6 Sling Lift
6 Sling Lift
(OP)
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!
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!






RE: 6 Sling Lift
RE: 6 Sling Lift
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 ?
RE: 6 Sling Lift
RE: 6 Sling Lift
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?
RE: 6 Sling Lift
RE: 6 Sling Lift
otherwise, there doesn't seem to be much point to adding the extra straps, depending on the stiffness of the thing being lifted.
RE: 6 Sling Lift
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!
RE: 6 Sling Lift
RE: 6 Sling Lift
RE: 6 Sling Lift
RE: 6 Sling Lift
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.