Mobile Crane Design

[KennyPCT]

Hi guys,

This is my first time here so apologies if this is in the wrong place or anything like that.
I am just out of University and have been handed a design task by my company for a mobile crane which will be operated offshore on a ship.

Does anyone here have any idea on how to perform dynamic calculations on mobile crane stability? My concerns are that although the static stability is acceptable things like wind, boom/mast movement and heel/trim of the boat due to waves will have a fairly significant effect on my CoG location and could therefore take it outwith the stable area I have plotted.

I have attached (below) a photo of my current most critical static CoG location with the blue lines representing the tipping point.

Thanks in advance!
Kenny

 

[Jboggs]

If you are just out of university and you being given an assignment that could result in human injury or even death, you should be closely supervised by an experienced engineer who would actually be taking final responsibility himself. I am also confident that there are some codes or standard practices involved here. Someone in your engineering chain-of-command should be able to help.

 

[Tmoose]

Are those casters or wheels at the ends of all the blue lines?

How high up off the deck can the CG be?

 

[LittleInch]

KennyPCT

Whoever thought of this idea is seriously unhinged and clearly has no idea about cranes.

From your OP and the picture my understanding is that you're going to use a standard mobile crane which has presumably been lifted onto a vessel and then propped up on its outriggers, though I've never seen outriggers at that angle before.

Telescopic mobile cranes are only designed to work on flat stable ground. They do not like and are not rated for any sideways load or movement or forces on the jib. Hence any load charts or calculations you come up with are not worth the paper they are written on.

To consign this idea to the bin where it belongs, simply call up the crane manufacturer and ask him for his advice.

If you really want to see what happens when you use mobile cranes on barges, look at and study this recent incident

http://www.eng-tips.com/viewthread.cfm?qid=392492

You really don't mess with cranes. for the sake of emphasis I'll say it again - mobile cranes, especially telescopic jibs have no real ability to withstand any sort of "dynamic movement" and you need to be coming up with good reasons why this idea is technically unsound and carries huge risks.

Purely out of interest - what is it supposed to be lifting?




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

 

[SnTMan]

Well, without commenting on the merit of the idea...

I don't see why you can't treat is as a quasi-static analysis using F=ma. All you have to do is account for all possible load positions and magnitudes, accelerations, roll angles, wind forces, guys leaning on it, etc, etc.

Sounds like a career to me

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[LittleInch]

Because the crane boom doesn't like it, that's why.

There's also the impact of the load on the vessel and what it does to the pitch one way or another.

We have no idea of the swing or is it a static lift, is the vessel ballasted??

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

 

[SnTMan]

LittleInch, "we" don't need to know these things, "we" are not solving this problem, the OP is

Regards,

Mike


The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[BUGGAR]

Contractors put mobile telescopic hydraulic cranes on barges all the time. And sometimes use them to place concrete in buckets with the barge heaving and the operator timing the landing of the bucket with the swell (Guy Atkinson, Dumbarton Bridge, circa 1985). But there are limits. Maybe talk to some Contractors instead of Engineers.
ps: we welded the outriggers to the deck.

 

[Lomarandil]

It's not unheard of to use a hydraulic crane for marine work, but lattice boom cranes are more common (because they're more robust for these sorts of things).

You say offshore in your original post. Hopefully you just mean marine work in general, in some sort of protected waterway with limited wave/wake/weather exposure. You really don't want to be operating a crane (not of this sort) in true offshore or deep ocean conditions.

The crane manufacturer should be able to provide you a "barge chart" which will offer reduced crane capacity for differing barge list values (and will include allowances for wind up to the operational limit, typically 30mph). If they can't/won't, you don't want to use that crane for this purpose. As Mike alluded, there are a lot of factors that can overload a crane, none of which end well. You don't want to be extrapolating or working from first principles on this.

Finding out how much your barge will list (based on weather, location, operational concerns like how much weight you're picking, the barge used) is the realm of a marine architect. See the incident that littleinch linked, and hire a marine architect consultant. That's one realm you especially don't want to mess around with without the right background and experience.

 

[chicopee]

As the vessel list and tilt, the load radius will increase and decrease, therefore the land based load chart has to be modified for water based operations. Tipping is one sort of failure shown on crane land based load charts, however, you also have structural failure of the boom which is also shown on these load charts. Wind speed is also also listed on these land based load charts. The best advise is stated by Lomarandil and that is to contact the crane manufacture. My additional advise for you is to take a crane safety course .

 

[KennyPCT]

To clear a few things up. This is not going to be shipped straight off to a working environment. I am going to take forward the design with the best of my abilities and then it will have an experienced engineer check all my work and finalise my design, correct my mistakes and so on so forth.
There are also design codes, practices etc. along with rigerous testing procedures prior to actually using such an item.

