Tankers hauling Russian crude through Denmark’s perilous shipping straits are increasingly rejecting the services of pilots when they do so, raising the chances of an oil spill off the country’s coast.

In the three months through July, 20% of all so-called shadow-fleet tankers moving Russian oil through the Danish straits declined the help of the experts who know the local waters, according to a data compiled by Bloomberg and Danwatch, a Copenhagen-based investigative journalism nonprofit. That proportion has increased from 4% a year earlier.

Crisscrossed every day by ferries and other maritime traffic, the straits can be hard to navigate. Sandbanks abound and there are strong currents and varied water depths. The use of pilots is recommended by the International Maritime Organization and has been commonplace for decades, helping to avoid any kind of major spill in all that time.

But Moscow, denied ready access to the world’s mainstream tankers by western sanctions, is increasingly relying on a fleet of ships that are older, poorly maintained, and have undeclared owners and questionable insurance. Those vessels, which now represent by far the biggest chunk of Russian oil transportation, are the main ones refusing pilotage.