Padeye and shackle 3

[rahul4engg]

Dear engineers,

As per DNV rules, padeye design is suitable for only one shackle size (shackle pin diameter and shackle inside width). Hence, I want to design a padeye (to be marked a SWL of 4.75 tonnes) suitable with 4.75 tonnes WLL standard shackle.

Qusetion 1)
However, when i am supposed to test the padeye, test load = 4.75 x 2.5 = 11.875 tonnes. hence, for testing, I will need to use a 12.0 tonne shackle, but the hole in the padeye is unsuitable for this.
Could anyone tell me if there are special shackles for padeye testing with smaller high grade pins, or is it allowed to overload the shackles for padeye testing?

Question 2)
According to DNV 2.7-3 "Offshore Portable units", padeye is designed with respect to RSF which comprises of skew load factors, sling angles and dynamic factors during lifting. corresponding shackle WLL is stipulated as WLL = 0.45 x RSF.
If I want to mark a SWL on the padeye, will it be correct to mark SWL = shackle WLL = 0.45 x RSF

Best Regards,
Rahul Barve

 

[JStephen]

Can you just use an oversized shackle, so that the shackle is also adequate for the test load?

 

[rahul4engg]

Dear Stephen,
Thank you for your quick response.

for 4.75T shackle (pin diameter = 22.4mm) the padeye hole diameter should be around 23.5mm.
For 12 T shackle (pin diameter = 35.1mm) the padeye hole diameter should be around 37mm

For design with respect to DNV, hole size of padeye cannot be larger than 106% of pin diameter.

Thus, after testing, for actual use, I will always need to provide a 12T WLL shackle and limit the load capacity to be 4.75 tonnes. this is quite inefficient.

Any thoughts ?

Rahul

 

[CANPRO]

I just took a quick look at the shackles in the Crosby Catalog...the proof load of a shackle is 2.0 times the safe working load and the minimum ultimate capacity is 6 times the safe working load.

So for the purpose of your test...the 4.75 ton shackle has a proof load of 9.5 tons and an ultimate load of 28.5 tons. You need to test to 11.875 tons, which leaves you with a factor of safety = 2.4 on the shackle. I'm not sure what your requirements/conditions are for testing, but in a controlled environment 2.4 F.O.S isn't bad. I would suggest using the 4.75 ton shackle for the test.

 

[JStephen]

My thought was to go with the 12T shackle in that case.
In our applications, those shackles are already part of the shop inventory, they do not have to be purchased.
The larger shackle may also be desirable for the additional throat capacity.

 

[rahul4engg]

thank you JStephen and CaneIT for your replies.

Jstephen: But can we then extrapolate this o say a 25 tonne SWL padeye and then we would have to provide a 50 tonne WLL shackle for the use. Hence, although the solution to use higher rated shackle is practical and will have no problems getting approvals, it doesnot sount right / efficient..

CaneIT: a quick food for thought: I am not shackle manufacturer. we have fabricated only the padeye. If I have to write the procedure for testing a 4.75 tonne padeye, shouldn't the shackle should be treated that it is "working" and not being tested. hence, as per shackle manufacturer's requirements, i can load the shackle only to 4.75 tonne.

Cheers,
Rahul

 

[CANPRO]

rahul4engg,

I think that would be a good question for the manufacturer of the shackle.

I think it all depends on your testing environment. I think the working load being 1/6 of the ultimate is meant to be used in typical working conditions where life safety is affected by the perfomance of the shackle. When testing the padeye I would hope that a failure in the padeye wouldn't cause any safety concerns, and with that said, a failure of the shackle shouldn't cause any safety concerns either...so I don't think that the "safe working load" of the shackle in your condition should be the same as the published safe working load that is meant to cover all bases.

 

[boo1]

Buy a Pad Eye Tester or use the smaller shackle for proof testing and throw it away after proof loading

 

[dhengr]

Rahul:
Please tell us what your engineering background and experience are. Then, think through this problem rather than just blindly following the code verbiage. Show us a sketch of the padeye (and equip.) you are talking about and define all the acronyms you have used (SWL, WLL & RSF). We do not all have a copy of the DNV code you are looking at, so it would be helpful if you could post the exact parts which are in question. The 106% pin size to hole size is in keeping with current code writing practice, so that no one needs to really understand the problem, or what they are doing, if they just blindly follow the code cookbook rules. The pin dia. vs. the hole dia., size difference, leads to a Hertz bearing stress problem (or failure issue); and the padeye width vs. inside width of the shackle at the pin leads to a pin bending problem and the variability of the reaction loading across the width of the padeye, with the max. at the sides and a min. at mid thickness of the padeye.

