SKIRT BASE RING ANALYSIS IN WELDED CONDITION

Dear Friends,

I have one concern on the analysis of skirt base ring in welded condition to the offshore platform by using PV Elite. Can we do the base ring analysis for wind/ seismic/blast & Transportation loads in welded condition with foundation/platform , instead of bolted with anchor bolts? If yes, than how to model the vertical vessel skirt support ring in welded configuration to foundation/ platform instead of Anchor bolted one?


In this regard, for blast load analysis in vertical vessels, using the user defined wind loads option, whether the wind profile height shall be the vessel height from the base ring or should be provided from the base elevation of the platform(in case the vessel is located at certain height on the offshore platform). However the blast load is acting on the complete platform.

Pl. put some light on the above issues.

Somesh,

PVElite does not support skirt base rings welded to steel substructure. I would model the base ring using a bolt circle diameter equal to the base ring outside diameter and a reasonable number and diameter of bolts. This would adequately represent the base ring design. I would then take the base moment and shear loads and carry out a hand calculation to check that the base ring/substructure attachment weld inertia was adequate.

The user defined wind loads option simply applies a wind (or blast) pressure to a height. If your vessel is 10 metres high and subject to a blast load of 0.25 bar then you would enter 10 metres in the "To" Height field and 0.25 bar (multiplied by shape factor) in the Pressure field. The vessel base elevation (or height above sea level) would only be used if the wind load were to be calculated in accordance with a wind design code.

Somesh,

In PV Elite there are two options for basering design and they are the Simplified Method and the Neutral Axis Shift method. For the normal scenario of concrete to steel, PV Elite can move the neutral axis utilizing "text book" methods. This action will generally (but not always) help reduce the concrete load and consequently the basering thickness, especially when the vessel has a high moment load.

The other option is the simplified method. In this case, there is no shifting of the neutral axis and the concrete strength parameters are not used in the analysis. You can create a simple model and make a few different runs, being sure to change the concrete strength values each time. You should notice that the required basering thickness does not change between each iteration.

Then of course as Stan mentioned above, you will have to do some hand calculations to insure the chosen weld size is adequate to handle all of the mechanical loadings on the attachment weld.