Padeye-Safe Working Load

[victor007]

Hi everyone,
There is some argument comes up at my office every so often about the safe working load to be marked on the pad eyes.
I have a structure to be installed offshore in GoM (Gulf of Mexico). The governing code to design the structure is API RP 2A-WSD. (By WSD I mean Working Stress Desing).
I run a 4 point lift static analysis on my structure and come up with the maximum un-factored static reaction of 100 kips in my sling. (By 4 point lift I mean 4 slings are hanging from one lift point/hook point located above of the center of gravity of the structure. The other ends of the slings are connected to 4 pad eyes at each corner of the structure). Per API RP 2A section 2.4.2.c, I pick up a DLF (Dynamic Load Factor) of 2.0, and design my pad eye per AISC-ASD. (By ASD I mean Allowable Stress Design).
The stress ratio for the combined stresses on the pad eye is 0.90. I proceed with the fabrication, and then I run an actual load test on the pad eye for 1.5x 100 kips, and it passes the load test at the fabrication site.
At this point I want to mark that pad eye for a Safe Working Load (SWL). What that safe working load would be?
Is it 100 kips, or 1.5 x 100 kips or 2.0 x 100 kips or 2x 100 kips/ 0.90?
The way I understand it, the SWL is 100 kips. I know technically and theoritically the Pad eye stress is going to reach just to the yield stress even under a 2 x 100 kips / 0.9 =222.22kips load, but that is not the number to be marked on the pad eye as the safe working load on the pad eye.
Please help me to see if I am missing something.
Thanks, 007

 

[TomasAron]

Victor,

From your description I would agree that 100 kips is the SWL.

However, I would strongly suggest not putting that on the lifteye. The reason is that 100 kips is the SWL for a specified angle with minimum out of plane bending. Unless you go through the effort of analyzing the structure and lifting eye for every angle of lift including some significant out of plane bending, I think that a SWL could be misinterpreted. Also I would read 2.4.2.e carefully because a few engineers I know take this to mean that your short sling and long sling should be able to take 100% of the load, which means that your padeyes should be able to carry 200 kips. Another option is to provide Installation drawings showing design assumptions such as sling lengths, lift arrangement, and shackle size/type.

Regards,
TomasAron

 

[victor007]

TomasAron, Thanks for your response.
Regarding the pad eye angles, I agree with you. As a matter of fact, the way I positioned the pad eyes on the structure, minimizes any out of plane bending. Basically I rotate the pad eyes on the fashion that each diagonal pair of pad eyes fall in a plane goes thru the center of gravity of the structure. Also my structural drawings clearly call out this positions for the pad eyes. From design stand point, per API RP 2A-WSD section 2.4.2.a, I have to design the pad eyes for 5% of the static sling load as a horizontal out of plane load applied at the center of the pinhole. Theoretically this 5% will take care of almost 2degree and 52 second rotation of the pad eye out of that ideal plane. [Arc Tan (0.05) = 2.86degree].
Besides the orientation of the pad eyes on the top frame of the structure, I also call out the location (X, Y, Z) of the lift point(hook point), location of the C.G. (X, Y, Z), and also the angles of the slings to horizontal on my structural drawings to minimize the risk of any loading operation different than my analysis.
Speaking of section 2.4.2.e, I am totally familiar with this concept, and I take care of that by running skew lift analysis with different load distribution ratios between the sling pairs. I did not bring it up in my original question because I wanted to simplify my case study. So in my original question, I assumed that the fabrication tolerance in the structure, pad eyes, and slings will be zero.

 

[Duwe6]

Mark your eyes just like Eye Slings 'chokers' are marked: 100Kips - VERTICAL. Make the Rigger do the calc's, because he will know the actual choke angles. Trying to outguess the future rigging is dangerous.

If you test at 1.5 x 100 Kips, the prudent value for SWL is indeed 100Kips.

 

[victor007]

Thanks Duwe6 for your response.
It seems we all agree on 100 kips value for SWL.
The challenge I am facing is coming from the Installation contractor who has the vessel to install my structure offshore.
I mark the padeyes with SWL=100Kips. According to them, they are NOT allowed to lift the structure if they know more than SWL=100 Kips load goes to the padeyes. In real world they have to install the structure when there is some seastate motion is going on. Because of the dynamic effect of that seastate on the structure, the padeyes will face a load more than 100 kips for sure (how much more? Depends on how bad or calm the seastate is). My calculations show the stress ratio in the padeyes will stay below the allowable stress even for 2 times of that SWL (Per API RP 2A section 2.4.2.c).
So they argument is if the padeye stress level is below the allowable stress even for 2 times of 100 kips, and also the load test shows the padeye can handle 1.5x100kips load, why we can NOT mark the padeye for a SWL more than 100 kips. They say the load test and the calculation show the pad eye is safe for loads way more than 100 kips.
Could you please help me to respond to their argument?

 

[boo1]

GL presents a nice marine layout for lifts, see attached page 17

or check

http://dnvgl.com/oilgas/

 

[victor007]

Thank you so much "boo#1" for sharing the GL document. I am familiar with marine lift concept and its governing codes, and I've been doing this job for a quite while.
All I'm asking for is some help to build a logic to respond back to the argument made by my installation company.

 

[Duwe6]

". . . test at 1.5 x 100 Kips, the prudent value for SWL is indeed 100Kips."

Assuming that the calc's bear this out, test these padeyes at a higher value. The lowest test I've ever seen/heard of was at 1.2 x SWL. I agree that 1.5 is much more prudent, even for a 1-time use. If customer demands 100kips x dynamic factor, make them give you the value for their Dynamic Load. Only they [the rigging master] will know what their cut-off is. And then proof-test at 1.5X that value.

Sounds like they want you to 'certify' these padeyes without them being actually proof loaded at their envisioned actual load.