Three billion tonnes of crude oil and refined products are shipped on huge tanker vessels worldwide every year. But recently, some of these ships have stopped. In fact, since March, many more than is usual have simply been anchored off ports like Singapore and Rotterdam in the Netherlands – and it’s not immediately clear why.

The International Energy Agency (IEA) has been monitoring the situation. It has tracked just under 94 million barrels of unrefined crude oil in what’s known as “floating storage” at the end of May. And Clarksons, a shipping analyst firm in London, says it has tracked an increase from 85 to 120 oil tankers at anchor around the world.

To measure these things, Clarksons tracks ships visually with satellite imagery and compares this to known information about how oil is being traded in the world. “It’s quite a bit of detective work trying to piece all those bits of information together,” says Simon Chattrabhuti, a market analyst at the firm.

The Clarksons figure covers a range of ships from 55,000 deadweight tonnage (dwt) to 440,000 dwt – including 55 Very Large Crude Carriers (VLCCs) of 279,000 dwt to 320,000 dwt and two Ultra Large Crude Carriers (ULCCs) of 320,000 dwt or more. The VLCC figure has risen by 18 since March.

These are big ships, and 90 million barrels is a lot of crude oil. Usually the amount stored floating is no more than about 70 million, according to the IEA. Why this increase?