The crane will also have operational parameters such as it must not be used unless in absolutely flat conditions for obvious reasons. However I want to be able to ensure that if there is any movement in the ship despite flat sea conditions I take this into consideration/allow for it as it is potentially a dangerous piece of equipment.

Tmoose Those are outriggers so it will be a flat cylinder at the end of the blue lines.

LittleInch: I appreciate your input and will be sure to take it into consideration, but I'm not about to tell my boss he is unhinged. I am here because I realise I am in over my head and hoped to recieve some help and advise from within the industry, I don't overly appreciate the brow beating when I'm already trying to reach out to a comunity of more experienced engineers. That being said, your concern is duly noted.

SnTman: I had considered this but my main concern is the angular movement of the crane. If you consider the crane lifting something at it's mosts extended point and then rotating round from it's displayed position to it's central position to me this is problematic as it has an angular movement as opposed to linear, or am I missing something?

Lomarandil: Thanks for the input, from the sounds of things I need to be passing this off to someone far more experienced or at least drafting in a marine architect as you suggested to deal with the specifics of the situation. In terms of the sea movement the crane will be specified as not operational unless on absolutely flat sea.

Other information regarding the replies here.
I do have a barge list which displays wind speeds, accelerations of the ship in x, y, z axes etc. The real issue is that I do not have the engineering knowledge to impliment these into my design correctly. I have static calculations which concurs with my 3D model and confirms the crane will not tip. However these do not allow for the above mentioned dynamic influences.

The crane will not be able to be welded to the floor as it's purpose is to be mobile.

The maximum load it will carry at it's maximum extension will be 650kg at 4.72m.

Thank you all for the input.

 

[Lomarandil]

As a first point, most of the responses here have probably been based on the assumption of a much larger scale -- say a 200tn Hydro crane picking at a radius of 20m. The scale you've now provided will inform different details of the approach, even as the theory stays the same.

Yes -- even in perfectly flat sea conditions (which is a fine place for theory, but not practical), the ship will list and roll dynamically as the crane is operated -- as the load is swung or the load radius changes. You can ballast to minimize this motion, but can't eliminate it.

That's why you need a "barge chart" for the crane, by the crane manufacturer that tells you what the reduced capacity of that crane is in various dynamic conditions. It's not just about tipping, the crane can fail in other ways too, including (and most notably) structural failure of the crane boom. This is one half of the equation -- for a given limit of dynamic conditions (say 1 degree of list), can the crane pick your 650kg at 5m?

Finding out if your ship is large and stable enough to keep the crane within your assumed limit during operation is the realm of the marine architect. Although if you give us a sense of scale for the ship (also the location of the crane on the ship), given your small load and radius, that may turn out to be negligible.

 

[LittleInch]

Kenny,

My apologies for what was a rather OTT statement - I was probably having a bad moment when I wrote this, but with the information you have now supplied it was a little strong and uncalled for and I retract the unhinged bit.... To be fair your OP did talk about boat movements and wind / waves etc so you can probably see where what we were thinking of.

I was also working on the basis that it was a much larger unit, more the size of the ones which failed in the Netherlands.

I was also unaware of barge charts for cranes until now. Learnt something new about that.

650kg at 4.65m is pretty small but I'm not sure re-reading the items what it is you are trying to work out and whether these items are really within the realm of the crane manufacturer / supplier.

In general with cranes so much will depend on the operator and what they can feel / see / judge to be an acceptable transient speed. The movement of the load should not be such that it increases anything by more than 10% other than for you the relative pitch and movement of the initially flat thing that you start with.

Flat calm seas are not that common, at least in a marine environment as opposed to a harbour or inland river / creek.

I think all you really need to do is look at where the loads are in steps of say 10 or 15 degrees of arc movement. Don't forget the change of load and the issue of how the counterweight affects things when you don't have a load on the end.

An idea of what the scale of the boat / barge is compared to the crane would help....

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

 

[SnTMan]

KennyPCT, I still think a quasi-static analysis is applicable. You do have to account for the worst combination of positions and accelerations as regards the possibility for overturning.

On Edit: As regards overturning only, it seems to me it's a fairly simple question: Is the resultant overturning moment due to the various forces caused by the various accelerations acting at the resultant center of mass greater or lesser than the resultant moment (due chiefly to dead weight) resisting overturning?

Things were done this way for more than a hundred years before computers, etc came along. Straightforward but messy.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[zeusfaber]

Make sure the Nav Archs are happy with what you are doing too.

In ship stability terms, as soon as you suspend a load from a crane, the load acts as if it were concentrated at the tip of the boom - playing merry hell with your Gm.

The UK's Marine Accident Investigation Branch reports on the death of some poor fisherman who has forgotten this at least once a year.

A.

 

[3DDave]

It's worse that concentrating it at the tip of the crane. As soon as it is swinging the weight increases based on the angular rate and the amount of swing and is represented in the tension of the cable.