Loading a tank vessel down safely is a matter of following directions. Tanks are loaded in sequence (it's possible to break most tankers in half by loading them badly. Steel has to bend, not a lot, but some, in order to stay in one piece), and volumes and densities have to be checked against the theoretical capacity of the tanks they're destined for. As an example, oil volume is measured at 60 degrees Fahrenheit as a standard. Oil actually shrinks and grows rapidly with temperature, like water, but more dramatically. A correction equation is calculated based on the logarithmic relationship between the cargo temperature and it's density- this Volume Correction factor is applied to measure the actual volume of Net Standard barrels or tons at 60 degrees when the temperature of the cargo is not actually 60 degrees. The stuff we carry solidifies around that temperature, so we don't want it to ever get below about 100 degrees, and 140 is better. As an example, there is a 700 barrel difference between observed vs. net barrels in today's cargo.
Moneywise, it's always better to load down than to carry a partial load- you're making a voyage, and you get paid more to carry more, generally. We like to load our tanks to 95% of their capacity, to prevent the cargo from sloshing in the tanks too much, which would result in us rolling around like a turd in a toilet bowl due to free surface effect. Black oil, the un- or -partially refined stuff that we carry, is dirty and nasty. Generally, you take the light aromatics out of the crude oil- the gasoline precursors, the diesel precursors, the naptha, water and dirt, and what's left over, a thick sludge, is fuel for ships and power plants. Without all the expensive stuff, black oil is quite light, compared to diesel and gasoline- a gallon of the stuff is almost a pound lighter than a gallon of diesel.
When you want to load down a tanker, you're going to either draught out or cube out to determine how much you'll carry. Draughting out means that you're loading the vessel as deep as you can, up to the maximum allowable. As your tanks fill, the boat settles deeper in the water, obviously. The draught is the depth of your hull under water. The depth to which you'll lower the boat in the water will be determined by either a draught limit, because there is shallow water somewhere in your journey and you don't want to touch bottom, or your maximum draught - preset maximum draughts based on the hull geometry, set by international safety standards to keep the owners from overloading the boat, causing you to swim home should the weather get saucy. Because we carry black oil in deepwater ports, we don't draught out for the most part.
Cubing out, on the other hand, is an issue for us. Cubing out means that you fill your tanks 'till they're full. For us, that's 95%, to allow for cargo expansion. At 96%, the high level alarms and a flashing yellow light go off, signaling that you need to wake up fast or your career and the local seabirds are in danger of having a bad time. At 98% capacity, the high-high alarms go off, sounding like the book of Revelation come to life, and a flashing red light strobes, signaling that you're within seconds of the danger of not being allowed to even work the register at McDonalds after you get out of jail.
So today we're ALMOST cubing out. There's actually a comfortable 6 inches between where the oil is sitting in the tanks, and where the oh shit alarms will get tripped. 6 inches isn't much. Point in fact, it would take about 2 minutes max to fill up that volume if one wanted to. For this reason, 'topping off' tanks is where the danger lies, and where a bunch of sailors, who appear functionally retarded outside their little steel world, come into their own, hovering over the displays, tanks and slips of paper accumulated, like so many soccer moms at a playground.
|Tank Top and control valves|
|Floating Pickle Jar|