In the living room of his house in Ho Chi Minh City's Binh Thanh District, Colonel Pham Xuan Hoa, former commander of the Navy's Brigade 171, has a large map of Vietnam with key locations in the Truong Sa (Spratly) Islands and the southern continental shelf marked in red.

An imposing and resolute figure, the retired sailor remains sharp. Though 30 years have passed, he remembers clearly every key event that occurred during his trip to survey the area for building the DK1 offshore platforms to station troops.

The DK1 rigs, short for "Scientific-Technological Economic Service Stations" in Vietnamese, are a collection of sea platforms for logistics and living facilities for the naval personnel.

When his unit was stationed in the Truong Sa Islands, every time vessels returned to base after patrolling the area from Song Tu Tay (Southwest Cay) Island in the north to An Bang (Amboyna Cay) Island in the south, Admiral Cuong would order them to make a detour through the southern Spratlys.

"Back then I didn't understand why, but later I realized his strategic vision," Hoa said.

To realize his vision, following China's invasion of Gac Ma (Johnson South) Reef in the Truong Sa Islands in March 1988, Cuong sought the construction of rigs on six submerged reefs in the continental shelf.

Of the six, Tu Chinh (Vanguard) Bank, with an area of 700 square kilometers and lying around 230 nautical miles southeast of Vung Tau, has a strategic position since it is the closest to the oil fields explored and operated by Vietnamese-Soviet company Vietsovpetro such as Thanh Long, Bach Ho and Dai Hung.

Colonel Pham Xuan Hoa, who has been involved with the construction
of Vietnamese outposts in the southern Spartly Islands since the beginning.
Photo by VnExpress/Phuoc Tuan.

In November 1988, Hoa and the deputy political officer of Brigade 171 were personally summoned by Cuong and tasked with a key mission: to survey, measure and determine the exact locations of the Ba Ke (Bombay Castle) Shoal, Tu Chinh, Phuc Tan (Prince of Wales), Phuc Nguyen (Prince Consort), Que Duong (Grainger), and Huyen Tran (Alexandra) Banks.

He told them: "You have completed the mission to defend the Truong Sa Islands, now this mission is the priority. It is very important and strategic as this area lies completely within Vietnam's continental shelf."

Not doing it would be a wrong perpetrated on the people and nation, he said.

Hoa realized that accessing and measuring the reefs would not be an easy task since all 28 of his ships were old and seen much service.

After several days of discussions the brigade's leaders selected vessels HQ713 and HQ688, their best ones, for the mission. But even they were obsolete, capable of speeds of only 8-10 nautical miles per hour, with inaccurate compasses and without a sonar or satellite navigation.

The northeast monsoon posed yet another challenge for the mission as it caused ships to drift off course, making it difficult to determine their position.

On the morning of November 6, 1988, the two ships finally left Vung Tau on a 513-nautical-mile voyage to the Truong Sa Islands.

Hoa recalled: "We thought from Truong Sa we would locate the first survey point at Da Lat (Ladd) Reef, from which we could survey and measure the coordinates of the other reefs. Without a sonar, the task force used fishing lines with lead weights to measure meter by meter."

HQ713 was damaged and had to stop for repairs, causing the mission to be four hours behind schedule. As the clock struck 10 a.m. and Hoa saw Truong Sa, the tide was low, revealing Da Lat Reef.

"So we no longer had to go to the Truong Sa Islands to determine the location of Da Lat Reef. The team members rejoiced since the mission had started smoothly."

From Da Lat Reef, the team proceeded to measure Ba Ke Shoal, moved to Que Duong, Huyen Tran, Phuc Tan, and Phuc Nguyen Banks and concluded with Tu Chinh Bank.

By November 15 the team had accurately measured 93 points on the six banks. The ships returned to Vung Tau the following day after completing the mission in just 10 days.

"The commander was thrilled and commended us," Hoa said.

A map of the reefs in the area was submitted to higher-ups, and in December 1988 defense ministry officials started holding talks with Vietsovpetro for building offshore platforms there.

Sitting across from the ministry representatives at the meeting were Ngo Thuong San, deputy general director of Vietsovpetro and former general director of the Vietnam Oil and Gas Corporation (now Vietnam Oil and Gas Group), and a team of experts in the design, construction and installation of marine structures.

