| Q.16 |
Would rotating pipe on board makes vessel rotate
together?
Does Chikyu actually use special techniques to react the torque? |
 |
Mr. Eigo Miyazaki
Technology Dept. |
|
As per your question, drill bit and pipe will be received frictional force
of geological layer when rotating drill pipe. This force is the torque
making Chikyu rotate in reverse. The question means what technique does
Chikyu employ to react the torque, but I would conclude first, there is
not such special technique.
”Top drive”
drilling equipment onboard to rotate drill pipe, has the torque about 17.4
t-m in
maximum. On the other hand, the force by wind, wave or current are made
vessel rotate, it will be the external force during drilling operation.
DPS (Dynamic
Positioning System) controls 6 azimuth thrusters to keep the position and direction. Chikyu has capability to keep its position
if it may receive such external force from the bow side within 30 degree.
The momentum by 23m/s wind, which is the maximum condition on normal drilling
operation, is about 1,700 t-m (torque). However only one of thrusters under
bow side has the 5,600 t-m (torque) in maximum. Therefore you can find
the frictional force by drilling is several orders of magnitude smaller
than the force to keep Chikyu position.
You may wonder how we can drill 7,000 meter using such a little power.
Despite of relatively slow speed as a rotating
machinery, 2 or 3 turns per second, the hardworking efforts quietly but diligently through 24 hours
and hidden technology inside the drill bit take us over to the deep Earth. |
| Q.15 |
I understand that deep drilling will require a lot of time at one
place, and D/V CHIKYU will not move during the operation. Food could be supplied by helicopter,
but how will D/V CHIKYU be refueled? Will a tanker come to supply? |
 |
Mr. Tashiro
provides shore-based supports for the operation of D/V CHIKYU |
|
As you know, it will take 3 or 4 months
up to more than 1 year to reach drilling depths of 5-7 thousand meters when D/V CHIKYU drills
using her riser system so that she will have to stay in one place on the sea. Therefore we have to carry
fuel, food, equipment, part etc. as well as conduct crew shift exchanges.
Alternation of crew; The crew and drilling technicians will rotate every 4 weeks.
However, it is difficult to maintain the operation if all of them alternate at once, so, in fact, crew
change will take place every 2 weeks. Crew will be transported by helicopter as it is very dangerous
to conduct ship to ship boarding.
Material supplies; Large working ships will be used as a supply boats to carry
fuel, supplies, chemicals, parts and food. These vessels will also bring drilling cuttings (rock pieces
filtered from recycled drilling mud) back to shore. The supply boats will stay in one position relative
to D/V CHIKYU by using a similar Dynamic Positioning System, and materials will be moved to D/V
CHIKYU using the deck cranes. |
| Q.14 |
Is there a possibility that mantle rocks could melt in the bottom of the
hole? If you drill from the seafloor to the mantle, decompression of the deep rocks would take place
as rocks are removed and the deep area exposed, could such decompression cause melting of mantle peridotite? |
 |
Dr. Hideki Masago
Science Planning Department |
|
As you pointed out, decompression at constant
temperature can cause melting (adiabatic melting). However, because we drill slowly and use drilling
fluid that equalizes temperatures, thermal equilibrium would be faster than any decompression melting
due to removal of overburden.
Basically because
1) the area affected is very small,
2) the decompression is very slow and
3) drilling fluid is being constantly circulating,
adiabatic/decompression melting is very unlikely to result from drilling.
|
| Q.13 |
I heard that every vessel has a vessel number in addition to the vessel
name.
What is the vessel number of D/V CHIKYU ? |
 |
Mr. Yoshio Ishozaki
Technology Dept. |
|
D/V CHIKYU’s vessel number
is “136960”.The "vessel number" is a number used to distinguish
each vessel whose gross tonnage exceeds 20 tons in Japan. This number is legally given by a public office
having jurisdiction over the registered domicile port of the vessel.
D/V CHIKYU is a Japanese vessel so that the "vessel number" has been assigned by the Kanto District Transport Bureau that has jurisdiction over Kanagawa Prefecture, Yokosuka City, which is the official anchorage for D/V CHIKYU.
Anyone can find the "vessel number" in the public office in case of D/V CHIKYU , the
Kanto District Transport Bureau that has jurisdiction over the vessel’s home port.
|
| Q.12 |
Isn’t Mantle a magma that melts like a mud? |
 |
Dr. Natsue Abe
Institute for Research on Earth Evolution / Research Program for Deep Subsurface
Environment
*She is not a CDEX staff but comes here to give us the answer
as a specialist for mantle research |
|
 |
| Photo(1) |
|
 |
| Photo(2) |
 |
| I study the core samples during the IODP expeditions. |
|
Mantle is a solid rock, and not magma(liquid). It is thought, based on seismic and other geophysical
information, as well as limited exposures of rocks thought to represent uplifted mantle rocks, that mantle
down to depths of about 600km under the sea floor is composed of a rock called Peridotite that is primarily
composed of the mineral Olivine (photograph (1)).Olivine in its pure form has considered a gemstone called
Peridot that has been prized for thousands of years. Below 600 km depth, the crystal structure of olivine
changes due to the extreme pressures, but the overall composition (magnesium and iron silicate) remains
the same.
Examination of the chemical composition and petrology of peridotite has revealed
that peridotite can be melted (both in the laboratory and in nature) and that if 5-25% of the original
peridotite is melted, the result is basaltic magma. Mantle is thought to circulate or flow inside the
earth, even it is solid rock, giving rise to the common misconception that "Mantle = magma".
Then, why does mantle (solid) flow?Solid material under great pressure can flow given enough time.
One example for illustration is the flow of glaciers. Glaciers are made up of solid ice, but they flow
very slowly as a result of the pressure produced by the weight of the glacier on the ice at the base
of the glacier. Mantle rocks flow and circulate inside the earth similar to the way a glacier flows.
This peridotite rises to the deep sea floor as a result of mantle convection under Mid-Ocean Ridges.
Partial melting of Peridotite creates basaltic magma in small batches through either temperature increases
or pressure decreases or a combination of both. The magma (liquid) reaches the sea surface sooner than
peridotite because buoyancy of the magma (liquid) is greater than the peridotite (solid). The magma (liquid)
erupts as lava, and cools to become volcanic rock (basalt) that makes up a large percentage of the oceanic
crust.
Nobody has obtained direct samples of mantle material even though the structure of the earth is roughly
understood through indirect, geophysical means and through theoretical modeling. In order to understand
the inside of the earth in detail and to test the hypotheses put forward based on geophysics and modeling,
it is necessary to use D/V CHIKYU and drill into the sea floor, to collect in situ samples of
the rocks that comprise the upper mantle.
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