Floating crane backwards stability? 4

[mementosun]

Hello guys,

I need to check the overturning posibility of a floating crane when its live load is suddenly dropped.
Do you guys have any ideas about this or any reference materails suggested?

Thanks in advance!

 

[SlideRuleEra]

This is a complex subject since it involves both the momentum from the sudden dropping of the load and buoyancy calculations for a hull that is shifting its orientation in the water. The classic book, "Naval Architecture", by Cecil H. Peabody will give you guidance for a tedious, precise solution. The book is in the public domain (in the USA) since it was published in 1917 (fourth edition) and can be downloaded, free, here:

http://books.google.com/books?id=wI1BAAAAIAAJ&source=gbs_navlinks_s

As an alternative, if the load is relatively light, compared to rated crane capacity, a series of statics calculations for various hull orientations should give you a reasonable idea of what will happen.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[racookpe1978]

Hmmmn.

If the floating crane floats stably when static and unloaded, then suddenly dropping a load from the static, loaded condition "simply" returns you to the original condition ... once the dynamic bobbing and dipping bouyancy waves are damped out.

So the critical point would be the first, maximum "dip" of the back deck towards the original water level. If a wave did overrun the deck when it is down at its maximum travel point, all other dips will be less. Also, but the deck in that area were covered and strong enough to not retain water in the hold, then the problem only reduces magnitude as the movement damps. The water, even if it reached the deck in the first dip, runs off. (However, any water trapped IN the hull by that first dip would make the problem worse.)

So the first movement down (in back) is caused by the inertia of the front of the crame going up, and being resisted by the increaing displacement of the back.

 

[mementosun]

Thanks guys!
I think I didn't explain my problem clearly. Sorry for my bad English! But still thanks for your information. They will be useful in my future work!

Actually, I want to check the stability of the crane mounted on the vessel. Of course, the worst case need to be checked, i.e. the crane is fully loaded at its minimum outreach position.

In german code DIN, it requires that the backwards stability of Crane need to be checked in the case when live load is suddenly dropped. When checking, a load of 0.3*(live load) applied upwards at the tip of crane was introduced. The moment produced by this load, crane's own dead weight and environment loads need to be algebraicly summed together to see if the crane is stable.

But with this method, our crane is definitely not stable. But I think the above mentioned method is not a rational one. So that is why I come here for help!
Any ideas would be appreciated. Thanks!

 

[racookpe1978]

Look again at the "assumed condition" of the DIN "rules" - I'm willing to bet a beer that those DIN rules are for a ground-mounted (pad and extension support) conventional crane.

 

[mementosun]

okey racookpe1978, you are right. I think I owe you a beer. The 0.3 is also stated in the code of some ship society (I can't remember where ). But that is what I can find, do you have any suggestions? I bet two beers you have.

 

[mementosun]

come on! anybody give me some hints?

 

[swearingen]

Of course the crane wouldn't be stable if you added 0.3 live load up on the boom - that's not the purpose of the load case. It's to capture the dynamic effects of the barge as it tips backwards. I believe this is a rather over-simplified way to approach this problem, but if it appears in the code, I'd imagine it's got some basis in reality.

What is the size and draft of the barge? What is your metacenter height?


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[mementosun]

Dear swearingen,

Thank you for your post.

You correct my misunderstading about this problem.
The assumed load should be used for stability check for whole crane structure with basement or barge and I just simply use it for crane's boom structure. Right?

So now, the problem should be how to check the backwards stability of boom structure when live load is suddenly dropped.

Because the stability of barge is not my work, I don't have details. I only know the length of the boom is almost 100m and the live load is 5000 ton. The boom is fixed on the barge. Sorry!

Of course the stability of barge is important, but just let's assume it is stable. Then do you have any ideas?
Thanks!

 

[DRC1]

If you drop a live load load You will craete an unstable situation where you get a severe roll away from the load. If the raduius is short and the load is significant, backward stability could control which will aggravate the roll. This can present a dangerous situation. We have barge mounted cranes and questions of capacity are always refered to naval architects who specialize in crane barges. I would strongly suggest you engage one for this problem.

 

[mementosun]

Dear DRC1,
Thank you for your suggestion! We already have one, that is why I want to check the boom overturning posibility,but thanks all the same.

If you woould kindly like to share some knowledge. I would appreciate it!