On 19 October China Coast Guard (CCG) 5402 arrived in the North Natuna Sea to relieve CCG 5302, an event I noted yesterday in a post on X:

It turns out there was more to this than met the eye, as Indonesian media has since reported that two of its ships deployed to "expel" CCG 5402 after it harassed oil and gas exploration activities in the country's Arwana Block. 

This exploration is being conducted by state-owned PT Pertamina using a Norway-flagged seismic survey ship, the MV Geo Coral.

According to a Maritime Security Agency (MSA, or "Bakamla") public affairs official, an MSA Tanjung Datu-class patrol vessel--assisted by an MSA aircraft and a Navy patrol vessel--moved yesterday to evict the ship from Indonesian waters:

"Based on this information, KN Tanjung Datu 301 moved towards the incident and detected the CCG 5402 ship at 05.30 WIB," said Rudi as quoted by ANTARA, Monday, October 21.

The ship, he continued, was detected at 125 degrees with a distance of 7.3 nautical miles (nm), entering the Indonesian continental shelf in the North Natuna Sea.

Before the eviction was carried out, he said, KN Tanjung Datu-301 communicated via radio with the Chinese ship. However, the CCG 5402 ship [insisted] that the region is part of the country's jurisdiction.

As of 05.38 WIB, KN Tanjung Datu-301 received ... assistance from the Indonesian Navy Patrol Vessel KRI Sutedi Senaputra 378 and the Indonesian Bakamla [MSA] maritime air partol aircraft.

"Together, the two Indonesian marine patrol boats carried out [shadowing] and succeeded in expelling the CCG 5402 ship out of Indonesia's jurisdiction in the North Natuna Sea," he said.

The MSA released video of the event, showing its ship challenging the CCG ship, which can be heard responding with the familiar assertion that "China has indisputable sovereignty over the islands of the South China Sea and adjacent waters": 

It's not clear exactly how Indonesia convinced the CCG ship to depart the area.

An examination of the automatic information system (AIS) tracking data provides even more context. It shows that harassment of Indonesia's survey activity began on 17 October with the arrival of CCG 5302 into the area, prompting the deployment of the KN Tanjung Datu 301 from the port of Batu Ambar the next morning. Later that day, CCG 5402 arrived, relieving CCG 5302. 

CCG 5402 closely shadowed the survey operations until the KN Tanjung Datu 301 finally arrived on scene on 21 October. Its Navy partner, KRI Sutedi Senaputra 378, was not detectable on AIS (which military ships are not required to use).

As of noon 22 October, CCG 5402 had passed out of Indonesia's exclusive economic zone into Vietnam's, and was still being closely watched by KN Tanjung Datu 301.

As an aside, new Indonesian President Prabowo Subianto was inaugurated two days ago on 20 October 2024. While the two events may be unrelated, it's worth noting that China often tests new leaders as a means of pushing the boundaries of their gray-zone aggressions.