LONDON—Up to 50 oil tankers are waiting to unload cargo in the port of Rotterdam, the highest number since 2009 and another sign that, amid a glut, crude is struggling to find a home.

The unusually high number of ships idling outside of Europe’s busiest port comes as onshore tanks around the world brim with crude and products like fuel or jet oil. That has sent buyers and sellers scrambling to find storage, using tankers and even empty railcars to stash the surplus.

The glut is affecting shipping in other ways, as producers send ships carrying oil and oil products from the Middle East on longer routes to Europe to give their cargoes more time to find buyers. That is bad news for the oil market, adding further pressure on a commodity that has fallen to around $30 from more than $100 a barrel in 2014. Brent crude, the global benchmark, was trading at about $36.50 a barrel on Monday in London.

There are currently between 40 and 50 tankers anchored in the waters outside Rotterdam, more than twice the usual number, said Sjaak Poppe, spokesman for the Dutch port.

“Storage in the Rotterdam area is nearing its limits,” he said.