Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
API MPMS 17.10.1 2014: Measurement of Cargoes on Board Marine Gas Carriers
A comprehensive guide to the standard’s scope, technical requirements, and implementation for custody transfer of liquefied gas cargoes in marine transportation
Scope and Purpose
API Manual of Petroleum Measurement Standards (MPMS) Chapter 17.10.1 (2014) establishes uniform practices for the measurement of liquefied gas cargoes onboard marine carriers during custody transfer operations. It applies to liquefied natural gas (LNG), liquefied petroleum gas (LPG), ammonia, and other cryogenic or pressurized cargoes carried in bulk by sea-going vessels. The standard provides end-to-end guidelines covering measurement system design, installation, calibration, calculation methods, and uncertainty analysis required to achieve accurate, traceable quantity determinations.
This standard is essential for cargo surveyors, terminal operators, ship officers, and auditors who must ensure that the quantity transferred between seller and buyer is measured with an agreed level of confidence. It complements other chapters of the MPMS, such as Chapter 17.2 (Measurement of Liquid Hydrocarbons by Tank Car) and Chapter 17.8 (Measurement of LNG Cargoes by Tank Gaging), but focuses specifically on the vessel-side measurement process for gas carriers.
Applicability and Relationship to Other Standards
API MPMS 17.10.1 is often used together with:
- ISO 10976:2015 – Refrigerated light hydrocarbon fluids – Measurement of cargoes on board marine gas carriers
- API MPMS Chapter 11.1 – Volume Correction Factors for Generalized Crude Oils, Refined Products, and Lubricating Oils
- API MPMS Chapter 12 – Calculation of Petroleum Quantities
- GIIGNL Custody Transfer Handbook – LNG-specific measurement guidance
Technical Requirements
The standard defines detailed requirements for each element of the measurement chain, including instrumentation, data collection, calculation algorithms, and reporting.
Measurement System Components
| Component | Specification | Acceptance Criteria |
|---|---|---|
| Level gauging (radar / servo / capacitive) | Accuracy ≤ ±5 mm (dynamic) and ≤ ±2 mm (static) | Calibration traceable to national standards |
| Temperature sensors (PRTs) | Range -200°C to +50°C, accuracy ±0.1°C | Three-point calibration at cryogenic conditions |
| Pressure transmitters | Range 0–25 bar (LPG) or 0–15 bar (LNG) | Accuracy class 0.05% of span |
| Density hydrometer / densitometer | Uncertainty ≤ 0.2 kg/m³ for LNG | Sampling method per ASTM D1298 or equivalent |
| Gas chromatography (composition) | Molar fraction < 0.02% mol per component | Calibrated daily with certified reference mixture |
| Flow computers / data acquisition | Resolution 0.01% of reading | Audit trail and tamper protection |
Tip: When selecting level gauging for LNG vessels, ensure the radar antenna is rated for cryogenic service and does not require purge gas that could contaminate the cargo vapor space.
Data Collection and Frequency
The standard mandates that all cargo tanks be measured before and after loading/discharge. Readings must be taken at evenly spaced time intervals (typically every 30 minutes) if cargo transfer exceeds 2 hours. Key parameters include:
- Liquid level in each tank (and trim/list corrections)
- Liquid temperature at top, middle, and bottom of each tank
- Vapor space temperature and pressure
- Cargo composition (by on-line GC or certified shore analysis)
Quantity Calculation Methods
Volume at reference conditions (15°C/60°F, 1 atm) is determined using the following sequence:
- Observed liquid volume – from tank capacity tables corrected for trim/list and temperature of the tank shell (CTPL and CTL corrections).
- Mass calculation – either directly from density or from the vapor balance method (mass = liquid mass + vapor mass).
- Energy content (for LNG) – using GCV (gross calorific value) per unit volume or mass per the composition analysis.
Important: The standard emphasizes that the vapor mass can be a significant fraction of total cargo (up to 1-2% for LNG) and must be accounted for in the custody transfer quantity, especially during cool-down and heel evaporation.
Implementation Highlights
Implementing API MPMS 17.10.1 effectively requires careful planning in both hardware selection and procedural documentation. The following aspects are critical for successful deployment:
Instrument Calibration and Traceability
All measurement devices must be calibrated at regular intervals (not exceeding 12 months) with standards traceable to international units (SI). Calibration of density and temperature instruments should be performed at conditions representative of actual service – meaning cryogenic temperatures for LNG and high pressures for LPG. The standard recommends documenting calibration certificates including uncertainty budgets.
