What Does Elephants' Feet Mean?
Elephants' feet, named after a geometrical shape or design typical of Turkmen rugs, are the ends of the main suction pipe in cargo tanks of tankers that are prone to corrosion degradation. Elephant's foot is a characteristic buckle failure mode for metal silos and tanks which increases elastic-plastic instability at the base boundary condition. This type of buckle failure occurs under high internal pressure accompanied by axial forces in the shell structure. This is a common failure mode for tanks under seismic loading.
Elephant's foot can cause oil spills, fires, explosions, accidents and even ecological disasters after a seismic event.
Corrosionpedia Explains Elephants' Feet
Metal cylindrical shell structures such as big welded steel oil tanks are vulnerable to an elastic-plastic instability failure at the base boundary condition. This is known as elephant's foot buckling failure because of its characteristics shape. This failure can cause serious damage to the tank and silo under seismic conditions.
The buckling modes of buried cylindrical steel tanks under seismic loading are generally of two forms:
- Elephant's foot buckling - outward bulging of the shell above the base.
- Diamond shaped buckling - occurs at either the lower or the upper sections of the shell.
The buckling failure form is different depending on the duration. After five years diamond shape buckling at the upper part of the tank is seen whereas after ten years elephant's foot buckling at the base of the tank is seen.
When a large oil storage tank suffers from an earthquake, the elephant's foot buckling failure mode is prevalent. During an earthquake, the pulse and convective pressure formed by the liquid in the tank lead to a rapid increase of dynamic fluid pressure inside the tank and local yielding. Elephant's foot failure is a result of interaction by circumferential tensile stress and axial compressive stress. Elephant's foot failure is prevented by making a correction of plasticity of materials.