Supertankers need a lot of time to stop and turn around. Similarly, shifting the world’s political stance towards Iran has taken a while to effect. And this tortuous process has given oil traders time to steer clear of the crude from one of Opec’s (usually) biggest producers.By the end of this year, Iran should add around 325,000 barrels of oil per day to the market, according to consultants Energy Aspects. This additional Iranian crude should soon steam its way (legally) around the world’s oceans. Given the need to move it, and other Opec oil, one might reasonably expect the supertanker business to hot up.Well, not quite. Spot day rates for very large crude carriers (VLCC) have fallen sharply this week from their highs of just a month ago. Last year was arguably a very good one for VLCC owners. Prices for shipping oil were double the five-year average.Those prices generate the kind of profits that tend to tempt greedier ship owners to buy another tanker. As a result, after five years of decline, tanker new-build orders have rebounded, and could hit historical highs this year, according to Arctic Securities. But worries about a potential tanker supply bubble have, understandably, put pause to the five-year surge in oil shipping rates.