Cargo Level Gauging & Overfill Protection — Two Alarms Between You and Oil on Deck

A tanker loads blind: the tank is sealed, the atmosphere is inert, and the only knowledge of how full it is comes from the gauging system. This guide follows the measurement from the radar antenna on the tank top, through the ullage arithmetic, to the two alarms — the 95% working alarm and the fully independent 98% overfill fork — and then into the topping-off discipline that makes the last metre routine instead of an emergency. Tap a step or press “Play”, then try to top off a tank yourself in the simulator.

ullage · alarms: closed · inerted · 3,000 m³/h

Step 1 / 9

A tank you cannot look into

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Key —

Tap a step

From alarm to oil on deck — a worked clockt = 60 × V ÷ Q

Common mistakefinishing at the bulk rate “to save time” — from the 95% alarm at 3,000 m³/h you hold 10 minutes; one phone call and half of it is gone.

The same 200 m³ of remaining space is four minutes at the bulk rate and twenty-four at a staged topping rate of 500 m³/h. The reduction does not slow the operation in any way that matters — it buys time back in exactly the window where overfills happen. That is the entire logic of topping-off discipline: the 95% alarm opens a ten-minute window at worst, and every stage of rate reduction stretches it. Run t = 60 × V ÷ Q for your own tanks before the loading starts — that arithmetic belongs on the loading plan, not in your head at 02:00.

Try it — top off the tank

You are finishing a 2,000 m³ wing tank; the shore can pump up to 1,500 m³/h. The shore end answers slowly — the actual rate chases your ordered rate with a lag — so act early. Two trainer faults let you freeze the radar and kill the fork to see what each layer of protection is worth. Get the tank stopped between the 95% and 98% marks. Sim time runs about ×20.

Ordered shore rate m³/h · the shore lags your order

1500

Manifold valve % open · never slam it shut

100%

Radar gauge trainer fault · − freezes it

LIVE

98% fork trainer fault · − kills it

space to the 98% fork180m³ · rate into tank1,500m³/h

t = 60 × 180 ÷ 1,500 = 7.2 min (to the fork)

the step-7 countdown, live — stage the rate down and watch it stretch; ignore the 95% alarm and watch it vanish

Radar ullage2.20m True level89.0% Rate in1,500m³/h To 98% fork7:12ship-min Line press4.5bar

Marks on the zoom — 95% H working alarm · 98% HH independent fork · deck at 100%. Cross the last one and it is oil on deck.

The level only ever rises — there is no undo in a loading tank. The radar drives the displayed ullage and the 95% software alarm; the fork is a separate creature that answers only to liquid on its prongs. Slam the manifold valve shut against a pumping shore and the line pressure spikes instead — the sim’s second way to have a very bad day.

READY — set the controls, then press Start

Set the ordered rate, the manifold valve and the two trainer faults to build your scenario — then press Start to load the tank and try to stop it between the 95% and 98% marks.

Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.