It seems to me that you are kinda missing the boat here, and not understanding what JStephen and CANEIT are trying to say. You are approaching this problem kinda bass-ackwards. You design the padeye cap’y. to meet the needs of the piece of equipment it is fixed to, and you define some max. sling angle from vert. which is allowable for your design. The min. padeye thickness is set by this part of the design and what stresses the equip. can tolerate. Its thickness at the hole can be increased with doubler plates (doughnuts) to match a larger shackle width. The padeye may be a constant thickness, but it doesn’t have to be. And, for goodness sake don’t assume you need full pen. welds all around. Then you pick a shackle which has a greater cap’y. than you need, you do not necessarily try to match the two, at a min. cap’y. Any testing you do is to prove the padeye and its attachment and angle of pull, not to prove the shackle. Do a little research on shackles and you’ll find they have a significant factor of safety btwn. their working load and failure load. Thus, (2.5)(padeye working load) will not likely fail an appropriate shackle, but it might fail a poorly designed or welded padeye.

Most riggers do not have sets of every size shackle, so a larger size shackle will almost always (more likely) work for them. And, the strength of the padeye is sufficient if it will lift its piece of equip. at the appropriate factor of safety, it is not defined by the shackle used. From the riggers perspective, there are ways to make a smaller pin (smaller shackle) fit a larger hole, but he must buy smaller shackles if you provide the smallest possible hole in the padeye. It is now incumbent on him to use a shackle which works with the hole in your padeye. But, the actual inefficiency, in the real world, is that you might force the rigger to buy shackles just to fit your padeye with the smaller hole, and wait for its delivery. Obviously, you could provide the smaller shackles right with your padeyes, but they will probably end up in the rigger’s tool box at the end of the lift. Then the same problem will exist the next time the load must be lifted.

 

[rahul4engg]

Sorry, if i am sounding like i am dragging this problem. But i feel that use of padeyes is very commmon engineering application and can warrant a lengthy discussion.

dhengr: I understand your practical suggestions. however, I do not understand the conclusion.
My consideration is that I need to get the marking on the padeye certified by someone like DNV. there are two ways of going about this that i can think of:
a) for a 4.75 tonne SWL (safe working load) padeye, provide a hole diameter good for 12.0 tonne WLL (working Load Limit) shackle.
problem for a: In future, to not meet the Hertz bearing stress problem,I will have to specify that "this padeye to be used with 12.0 tonne WLL shackle". Although, it looks practical, i feel this is as an inefficient system. however, I would like to know if this is the industry practice?

b) for a 4.75 tonne SWL padeye, provide hole diameter good for 4.75 tonne WLL shackle.
problem for b : however, when I am testing this, there is a requirement to test it to 2.4 times 4.75t. Agreed that the shackle will not break, but question is whether I am allowed (or rather, will it be acceptable for DNV or ABS surveyor) that 4.75t WLL shackle is loaded to 10.0 tonnes? the point is that manufacturer of shackle is different from padeye fabricator. hence, both are not tested at a time.

Also, those who are not familiar with the DNV code, it is free to download at following website:

http://exchange.dnv.com/publishing/stdcert/Standard2-7-3.pdf,

kindly refer to Design/3.8.4 and appendix A for the padeye design.
SWL = Safe working load, WLL = working load limit, RSF = resultant sling force.

boo1: yeah , i agree that padeye tester made of higher strength material with pin size equal to one or two sizes smaller than standard shckles would do the job. thanks for your input.

Cheers,
Rahul

 

[Ussuri]

Normally, if you are designing a 4.75 tonne padeye to suit a 4.75 tonne shackle then that is the size of shackle you use for your load test. The WLL (working load limit) on the shackle is 4.75 tonnes, but the load test is not a WLL case. Its a load test.

You need to design your padeye for the most onerous condition which may be static WLL, a dynamic condition or the load test condition.

You do not usually just increase the size of the shackle from 4.75 tonne to 12 tonne just for the load test.

Typically the shackles have a factor of safety on them of 5:1 or 6:1. So for a 4.75 tonne shackle the MBL (minimum breaking load) is 23.75 tonnes. If your one off test load is 11.875 tonnes then you still have a factor of safety of 2 on the shackle.