Add another black mark to the growing list perpetrated by the dark fleet: Burning high-sulphur fuel oil without scrubbers.

Ships carrying sanctioned Iranian, Russian and Venezuelan oil are taking on the bunkers — the cheapest available on the market — and likely doing so without a system used to remove pollutants from the exhaust gas, Reuters reports.

“A lot of shadow vessels have no scrubbers but they buy high-sulphur fuel oil when they are in Russia,” a source told the newswire. “So, they are breaching the IMO’s sulphur limit.”

Scrubbers were the talk of the shipping industry in the run-up to New Year’s Day 2020 when International Maritime Organization regulations kicked in limiting sulphur dioxide emissions.

Owners could either buy more expensive low-sulphur fuel oil or keep using the cheaper high-sulphur option and install a scrubber on board.

Enforcement of the regulations was up to IMO member states, but Reuters notes Russia and Iran are providing the oil and not enforcing the rules. It said ships will not be caught unless they call in countries willing to enforce.

It added that 10 ships had been detained through May and that nine had made previous calls in Russia.

The ships involved in sanctioned oil trades are referred to as the dark or shadow fleet.

Dark fleet owners are often registered in countries with opaque corporate disclosure laws or through convoluted ownership structures.

Vessels are often older, flagged in non-traditional jurisdictions and use unconventional insurers if they are insured at all.

They also use classification societies outside the mainstream, leaving ships poorly inspected.

Those factors together led to an estimated 50 incidents including collisions and oil spills last year, according to figures from Allianz.

Fuel spreads have varied significantly since the IMO 2020 regulations kicked in, reaching $400 per tonne during 2020, before cooling.

In Singapore, the world’s largest bunkering hub, the spread between high and low-sulphur variants was $61 per tonne on Wednesday, figures from Integr8 Fuels show, with dark fleet owners also potentially avoiding the multimillion-dollar costs of the scrubber and installation.

In Fujairah, the difference was $76 per tonne and in Houston $109 per tonne.