Environmental Engineering Reference
In-Depth Information
For these reasons, correction factors have been proposed for the strength parameters and
cone geometry. For equal end area cones only
q
t
is normalized. It is noted that cone resist-
ance
q
c
is corrected to total cone resistance
q
t
as follows (Robertson, 1990):
q
t
q
c
(1-
a
)
u
(2.4)
where
u
is the pore pressure measured between the cone tip and the friction sleeve and
a
is the net area ratio.
New classification charts have been proposed based on normalized data (Robertson, 1990).
Operations Offshore
General
Offshore shallow-water investigations, such as for ports and harbors, normally involve
water depths of 3 to 30 m. Jack-up rigs or spud barges are used and the cone is pushed
from the vessel by conventional methods. A drill casing is lowered to the seafloor to pro-
vide lateral support for the CPT rods.
For offshore deep-water exploration, such as for oil-production platforms, the CPT is
usually operated in conjunction with wire-line drilling techniques
(
Section 2.3.5)
,
with
equipment mounted on large vessels such as shown in
Figure 2.42.
The major problem,
maintaining adequate thrust reaction from a vessel subjected to sea swells, can be over-
come by a motion compensator and the drill string. Thrust reaction can be provided by
weighted frames set on the seafloor as shown in Figure 2.42.
Seafloor Reaction Systems
Underwater cone penetrometer rigs that operate from the seafloor have been developed by
several firms. The Fugro-McClelland system, called “Seaclam,” operates in water depths up
to 300 m. A hydraulic jacking system, mounted in a ballasted frame with a reentry funnel,
is lowered to the seabed
(
Figure 2.43
).
Drilling proceeds through the Seaclam and when
sampling or testing is desired, a hydraulic pipe clamp grips the drill string to provide a
reaction force of up to 20 tons. A string of steel rods, on which the electric friction cone is
mounted, is pushed hydraulically at a constant rate of penetration. Data are transmitted
digitally to the drill ship.
ConeTec have developed an underwater CPT that can operate in water depths up to
2500 ft that can penetrate to 30 ft below the mudline. It is lowered over the side of a steel
vessel and set on the seafloor. Fugro also has an underwater ground surface CPT which
presently has a penetration of about 6 m.
Sampling and
In Situ
Testing
The various underwater sampling and
in situ
tools using the Fugro Seaclam are illustrated
in Figure 2.43. Some sampling and testing is obtained by free-falling down the drill pipe.
Fugro have developed the “Dolphin” system for piston sampling,
in situ
CPT, and vane
shear testing to water depths of at least 3000 m. Tools that require controlled thrust for
operation, such as the cone penetrometer and piston sampler, employ a mud-powered
thruster assembly at the base of the drill string. Vane testing and push sampling do not
require the use of the thruster.
The Dolphin cone penetrometer is illustrated in
Figure 2.44,
and a plot of cone resistance
data is shown in
Figure 2.45.
Penetration distances are 3 m or may refuse penetration at
less than 3 m, and tip resistance and side friction are recorded. Drilling advances the hole
to the next test depth and the CPT repeated. The remote vane shear device is shown in
