why tanks are hydrocaarbons storage are designed spherical shape

Everything else being equal, spheres are stronger.

pressure distribution over a sperical surface is uniform compared to other surface. - ideal from design perpective

for a given volume of fluid, the amount of material needed to build a sperical container will be the least

a spherical surface does not allow for the build-up of stress concentration

having said this most storgae tank that i've seen tend to be cylinderical - the next best option, considering problems faced while fabricating spherical containers

Ideally (in one respect), all storage vessels would be spherical (for reasons already stated).

Ideally (in another respect), all storage vessels would be cuboids, so that they would occupy less volume (outside) for the contained volume.

Cylinders (with hemispherical ends) are a good compromise.