, uncertainty estimation, and preparation of uncertainty budgets. In Russia, the main standards are GOST R 8.595, while internationally — OIML R 117, API, ISO 17025, and others.

Special attention is given to the m-factor — an individual coefficient that characterizes the behavior of a measuring channel (especially in mass flow meters). Adjusting the m-factor based on calibration results at 3–5 points helps minimize systematic errors, especially under seasonal temperature fluctuations.

CTU calculation algorithms include corrections for density, temperature, and pressure. Accuracy is ensured not only by hardware but also by software logic. Pro meter uses mathematical models and data verification tools, enabling quick detection and correction of deviations.

In today’s conditions, metrological support becomes an integral part of engineering and digital design. It ensures traceable results, trust in measurements, and their legal significance.

6. Stages of CTU Project Implementation

The implementation of custody transfer systems for oil and gas (CTUs) goes through several key stages, each requiring strict compliance with regulatory requirements, coordination between stakeholders, and thorough engineering work. The quality of execution at these stages directly affects the accuracy, reliability, and legal integrity of the measurement system.

The first step is the development of technical specifications (TS), where the objectives, accuracy requirements, product parameters, and operating conditions are defined. Based on this, a project is developed, including instrumentation diagrams, equipment specifications, layout, and documentation for calibration and operation.

Next, contracts are signed with contractors, suppliers, and laboratories. These documents include specific metrological requirements: calibration, procedures, permits, and certificates.

Once equipment is delivered, installation and commissioning begin — including cable routing, sensor connections, and computer setup. All works are monitored by technical supervision and documented accordingly. The initial calibration of the entire system follows.

The final stage is pilot-industrial operation, during which the system is tested under real conditions, parameters are fine-tuned, and deviations are analyzed. Upon successful completion, the system is officially commissioned.

Pro meter supports its clients at every stage, providing technical, metrological, and documentation support. This approach reduces risks, simplifies certification, and ensures stable CTU operation throughout its entire service life.

7. Maintenance

Maintenance of custody transfer units (CTUs) for oil and gas is a regulated set of procedures aimed at ensuring the accuracy, reliability, and proper functioning of measuring instruments and auxiliary equipment. Regular maintenance guarantees stable system operation and prevents measurement deviations that could lead to disputes between contracting parties.

Maintenance is divided into the following levels:

TO-1 (monthly): visual inspection, communication and alarm signal checks.
TO-2 (quarterly): archive checks, signal matching, data analysis.
TO-3 (annual): calibration, m-factor verification, firmware updates, alarm diagnostics.

In addition, current and unscheduled maintenance may be performed after repairs or upgrades.

Maintenance activities are documented: calibration protocols, inspection acts, commissioning journals, reports on equipment and communication channels. These records are stored in the technical archive and form the system’s digital history.

Special attention is paid to equipment readiness for calibration: tag verification, algorithm relevance, cleanliness of measurement lines and sensors, availability of standard substances, and calibration software.

Pro meter has developed solutions for remote and multi-line CTUs, including automated data collection, parameter monitoring in real time, and integration with cloud platforms. This approach simplifies audits, facilitates calibrator work, and increases transparency in system operation.

8. Cooperation with Metrology Laboratories

Effective interaction with accredited metrology laboratories is essential for the reliable operation of CTUs. The accuracy and legal validity of measurements depend on the proper organization of calibrations, verifications, and the approval of methodologies.

At the design or modernization stage, contracts with laboratories are concluded. These contracts define the list of equipment to be verified, deadlines, documentation formats, and procedures for resolving disputes. They must also include options for emergency on-site service in case of equipment rejection.

For complex, non-standard, or unique solutions (e.g., working with viscous oil or unstable conditions), verification procedures are developed jointly with the laboratory and the client. These procedures go through expert review, approval, and official registration.

A key requirement is that the laboratory’s accreditation scope must cover the relevant types of instruments. If necessary, it can be expanded for a specific project. Pro meter assists in preparing documentation and supporting the accreditation process.

Modern CTU systems move towards digital documentation: electronic protocols (PDF, XML), digital signatures, and integration with regulator databases. This improves data processing speed, reduces errors, and simplifies storage of technical documents.

Thanks to its well-established interaction system with laboratories, Pro meter projects pass verification and commissioning without delays, fully compliant with Russian and international standards.

