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Riser drilling technology is used for oil field offshore drilling
(Figure 1). It consists of Riser Pipe (Photograph 1) of a large di-ameter,
that connects the Drill Ship on the sea and the seabed,the Blow Out
Preventer (BOP: Photograph 2) that prevents blow out from abnormally
high-pressure layers under the seabed,and the Drill Pipe (Photograph
3) to drill into the strata. Special fluid called "Mud" that is physically
and chemically adjusted is pum-ped into Drill Pipe (injecting flow)
by pumps on the ship, and the fluid is sent to the bottom of the pipe.
The fluid is blown out from Core Bit at the front edge of the Drill
Pipe, and returned through a gap between well bore and Drill Pipe
within the inside of the borehole and also through the gap between
Riser Pipe and the Drill Pipe (returning flow) when returned from
the seabed.
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This is called mud circulation applied in drilling to prevent the well bore
from collapse and to maintain the pressure balance within the borehole.
It is a significant key to drill into strata sev-eral thousands meters under
seabed.
The Riser Pipe is a steel pipe of a diameter of approximately 50 cm, and
is equipped with attached line and floating buoyant ma-terial (partially).
The floating buoyant material acts to reduce weight in water.
This Drill Ship is equipped with a Riser Pipe of a total length of 2,500m
that is hanged from Derrick 100m high (tower at the center of Drill Ship),
and it weighs up to 3,500 tones in air (weight in water is 900 tones) including
BOP instal-led on seabed. This Riser Pipe receives, in addition to selfweight,
other forces such as bending, tension or compression caused by motion of
the vessel. Therefore, in order to provide sufficient strength against these
forces, high-tension steel will be used. Strength of the whole Riser is
calculated by computer simulations. Moreover, an actual Riser model of 1/100
scale (to-tal length: 25m) was prepared, and the simulation itself is being
evaluated, applying vibration with a vibrating machine.
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BOP is provided at the bottom end of the Riser Pipe
on seabed. This is a safety unit that closes the borehole to prevent
any fluid (liquid such as oil or water, gas such as natural gas or
hydrogen sulfide) contained in the strata from rising up to Drill
Ship through Drill Pipe or Riser Pipe when the Drill Pipe reaches
ab-normally high-pressure strata. This unit in several layers of total
height of up to more than ten meters. |
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