"We were moved to learn about the navy soldiers’ fighting spirit and heroic sacrifice at the Gac Ma battle in March 1988," San said.

The ministry initially suggested building the platforms' foundations using the transport ministry's gravity method in which pontoons were filled with concrete and sunk to the seabed. But the company feared it would be unstable and unsuitable for permanent structures at sea because of waves, winds and currents and coral-covered beds.

Vietsovpetro was eventually chosen for the project as it had the means and long experience in building fixed offshore drilling platforms.

In January 1989, the Ministry of Transport’s Design Research Institute was tasked with building the foundations for the Phuc Tan and Ba Ke platforms using the gravity method, while Vietsovpetro was to design and construct the Tu Chinh platform's foundation using the pile foundation method. The task of designing and constructing the platforms' top parts was given to military engineers.

To build the platforms, specialized ships needed to work at sea far away from logistics bases. Furthermore, they had to design the platforms and their foundations without having seabed, meteorological or oceanographic data pertaining to the frequently storm-ravaged area.

"The mission was very difficult and challenging," San said.

He managed to get approval from the company's Soviet owners to use the specialized drills and rig bases they had besides ships NPK, Sao Mai 01 and Phu Quy for the mission.

Senior Colonel Nguyen Quy, former head of the army High Command of Engineers’ technical department, was named the commanding officer overseeing the design, manufacture and construction of the first batch of DK1 platforms.

"The government tasked the Ministry of Defense with designing the top part of the platforms because only military agencies know what living conditions soldiers need," Quy said.

The task of designing the platforms was handed to the Military Engineering Institute of Technology, while the manufacturing was done by X49 Factory in Hanoi.

The institute invited a number of agencies to make independent calculations and compare the results to ensure reliability.

Once the design of the platforms' top blocks was completed, X49 proceeded to manufacture four platform blocks weighing hundreds of tons in early 1989.

Quy, with his experience in building elevated houses on the Truong Sa Islands, trialed assembly of the platforms to ensure the components fit perfectly before having them marked and transported to Vung Tau and Ho Chi Minh City.

"How to avoid confusion? I thought about this then decided to add numbers 1, 2, 3, 4 after DK1. Later platforms were numbered similarly all the way to 20, 21."

On reaching Vung Tau and Ho Chi Minh City, the platforms were assembled under the supervision of X49 and the Military Engineering Institute of Technology.

Vanguard Bank platform

Two Vietnamese soldiers sail near the Tu Chinh 1 platform near
the Tu Chinh (Vanguard) Bank in the southern Spratly Islands in 2012.
Photo by VnExpress/Thanh Nguyen.

In June 1989, the High Command of Engineers, the Ministry of Transport and Vietsovpetro began transporting out to sea in batches the platform blocks and the equipment and materials required for building the foundations.

To reach Tu Chinh Bank where the Tu Chinh 1 platform was to be built, Vietsovpetro's three ships had to brave stormy weather for three days, causing many of the construction workers to suffer from seasickness. But once the vessels reached the bank on June 17 the engineers and workers immediately got down to work.

The engineers started by using sonar to survey a relatively flat area of seabed 14 meters deep that had been identified as the site for the first rig, DK1/1.

Unfortunately, there were three 30-meter-deep, 12-meter-wide trenches running north-south at the site, which they realized could pose a risk to the platform should it be built there.

It was then decided they needed to find a more suitable, flat area to the east of the original site. Though it was already dark, the team chose to work through the night, determined to quickly find a new location for the rigs.

And by 10 a.m. the next day Vietsovpetro managed to identify two new sites for the Tu Chinh 1 and Tu Chinh 2 platforms.

With the location finalized, the Sao Mai 01's crew proceeded to drill into the layer of coral rock on the seabed to collect samples.

The platform's foundation was then installed facing north to minimize the impacts of storms blowing from the northeast, and three steel piles were driven 10-12 meters into the coral rock with air hammers. The fourth pile however only managed to go four meters deep. When the engineers tried using another air hammer with more impulse, the pile snapped after just a few strikes.

The project's steering committee decided to cut the fourth pile at the 7.5-meter mark and weld a steel beam inside it, which enabled it to endure the air hammer's strikes, and the pile eventually went eight meters into the coral.

After calibrating the heights, the team used a crane to install the connecting frame, the platform's top block with the living quarters, a stairway, and a dock before reinforcing all four steel piles with cement.

