Authorities intensified their search Thursday for three crewmembers of a dredger that collided with an oil tanker in Singapore waters the previous day.

Shipping lanes around Singapore are among the world’s busiest. The navigational challenges include a narrow strait, frequent limited visibility and thousands of vessels sailing through daily.

“It’s what we’ll call a chokepoint, but also one which is characterized by dense sea traffic,” said Collin Koh, a maritime research fellow at the S. Rajaratnam School of International Studies.

When visibility is poor, “crews on watch stations may miss the other vessels, especially when you’re talking about smaller ones coming your way, or right in your path,” he said.