This is all part of a Chinese navy effort to enable its most modern ships to carry out long duration operations. For the last six years the Chinese have been sending flotillas (containing landing ships, destroyers, and frigates) on 10-20 day cruises into the East China Sea and beyond. Because of this, China began sending small detachments (two warships and a supply ship) to join the anti-piracy patrol off Somalia. This means sending a new detachment every three months, on a trip that takes it across the Indian Ocean and back.
The Chinese have been working hard on how to use their new classes of supply ships. These are built to efficiently supply ships at sea. This is called underway replenishment and it means transferring fuel and other supplies to moving ships, and the Chinese were seen moving fuel to two ships at once for five years now. This requires skill and practice and the Chinese are out there obtaining both, so much so that it’s become a regular practice.
Modern at-sea replenishment methods were developed out of necessity by the United States during World War II because of a lack of sufficient forward bases in the vast Pacific. The resulting service squadrons (Servrons) became a permanent fixture in the U.S. Navy after the war. Ships now normally stay at sea for up to six months at a time, being resupplied at sea by a Servron. New technologies were developed to support the effective use of the seagoing supply service. Few other navies have been able to match this capability, mainly because of the expense of the Servron ships and the training required to do at sea replenishment. China is buying in to this capability and it enables them to keep warships at sea for long periods.