The bottom of the deep sea, and even further down where a subterranean world spreads deep beneath the seafloor, is a world of high pressures. When sediment is collected with the usual methods and samples are brought to the surface, the pressure drops to levels far removed from their natural state and often they don’t retain their original form. In order to get raw data from this subterranean world, the need was felt for a device to collect samples which preserve the high pressure. This device is the Hybrid Pressure Core Sampler, the development and operation of which we have been successful in.
（Published online March 2013）
| Interviewee: Yasuhiko Mizuguchi Engineering Group, Operations Department, Center for Deep Earth Exploration (CDEX), Japan Agency for Marine-Earth Science and Technology |
The subterranean world, a world of high pressures
Water pressure is the pressure exerted in the water by the weight of the water. For subseafloor areas in the deep sea the weight of the layers of earth are further added to that. The pressure generated by this is several ten times or even several hundred times the level it is on the surface. The subterranean world spreading beneath the deep sea is a world of high pressures.
A variety of things is happening in the subterranean world. Various microorganisms slowly grow, and gasses dissolved in groundwater move to the cold surface of the sea. Sometimes soft underground mud gushes out like a volcano, sending out methane with the mud (mud volcanoes). These subterranean cycles are deeply intriguing, but we have very little concrete knowledge about them. This is because even if we can drill down, no device was in place that collects samples while preserving their high subterranean pressure.
Yasuhiko Mizuguchi of the Center for Deep Earth Exploration (CDEX) tells, “The Hybrid Pressure Core Sampler can go up and down in the drill pipes used by the CHIKYU, and can collect samples that hold their high underground pressure.” A drill pipe used for deep sea drilling is cylindrical and hollow, with a drill bit digging through sediments attached at its end. When it digs through the subseafloor sediments, the center is dug out and samples are contained in a plastic sample container (the inner tube). The sediments dug out in this cylindrical shape are called cores. By bringing up the container with the sample through the drill pipe, sediments can be collected.
The problem is that when the sample is recovered on board ship, the structure of the sediment sometimes, depending on the circumstances, falls completely apart. For instance, methane hydrate retains its solid form under high pressures and low temperatures, but when it is brought up from the seafloor and the pressure drops, it reverts to its gaseous state of methane gas and starts to foam violently. The structure of the sediment is destroyed or even pushed out, sometimes resulting in an empty sample container. To avoid this, the excavated sediment must be tightly sealed to preserve the high pressure of its original location.
The Hybrid Pressure Core Sampler
Building a new device or system is not just a technical matter however. Engineers, who know in what way this equipment is used on site, need to design it with an understanding of its operational aspects as well. This is the work that Mizuguchi was engaged in. Firstly, the Pressure Core Sampler must be able to go up and down in the drill pipes used by the CHIKYU. Then the material also needs to be able to maintain high pressures. An existing system was miniaturized to fit in the pipes, and steel was used for the tube holding the sediment. A ball valve is used to seal off the top and bottom of the tube to preserve the pressure. Explains Mizuguchi, “There are also methods where the tube is closed by sliding on a cover, but these can’t be checked prior to use, on land. So you can’t really call it a device for research purposes. The sliding cover method also poses obstacles to experiments that relate to its structure. The weaknesses and limitations of this device have to be taken into account.” The ball valve system that was employed uses a metal ball with a hole that seals the pipe by rotating 90 degrees.
The series of movements in the Hybrid Pressure Core Sampler are mechanically operated during the process where it is brought up on board ship through the drill pipe. “In the improved Pressure System the valve is now closed with a spring. Some methods use things like links for its operation, but operating it simply, in a mechanical manner, has the benefit of few breakdowns,” explains Mizuguchi. When the Hybrid Pressure Core Sampler is brought up in the drill pipe, first the seal at the top (sealing cover) is closed, and then the ball valve at the bottom closes. The sealing off itself is now complete, but then a nitrogen gas valve placed at the top opens for a moment, and, just as if a piston is pushed in, the pressure on the inside is increased. By increasing the internal pressure a little the valve is pressed outwards from the inside, thus sealing it even more securely.
（Please see the mechanism by clicking the arrow below）
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