I’ve gotten it into my head, somehow, that if a laden supertanker were to encounter one swell at it’s stern, and the following swell at it’s bow, and these swells were high enough, the ship would break in two.
Sounds logical to me, given the length of these things, and the tremendous loads they carry.
True?
Peace,
mangeorge
why wouldn’t it just be pushed in the water perpendicular to its direction of travel? I bet they are flexible enough to take whatever kinds of roll, pitch and yaw the sea has to offer.
The vessel’s structure is designed to be strong enough to cope. I can’t think of any example of a vessel simply breaking in half in this way. Certainly however, any number of vessels have suffered some degree of structural failure in a seaway*.
There was signficant speculation that the ore/bulk/oil carrier “Derbyshire”, which sank suddenly in a typhoon, broke in half but the final analysis of the wreck suggested otherwise.
*Someone will post the link to the youtube clip of John Clarke’s take on the “Kirki” in a minute.
Not a supertanker, but rogue waves are one cause attributed to the wreck of the ore ship SS Edmund Fitzgerald. There’s some controversy about how much the ship’s structural integrity had been compromised before the waves hit.
What you describe was of the early theories of what happened to the Edmund Fitzgerald:
Your premise is fine. Ocean reality dictates that it (probably) will never occur. Even in large storms, it would be exceptionally rare for a wave interval to be over 500 feet while simultaneous wave height would be over 50 feet (which would be my rough guess as to how long and high the waves would have to be to hold the ship ar three points, much less at two.
Your principle is reflected in Great Lakes construction. Salties (ocean going ships) and traditional lakers are both built to lengths of about 730 feet, the greatest length admitted to any lock on the St. Lawrence Seaway. Salties, however, do not load to their full capacity in Duluth, Thunder Bay, or Chicago because they would not be able to get through the canals or the river system. Instead, they take on partial loads at the Great Lakes ports, then top off when they get down to the ocean. Lakers are built much shallower so that they can carry their full loads through the rivers and locks, but they cannot venture onto the ocean because, at a length of 730 feet and a beam of 75 feet, with a depth* of just under 40 feet, they would, indeed, run the risk of breaking in half in a storm. In contrast, the salties with lengths in the 700 foot range typically have a depth* around 50 feet. Supertankers are longer, but deeper, yet.
- Depth is the distance from the spar deck (the top of the cargo area) to the bottom of the keel. The 39 foot deep lakers have a draft of 28 feet. The 50 foot deep salties only load to a draft of 28 feet, but will ride much lower when loaded.
(The newest lakers, the “thousand footers,” are built much closer to the ocean designs, but they are too large for the Seaway locks and are confined to the upper lakes.)
ETA: the 730 foot length applies only to salties designed for Great Lakes service. Ships that will never venture into the St. Lawrence, of course, are much larger.
First of all, I don’t think you understand the terms you are using. A swell is a formation of regular waves generated by energy absorbed from wind forces far away from the measurement point and resulting in a quasi-sinusoidal soliton; that is to say, the motion of the waves is close to a sinusoidal function, and the formation is self-supporting, maintaining a regular waveheight and period until it interacts with bottom geometry as it approaches shore. Swells tend to fall into a well-defined band of waveheight versus period, and the more signficant the amplitudes get the longer the period.
The scenario you are trying to describe, I think, is where you have the hull supported near the stern on one wave crest and near the bow on a successive (or leading, in the case of a following sea) wave crest, so that the hull is essentially a simple beam supported at the endpoints. The problems with this concept are that the ship is not a simple beam and it isn’t supported at points. Large tankers and other large vessels are designed for a significant amount of flexure, especially in bending; indeed, it would be impossible to make a large hull out of steel (or any other material) that does not bend. The trick is to make the structure such that it isn’t forced to bend too much at any given point, so that any flexure is distrubuted across a lot of material.
As for how the ship is supported, it doesn’t sit atop the water, even in waves; the hull is submerged sufficient that the draft displaces a volume of water equal to the supported weight. Since a hull is mostly empty space, a conventional monohull ship has to have a signficant amount of hull surface submerged, and all of that provides vertical support. There are dynamic effects which cause deviations from static buoyancy, of course, but a ship this size even a rough ocean–say, WMO Sea State 7 with 20-30 foot significant waveheight at a ~15 second period–isn’t going to be tossed around like a cork. It’ll see not insignificant heave (direct upward motion) and some amount of pitch but it’s not going to be perched on a wave like a rubber ducky. If a wave formation aggressive enough to lift the ship up were impinging (which would be far too short a period for normal swell) the conditions would be such that the screws would not be able to remain submerged, the ship would lose weigh, and would be forced into a beam heading (parallel to the waves). This would be very bad from both a stability point of view and for the roll forces developed, but the kind of seas in which this would happen would not be traversed by an experienced tanker captain with a hundred million dollar cargo.
Most large tankers operating today have a double bottom (sort of a second sealed hull under the main hull) and most tankers currently being build are double hull designs, in which one sealed hull is actually built inside of another. In both cases, this is like making a box section, which helps give the tanker more rigidity and controlled flexure, and also helps to prevent leakage. This isn’t to say that tankers are foolproof; the large number of major oil spills in the last couple of decades testifies to the fallibility of man and machine in the face of Nature, and the first law of seafaring is that nothing that goes to sea is ever certain to return intact if at all. But a hull isn’t going to just break the way you suggest; when failures do occur (aside from navigational failures like running aground or into flotsam) they typically happen because of poor maintenance, fatigue failure, or some metallurgical defect rather than a failure of the designers to anticipate maximum operating conditions experienced in regular operation. Indeed, given the number of large tankers (over 1000 long tons in deadweight) that operate almost continually and the fact that most tankers are registered to countries that do not uniformly impose high inspection and maintenance standards, the fact that there are so few major spills and almost all are due to navigational errors indicates that tankers are, as a class of vessel, designed with high structural margins to any sea conditions in which they operate.
