Forces at work

 I've been on the HQ again for a week now, and while the bloom is off the rose, I find myself more open to appreciating the things we have and the things we don't have on here than I did when I was on The Loaner last week. This is because the things that The Loaner has are mostly mold and assorted fungi, filth and poorly-maintained and designed shit on deck, while the things that the Loaner doesn't have (hygienic living conditions, clean water and a safe work environment) remind me that our has/doesn't have matrix is quite heavily weighted in my favor).  My partner B is on there now, but he's having a jolly good time as The Loaner is presently Out Of Service. There's structural welding to be done, and a tank vessel must be Gas Free (contain no explosive vapors in the area or adjacent areas to compartments where welding is occurring, so no cargoes to load. 

     Smaller tank barges are built such that sometimes a point-load  (say a tugboat bumps us a little too hard on the side one night for example, knocking me out of my bunk and all the dishes out of the cabinets in the galley)  will cause a hairline crack in the framing inside the hull. There are multiple reinforced frames in a small area in the double hull of a tank barge, so a single crack won't actually cause trouble at all, but they're inevitable with use, and periodically any cracks will be rewelded by a specially certified welder, generally speaking once every few years. The HQ's I've had have had anywhere from 4 to over 50 cracks found and repaired in the span between 5-year mandatory shipyard visits in their life cycle

     Every time you load or discharge a tank vessel, you deform it temporarily. Steel must bend if it is not to break, and even very heavy steel gets fatigued if you bend it enough times. 

           A ship or tank barge uses a combination of structural forces to combat loading stresses. Like most vessels there is web-frame construction, where frames, ribs, stringers, beams, etc distribute loading stresses. Think of the framing of a traditional wood sailing ship- frames and planks, with heavy beams that run between the frames at various points to connect them- it's not that different from that. Tank vessels also have stressed-skin construction, where some of the load stresses are passed around and through the hull plating too. Think about a sheet of plate steel. It's not that hard to bend it when it's flat, right? Well, try to stretch it like taffy. The steel is going to resist. It's strong in that direction, if you try to yank it laterally. Steel has a grain to it, almost like wood. Bend it in two different planes at once, in a complex curve, and it is going to resist bending in any direction after, unlike flat sheet steel. 

   So, tank vessels are surprisingly strong and supple, too. You can bend the shit out of them.  I mentioned this before when I talked about Happy Bananas and Sad Bananas a few posts ago.    Now, granted, you can break a tank vessel, by loading or discharging it in an extremely stupid way, set up the hull for failure, and stress it beyond what  the steel is capable of doing, but you're going to have a VERY sad banana for quite a while before it finally calls it a day and you have partial Tanker Mitosis happen. 

You made the banana too sad, friend. 

    So, all that stress and strain is well and good when you keep the forces in between the goalposts, within your design stress limits. 

         The next time you see an oceangoing oil tanker, notice the markings on the hull. 

 

     You'll see the Plimsoll line: 

 Oh, those letters are short  for this: 

   Because water density varies a LOT with salinity and temperature, if you were to put the max safe load on a ship, as the ship sits deeper in the water, the final draft of the ship is affected by the salinity and density of the water you're sitting in.  As a quick example, with a  medium sized ship with a max 40,000 ton cargo capacity, you can expect your maximum safe load to put the water's edge on those marks.   The other side of this coin is that you need to have some buoyancy and you WANT to have some freeboard (the height of the deck above water)  to help you preserve your buoyancy. Between fuel, cargo and the ship itself, if a little water gets inside where you don't want it to be, it's really cool and neato if you don't immediately sink. So, if you, say, overload your ship in Santa Catarina in Brazil, a popular port with almost fresh water at most  terminals, you're going to be coming close to impersonating a submarine by the time you get to Copenhagen in February. 

       If you go over your mark and put it underwater, your insurance agent is going to be very happy if you break your ship, because that's your problem now and not his, but that's only if the local Port State Control doesn't see you doing it, at some point because they have every right to stop you and try to save the poor bastards on board who didn't know you were trying to get them killed.  Yes, this is a regularly occurring problem. The marks exists because greed has killed more sailors than storms. 

   Now, ships also have external markers noting where internal bulkheads and reinforced areas are. This is because tugboats will be used to nudge the ship into position at almost every dock it will ever land alongside, and sometimes the tugboat will need to use a LOT of force across a very small area of hull to shove the entire ship sideways through the water.  As a result, ships are either built with special reinforcing at certain areas in the hull, or more likely the builder will just mark where the heavy transverse bulkheads (framing) that connect one side of the ship to the other are. These stronger areas in the hull will be made pretty obvious if someone values their ship and don't want it caved in.  

see if you can spot a good place to put a tugboat to shove you sideways

         Now, smaller oil barges, on the other hand, have a special niche in transportation. They get into smaller places and shallower water, usually, than ships.   The HQ, for example, at 300'x50'  won't exceed 13' of draft, which is about the same draft as the 3000 and 4200hp tugboats that are designed to move us about.  One corollary to this, is that sometimes our tugboat operators have to put us in places that are pretty tight- in the corners and niches where ships won't fit. As a result, sometimes the tugs move us by getting 'in the notch,' in a specially reinforced divot in our stern that fits the tugboat's bow nicely, and from which the tug can push us forward

this is actually much deeper than the HQ's notch, but you get the idea

Or push us "On the hip" 

Or even backwards, which we call, unoriginally enough "Heads to tails"

 All depends on where we're going, how we can fit in the berth, and if we can get our cargo hoses to meet the berth's oil pipelines.  BUT, in all those cases, the tugs have to get their lines made fast to push or tow us in a particular way.  On the hip or heads/tails, the tug has a stern line that runs to us which has to be super tight, tight enough to stretch the tug's bow line by pulling the bow outwards as the stern line gets tighter, until both lines are tight enough that the barge and tug move almost as a single unt, where the tug can shift their rudders over and shove the barge, and the relative positions to each other of the two vessels moves less than a few inches. This puts ENORMOUS force on the sides of the barge where the tug is pressing up against the barge... a point load, in other words. Point loads sometimes cause nearby welded surfaces to pop apart, and with  a sound like a shotgun being fired, a hairline crack will form at a weld somewhere between the outer and inner hull. 

 Shit happens, in other words.  cracked steel bracing and weld failures are designed to be accounted for. They're almost inevitable, and the hull design must be rugged enough to shrug off a bunch of them. 

 So, while I am experiencing nirvana in the form of my first watch completely off in a dog's age, B is sitting while the ABS (American Bureau of Shipping) welders are giving some TLC to The Loaner.  B will be back aboard a week from today, in fact, which will also mark the midway point of this tour on the HQ, and the start of week 9 since I've been out here. Oof.