Software and Data Integrity
Flow computers and data loggers must comply with the AIAG / API MPMS Chapter 21.1 requirements for electronic data interchange. Key implementation steps include:
- Ensure all calculations use the latest published algorithms (e.g., GPA 2145 for compressibility factors).
- Maintain a digital audit trail of all measurements, intermediate calculations, and final quantities.
- Use a certified third-party to validate the calculation software against reference test cases.
Best practice: Many operators implement a dual-redundant measurement system (e.g., radar + servo level, and two independent temperature strings) to meet the uncertainty targets and provide backup in case of instrument failure.
Compliance Notes
Adherence to API MPMS 17.10.1 is not mandatory under all jurisdictions, but it is widely referenced in commercial contracts for LNG and LPG trade. Compliance involves both technical and administrative elements.
Required Documentation
- Cargo Measurement Report – timestamped, signed by the surveyor, containing all raw data, calibration certificates, and calculation details.
- Uncertainty Statement – combined uncertainty of the delivered quantity (k=2, 95% confidence interval), typically ≤ ±0.25% for LNG and ±0.30% for LPG.
- Deviation Log – any deviations from the standard procedures (e.g., use of an alternative vapor pressure correlation) must be documented and approved by both parties.
Non-compliance risk: Failure to adhere to the standard can result in cargo quantity discrepancies that may lead to payment claims, arbitration, or loss of contract. In extreme cases, incorrect measurement may cause tank overfill or pressure excursions, posing safety hazards.
Audit and Verification
Buyers and sellers often require periodic audits of measurement systems by independent third parties (e.g., API-accredited inspection bodies). The audit scope includes:
- Checking that all instruments are within calibration intervals and that calibration records are complete.
- Verifying the correct calculation of trim/list corrections, CTL factors (from API MPMS 11.1 or ISO 831), and vapor volumes.
- Reviewing the uncertainty analysis for completeness and appropriate component contributions.
Q: Does API MPMS 17.10.1 apply to both LNG and LPG cargoes?
A: Yes, it covers all liquefied gas cargoes that are transported in bulk on marine gas carriers, including LNG, LPG, ammonia, ethylene, and other refrigerated or pressurized products. However, some specific calculation methods (e.g., energy content for LNG) are detailed in annexes tailored to each cargo type.
A: Yes, it covers all liquefied gas cargoes that are transported in bulk on marine gas carriers, including LNG, LPG, ammonia, ethylene, and other refrigerated or pressurized products. However, some specific calculation methods (e.g., energy content for LNG) are detailed in annexes tailored to each cargo type.
Q: How does this standard define the uncertainty requirement for custody transfer?
A: The standard states that the combined expanded uncertainty (k=2) of the delivered quantity should not exceed ±0.25% for LNG and ±0.30% for LPG, based on all contributions from level, temperature, density, composition, and volume correction factors. A detailed uncertainty budget must be prepared and maintained as part of the measurement system documentation.
A: The standard states that the combined expanded uncertainty (k=2) of the delivered quantity should not exceed ±0.25% for LNG and ±0.30% for LPG, based on all contributions from level, temperature, density, composition, and volume correction factors. A detailed uncertainty budget must be prepared and maintained as part of the measurement system documentation.
Q: Is it mandatory to use API MPMS 17.10.1 for every gas carrier loading?
A: While not a legal requirement, most commercial contracts for LNG and LPG spot cargoes and term contracts reference API MPMS 17.10.1 as the method of measurement. Terminal operators and shipowners are strongly encouraged to adopt it to ensure consistency and avoid disputes.
A: While not a legal requirement, most commercial contracts for LNG and LPG spot cargoes and term contracts reference API MPMS 17.10.1 as the method of measurement. Terminal operators and shipowners are strongly encouraged to adopt it to ensure consistency and avoid disputes.
Q: What is the role of the cargo surveyor under this standard?
A: The cargo surveyor is responsible for ensuring that all measurement procedures are performed correctly, data is recorded accurately, and calculations follow the standard. The surveyor issues the Certificate of Quantity and quality, which is the basis for the bill of lading and final invoice.
A: The cargo surveyor is responsible for ensuring that all measurement procedures are performed correctly, data is recorded accurately, and calculations follow the standard. The surveyor issues the Certificate of Quantity and quality, which is the basis for the bill of lading and final invoice.
This article is based on the 2014 edition of API MPMS 17.10.1. Users should verify the current edition and any published addenda or errata before implementation. Published 2026.