Despite Amarin's headwinds (loss of patent and slow rollout/pushback in Europe), I would think it would represent a "low risk" acquisition for a large BP.
Call it $6B ($15/share). You have to figure on an absolute minimum of $500M of peak cashflow for Vascepa ($300M+ U.S., $200M+ RoW).
So even if things don't go "well", and they can't fix anything, you're looking at a reasonable (albeit low) ROI. And again, this is assuming things don't go well - we don't get France, Italy, or Germany. And no additional indications or increase revenue in U.S.
But you also have to assume that there would be a tremendous number of things they could do to maximize revenue. I see it almost as a "blue chip" acquisition, with lots of potential upside.