Liquid Cargo Measurement: Ullage, Sounding & Draft Survey

Close-up of a ship's draft marks showing measurements in meters and decimeters

In the bulk cargo shipping industry, shortage claims are a common issue faced by insurers, reinsurers, and cargo owners, and they should advocate for the adoption of best practices in cargo surveys to mitigate risks and ensure fair claims resolution.

Cargoes in bulk often experience weight loss during carriage due to natural shrinkage, evaporation, and similar factors. Additionally, the loss amount may increase during handling operations at both ports as a result of various factors, including:

  • Spillages from hoppers.
  • Overloading of cargo receiving equipment.
  • Defects in the weighbridge causing incorrect shore measurements.
  • Differing measuring methods upon loading and discharge.

When the cargo weight discharged is less than that declared in the Bill of Lading (BL), a shortage claim is likely.

Importance of Accurate Cargo Weight Determination

Accurate cargo weight determination is not merely a procedural necessity; it is a legal imperative.

In most bulk cargo commercial transactions, the parties consider the quality and quantity “final at loading.” This means the receiver will pay for the cargo based on the BL weight and may seek compensation in the event of weight loss.

Carriers may argue that such losses constitute a “paper loss” and that the vessel delivered all the cargo received on board. However, this argument might not suffice to defeat a claim, making it crucial to have accurate and reliable methods for cargo measurement and documentation.

The accurate determination of cargo quantities through reliable methods such as ullage, sounding, and draft surveys.

Let’s explore these methods in detail:

1. Ullage method

The ullage method measures the space between the surface of the liquid in a tank and the top of the inner surface of the tank. This method is typically used when the tank content is highly viscous and/or the tank is filled to or near the maximum capacity.

Procedure:

  • Tools Used: Ullage tape or electronic gauge.
  • Steps: Lower the ullage tape or electronic gauge into the tank until it touches the liquid surface. The distance from the liquid surface to the top of the tank is measured.

 

Marine surveyors in safety gear conducting volume calculations on a tanker ship using specialized equipment.
Diagram comparing the Ullage Method and Sounding Method for measuring liquid levels in a tank
Illustration comparing the Ullage Method and Sounding Method for measuring liquid levels in tanks. The ullage method measures the free space above the liquid, while the sounding method measures the liquid depth. Both methods are essential for accurate liquid cargo measurement. Source: Marine Insight

2. Sounding method

The sounding method measures the depth of the liquid by determining the distance from the liquid’s surface to the bottom of the tank. This method is used to calculate the volume of liquid present in the tank.

Procedure:

  • Tools used: Sounding tape or electronic sounding device.
  • Steps: Lower the sounding tape or electronic device to the bottom of the tank until it touches the bottom. Measure the depth of the liquid from the surface to the tank bottom.

3. Draft survey

The vessel draft survey measures the displacement of the water both before and after the loading or unloading, with the resulting difference between the two displacements representing the weight of the cargo.

A draft Survey must be conducted at both the loading port and discharging port (and at intermediate ports) and by all parties involved/interested (cargo owners, shipowners, insurers, etc.) due to it margin of error in readings.

Procedure:

  • Tools used: Hydrometer, vessel’s hydrostatic table.
  • Steps:
    1. Read six draft marks of the vessel and calculate their mean, corrected for the vessel’s trim.
    2. Sample the water where the vessel floats and determine its density with a hydrometer.
    3. Use the vessel’s hydrostatic table as a reference to provide its weight displacement.
    4. Determine deductible weights by measuring all weights onboard (e.g., ballast, fuel, fresh water, stores, etc.).
    5. Subtract the deductible weight from the vessel’s corrected load displacement weight.
    6. This process is performed by an expert marine surveyor, witnessed by on-site observers and recorded on video.

Comparative Table: Ullage, Sounding, and Draft Survey Methods

MethodPrimary UseApplicationsChallenges/Considerations
Ullage MethodMeasuring ullage (empty space) for viscous liquids or nearly full tanksHighly viscous liquids, tanks filled to/near maximum capacityTank deformation, temperature changes, presence of sediments; regular calibration of tools required
Sounding MethodMeasuring liquid depth to calculate volumeVarious types of liquids, volume calculationTank shape, temperature changes, presence of contaminants; regular calibration and maintenance of tools needed
Draft SurveyDetermining cargo weight by measuring water displacementBulk cargoes, weight verification at loading and discharge ports

Water density, ballast quantity changes, consumable changes, trim and deformation corrections, sea conditions, 

Hogging and sagging

These methods provide crucial evidence to challenge the shipper’s declared cargo weight figure in a BL.

By comparing the figures in the two draft survey reports (i.e., at the port of loading and before discharge), you’ll prove your vessel delivered approximately the same weight received.

 

Can there be potential errors in the results of these methods?

 

Yes, potential errors can occur in these methods due to various factors. For the ullage method, errors may arise from improper calibration and ignoring tank conditions like deformation and temperature changes. In the sounding method, improper maintenance and environmental factors such as tank shape and contaminants can impact accuracy. Draft surveys can encounter errors from incorrect readings of conditions like water density, vessel trim, and changes in ballast quantity and sea conditions.

⚠️ Draft surveying requires a high level of technical competence and expertise. Ensure maximum accuracy by hiring a marine surveyor with the right qualifications and background.

Trade Allowance: the acceptable shortage percentage

It is a commercial practice that has been widely accepted by stakeholders and Courts.

Purpose
Corrects measurement inaccuracies/errors within a reasonable percentage. It accounts for minor variations in cargo measurements that may arise due to differences in measurement techniques, equipment calibration, or handling losses during loading and unloading processes.

Negotiation and agreement
Stakeholders can and should agree on a trade allowance for the specific cargo and voyage, to avoid disputes and ensure clarity for both parties.

Only when contractually agreed will a trade allowance be binding.

Be aware: This allowance differ depending on the the governing laws, the countries, and courts. In some countries no allowance is accepted.

For expert legal advice and to ensure the highest standards in marine surveys, contact us today.

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