Shore tank measurement and tank gauging at oil terminals

A shore tank quantity is a volume read from the tank's calibration table at a measured level, corrected for temperature and converted to tonnes with a density. The level comes from a manual dip or an automatic tank gauge, and the quantity that changes hands at the terminal, the custody transfer figure, is only as good as that calibration and those readings. This article covers the shore side. The ship's side, with sounding tables and trim corrections, is in ullage report and sounding tables.

Tank calibration: calibration tables and the strapping table

A calibration table (also called a tank table or strapping table) gives the volume in the tank at each level. It is made for one tank and one reference point. OIML Recommendation R 71 covers fixed storage tanks used for measuring quantities of liquid and states that the calibration table is a component of the measurement, so it has to be determined accurately.

Strapping is the usual way to get the table. API MPMS Chapter 2.2A describes how to calibrate upright cylindrical tanks by manual strapping, from the measurements of total and incremental volume to the computation of the volume at each liquid level. ISO 7507-1 gives the strapping method as a working, reference or referee method, and says it does not apply to tanks that are abnormally deformed, such as dented or non-circular ones. We cite both by number and do not reproduce their procedures.

FOSFA's Code of Practice for Member Superintendents asks the surveyor to obtain the calibration data from the terminal: capacity, dimensions, total heights and bottom allowances, plus any deficiency such as a listing tank or an irregular bottom. The bottom allowance is the correction of the table for bottom irregularities. The NIOP Trading Rules (2025) require each storage tank to be calibrated by a recognised authority and recalibrated if it shows a history of differences.

Manual gauging and automatic tank gauging

Manual gauging. API MPMS Chapter 3.1A covers manual gauging of tanks and marine vessels, including free water, checking the length of the gauge tape and the influence of bob weight and temperature on it. FOSFA's code recommends a steel dip tape with an integral stainless steel dip weight. The dip is taken at the dip hole the calibration data refer to, the tank bottom must be covered for the volume calculation, and the contents should be stable before dipping. Ullage from a calibrated level is the alternative, but dip is preferred. Free water is found with water-detecting paste, treated paper or electronic means, and the code warns that the boundary between free water, emulsion and oil is often indistinct.

Automatic tank gauging (ATG). API MPMS Chapter 3.1B covers level measurement of liquid in stationary atmospheric tanks with automatic gauges, including accuracy, installation, commissioning, calibration and verification, for gauges that measure either innage or ullage. OIML R 85 sets the metrological requirements for automatic level gauges. Whether the ATG reading or the manual dip settles a custody transfer is a matter of the contract and the terminal's procedure. As a matter of practice, record the manual dip next to the ATG reading at the moments that count.

Temperature and density

A volume means little without a temperature. API MPMS Chapter 7 covers temperature determination, and describes more than one method for the average static tank temperature in custody transfer. FOSFA's code lets the surveyor use probes at various levels or draw up a sample and read it by hand; no glass thermometer may go inside a tank with oils and fats. NIOP takes the average of at least three levels.

For oils and fats that are heated, FOSFA's heating guidelines add that the oil is not considered evenly heated if the top and bottom temperatures differ by more than 5 °C. A layered tank gives a layered density, so a single spot reading can mislead.

The observed volume is then corrected to the reference temperature. The petroleum measurement tables of ISO 91 use 15 °C and 60 °F as reference temperatures, and API MPMS Chapter 12.1.1 covers the calculation of static quantities in upright cylindrical tanks and marine vessels. How the correction factor and the density work together is in volume correction factor and density at 15 °C. The API and ISO tables were written for petroleum; for vegetable oils the contract names the factors to use.

Ship and shore: the difference

At loading, the shore tanks feed the ship, and the shore figure is the basis of the bill of lading weight. At discharge, the shore tank receives the cargo and gives the outturn. Either way two independent measurements of one cargo are compared.

A fictitious example

The shore tanks deliver 5,012.400 MT at loading, the figure on the bill of lading. The ship's own figure on board afterwards is 5,001.150 MT. The difference is 5,001.150 − 5,012.400 = −11.250 MT, which is −11.250 / 5,012.400 × 100 = −0.224% of the shore figure. A threshold of 0.5% would be 25.062 MT, so this difference is under it. Compare in tonnes, not in rounded percentages, when the figure is close to the line.

What counts as acceptable is set by the contract, the charter party or the house procedure, not by a universal rule, and some of the difference is noise on both sides. FOSFA's checklists have the surveyor take shore tank figures as provisional and final and calculate the receipt, so a protest on provisional figures should say so. The outturn side, with trade allowances and the protest, is in outturn report and cargo shortage. How a ship's own measuring pattern is tracked over voyages is in vessel experience factor.

Custody transfer

Custody transfer is the point at which a measured quantity becomes the basis of payment or of liability. For a terminal that point is the dip or ATG figure at the start and end of a transfer. The documentary needs are simple: a valid calibration table, the method and instrument used, the readings with date and time, temperature and density, and the calculation. FOSFA's checklists put the same items in a fixed order: allocation, dip gross, water, temperature, sample.

When a tank needs recalibration, and when you can refuse it

A tank comes with a book of calibration certificates, and the surveyor looks the figures up in it. Tanks settle, bend, expand and shrink over the years. The reference height, the point from which every dip is taken, is the first thing to check at each measurement. If it has moved by some centimetres, or the tank keeps giving odd differences, the tank has to be recalibrated.

The rule is recalibration about once in 15 years, but in practice it often does not happen, because nobody asks while the figures look normal. After a few odd differences the terminal usually looks into it anyway. It can also be demanded: a receiver can refuse a tank whose reference height does not match the book ("I do not accept this tank"), and a seller can say the tank was not properly calibrated. Raise a tank that is not in order before the transfer, not after.

Practical rule

Before you take a shore figure, write down the table's identification and date, check the reference height at the dip hole against the table's value, and note when the contents last moved. If the reference height is off, or the tank was filling five minutes ago, say so on the record and dip again later. NIOP's rules likewise have the reference height noted before an innage gauge.

Based on the working practice of our sector specialist: 25 years in the port, 20 of them as a cargo surveyor. At modern terminals a tank radar or other automatic gauging system takes the measurement, and the surveyor attends and checks the readings with the terminal. Seagoing ships are almost always measured by ullage, shore tanks and tank barges by sounding, which is more precise.

From nomination to certificate

A shore figure ends up on a certificate, so it matters that it is entered once and stays traceable. FOSS, short for Field Operational Services System, is the software from Operium for the document flow from nomination to certificate: offline entry on board, built-in calculation methods as used in the sector, document generation, a client portal showing nomination, progress and documents, and an audit log of who entered which value. See FOSS or get in touch.