6 Sling Lift

[engpes]

I have an offshore lift application that is requiring a 6 sling lift. The lift is a traditional single point basket lift with 4 slings connecting to the rectangular box corners, but with an additional 2 slings connecting to the basket geometric center.

I am trying to find a standard / rule of thumb for determining the tension in the middle cables. I can do the statics to determine the sling tension, but in the real world all of the slings may have slightly different lengths.

I know DNV says to calculate sling tensions based on "n-1" slings, but this application seems a bit unique.

Any insight would be greatly appreciated.

Thank you very much!

 

[ishvaaag]

As a brute approach, decide for a maximum initial tolerance in more or less of the wanted lengths, and then build a table of the cases to investigate. Till 6 slings with symmetries this should stay manageable.

 

[rb1957]

if the thing being lifted is reasonably rigid, then won't the strap loads redistribute to maintain a static balance ?

i mean, the vertical reaction at each of the six pickups is W/6; if one is slack, then the vertical reactions will redistribute, on one side the two remiaing reactions will be 1.5*, no? this assumes that you're adding two independent straps.

could you attach the six straps, take the strain and then adjust the two new straps (turnbuckle, strap tensioner) so that they are taunt, then continue the lift. assuming teh four corner straps are known to be taunt and equally effective. how well do you know the alignment of the CG and the lift point ?

do you worry about wind loads ? i'd guess that the lift is slow enough that inertia loads are negligible ?

 

[MiketheEngineer]

I take real simplistic approach to these things... Can one or two straps carry the load by themselves. IF so - you are covered... You will NEVER be able to predict the exact loads on 6 slings. Some might be loose while others are carrying everything!!

 

[dhengr]

Engpes:
A sketch with plan and two elevations showing C.G., loads and dimensions, slings and hook, etc. would help define your problem, so others could understand it well enough to give some meaningful comments. What is a “traditional single point basket lift,” one hook and 4 or 6 slings, show that too? Note that the geometric center is likely not the same as the C.G. And, why would you add 2 slings to the geometric center? Just as a safety feature, and maybe never loaded? You are dealing with an indeterminate system once you get beyond 3 slings, since they would carry the box statically. If the 4th sling is a little longer than the other 3, it may carry very little or no load, depending upon the stiffness of the box. Then, as you start lifting, the box will rotate until the C.G. is directly under the hook. This changes your original geometry to accommodate sling lengths and stretch, etc. You are dealing with a highly indeterminate system when you add the fifth and sixth slings. You will have to know the exact stretched length of all of the slings, and the stiffness of the box you are lifting, not an easy feat. With a few assumptions, you should be able to look at the north and west elevations and write enough static equations to get a good approx. of the loads on the 4 corner slings. Rb1957 suggested turnbuckles or some such and these will probably be needed for several of the slings to really share, or adjust, the load. The factors of safety are high for these kinds of lifts, in part because of these uncertainties. The 5th and 6th slings certainly cover the DNV n-1 criteria, just snug them up. If one of the slings fails (the n-1 criteria), then the question becomes, can the remaining slings hold the load. But, more than that, is your skid or box strong enough, stiff enough, to tolerate this loading condition?

 

[jorton]

I agree with Mike. No two slings are the exact same length. If picking a rigid box with a straight hitch to each corner, only two of the slings will carry the majority of the load. Of the other two, only one at a time will carry just enough load to balance the box - it will likely bounce back and forth between the two if everything is balanced well. Of course these other two are needed for stability. Adding additional slings cannot be relied on to reduce the loads in the first two slings.

 

[rb1957]

if the sling lengths were adjustable (eg with turnbuckles) then all could be equally effective (or near enough).

otherwise, there doesn't seem to be much point to adding the extra straps, depending on the stiffness of the thing being lifted.

 

[engpes]

Jorten / Dhengr,
I agree with both of you. This is an existing basket that has been built and already in service for some time now. The basket does have 6 lift lugs / slings as explained above and the user wants to increase the capacity.

Like you both mentioned in a perfect world (non-existant) all 6 slings are equal length, but in the real world only 2 of the slings will carry the majority of the load because of the slight differences in length.

I was just curious to see if there is any generally accepted practice for taking some credit for the additional slings (5 & 6). I know that calculating the stiffiness of the box and stretch of slings would be very difficult to simulate accurately.

All lifts I have done in the past are all 2 or 4 slings, and I have never seen this type of application.

Thank you very much for all of your comments!

 

[ztengguy]

Can you use cables with rolling blocks, and connect to all four corners, and the other ends to the center lugs, that should distribute it out somewhat even, and utilize all 6 lugs.

 

[TLycan]

According to the BS12079 for designing of the offshore containers you are not allowed to use more that 4 padeyes or slings. and even when you use 4 slings your design is performed as only a 3 slings are working. Also the Code have a table,along with an equation, containing the weight of the container and the corresponding sling load.

 

[desertfox]

Please post a sketch along the lines of dhengr's post

 

[Hoaokapohaku]

In the U.S. OSHA regs are same as TLycan notes for the Brits, and apply in all cases, not just design for offshore containers. Max 4 legs, only 3 considered for load carrying capacity. The recommended practice is to use only two legs for determination of capacity because anytime there are more than two, the load may be carried nearly completely with two legs and other leg(s) acting only as load balancers. There are definitely no gains to be made in basket capacity by adding more slings.

If the basket itself is capable of carrying heavier loads, but only the rigging lacks the capacity, why not just use higher capacity slings?

Just for general information, slings are generally sold as matched sets. Of course they are not manufactured identically, but after manufacture they are measured and matched with other slings of the same length to produce the sets. As long as all the slings are from a matched set, and given the minimum 5:1 safety factor, differences in sling length are not a significant factor. A much larger factor is differences in the effective length of the slings actually in use because of differences in the way the slings lie in the hook, bridle, shackle, etc.