I think it is that tugs are much, much smaller than these cargo ships and not likely to cause a bridge collapse if they drift into the base of a bridge.
I’m not talking about a tug hitting a bridge. If a tug is pulling a Dali-class cargo ship and suddenly loses steering, the results may well be the same as if an un-tugged Dali loses steering. The large ship is still drifting out of control.
I think one of the problems here is that folks don’t get that ships (and space craft) are not cars.
I am not sure that the notion is the tugs drag the ship out but rather escort the ship out and intervene if there is a problem. And I think there would be more than one towing such a large ship so, even if one tug was disabled the other could still manage some control sufficient to miss a bridge (if towing was the thing).
I absolutely could be wrong about that. They could do either I think. Also, even if the ship is being towed I’d imagine the ship’s engines would be on and ready to go at a moment’s notice (understanding that these are BIG engines and ships and they do not respond quickly).
ETA: Thinking more, I do not know if a tug could manage to push a ship with its bow or get a tow line hooked up and do something while such a big cargo ship is going 8 knots. Seems dangerous too.
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The picture is wrong. The depth around the south support of the main span is 30+ feet for at least 100 feet past the base towards shore (it does get shallower faster on the north side).
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The bottom around here is pretty soft (mud).
It’s not granite. A ship with that much momentum won’t be stopped instantly (see: the Ever Forward with a draft of 42 feet running completely aground near the Chesapeake Bay Bridge in a 25 foot depth - needed dredging and cargo offload plus very high tide to be tugged out).
I don’t know for sure either, but it’s real clear there’s a huge difference in any ship between pushing the ship’s bow left or right so it’s pointing the direction you want it to go, and the actual velocity vector changing so the ship is going where it’s now pointed, rather than still pretty much going towards where it had been pointed.
Ultimately, the way to reduce these monster forces is to move the ships at nearly zero speed. Cruise ships with multiple azipod props at the stern and multiple large transverse (or azipod) thrusters at the bow can maneuver with great control and poise at fractions of a knot.
A single-screw single-rudder cargo ship needs to be traveling WAG 8-10 knots minimum versus the water to have enough waterflow over the rudder to control heading, which as noted above, eventually controls track.
Current and wind are both problems for ships that require increasing speed to increase control margin. But whose presence may also reduce control margin laterally as a bunch of side rudder is needed just to offset the wind’s or current’s tendency to turn the boat’s heading. Meanwhile any current (but not wind) adds or subtracts some vector from their speed over the bottom which is what determines their actual gross inertia and damage potential if they hit something.
According to Wikipedia, the largest commercial harbor tugs out there are capable of a bollard pull of 60-65 tons. The Dali weighs in at around 100,000 tons. If the Dali is moving forward at 8 knots, I don’t think you can effectively point the tug perpendicular to it, because then your tug would be moving sideways at 8 knots. I suspect you’d have to keep the tug pointed at least somewhat in the same direction as the Dali is moving, which would mean it can’t exert all of its 60 tons of force perpendicular to the Dali’s path of travel. Even if I’m dead wrong, the most a tug could manage is to push sideways on the bow/stern of a 100,000-ton boat with ~60 tons of force. We’re not trying to shove the entire boat sideways, just rotate it a bit - but even so, it’d take maybe 30 seconds to give the bow or stern a lateral velocity of just 1 knot, and that’s assuming your tug is already optimally positioned to do the required pushing the moment an emergency arises. And as @LSLGuy points out, it’ll take more time than that for the ship to stop skidding and actually start traveling in the direction the bow is now pointed.
Then there’s the cost. As was pointed out in the other thread, the charge for an hour of service from a tugboat in Baltimore Harbor is $3100, with a minimum two-hour charge, and you’d need at least two tugs. So you would add $12,400 to your cost of arrival, and another $12,400 to your cost of departure. For every ship.
It’s about 5 miles from the Port to the Key bridge. Last week, Dali covered the distance in about 50 minutes; if speed is restricted to 1 MPH, the same trip will take five hours, i.e. an extra ~four hours. Assume the same thing happens whenever it reaches its destination port - so instead of a 10-day journey (240 hours), it’s more like 248 hours, about four percent longer. The crew costs for those extra eight hours don’t amount to much, but now every ship is moving about four percent less cargo per year than it used to. We will need four percent more ships to move all the goods we’re currently moving.
Yes, of course.
Any idea we propose to fix what’s perceived as the excessive (this week) risk of out-of-control ships will wreak havoc on capital and operating costs and productivity Which together form a self-reinforcing financial “doom loop”.
We got into this place in state space by prizing cost-efficiency above almost all other parameters. In any optimization problem a decent rule of thumb is if you change your goal function materially, whatever parameter you had been optimizing most for under the prior goal function will take it in the neck under the new function.
If the bridge piers are on islands that extend far ahead and behind the pier in the path of an approaching ship shouldn’t that prevent such accidents? The sides of such islands are limited by the distance between piers to allow a ship through, but if they extend far enough ahead and behind it there’s no way for a ship to enter the passage with a great deal of lateral velocity that could allow the piers to be damaged from the sides. A ship could still get stuck in there, and possibly an act of extreme incompetence or intent to cause damage could disable a ship under the bridge creating a huge problem.
