What does fuel oil, lube oil, slops, ballast, gray, black and potable water all have in common? All are stored on board ship in tanks. Tankage is an inherent structural necessity for large ships and represents a significant portion of overall vessel volume.
Tankers are almost entirely comprised of tanks since the cargo; refined oils, crude oils, chemicals or compressed gasses, are transported in liquid form within large cargo tanks. A container ship or Roll On / Roll Off carries her cargo in holds. These holds, as I am proving this week, are surrounded on both sides and below the lowest hold by tanks. Fuel oil and ballast water require the most space whereas the other aforementioned liquids occupy much smaller tanks within the engine room.
Since my ship is not a tanker, and as the Cosco Busan proved in San Francisco Bay, carrying fuel oil on the skin of the ship, (Unsegregated by a void from the ocean besides the shell plating of the hull) is a necessity. All along the length of the bottom of the vessel fuel oil tanks are staggered in between ballast tanks.
Ballast serves two purposes. The first and most important is to ensure positive transverse stability through the addition of weight below the ship's center of gravity. The second purpose is to allow myself to control the vessel's trim, heel and drafts.
Since ballast falls into my area of responsibility, and since the annual ballast and void/cofferdam inspections are due this month, I have been using our time in the yard to pump out, open up, inspect, reseal and refill each and every ballast tank on board. Due to the size of this beast this has meant spending hours each day within the dark, damp and at times rank confines of 27 steel mazes, usually by myself.
Inspections are necessary because coatings, or the specialized and very expensive paint that covers the tank's innards, must be maintained or the tank will oxidize from the inside out. Observing the condition of the coating on a regular basis ensures that during the next dry docking trouble tanks can be addressed before structural damage occurs. Additionally I'm looking for any signs of stress such as steel fractures, broken coatings and deformation. You'd be surprised how easy it is for an assist tug to put a dent in the side of a steel ship.
Tank time was an experience I became familiar with early in my career. As a cadet I mucked out chemical tanks in between differing products and spent two weeks on the same tanker blasting mud and scooping rust scale from the bottom of ballast tanks. Working in remote corners of double bottoms holding a flashlight and fully charged fire hose struck me at the time as a unique way to spend my nineteenth summer.
Now it's my bread and butter and I can't say I love working in confined spaces requiring testing for sufficient oxygen and the presence of noxious gasses but then again, it is interesting as hell. The structure of a ship is exposed in tanks and the more you get to know each one the more efficiently you are able to work the ballast system.
Every time I crawl through a manhole wearing rubber boots with a head lamp strapped on, a radio and an O2 meter dangling from my neck I can't help but think about mining coal. Where else on earth could it be this dark besides the bottom of a mine as it is in the double bottom of a ship?