ABOARD XIANGYANGHONG 09, June 22 -- China plans to upgrade its submersible Jiaolong before 2019, scientists said Thursday.

The submersible ended its trial phase last week after completing the country's 38th and its fourth oceanic scientific expedition, which lasted four months.

It will return to east China's Qingdao on Friday aboard mothership Xiangyanghong 09.

Jiaolong made dozens of dives during the expedition and descended to nearly 6,700 meters in the Yap Trench in the west Pacific, close to its record depth of 7,062 meters in 2012.

Scientists observed and photographed large marine creatures and the seabed, and collected seawater, rocks and samples of marine life from the deep sea.

The 39-year-old Xiangyanghong 09 is also set to retire after carrying Jiaolong for hundreds of dives since 2009.

A new mothership for Jiaolong is under construction, and is scheduled to be launched around the end of 2018.