Drilling for combustible ice at 2,000 meters deep sea, "Manatee II" is really awesome!

At around 23:00 on April 7, 2021, my country's first "Manatee II" large-hole deep pressure-maintaining coring drilling system, led by Hunan University of Science and Technology, was carried on the Marine Geology II research vessel and successfully drilled 231 meters deep in the South China Sea at a depth of more than 2,000 meters, obtaining combustible ice samples at this depth, refreshing the world's deep-sea drilling depth.

Why do we need to drill in the deep sea? How does the seabed drilling rig "Manatee II" work? Where is the technological breakthrough? Today, let's learn about the seabed drilling rig, a difficult and powerful tool of a great country.

How difficult is it to get underground?

The interior of the Earth consists of the crust, mantle, and core from the outside to the inside. The crust is a thin layer on the surface, and its interface with the mantle is the Moho discontinuity. Its average thickness is only 17km, which is far less than the thickness of an eggshell relative to an egg compared to the average radius of the Earth (6378km). So far, humans have never been able to break through this thickness.

The general process of drilling down on the ground is to continuously add new drill rods on the basis of existing drill rods to achieve further drilling. As the drill bit drills, the drill rod will spray high-speed water to flush the broken rock and soil to the ground, thus drilling a deep hole in the ground.

When exploring for minerals, rock and soil samples at a specific depth are required. At this time, it is necessary to first drill a hole to a position close to the required depth, then stop drilling and replace it with another drill bit - a coring drill bit. The corresponding drill rod (also called drilling tool) is called a core tube.

The typical core drilling technology now is called rope coring technology, which is to lower the core tube through the wire rope, drill the core and then retrieve it.

There are many factors that restrict existing technologies, such as:

① It is very cumbersome to constantly connect new drill pipes;

②When the temperature is high, the drill bit material may become soft, affecting the working ability of the drill bit;

③ The rock and soil pressure is high, and the deep hole will collapse if the hole wall is not protected in time;

④ The longer the drill rod, the harder it is to control. It is also easy to be twisted off or pulled off by its own gravity, making it difficult to effectively transmit the torsional force of rotary drilling;

There are many other factors that make it difficult to enter the ground. However, due to the needs of the oil industry, land drilling technology has actually become relatively mature. A land drilling rig developed in my country, the "Crust No. 1", can already achieve a drilling depth of 10,000 meters.

What's on the seabed?

We have been taught since childhood that my country has 9.6 million square kilometers of land territory. Many people actually don’t know that my country also has about 3 million square kilometers of maritime territory. At present, my country’s maritime areas specifically include 77,000 square kilometers of Bohai Sea (our inland sea), 315,000 square kilometers of Yellow Sea (adjacent to North Korea and South Korea, based on the exclusive economic zone principle, my country has jurisdiction over approximately 80% of the Yellow Sea), 508,000 square kilometers of East China Sea (adjacent to Japan, based on the exclusive economic zone and continental shelf demarcation rules, my country has jurisdiction over approximately two-thirds of the East China Sea) and approximately 2.1 million square kilometers of maritime territory demarcated by the nine-segment line in the South China Sea.

The ocean contains almost all the resources that exist on land, and also contains some resources that do not exist on land. At present, people have discovered the following six categories: ① oil and natural gas; ② solid minerals such as coal and iron; ③ seashore placers; ④ polymetallic nodules and cobalt-rich manganese crusts; ⑤ hydrothermal deposits; ⑥ combustible ice.

The seabeds of the East China Sea and the South China Sea are rich in oil and gas resources. According to estimates, the amount of combustible ice resources in the South my country Sea alone is 70 billion tons of oil equivalent, which is about half of the total amount of oil and gas resources on land in my country. Due to the harsh seabed environment, my country still lacks the technical means to conduct deep-sea resource exploration, and the investigation of deep-sea areas is still insufficient. It is still difficult for people to know how many seabed minerals there are in the vast sea.

Manatee II deep-sea drilling rig: Drilling into the sea and into the ground, breaking the record with a drilling depth of 231 meters

In order to explore deep-sea resources, my country has developed submarine drilling rigs, but submarine drilling rigs need to go into the sea and into the ground, which is easier said than done. After more than ten years of research and development, my country has gradually reached the international leading level in the field of submarine drilling rigs. In 2015, the Hainiu submarine drilling rig had achieved a 60-meter deep seabed drilling.

In order to explore the methane hydrate resources in the deep sea, in addition to deepening the drilling depth, in order to ensure that the methane hydrate in the extracted cores is still an icy solid substance, it is necessary to maintain the pressure on the sample during coring. Therefore, the Manatee II deep-sea drilling rig was further developed.

