Report issued on MOL Comfort Loss

Whipping, as it is called, is now being introduced as a load case scenario to account for in global loading.

 

Sure dont you remember all the single hull oil tankers than cracked..lol

(so they had to build paper thin double hull rust buckets, only thing that did was to create work for shipyards and finally the USCG are looking at it as a negative in that over their life cycle they cause more pollution)

 

They just dont make waves like they used to do they..lol

 

Thank you Ad Hpc, I am pleased to learn that. It must be quite tricky to work out the loadings with uneven container weights?, but some broader criteria for that could be arrived at, to allow reasonable calculation should be suitable. Motor vehicles have used HTS for vehicle bodies for quite a long time. Whilst perfectly good at saving weight, I understand there can be more corrosion problems (ie faster), and more prone to notch stress fracture, depending on the exact alloyed steel. However, I'm sure this is well accounted for in the marine hull grades.

Once again, good to learn of the inclusion of a 'Whipping' factor.

 

It has been known from the early days of welded ship construction that abrupt changes in structural geometry, like going from relatively thin plate to a strake of heavy plate, provide regions where cracks can start and then spread to other parts of the hull girder if not arrested.

Container vessels, due to their arrangement, must use a heavy box girder running along either side of the container holds at the main deck to provide strength to the hull in way of the large openings which provide access to the cargo holds. The sides of the hatch coamings may also be constructed using thicker plate to provide additional longitudinal strength.

In the large container ships currently being built, it is not clear what thickness of plate is used in the box girders and hatch coamings, but according to this study:

http://www.antiport.de/doku/gutachten/ulcs.pdf

a notional large container vessel of approx. 12,500 TEU capacity might have strakes as thick as 65 mm in the upper parts of the hull girder. Joining such thick plates together effectively takes more preparation and welding than joining thinner plates. The Class NK report did not disclose the details of the scantlings of the MOL Comfort, but it would be reasonable to assume that even with the HTS steel used in her construction, the upper strakes of plate in her hull were > 25 mm thick, perhaps as much as 50 mm thick.

To compensate for this additional hull weight, perhaps her designers took out too much material from the bottom scantlings, where the fatal crack appears to have started and where additional steel was added to the surviving vessels.

 

Using FEA to design complex structures, like those found even in simple ship designs, is still more of an art than a science. Even with the fantastic capabilities of modern computers, modelling all of the complex structural details one encounters in a hull girder, like all of the brackets, stiffeners, and other small pieces parts, would take an incredible amount of time. What usually occurs is that a gross model of the entire hull, or a portion of the entire hull, might be modeled initially, and then other, more detailed models of particular regions of interest might be developed and analyzed.

Analyzing the structural response of a container vessel, which is subject to shear and bending, in both the longitudinal and transverse directions, torsion, warping, and then the transient modes of loading, like whipping and slamming, can pretty much exhaust a design department. Not all of the responses of a container ship hull girder are equally understood, and research continues. Using simple beam models, like with other vessels, may not be entirely appropriate, and much research has been devoted in the past two decades or so to trying to determine what happens to the ultimate strength of a hull girder if a portion is damaged, say due to buckling of the structure or due to a defect in fabrication.

The cook book model of class society scantling rules may no longer be entirely suitable for designing such complex structures.

 

Its starting to sound like a racing yacht has a more reliable structure than an experimental containership?
Maybe the containership is a racing boat, lets build it as thin as fast and as cheap as we can as it only has to last....years..lol

If the next one is thicker does that mean the previous one was guesstimated?

One of GL-DNV statements was that they lost several builds due to being out bid for a thinner vessel, they didnt say it was the MOL Comfort but certainly intimated it was.

( Makes you wonder what was removed from the Airbus 380 to make it light enough that it stood some chance of ever making a profit?)
(Outboard engines don't have reverse thrust for one..)

 

Thanks, that certainly sounds like the situation with modern large box ships.

 

Aahh...you're getting confused now between metallurgical features of a material and how it behaves owing to its arrangement when fabricated, i.e its structure stiffness.

An abrupt change in section is owing to the stress concentration of the change of section, owing to the change in stiffness. Not because the plate is thicker or thinner. It could be both or a mixture...

Additionally fracture in steel is brittle in nature. And these cracks propagate from thin to thick sections not the other way around. Steel is a brittle material when it fractures not ductile like aluminium.

Also in those riveted days, the rivets if not done correctly were voids/crack ready made just waiting to propagate. Thus if poor workmanship, then it cracked...and if poor detail design (stiffness) it cracked. Nothing has changed there other than now there is the SCF of the welded joint itself to take into consideration!

Not really, since that is their job! You're making a very broad generalised statement there without foundation.

 

testing in software is just testing a known model, that usually works but who says its true when you could make a million different types of metal and then it could have a million different fatigues done to it before failure not to mention several outcomes from being welded, was it a hot day or a cold day?
Is the steel that ends up floating the same as the sample that came from the foundry?

 

I don't think so.

The structural analysis techniques which are required to analyze a modern container ship require more than just a nodding acquaintance with standard beam theory.

Up until computers became commonplace, there was no practical way to investigate many of these structural questions, let alone optimize hull structures for strength with minimum weight.

The problems faced by the designer of an open container vessel might be covered in a course or two of intensive structural analysis techniques, but I doubt the average naval architecture curriculum spends much time dealing with the torsional response of hulls, warping, whipping, and the like. Most of what I learned about such topics was learned after I graduated from school.

That we are seeing container ships breaking up at sea means that the class societies, the design offices, and the shipyards do not have all the bugs shaken out of the design and construction of these vessels.

The class societies are slowly beginning to realize that building large, open vessels is really pushing the envelope of current knowledge and experience, which I think is why DNV-GL is trying to get a reluctant Class NK to meet with the other members of the IACS to come up with a solution to these problems before they get worse.

Big box boats are here to stay, because that's what the owners want to service the trade routes from the Far East to the Americas and western Europe. The underwriters are looking to the class societies to provide them with assurances that they have not taken on undue risk in insuring these vessels and their cargoes, which, on each voyage, represent an exposure of a half-billion dollars or more in potential losses if there is a casualty.

 

Never said it didn’t. Also you’re assuming that NA only perform standard beam theory calc’s for structures!

Your misplacing doing details via FEA with simple over engineering in its place to ensure fit-for-purpose of previous years.

If it is their job then they do. I didn’t spend much more than a few hours of lectures doing aluminium, or the importance of weights or propellers or any other, in the many subjects one needs to learn.

Every NA starts with basic knowledge, the degree. It is the apprentiship as such. One only learns more details and often their pet subjects (or where they end up working) post-graduation…to suggest otherwise is missing the point of what a degree is and how one learns through experience on the job.

A vessel goes down everyday somewhere in the world. Only if it becomes “news worthy” does the general/wider public become aware; generally no one is ware of such. Having a high value cargo and being caught on film…is clearly a no brainer for publicity!

As I noted from the outset, Japan does things its own way, hence the drip feeding and not so open and baring all of ones soul, as such a report must be. That’s not the Japanese way! Which is also why eluded to what IACS et al, will say regarding the report and its findings.

I have been on the committee reviewing the Common Structural Rules, it is a very long process with many aspects to consider. Not the kind of exposure and scrutiny that Japanese are used to. Which is clearly evident in their less than open and at times obtuse report.

 

"A designer knows he has achieved perfection, not when there is nothing else to add, but when there is nothing else to take away."

... of course ignominy is sometimes found where just a little more that that is taken away....