Effect of Sudden lowering of Suspended pontoon

[MahReda]

Dear Sir's

I have a pontoon with length of 85 meters which is lifted by two floating cranes as shown

my question is when the floating crane is in heaving motion (go up and down suddenly due to do waves) how can i calculate this motion impact on wires and on the pontoon itself

it's something new for me so excuse me if i give little information ..... but you can ask me if there is any info you need

 

[LittleInch]

Find out from the crane supplier what the vertical motion and acceleration is during certain wave heights and periods. Then include these additional loads with suitable factors of safety.

Or just times the static load by 3.

Try looking up DNV or Norsok standards for guidance.

A big floating crane won't move that fast unless you're trying to do this in a typhoon.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WARose]

Seems to me like you would need to know the worst case wave event. (Or what you would limit them to in terms of it being out of plumb while it was being lowered. That would probably control over it being hit by a wave since it would be completely supported by the cables at that point. In the water (while being hit by a wave), you'd probably get some relief on the cables from the buoyancy.)

You could probably take all the guess work out of it by have a single cable (per lifting ring; not sure if that is 2 cables per ring or a single cable) able to carry a large portion of the weight + a healthy impact factor.

 

[JStephen]

I don't think there's going to be a simple solution, unless, as LittleInch suggests, that information is available from the crane manufacturers.

A proper analysis would have to consider the motion of the cranes/boats/barges due to the wave, but also the elasticity in the hoisting lines, booms, etc., and I would doubt any of that is available to the rigger.

If this is two cranes and two barges/hulls, you'd also need to consider the possibility of out-of-phase motion of the two. And I assume when they go up and down, the load also swings right and left, and therefore swings nearer and farther from the crane.

Having watched a few YouTube videos of hoisting disasters, I'd say it might be worthwhile getting somebody with more experience involved if it's a critical lift.

 

[MahReda]

LittleInch

I agree with you that big floating cranes won't move that fast, but in our case it's moving that fast in somecases

The overload sensors shut down all motors when this happen even the pontoon weight is 460 tons and both floating cranes have sum of 1000 tons lifting capacity

Thank god that the floating cranes and all rigging equipment have suitable factor of safety

I can obtain the acceleration of the floating crane easily ( from DNVGL rule or from manufacture as you said)

Now let's assume that i have the acceleration and it's value is XX m/sec2

What formula i can use to calculate this added load ?

regards

 

[MahReda]

JStephen

This lift had already performed 3 times and it's done safely

I just wanted to investigate it more with more worse cases

 

[JStephen]

F=ma

 

[MahReda]

JStephen

Thanks

Just to make sure

lets assume that acceleration = 2 m/sec2

the pontoon mass is 460 tons

added mass is = 460 * 2 = 920 newton = 93.7 tons

Total effecting mass = 460 + 93.7 = 553.7 Tons ??

i made this calculation before and i had doubt about it because as LittleInch said "Or just times the static load by 3"

 

[LittleInch]

That assumes a steady fixed acceleration. Whether that is applicable here is doubtful and takes no account of vectors or sway.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dik]

Can the rigging be revised to more redistribute the loadings so that all lifting lines are shared between the two cranes?

Dik

 

[MahReda]

LittleInch

Sway is anther problem which will be required to be investigated also

but from what you said is if the acceleration is steady fixed the above calculation is considered correct ,right ?

 

[MahReda]

dik

i don't think is possible as there is limit of 12 degrees between the crane center line and the wire

 

[dik]

Thanks didn't know... not into rigging, but, was thinking that there is a manner to redistribute the loading.

Dik

 

[LittleInch]

Yes, but my point is that a steady acceleration is not the best way to consider such a movement and you need to add in the considerable dynamic effects and shock loading. you need peak accelearation forces and also factor in momemtum once this 450 ton load starts moving. You need to absorb the kinetic energy and that translates to load or force on the restraining equipment.

The consequences of failure are extensive so that's why you end up with large safety factors like 3 times static load.

Marine lifts of this nature in swell are difficult to accurately predict and need a great deal of experience and forethought and the ability for someone to say STOP when sea conditions are outside of the envelope of safe operation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MahReda]

Dik

The 460 tons are quite uniform distributed along 85 meter long

there is nothing we can do about it

the space between the two floating docks is restricted by the pontoons width

 

[MahReda]

LittleInch

Thanks for help

I'll start search about some missing values

 

[dhengr]

MahReda:
What governing safety restrictions exist to limit your operations when the winds and waves are this high (too high)? Should you even be performing a lift under these severe conditions? I would still involve the crane owners/manufacturers in this discussion/analysis, they know much more about their equipment crane/barge combination than any of us do. You have provided very little meaningful info. about what is really going on here. Exactly what conditions are causing these shock loadings? Are the two crane/barge combinations of equal cap’y. (500t each) and/or do they carry the same proportion of the load as a percentage of their cap’y? You say.... “The overload sensors shut down all motors when this happen even the pontoon weight is 460 tons and both floating cranes have sum of 1000 tons lifting capacity.” 1000t/460t = 2.2 is not a particularly high FoS for this kind of lifting work. I’ll bet both cranes don’t shut down at the same time, during the same incident. If crane ‘A’ is in a wave trough, while its end of the pontoon is high on a wave, during a lift, then in a matter of a few seconds the relative elevations switch, the lifting lines will go from slack to being jerked into high tension, with many parts of wire rope elongation, boom deflection, barge buoyancy considerations due to added load, etc. etc. Now, at the same time, consider the fact that the pontoon has unloaded slightly at crane ‘B’ and/because it is actually floating at its far end, which is also moving vertically. Now, the static loading at crane ‘A’ is actually higher than you had first calc’ed., due to the sum of these actions. It is not hard to imagine that one of the cranes might be overloaded for a short period of time, and its sensors might trigger a shut down. But, this shut down might not prevent the additional overload.

We get all tangled up in trying to calc. some acceleration or shock loading or dynamic loading, with so many variables involved in that analysis, that the Good Lord could not come up with an answer; and then we don’t even really understand, nor can we explain, what is really happening. I don’t know how you select the variable values accurately, with any confidence, let alone calc. an answer which you think is correct. The crane sensors are telling you something, listen to them.

 

[WARose]

Thinking a bit more about your problem, I looked in a structural dynamics text I have: 'Structural Dynamics...', by: Tedesco, et al. Chapter 21 discusses wave loads on structures. Problems 21.10 & 21.11 (for which the solutions are not given in the text) are similar to your situation. (Except the size of the submerged box is much smaller.)

The solution technique depends on the size of the submerged object, and (of course) the height of the max. surge wave. (Which goes back to what I was saying before.)