While doing the research and fact-checking for the Practical Stability For Tugs post, I realized that I normally don’t think too much about the stability of my tow. We move oil and I don’t load the barge or pump it off, that’s what the tankermen are for. But there’s only so much that the tug’s captain and mate can do, and I have enough on my plate already without having to worry about the barge beyond the usual concerns: Did they plan their load so the barge will be trimmed properly and will handle okay on the wire as we tow them up Ambrose Channel or through Narragansett Bay’s East Passage? Is it properly secured for sea? Will they remember to pick the bridles up on deck before we push them into the winter ice of the Hudson River, or through the lobster pot-minefield that is Long Island Sound?
On a day-to-day basis, with conventional tugs and barges, you shouldn’t have to involve yourself much with the stability of your barge unless you can clearly see that something isn’t right, like the barge has a list, is improperly trimmed, the applicable load line is submerged, or something else of that nature. Tank barges, especially when loaded, are inherently stable. But there is a time when your knowledge of basic stability principles could make all the difference between a tolerably bad outcome and a catastrophic one. That would be after a collision, allision or grounding results in a holing of the barge, wherein your knowledge of what is known as damaged stability will likely decide your fate. If you have no understanding of damaged stability then you’re riding on luck alone.
For tank barges, in particular, the survival strategy you must take hinges primarily upon where the damage was sustained, the extent of the damage, and what condition of load the barge is in at the time. To complicate matters, there will be several potentially conflicting imperatives to be considered: maintaining enough transverse (or lateral) stability to avoid capsizing, maintaining enough reserve buoyancy to remain afloat, minimizing cargo loss and the pollution that results from it, and not exceeding the hull’s longitudinal stress limits. Reserve buoyancy is defined as the volume of intact space remaining above the waterline. Tugs, towboats and fully-loaded single-skin barges generally have very little reserve buoyancy and, being made of steel, have no inherent buoyancy whatsoever. Once you lose your reserve buoyancy you’ll sink. Most modern double-hulled barges have a considerable amount of void spaces and/or ballast tanks, thereby giving them significantly more reserve buoyancy than the single-skin barges that are rapidly being retired.
The Cause: The following damage-causing collision, allision or grounding scenarios are what put you in the damaged-stability state. Please note, as with all things tug-and-barge related, the term head-on is relative and refers to the working bow of the barge. Barges may be pushed ahead, towed alongside, or towed astern either bow-first or stern-first as necessity dictates. Therefore the working bow may actually be either the true bow or stern of the barge, depending on the make-up of the tow.
Scenario #1 – Head-on or nearly head-on: If you must hit something solid, it’s best to hit this way. Since tank barges almost always have ballast tanks or voids at both the bow and stern, and they’re usually kept empty, this is the preferable location for absorbing the kinetic energy of a collision, allision or grounding. As with a car, think of them as the sacrificial crumple zones that protect the more vital areas. These voids should run the full width of the barge and, if flooded, will affect longitudinal stability but not transverse stability. So the bow (or stern) will increase in draft, and you’ll have added free surface area, but you’re unlikely to develop any significant list and capsizing should not be a threat. In the unlikely event that there is watertight longitudinal subdivision of these voids you would normally want to leave them cross-connected so that the undamaged compartment would automatically counter-flood along with the damaged side. That first transverse bulkhead, dividing the bow’s void space from the first set of cargo tanks, is commonly referred to as the collision bulkhead.
Scenario #2 – Sideswipe or T-Boned: This type is the least survivable and you should avoid this kind of collision/allision at virtually all costs, especially at speed. It was the last-second attempt to swerve around the iceberg that opened up the Titanic lengthwise and sent her to the bottom. The loss of even two adjacent compartments on the same side can sometimes be enough to sink you. If, despite your best efforts, you find yourself in this position you must be prepared to act very swiftly to avoid losing the barge altogether, up to and including intentionally grounding it on a level or nearly-level bottom if you can get to some in time.
Scenario #3 – Bottom Damage: This could occur from an unintentional grounding or possibly running over a submerged or partially-submerged object like a large log, telephone pole, shipping container, vessel, jet engines stuffed with Canada Geese, and so on (you never know what you’ll find in the Hudson River!). Outcomes are variable depending upon the extent of the damage. A lot of it can severely degrade the longitudinal hull strength of the barge, possibly causing it to break in two if mishandled even slightly.
The Response: Under normal conditions, just as with the tug, the crossover valves between a barge’s tanks that have any free surface area must be kept closed at all times to minimize free surface effect and prevent unintended hydrostatic balancing. But in the event of damage that results in flooding and/or loss of cargo, the following general rules must guide your actions. Fortunately, they’re both simple and consistent.
1. Loaded Barge – the crossovers should be immediately opened between a holed tank and its opposite. Cargo flowing out of a holed tank will soon cause a list to the opposite side. Opening the crossovers equalizes that flow and keeps the tank levels more-or-less equal, thereby preserving transverse stability.
2. Light Barge – the crossovers should normally be left open so that, in the event of damage, the tank(s) on the opposite side from the damaged and flooding tank(s) will automatically counter-flood, thereby preserving transverse stability. If, for whatever reason, they’re not normally left open then you would want to immediately open them.
Never forget that the primary safety concern should always be to protect the crew. To that end you must be prepared to quickly break tow if you have any doubts about the barge sinking or capsizing and taking the tug down with it. Things can go from bad to worse in a hurry, so you may even be forced to cut yourself away. If you can stay with the barge and you’re in push gear you’ll probably have to slack off on it, as well as your safety lines, to allow for the change in the barge’s trim. If the barge is being towed alongside be prepared to check your lines as needed to keep them from getting too tight as the barge gains list. When alongside you must be particularly careful because it is then that you’re at your most vulnerable. If the barge starts to roll on you it can quickly submerge a rail and allow down-flooding of the engine room if a door is left open or isn’t fully dogged down. Err well on the side of caution with this.
While the general principles may be the same, ATB’s are generally both larger and much more complex than the smaller conventional barges. Careful advance preparation of a damage control and response plan, requiring detailed knowledge of the specific tank, ballast and piping layout of a given barge, is needed if you expect to make a decent go of it. More on that later…..
For more on stability see the posts Tug Valour Investigation Report, Close The Crossovers!, Your Stability Letter: Read It & Heed It, and Practical Stability For Tugs.
Commentaires