Question:
A vertical vessel supported on a skirt at a specific elevation, which currently results in a shear load of 83,786 daN. However, when the skirt elevation is reduced by 200 mm, the shear load increases significantly to 282,726 daN, which appears to be unrealistic and unexpected.
In contrast, when a similar 200 mm elevation reduction is applied to a bracket-supported vessel, the shear load remains largely unchanged, unlike the behavior observed in the skirt-supported model.
Why?
Answer:
Wind loading:
If one was to considering wind load instead of seismic. The reason for the difference in results would be due to the difference in modeling.
In a vertical vessel on skirt, only the part above the skirt is subject to the wind; increasing the height above the skirt will increase the wind load.
In a vertical vessel on brackets, both parts (above and below the consoles) are taken into account in the wind load; moving the brackets without changing the overall height has little effect on the wind load.
Seismic loading:
It is not easy to compare the vessel on skirt with the vessel on brackets because the modeling of the upper part is very different, particularly because of the additional mass added to the top of the skirt (due to the mass of the part under the skirt).
This mass does not exist in the vessel on brackets because the mass of the lower part is directly absorbed by the brackets (therefore not in the equivalent beam model).
Furthermore, in the earthquake calculation, the loads depend on the delta factor related to the distribution of masses along the height as a function of displacements. Changing the position of the skirt changed the displacements (because the external loads also moved in the beam model).
The delta factor increased, which leads to a significant increase in horizontal loads due to the earthquake and therefore in shear at the base.
If you remove/modify the external loads, this will change the shear.
(11715123)