Vessel Foundation Verification

[StructuralNomad]

Dear Community Members,

I am verifying an existing vessel(horizontal) foundation for new vessel loads supplied by the vendor. The vessel has 24" diameter inlet/outlet nozzles with forces/moments along three principal axes.

Ironically, the existing foundation design does not consider the nozzle forces and moments in the design. (Only gravity, environmental, thermal load combinations are considered). I have also come across client specification which recommends fifty percent of nozzle loads to be considered in the design of saddle connection to the foundation. In my opinion the same fifty percent loads should also be considered in the verification of vessel foundations. Please correct me if my assumption is wrong.

Can you also direct me to a reference for the design of vessel foundations?

Best Regards,

 

[JStephen]

It seems to me you'd have to look at the loads in question and where they came from to make a reasonable determination.

Some specifications will require nozzles to be adequate for certain arbitrary loads as a check of the nozzle design, and I wouldn't assume those loads needed to be transferred into the foundation, as they aren't actual expected loads.

You could have nozzle loads that were balanced by other nozzle loads. For example, if you connected two nozzles on top of the vessel with a run of rigid pipe, you could develop large loads at the nozzles due to thermal differences without generating any net load that had to be transferred to the foundation.

You could calculate the estimated settlement of the vessel and foundation, apply that deflection to the attached piping, and generate large forces acting to lift the vessel. But, there wouldn't be any reason to try to design the foundation for that kind of uplift force, it would actually just result in a reduction of the settlement.

You could have overly conservative loads for various reasons. For example, piping loads may have been calculated assuming the vessel nozzle was "fixed", and be much reduced if any flexibility in the system is accounted for. You could have nozzle forces that would add to the overturning moment of the vessel/foundation, when in fact that piping would help prevent overturning, and I'm not sure how that situation should be handled.

In the definition of the nozzle loads, they are sometimes located at nozzle faces, sometimes at junction of shell and nozzle. Also review the defintions to see how the pressure on the open area of the nozzle is handled, whether that is included in the nozzle force, whether the resulting axial tension in the attached pipe is included in the nozzle force, etc.

 

[StructuralNomad]

JStephen,

Thanks for your response. The forces given for design were based on the pipe stress analysis which included the vessel modelled as stiff element with spring stiffness to account for foundation resting on soil. The forces were given at the nozzle face. This being the case, the forces on the nozzle face do act on the vessel foundation.

Best Regards,

 

[SAIL3]

JS gives a good description of the assumptions used for each component. As you know this is a really grey area in how far does one take the assumed piping loads thru the vessel and fdn.IMO, it is an upper bound solution in assuming a fixed condition as far as the piping is concerned. I would tend to design the nozzles locally for the given piping loads and assume that there is enough "give" in the fdn that these consevative loads never get a chance to materialize in the fdn or the vessel as awhole. These loads can become quite large from the piping. Not entirely sure if I would look at the vessel itself and it's tie-dn for these loads, maybe in order to sleep at nite , I might use 50% of these loads at least for the tie-dn of the vessel.

 

[StructuralNomad]

Thanks for the reply SAIL3.

Agree that the extent of force transfer to the foundation is a grey area. In a new design, it is more often OK to err on the conservative side. But for verification of an existing foundation for new loads, it is a bit like clutching a straw.

In this instance the existing foundation had a bit of fat to cater for higher loads that what it was designed for.

Best Regards,