9. Operational Practice and Equipment Performance Analysis

The longevity and accuracy of measurements depend not only on correct installation and calibration, but also on operational conditions. Analyzing real-world projects helps identify bottlenecks, optimize technical solutions, and improve CTU reliability in the long term.

For example, prolonged operation of CMF-type mass flowmeters in cold environments revealed instances of measurement instability. The cause was condensation and crystallization of water inclusions in pipelines. Solutions included relocating equipment, insulation, and adjusting the m-factor.

Coriolis meters are sensitive to temperature fluctuations and contamination. In some cases, their readings significantly impacted the final mass value. Regular calibration, lab sample comparison, and cleaning of channels are essential.

Provers require special attention to temperature stability, the availability of reference standards, and strict adherence to calibration schedules. Even small volume deviations can significantly affect the k-factor of turbine meters.

Pro meter’s practical experience demonstrates the benefits of duplicating signal channels (A and B), automated m-factor monitoring, and maintaining digital parameter archives. Digital MF graphs are also used to log deviations, supporting regular internal audits.

This approach not only improves measurement accuracy but also builds a culture of metrological discipline at the site, reinforcing trust between operators and regulators.

10. Environmental Aspects

Environmental safety is an essential component of modern oil and gas metering projects. Any product leak, system failure, or signal disruption can lead not only to financial losses, but also to serious environmental consequences. This issue is particularly critical at port terminals, tank farms, and sites near bodies of water.

Pro meter’s experience includes cases where a leak through a shut-off valve or a failed control valve led to product release. Consequences included work stoppages, fines, and reputational damage. Conclusion — project designs must include emergency scenarios, valve position control (NO/NC), safety valves, and remote control systems.

The company’s design principle is: “no overfilling, no flooding.” Projects include: automatic valve shutdown during power loss, emergency tank drainage, real-time monitoring of metering systems, and event logging with timestamps.

Additionally, environmental requirements cover corrosion-resistant materials, presence of drainage and purging systems, and placement of equipment in protected containers or safe zones.

Pro meter also recommends the development of an environmental protocol — a document outlining emergency scenarios, response procedures, and responsible parties. This increases readiness and reduces environmental and business risks.

11. Science and Development

Pro meter views metrology not only as a production task, but also as a field for scientific exploration and technological innovation. The development of new algorithms, error modeling, digital simulators, and training are all part of a comprehensive approach to measurements in the oil and gas sector.

Over the years of operating CTUs in various conditions, Prometer has accumulated a unique database used to build mathematical models and improve calculation accuracy. Methods for m-factor linearization have been developed to account for the instability of measurement instruments under changing external conditions.

As part of its scientific work, Prometer collaborates with technical universities in Latvia, Russia, and Kazakhstan. Topics of graduate and master’s theses include:

Optimization of the m-factor using archived data
Influence of low temperatures on measurement accuracy
Creation of digital twins for CTUs
Development of automated maintenance planning systems
Creation of training systems and simulators for operators

Many ideas have already been implemented, such as:

A digital maintenance log with online access
Automated report generation for verifiers
MF-graph libraries for various product lines
Integration with cloud-based SCADA databases

Pro meter also publishes articles, participates in industry conferences (e.g., Neftgas, API Standard Week), and prepares educational materials. Thus, science becomes a direct tool for improving accuracy and building trust in custody transfer operations.

12. Conclusion and Recommendations

Metrological support of custody transfer operations in the oil and gas sector is an essential part of the industry’s professional and legal landscape. Its role goes far beyond verification and calibration — it includes design, operation, environmental concerns, scientific research, and digital technologies.

A well-structured measurement system with high accuracy, stability, and legal robustness is the foundation of trust among suppliers, buyers, regulators, and society. Metrology ensures result comparability, reduces risks, and streamlines the work of all participants.

Based on Pro meter’s experience, the following key recommendations can be formulated:

Always start the project with a clear technical specification, including metrology requirements;
Include mandatory verification, calibration, and support in contracts;
Use digital archives and analyze parameter dynamics;
Maintain cooperation with accredited laboratories and consultants;
Plan maintenance from the early stages of the project.

The future of metrological support lies in the integration of measuring instruments, automation, digital twins, and cloud-based monitoring systems. Pro meter actively applies these approaches in its projects and is open to scientific and technical collaboration.

We thank you for your interest in metrology in the oil and gas sector and invite you to engage in dialogue, share experience, and collaborate on joint developments.