The goal of the "Manatee II" submarine large-hole deep pressure-maintaining coring drilling system is to develop a submarine large-hole deep pressure-maintaining coring drilling system with an operating water depth of not less than 2,000 meters, a drilling depth of not less than 200 meters, and a pressure-maintaining success rate of not less than 60%, which can effectively meet the exploration of submarine natural gas hydrate (combustible ice) resources in my country. The "Manatee II" drilling rig body is 7.6 meters high, 10 meters in waist circumference, weighs 12 tons, and weighs 10 tons underwater. It is currently the heaviest underwater geological exploration and scientific research equipment in my country. It looks like a behemoth, but on the seabed, "Manatee II" is as flexible as a loach. On April 7, 2021, Manatee II set a new world record with a drilling depth of 231 meters.

How can "Manatee 2" drill so deep?

In order to enable the seabed drilling rig to drill holes deeper than 200 meters, many technical problems need to be solved. The two typical problems include:

(1) Drill pipe loading, unloading and storage technology under telemetry and remote control

The main difference between deep holes and shallow holes is that more than one drill pipe is required. Many land drilling rigs have the function of automatically installing and removing drill pipes, but the operating conditions are much better, the operation process can be directly observed, and the drill pipe can be installed and removed manually if problems occur.

The automatic storage and unloading system for drill pipes and core tubes of the Hainiu (the first generation product of Hainiu II)

The Manatee II uses a turntable rack to store drill pipes and core tubes, and cooperates with a manipulator to realize the automatic installation and removal of drill pipes. The Manatee II has a large number of telemetry and remote control sensors with high accuracy, which can collect a large amount of equipment data. A deep-sea camera for monitoring has been added, and the reliability of the design has been verified.

(2) Hole alignment and rig leveling and anti-skid technology during drill pipe connection and unloading

After a deep-hole drill rig has fully drilled a core tube, it is necessary to use a wire rope to pull out all the drill rods from the borehole, replace them with another empty core tube, and then put the drill tool into the hole. This process is very time-consuming and tedious, and it tests the accuracy of the hole alignment.

Moreover, it is difficult to fix the seabed drilling rig directly on the seabed. If the drill bit slides sideways after being lifted out of the hole, it will affect the next drilling. Therefore, the deep-sea medium-deep hole drilling rigs operating on the seabed require absolutely stable support. In order to achieve this function, the design of the legs of the Manatee II drilling rig has undergone a very detailed design, and adopts a three-legged design, which can achieve leveling and stable placement on the seabed.

How to "feel good" about excavated methane hydrate - new pressure-maintaining drilling tools

All the drilling tools required for each operation of the submarine drilling rig are lowered to the seabed together with the submarine drilling rig. After each drilling, the inner tube salvaged from the bottom of the hole is first stored in the drilling tool warehouse on the submarine drilling rig body. After the entire drilling is completed, it is lifted to the deck of the mother ship together with the drilling rig.

In order to keep the excavated methane hydrate samples in their state on the seabed, their pressure needs to be maintained, so a mechanism needs to be constructed in the core tube to maintain the surrounding pressure.

By designing a pressure-maintaining rope coring drill, the sample pressure was maintained. Within 6 hours, the pressure loss rate of this drill in a high-pressure state (10~30 Mpa, corresponding to the water pressure at a depth of 1000m~3000m in the deep sea) was 10%, effectively improving the sample quality and further improving the accuracy of the evaluation of seabed natural gas hydrate reserves.

Some time ago, in order to ease the relationship with our neighbors, we advocated the joint development of resources in the relevant sea areas. However, in fact, the technical threshold for the development of deep-sea resources is very high. Without the technology, you can't do it. Without the diamond drill, you can't do the porcelain work.

It is precisely because we have powerful national equipment like the Manatee II that we are at the forefront in the exploration and utilization of deep-sea resources. As our navy advances beyond the first island chain, our marine equipment will make a significant contribution to the security of our country's resource supply in the future.

At this point, the editor is inspired to compose a seven-character poem. The Manatee II explored the sea and successfully drilled 231 meters in the South China Sea at a depth of over 2,000 meters, setting a new record for the world's deep-sea submarine drilling depth. It is:

Look at the deep waves of the South Sea, where the iron ox of the sea is digging a mysterious hole.

The top of the cliffs and the deep valleys are not to be feared, what we need is to obtain the dragon scales for the nine states!