Pull-In Winch Support Frame

[Ussuri]

I am currently designing a steel support frame for a pull-in winch. The winch will be used to pull a 100 Tonne caisson into a production platform. The winch itself is vertically spooling so needs to be positioned directly above an opening. Hence a steel grillage is required to support it.

The pull-in condition is a temporary load case with the pull-in operation taking no more than 5 hours.

I am designing the steel using BS5950 (which is not really suitable but its the code I know best) but I have a problem with the partial factors of safety. BS5950 applied a factor of safety of 1.6 for imposed/crane loads. This is giving me a maximum moment of 1500kNm and a max shear of 1000kN. The associated stress in the member is about 180N/mm2. I could look at in terms of working stress, but I am not that up to speed with the approach (I'm an ultimate limit state man).

I would like to reduce this factor of safety in order to reduce the section size but I cannot find any guidance on a suitable figure to use for this exceedingly temporary condition.

Any suggestions or comments?

 

[mrMikee]

A factor of safety of 1.6 is about what a building code would specify for a once in 50 year occurance. In your case you know what the load will be even though it will be temporary. I doubt that a building code or a steel design spec would apply to your situation however, so it might be a good idea to look for an industry spec. I would be reluctant to go with a smaller FS but maybe someone else with more specific experience with your application can give better advice.

One concern that I would like to point out though is that there can be significant side loads when using winches or fixed hoists. Whether intended or not side loads can result in very high stresses and should be accounted for.

Regards,
-Mike

 

[StephenA]

What type of winch is it. Linear, capstan, or drum? If drum, loads will be appled eccentrically as the drum fills up from side to side. Is there a significant fleet angle which may give side loading.

If I remember correctly, allowable stresses from temporary works loads can be increase by 25%, but you would need to check this.

 

[Ussuri]

It's a vertically spooling drum winch. The winch has a maximum pull of 192Te on bottom wrap reducing to 100Te on top wrap. This is with the drum holding 500m of 64mm dia wire. We are not going to be pulling that far, maybe 100m, so we are at the larger eccentricity. Fortunately though the wire passes through an opening in the skid base so the line of action of force is never outwith the footprint hence overturning isn't a problem. The fleeting angle is about 3 degrees from centre so we do have some lateral loading.

I am coming to the conclusion this whole assembly will be easier to check using an allowable stress code.