Civil Engineering Reference
In-Depth Information
into the prevailing surface sea and wind to minimize vessel roll, pitch and
heave. Satisfactory drilling and sampling can be done in seas of 1.8
-
2.4 m
(6
8 ft) height.
The supply vessel should have sufficient quarters for both the vessel and
drilling crews, and two drilling crews are provided so that work can be conducted
on a 24-hour basis once a boring is started.
Many vessels have been modified to include a centrally located drilling well
through the hull. The central location minimizes the amount of vessel motion
and provides ample work space around the drilling rig. If such a well is not
available, drilling can be done from a temporary deck cantilevered over the
side. The best location for the platform, from the standpoint of vessel motion,
is at the mid-point of the side.
-
4.2.2 Drilling Equipment and Method
Nowadays, the offshore industry has moved to using deep-water platforms
designed and built for depths approaching 150 m (500 ft); one has even been
designed for a water depth of about 300 m. Pile foundations frequently pene-
trate 90
120 m into the sea bottom. The combination of water depth and bottom
penetration means that drilling equipment for soil borings should have a depth
capability of 300 m or more.
A convenient rig has utilities that include hoisting drums and a rotary pump.
A drilling operation may be improvised from a power swivel or tongs to pro-
vide rotation, a crane to provide hoisting capacity and a pump for circulation.
Weight on the bit must be provided through drill collars. Telescoping
bumper subs are sometimes used to keep the bit on the bottom as the vessel
heaves, or a motion compensator can be used for the same purpose. A weight
indicator on the drilling line permits the driller to observe and to control the bit
weight and to prevent damage to the drill pipe.
In the early days of offshore soil exploration, it was customary to set a cas-
ing or conductor from the drilling deck into the sea floor. This provided a means
of recirculating drilling fluid and allowed repeated entry into the hole to drill
and sample by conventional land methods. However, as work moved into
deeper water, this procedure became quite slow and costly. The longer time
on site invited more weather interference. Therefore, it became desirable
to develop new and faster drilling and sampling techniques; the so-called
“
-
wire-line
”
methods were the result.
4.2.3 Wire-Line Sampling Technique
In the wire-line method, the drill pipe itself serves as the conductor; no other
pipe is used. An open-center drill bit is used on the lower end. A boring is
advanced by ordinary rotary methods; drilling fluid pumped through the drill
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