With the rig barely visible on the horizon but the Chinese ship looming close behind, the Vietnamese patrol boat, CSB-8003, blasted a two-minute recorded message in Chinese, from loudspeakers on the back of the boat. These waters belong to Vietnam, the message said, and China's placement of the rig had "hurt the feelings of the Vietnamese people."
About six hours after the encounter on July 15, one of the last in a 2-1/2-month standoff over the rig known as HD 981, China began moving the rig north toward the Chinese island of Hainan and out of waters Vietnam considers its exclusive economic zone. Three weeks later, analysts are still debating whether China, facing international pressure, blinked in its standoff with Vietnam - or whether this was just a tactical retreat before a more aggressive campaign.
While Vietnam claimed success in forcing the departure of HD 981, China National Petroleum Corp., which managed the project, said the rig had completed its exploration work and was moving as planned.
The relocation of the rig just before the approach of a typhoon in the area also prompted speculation that the storm may have forced its early departure. But the $1 billion rig, which is owned by the state-run China National Offshore Oil Corp., was moved to a spot about 60 miles southeast of Hainan Island that is also exposed to typhoons.
While the Vietnamese Coast Guard celebrated the departure of the Chinese rig, some officers said they were worried that the episode represented a more aggressive attitude by China.
"From the moment that they installed the rig near the islands, the Chinese began more and more and more attacks, in words and in actions," said Lt. Col. Tran Van Tho of the Vietnam Coast Guard as he stood smoking a cigarette on the deck of CSB-8003. "Why? It is a part of a Chinese strategy to control the sea. This is a first step to try to make a new base to expand farther south. This not only threatens Vietnam, but the Philippines and other countries. This has been organized systematically, as part of a strategy. It is not random."
Lyle J. Goldstein, an associate professor at the at United States Naval War College's China Maritime Studies Institute, said that China has long taken an assertive stance toward its claims in the South China Sea, but was now much more able to uphold them.
"If anything is changing it is that China has capabilities to enforce and explore more carefully and it has money to field the cutters - that to me is what driving the situation," he said.
Vietnam invited groups of foreign reporters to embed with its Coast Guard vessels in an effort to focus international attention on the standoff over the rig. On the water with CSB-8003, the superior numbers of the Chinese vessels were clear.
On its two-day trip from Da Nang in central Vietnam, CSB-8003 encountered some 70 Chinese vessels, including fishing boats, Coast Guard cutters, patrol ships from other Chinese maritime organizations and two vessels that the Vietnamese Coast Guard identified as Chinese Navy missile corvettes.
Vietnam says there were about four to six Chinese military vessels among the more than 100 Chinese ships that patrolled around the rig, along with the Chinese Coast Guard, other maritime agencies and dozens of fishing boats.
As recently as two years ago, many observers said China's policy in the South China Sea was dominated by an array of poorly coordinated agencies.
Some encounters showed organizational ability, as when Chinese ships harassed the Impeccable, a U.S. Navy surveillance ship, in the South China Sea in 2009. But many analysts argued that the Chinese Navy, China Marine Surveillance, the Bureau of Fisheries Administration, local governments and state-owned energy companies operated with high levels of autonomy and fueled regional tensions as they sought to increase their own influence and opportunities.
The standoff over the rig shows how things have changed. "The idea that China lacks a coherent policy, that's clearly not the case with this oil rig," said Ian Storey, a senior fellow at the Institute of Southeast Asian Studies in Singapore. "It shows a high degree of interagency coordination involving civilian maritime agencies, the People's Liberation Army and the oil companies."
Efforts to streamline China's maritime law enforcement agencies saw significant advancement last year when four of them were joined under the State Oceanic Administration to form a unified Coast Guard.
The placement of the rig indicates the will of China's leadership to push maritime claims, Storey said. "Clearly this was sanctioned at the highest level of the Chinese government," he said. "This is another indication of how Xi Jinping has very quickly consolidated his power in China and is calling the shots."
Chinese energy companies backed away from plans to explore for oil and gas in the South China Sea after Vietnamese protests in 1994 and 2009. Now it is not so hesitant. HD 981 should be seen as a starting point for future exploration, said Su Xiaohui, a researcher at the China Institute of International Studies, a research institute run by China's Ministry of Foreign Affairs.
"China is sending out a signal to the related countries that it is legal and natural for China to conduct energy exploration and development in the South China Sea," said Su.
The Chinese placement of the rig caught Vietnam off guard, and set off protests and riots targeting Chinese-owned factories in Vietnam. Factories owned by Taiwanese, Japanese, South Korean and Singaporean firms were also hit. Four Chinese workers at the Taiwanese-owned Formosa Plastics steel plant were killed by rioters in May.
The rig was first parked about 120 miles off the coast of Vietnam and 17 miles from the farthest southwest islet of the Paracels, islands held by China but claimed by Vietnam.
Both sides have exchanged accusations over who had been the aggressor in the standoff over the rig. In June, China said that over the first month of operations, Vietnamese ships had rammed Chinese ships 1,400 times. But Vietnam appears to have suffered the worst of the skirmishes at sea, with more than 30 of its vessels damaged in collisions during that same period.
The most severe clash was on May 26, when a Vietnamese fishing boat sank after a collision with a Chinese fishing boat. Video later released by Vietnam showed the much larger Chinese boat ramming the wooden-hulled Vietnamese vessel.
The movement of the rig to waters farther north will help defuse the conflict between Vietnam and China. But the broader issues over sovereignty in the South China Sea, and who has the rights to extract oil and gas in the region, remain far from resolved.
At talks among senior diplomats from the Asia-Pacific region on Saturday in Myanmar, Secretary of State John Kerry reiterated a suggestion by the United States that countries in the region refrain from taking steps that would further heighten tensions in the South China Sea, including expanding reefs into islands by artificial means to claim them as territory, a practice China has pursued.
"We need to work together to manage tensions in the South China Sea, and to manage them peacefully, and also to manage them on a basis of international law," Kerry said at the regional forum of ASEAN, the Association of Southeast Asian nations.
The proposal has been rebuffed by China, which recently announced plans to build lighthouses on five of the Paracel Islands. China and ASEAN "had the ability and wisdom to jointly protect peace and stability in the South China Sea," China's foreign minister, Wang Yi, said, according to a statement posted on the Ministry of Foreign Affairs website. In addition to China, Vietnam, Taiwan, Malaysia, Brunei and the Philippines also claim parts of the South China Sea.
China announced last month that it would place four more rigs in the South China Sea, and Vietnam's inability to block HD 981 will likely give China confidence about its ability to drill in contested locations.
"I think China feels it got its point across," said Bernard D. Cole, a retired U.S. Navy officer and a professor at the National War College. "I would not all be surprised to see them do it again."