If the earth is flat nothing can appear behind the horizon. The oil rig is behind the horizon therefore the earth is not flat.
Flat earthers will invent many new, never before heard of, physics explanations for this. The reality is that none actually works. Perspective/compression/convergence cannot obstruct or rearrange the ordering of things. Refraction works in the opposite direction, just ask them for a model of the refraction in conditions that are possible and common, they won’t.
Some even claim diffraction causes the blockage and refer to “angular resolution” and “vanishing point”. A reduction in angular resolution does make things disappear, it just makes fine details unresolvable. “Valishing point” is an artistic term. There isn’t an actual vanishing point, it’s that things are too small to resolve different objects. The relative positions do not change due to these effects.
This is the predicted view from 1.1 foot elevation on flat earth. Yet this is never seen.
Watch my short video covering this here: