I think the oil fire in Sebastopol, is to deny fuel to the Russian navy. Otherwise the navy could interfere in ground operations in Crimea.
So, I think the attack will be to the south. Get near to the coast and then start bombing The kerch bridge.
They will take the shortest route to the coast which is Melitopol. If the Russians perform badly the Ukranians might get to the east of Berdyansk.
The road from Mariupol, runs north to heavily defended Donetsk so the Ukranians will stop west of Mariupol and then form a line back to Ukranian territory. Such a line will 1,000k long and hard to defend.
With the bridge destroyed the Ukranians will threaten Sebastopol and at that point offer to negotiate.