I had a long couple of days but we're off for the night here at the HQ.
When nothing goes right, not a single thing beyond that you didn't fall down or crap your pants? No, I guess it wasn't that bad, but there was an unusual aggregation of shit going south this week.
The HQ has been modified several times to change how and what it can load and pump off. Originally it was designed to carry two separate grades of oil that didn't need to come in contact with each other- each with it's own pipelines, manifold and multiple block valves, so that all the tanks *Can* be made common and can be used to carry just one product, or we can subdivide, and where the pipelines meet, we have multiple block valves so there is never less than 2 block valves keeping for example, gasoline and diesel fuel, so if one fails they still can't shake hands.
The HQ had a 3rd manifold and pipeline added so we can carry 3 grades separately. Loading or emptying the tanks pushes the hull deeper in the water or more shallow, and which tanks we use, and on which side (every tank has a number and the side designation- the bow tanks, called the #1's. have port and starboard sides, divided at the hull's centerline. So filling one and not the other rolls the hull over some. As we move aft, we pass the number 2's, 3's, 4's, etc. This is important because we usually load two products at a time and often in unequal volumes... and sometimes we load such small volumes that we have to just use one tank, because we need a minimum volume of fuel in the tank in order for the cargo pumps to catch prime.
So we have to load our oil with a mind towards list (port and starboard tipping), trim (forward and aft tipping) and hull stress too.
Tankships are made of steel, which is quite a bit more flexible than you'd think when you're loading tremendous mass on it. Ships MUST be flexible, as the enormous amount of energy the hull is dealing with- gravity, buoyancy, not to mention that the loads aren't static- they change as the hull bobs around, the metal MUST bend a bit to bleed off the forces involved, or it will shatter like a dry stick. So I also gotta load with a mind to not overstress the hull too. It has a limit on how much it wants to bend. If I load, say the aftmost port tank and the forewardmost starboard tank... have you ever snapped a Kitkat bar in half?
Same reason trees are made to sway in the wind. Good one, God.
For maximum versatility, some of our tanks are dedicated to one type of fuel or another. #1's are for ultra low sulfur, #2's, are for low sulfur, 3's are for diesel, etc... each pair of tanks is reserved for one grade of fuel. This is done because sulfur content is critically important. Plenty of companies have had 7-figure fines for burning the wrong fuel in the wrong place... yeah, the ocean has emissions control schemes. Some countries have more strict regulations than others, and there's a global limit on how high a sulfur content you can burn at all. Sulfur provides lubricity to the parts of the engine that are exposed to the fuel, but it's also a deeply noxious gas in terms of emissions. There's a balancing act there. We don't lump all the like product tanks together because sometimes we load just one product, and we have to distribute that weight evenly for trim, list and stress, too, especially because the other tanks that carry other products will stay empty. There are plenty of times I will, for example, load tanks towards the midships and fill just one tank on each side- for example, I will fill #2 starboard and #5 port. Ballast is often used to even out hull stresses, tankers being double hulled, the space between the outer hull and the cargo tanks can be flooded with water to weigh the ship down or even out stress... but the HQ is the right combination of built heavy and built for non-oceangoing service, so we don't need water ballast while bunkering in protected bays, lakes or sounds. Keeps things simple.
Now, next thing is ensuring I can empty the tanks. Each tank has a sump, a low point, where the suction pipe is. The sumps are located in an aft corner of the tanks, and closest to the centerline. The tanks have a flat bottom, so the higher the difference between bow and stern draft of the hull, the more that the oil flows 'downhill' so the dregs come off faster... but it's black oil, thick stuff, and it clings to surfaces, so it's impossible to get rid of the last little bit. In winter, when the hull is cold, the cold steel makes the oil solidify, so any sort of downhill difference becomes even more important so as not to end up with 'heavy bottoms,' or a deep sludge of solidified fuel. By end of February, this is almost inevitable to greater or lesser degrees.
Now, we have loading programs that help us calculate hull stress and predict trim and list to help us load and discharge safely... It can also be done by hand using a calculator, pen and paper and some charts the builder gave us when the hull was new, but the truth is that the programs are a backstop against experience and understanding. Now, we MUST use all tools in our toolbox to do our job right- that is, if you have two ways of measuring something, two is one and one is none- that is, you are obligated to use every tool appropriate to the situation... and the present HQ is almost an identical hull to my last HQ, so I have 15 years of experience with this hull and its' idiosyncrasies. I know what load plans work well and what doesn't, off the top of my head... but more important, I know that when I am NOT dealing with a gimme of a cargo load, which is about 15-20% of the time, I know the process and the parameters that keep things bulletproof in terms of safety...There's still an experience factor though, a point where the computer can tell you stress levels on the hull but it can't tell you if you'll be able to do a job if the volumes change and a ship decides they don't want all the fuel they ordered and now you have, say, 400 more tons of fuel than you had expected, causing a port list, when you had loaded based on the presumption that you'd have an even keel and that tank empty when you started pumping off another tank.
Just an example. Just like my post from last week, my poo writing skills make this sound much more interesting than it is. Ever try to cook in a skillet but you went a little light on the oil and you have to tilt the pan to get some more from the edge of the pan? Same same.