None of which changes anything about what I said. There exists some amount of time in which a response is realistically possible. I don’t know what that amount of time is, but it exists, and the engineers whose job it is to figure it out, can figure it out.
Whatever that time is, at that time before reaching the bridge, the ship should already be aimed correctly to pass between the supports.
That’s half the battle.
To be fully safe, by that determinable point the ship should aimed to pass through safely if nothing goes wrong past that point as you say, and also to pass through safely if everything goes wrong at any point past that point.
Aye, there’s the rub.
For an e.g. rudder hardover in a narrow channel with unfavorable wind and current, they may need to within a shiplength or 2 to make it out the other side before leaving the channel and hitting something. For a big ships at a whopping 8 knots, a shiplength is ~90 seconds.
Not in Baltimore but there are harbors or rivers with a series of bridges & important shoreside infrastructure close enough together you’d need to treat them as a unit. By the time the bow is close enough to the far end of the gauntlet to be collision-proof, the stern is still in the thick of things.
You continue to try to ‘logicfy’ the notion that a tug redirecting the out-of-control vessel is “The Solution” without any reference to actual knowledge or fact, including multiple videos from a tug captain challenging that notion. Here you can see the track of MV Dali (I would link to MarineTraffic.com but it requires a membership to log into to see past tracks and lacks the annotations) and it can be seen that the vessel was essentially on track to pass between the bridge piers until ~2 minutes before impact, at which point the pilot made the call to drop the port-side anchor and a “Mayday” call was issued. While we don’t know what caused the ship to veer off in that direction, without steerage a boat will go wherever the water and its residual momentum is taking it (which is one of the first things you learn in boat handling), so it isn’t as if the notion that people have that a tug can just come along and ‘dig in its heels’ to easily push the much larger container vessel back on course is not reality; the fact is that the tug would have to get to the bow, keep the same forward motion as the container vessel, and have enough thrust left over to push the vessel back over despite momentum and fluid resistance. Tugs do not normally operate in this way (pushing a boat sideways with significant forward speed) and from the linked videos you can see how difficult and hazardous it is to do this even when a tug has time and room to get into position.
Something else that people do not seem to grasp is just how narrow the shipping channel is under the bridge. Like most river shipping ports, the channel has to be regularly dredged so it is intentionally kept narrow, and while you can’t see the locations of the piers on the map from scaling the span between them looks to be less than 400 meters, so even if the vessel were centered it would pass less than 200 meters from a pier, and of course vessels don’t travel straight down the center. So a loss of steerage at ~7 kts a few hundred meters from passing by the piers, combined with whatever impetus to veer sideways makes it almost impossible that a tug could have redirected the ship even if it had been able to get into place. And the notion of having tugs pull the unpowered container ship all the way out to sea is beyond absurd even if there were sufficient tugs (and trained crews) available to do so.
Avoiding this kind of catastrophe shouldn’t fall on the band-aid of having tugs buzzing around ready to intervene in some physics-defying fashion at a moments notice. It should be a combination of adequate protection for bridge supports from modern vessels combined with adequate preventative maintenance and fuel quality to ensure that a loss of steerage doesn’t occur. Those ships run for many tens of thousands of hours on those engines between major overhauls because those engines are designed to be highly reliable; this kind of failure shouldn’t have occurred coming right off port unless something was seriously wrong with maintenance, fuel, or another preventable issue.
Stranger
Yes an no. I agree it seems unlikely to impossible for a tug to run to the rescue of such a ship with two minutes warning.
This particular bridge had zero protection from such a crash and the channel under the bridge was narrow (very little margin for error). Also, the port is not far from the bridge. Maybe it would have been prudent to have tugs tow ships through the passage under the bridge. In one of the videos you posted above the tug captain said he worked a port (New Haven) which did exactly that.
Given the damage and cost of even once in 50 years it might make sense.
That said, I suspect the replacement bridge will be much better protected from such an occurrence.
Ok, thanks. I realize that a soft bottom isn’t going to stop a ship immediately, but the ship drifted for quite a while, and one would think 20 feet would be enough to stop or at least slow it over that distance. But not if the actual depth was greater.
So, to get back to the OP, maybe the Golden Gate bridge would survive? Any other famous bridges?
I’d think the bridge piers would need to be protected to survive a ship like this. In other words, something around the pier the ship strikes first and not the pier itself (or, at least, dramatically reduces the impact energy by the time the ship hits the bridge pier itself).
That said, I look at something like the Brooklyn Bridge piers and they certainly look robust. Could that bridge survive such a collision? I have no idea.
I think it’s probably safe to assume that something upwards of 50% of those claims of expertise are completely false (anyone with no experience beyond the armchair can pen a comment that starts ‘As a [whatever profession]’), and a significant portion of the remainder are grossly exaggerated (like they say they are a Naval marine engineer, but in fact they were part of the cleaning staff on a car ferry or something).
I would hesitate to assign an exact figure to letters exaggerating expertise. I have no doubt it occurs often. But you do have to be a NYT subscriber, which is admittedly little protection, on par with Internet comments in a prestigious forum like the SDMB.
Well, as an experienced naval engineer, I think bridges should be built stronger.
As an experienced anti-ship hijacking expert I think bridges should have anti-ship missiles.