Difficulties With Half Skirt Calculations

Is this a solo project or is it for an organisation that has procedures, "as built" data on previous vessels, and the requirement to adhere to published standards?

• Note that the weight of the contents can vary with the cube of the diameter for a spherical vessel, and this will have an impact on the strength needed, and the thickness selected, of the components.

Thank you for your reply.

This is not a solo project and we do have to adhere to published standards (vessels will be as per ASME section VIII, PED 2014/68/EU and/or AD 2000, depending on project requirements). But this part of the design is, as far as I am aware, not set in these standards, else I would probably already have a solution by using the standards.

I do understand the weight of contents will vary, I do already have a full calculation for the skirt plates themselves based on the forces from operating, wind, earthquake and transport cases, and whichever force is highest I will use to determine the thickness of the baseplate (the bolt force I mentioned in my first post).

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Is there a Mentor at the facility that can advise - the individual that carries out the "second person check" on the calculations, according to adopted procedure?

There is and I have had some discussions with him to find a solution but the only things we can come up with are some assumptions which we ourselves can not fully substantiate and also are not fully satisfying when trying them on different designed vessels.

I was hoping one of you might have some more experience in this or some ingenious ideas. ;-)

Out of my area of expertise, but this might help...

https://cr4.globalspec.com/thread/73238/Seek-help-and-advise-for-open-Half-Skirt-Design-support-for-Vertical-Vessel

Thank you for your input, however this does not help me in this case.

I will however look into that topic later again to share my thoughts on yhinganetone's problem.

I do not believe that you can perform a simple calculation to resolve this issue.

I believe that, unless there is a complete, verified method established by a competent engineering firm, finite element analysis should be used.

I am aware that these weakened PV skirts are a design used in the EU, I do not believe that they should be used in any earthquake prone areas. A full skirt design should be used IMHO. There is nothing about the piping interconnecting the bottom head that would prevent a full skirt design.

A company guideline probably exists for these weakened PV skirts. They should only be used for small diameter vessels with limited loadings.

A good engineering firm never lets the client place crippling limitations on design

Thank you for your input, I get what you are saying, it is more a European practice to have half skirts, large clients such as Shell do in some cases even prescribe the use of them, however we do only use the for "small" vessels (O.D. max 2m).

For these sizes they do give a stable supporting, also in earthquake prone areas. In most cases, transport loads are the highest on the skirt itself (sea transport).

Perhaps I'm being slow, but I can't visualise your vessel, hence understand the problem.

Does the drawing show a plan of the baseplates, the curved lines showing parts of the shell (or skirt)?

When you say bolt load, do you mean the maximum bolt pullout force, calculated from the operating, wind, earthquake etc forces? The bolt design will be greater, but if, as is usually the case, it's bolted down to a concrete slab, there's no need to take account of that in the baseplate design, as it just causes local compression of the baseplate material.

If all that is correct, it shouldn't be too hard to estimate the maximum bending moment in the baseplate, if necessary making very safe assumptions. Even if you overdo it a bit on thickness, it's not likely to have much of an impact on total cost.

The plan shows the top view of skirt (curved lines) on the baseplate.

And yes, the bolt load is the pullout force calculated from the operating, wind, earthquake and transport loads (in most cases). We do however put our vessels (which can often be considered small) on steel structures (part of a skid based package) and not on concrete foundations.

This was discussed in 2010 ......

https://www.eng-tips.com/viewthread.cfm?qid=270533

The design guideline is SHELL DEP S22.005.... The COADE discussion linked below suggests that they be modelled as weakened "legs" by the COADE software.

http://forums.coade.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=36902

Thanks for your replies MJCronin.

We do already see the halfskirts as legs in our calculations, but that is not the problem I am trying to solve. It is the length of material I need to take into account to determine the sectional modulus of the baseplate which is affected by the moment on the bolt by the vessel.

I do have something that might be usable and that is to see it as a round hole pattern around the vessel, which gives me the option to use the following section of the EN 1993-1-8, section 6.2.6.5 table 6.6 and figures 6.10 & 6.11 (sorry I only have a Dutch version of this standard so I put some translations on it myself...)

This would give me the following calculation:

What are your thoughts?

My view is you're overthinking this. I don't think you'd go far wrong if you took the moment arm as the perpendicular distance from the bolt to the skirt. If you want to be fancy you could write a formula for the width (in the horizontal plain) of the section as a function of distance from the bolt, and find the point of maximum bending moment. Then add a safety factor!