To improve the platform's stability, the steering committee also decided to pump another 250 tons of mortar and cement into the steel tank preinstalled at the bottom of the foundation.

On June 27, 1989, the first platform in Tu Chinh Bank was ready. Located in 23 meters of water, the 25-square-meter platform stood 11 meters above the surface at high tide and was 39.5 meters tall overall.

At 3:45 p.m. the Vietnamese flag finally flew high above the Tu Chinh 1 platform amid the joy of the Vietsovpetro engineers and the commanders and officers in the military engineering unit.

According to San, in addition to waves, winds and storms, the high porosity of the corals, their brittleness and low material bondability and complex currents also posed challenges to the construction of the rig.

Vietnamese soldiers sail near Phuc Tan A platform near the Phuc Tan Bank
in the southern Spratly Islands, 2017. Photo by VnExpress/Huu Khoa.

Around the same time, the Ministry of Transport and the High Command of Engineers also completed platforms DK1/3 in Phuc Tan Bank and DK1/4 in Ba Ke Shoal. But their pontoon structure kept wobbling even after completion, forcing the soldiers stationed on them to move onto ships whenever the waves were too high and move back once the sea became calm.

Quy explained: "Theoretically, after pumping concrete into the pontoon to make the foundation heavier it cannot be toppled. However the seabed is not a football field or land; it is coral."

In July 1989 then vice chairman of the council of ministers, Tran Duc Luong, chaired a meeting to review the first phase of the DK1 construction as the head of the project's new steering committee.

At this meeting Quy spoke about the need to reinforce the foundations and provided accurate hydrological data and seabed survey results for selecting a suitable location where drilling could be done to better design and install pile foundations.

Cuong warned against using the pontoon method, saying it could endanger the soldiers’ lives. As a result, subsequent DK1 platforms were all built using Vietsovpetro's pile foundation method.

The moment the first DK1 platforms reached completion was also when foreign powers started eyeing them.

In September 1989 many foreign military ships turned up to survey the banks. The soldiers stationed at the platforms therefore had to closely monitor their activities and be combat ready while still performing their day-to-day activities in the rough conditions.

In late 1989 the Tu Chinh B (DK1/5) platform was built and became the second in Tu Chinh Bank. By the end of 1990 the Phuc Nguyen A (DK1/6) platform in Phuc Nguyen Bank was ready. To build the two, many soldiers had to walk the thin line between life and death.

A storm hit the site just as the team was starting to drive the latter platform's piles into the coral, and the installation of the four piles took 22 grueling hours of struggle instead of the normal two hours.

On October 26, 1990, a foreign ship headed straight for the construction team, forcing the naval ship escorting the team to fire warning shots and chase the intruder away.

Through a slew of difficulties, subsequent platforms continued to be built with names such as hut, bungalow, village on the sea, and the "god's eye" watching over the East Sea.

Phuc Nguyen A outpost near Phuc Nguyen Bank, 2012.
Photo by VnExpress/Thanh Nguyen.

Quy, who was involved with the construction of all platforms until DK1/16 in 1996, recounts that for later platforms the government leased a Russian ship and used ultrasound scanning to map the seabed over a diameter of one kilometer to identify relatively flat areas for construction.

Before building the foundation, the construction team would drill to make a map of the cross-section to analyze and identify points that were soft, hard or hollow.

The foundations and piles were also improved to be able to withstand strong waves and winds of up to 109 miles per hour.

Quy recalled: "At first we were very vague about rig construction, but the good thing is we learned from the experience, made upgrades, and they are now basically completed. The next generation should try to maintain them.

From the mainland, to go to Thanh Long, Bach Ho, Dai Hung and some other oil fields, we need to go through Tu Chinh. We need to keep it at any cost. If we lose Tu Chinh, we will lose everything: all the oil fields and all of DK1."

San says Tu Chinh Bank is part of a geological structure that the petroleum industry calls the "Tu Chinh-Vung May potential petroleum-containing sedimentary basin," one of the oil-containing sedimentary basins in Vietnam's continental shelf that PetroVietnam has mapped, surveyed, assessed potential reserves, and solicited foreign investment.

"DK1 platforms, in addition to their value in collecting scientific information on marine, marine economy, are also landmarks that testify to Vietnam's sovereignty over the continental shelf and its oil and gas."