Stranger
tomndebb and Stranger: Your posts are lucid and informative, and made interesting a subject about which I cared little and knew less. You have successfully worn away one tiny little corner of the immense mass of my ignorance, and once again reminded me of why I love the SDMB. Well done.
Well, I am not a naval architect, though I will check with one tomorrow, but I don’t think a modern ship has ever been suspended between two waves and snapped. However, it is more likely (how likely is unknown) that a ship might encounter an extreme wave and end up with a portion of the ship suspended in mid-air. Such extreme waves were once thought to be myths or at least extremely rare. Turns out they are neither, though apparently encountering one and living to tell about it is extremely rare…
The European Space Agency recently conducted a world-wide census using satellite borne synthetic aperture radar and showed that such waves (generally defined as greater than 50 ft in height) are surprisingly common. Here are a couple of links showing pretty pictures and some discussion of extreme waves.
I was surprised to see that there is some discussion about extreme waves knocking low-flying helicopters out of the air.
I apologize for the quality of the above links. Google created them and I can’t vouch for the sites. I can say that extreme waves exist (I am an oceanographer working on wave models) and there is at least an assumption that the waves do cause ship casualties.
Very large waves certainly exist. They require patience, they tend to be the random sum of many smaller waves, the right winds and the right currents. The Angulas current off South Africa is infamous for them.
Sometimes ships simply sink. Sometimes without warning. Extreme waves are one possibility.
I pretty much understand the term “swell” just as you describe it.
When I said “the length of these things, and the tremendous loads they carry” I was referring to the tanker, not the swell. True, I know little of the actual dimensions of these swells. But:
If two swells were 40ft high and 90ft apart, and a ship 100ft long were to span these two swells evenly, that would be the kind of situation I’m thinking of.
Dunno if it would indeed break, though.
Sometimes the front falls off.
Is that a serious interview?
Because it’s awesome.
It is certainly possible to hang a ship up by either end such that it breaks in the middle. It happens when they go aground. However, sea conditions as you posit just don’t happen (no way do you get swell that high, with such a short period) and vessels are built to withstand such conditions as do actually arise.
Your scenario isn’t too far-fetched in wartime, though: some torpedoes are designed to detonate under the hull of an enemy ship and “break its back” via a lack of support.
Well, you don’t understand the definition of a “swell”, because you continue to use it interchangably with “wave”; a swell is a collection of more or less uniform waves that travel together, generated by a distant wind source. You are speaking of the height and distance between waves, or more precisely, wave peaks.
The situation you describe, i.e. “two [waves] … 40ft high and 90ft apart” would not happen in nature, and certainly not in swell. At 40 ft significant wave height, the probable wave period is around 16-20 seconds in the most aggressive case. This would be roughly a WMO Sea State 8 or 9, which are conditions too rough for any commercial shipping.
There’s one in every crowd. I like the sonar whale sketch better, though.
Stranger
I would not call waves that were 40 feet high and only 100 feet aparts swells; I’d just call them waves. (I’d also call them pretty nearly imaginary as I do not think there is any way to create that situation on this planet.)
That nitpick aside, you’ll note that your first question was in regard to supertankers. Those are 1,000 feet long and the sheer weight of them keeps most of their hull in the water (across multiple waves, if need be) rather than allowing them to be suspended between just two (or across three) waves. They also have depths in the range of 90 - 100 feet that provides a lot of surface for waves to support while providing a lot of longitudinal support.
In the days of wooden ships, hogging and sagging were real concerns. (They would more often be the cause of stress failure over time than a single catastrophic wave event, but that did not make the ships less imperilled.)
In 1914, the SS Oklahoma sank abruptly in heavy and the survivors reported that it appeared that the ship had been caught between two waves. however, it was only 419 feet long with a depth of only 28 feet, so 30 foot waves would have provided a lot of hogging and sagging stress to that ship. (NYT archival story, .pdf and may require registration.)
It is the work of the genius of a bloke by the name of John Clarke. His interviews with straightman interviewer Bryan Dawe are an Australian institution. Clarke plays various politicians and the like in a wonderful deadpan style with no attempt at impersonation of voice or mannerisms. It’s very clever.
Enter “clarke dawe” into Youtube for lots of them. The issues and people tend to be a bit Australia-centric, but the bullshit of politicians is universal, so you should get a laugh out of this stuff no matter where you are.
And that interview is (per the footnote to my first post) a compulsory link in any thread on the 'Dope on the subject of vessel casualties.
OK, what about a wave that’s so large that it lifts the supertanker out of the water so that the bow and the stern are both out of the water while the middle is basically supported at the wave crest? Is that far-fetched or possible? I understand it would have to be an awful big wave, would one of those rogue waves fit the bill?
Is this a good time to mention the America’s Cup? 
By the way, John Clarke was inducted into the Australian TV award’s Hall of Fame last night. Good on him!