Iranian oil exports have risen sharply in September in defiance of U.S. sanctions, three assessments based on tanker tracking showed, throwing a lifeline to the Islamic Republic and its collapsing economy.

Exports have shrunk from over 2.5 million bbl/d since the U.S. withdrew from a nuclear deal with Iran and reimposed sanctions in 2018. Still, Iran has been working to get around the measures and keep exports flowing.

Data from TankerTrackers and two other firms, who asked not to be named, indicated exports are rising in September, although the figures fall into a wide range of between 400,000 bbl/d and 1.5 million bbl/d, reflecting the difficulty in tracking the shipments.

"Exports are way up right now. We are seeing close to 1.5 million bbl/d in both crude and condensate so far this month," Samir Madani, co-founder of TankerTrackers, told Reuters. "These are levels we haven't seen in a year and a half."

The amount was twice that of August, TankerTrackers data showed, and around 11% of it was ultra light crude, known as condensate. Reuters could not independently verify the data. The range for August's exports is 300,000 bbl/d to 750,000 bbl/d.

Data from TankerTrackers, which tracks shipments and oil storage, showed almost half of Iranian exports were picked up by foreign vessels via ship-to-ship transfers, making it difficult to determine final destinations.

The two other firms also see an increase in September, although not to the same extent.

One of them said it was seeing an increase of at least 100,000 bbl/d in September to 400,000 bbl/d. That is a sizeable volume compared with a low point in May 2020 when exports fell to 100,000 bbl/d according to tracking firm Kpler.

Iran's oil ministry did not respond to a Reuters request for comment on export levels.

Oil Minister Bijan Zanganeh defended last week his efforts to boost exports of crude despite U.S. sanctions and said oil documents were forged to hide the origin of Iranian cargoes.

"What we export is not under Iran's name. The documents are changed over and over, as well as specifications," he was quoted as saying in parliament by state oil company NIOC's website.

Hard to Track

Satellite data has made the task easier, but tracking tankers is both art and science and estimates of how much oil a country such as Iran exports often differ.

In October 2018, when the sanctions were reimposed, initial estimates of Iran's crude exports also varied by more than 1 million bpd as ships became harder to locate.

Adjustments to tanker schedules and week-by-week variations complicate assessments. Since the U.S. reimposed sanctions, Iran has stopped providing production figures to OPEC, making it harder to assess its output.

Tankers loading Iranian crude sometimes switch off their AIS signal, an automatic tracking system used on ships, and switch it back on at a later stage of their journey, oil industry sources say, making tracking them harder. Iran has declined to comment on this.

Refinitiv Eikon, which shows Iranian exports of crude and condensate at around 383,000 bbl/d in August to destinations including China, Turkey and the United Arab Emirates, has reported no crude and condensate exports so far in September.

With the sanctions reducing sales, Iran has been storing unsold oil in tankers at sea. While onshore storage has increased, the volume in floating storage has fallen in 2020, suggesting Iran has found end-users for some of the oil.

The floating storage volume has fallen by about 20 million barrels, separate data provided by OilX and Kpler showed, although each firm had a different estimate of the total.

"Floating storage decreases as more vessels discharge into China, sometimes more than a few months after they loaded," said Alex Booth, Kpler's head of market research.