Civil Engineering Reference
In-Depth Information
functionality during mobilization of equipment on board the survey vessel.
The functionality checks should include, but not be limited to, the signal
response of sensors, the data-acquisition system and a wet test of essential
subsea equipment.
In-situ equipment with electronic transmission should be designed to sustain
the water pressures expected in the field.
During testing, zero readings of all sensors should be recorded before and
after each test. The specifications for in-situ test equipment are made for the
most commonly used tests. For other in-situ testing, equipment specifications
and procedures should be established prior to mobilization.
Records of experience with the use of the equipment, routines and proce-
dures for interpretation of measurements for assessment of soil parameters
should be documented and should be made available on request.
In most cases, the in-situ test tool can be inserted into the soil from the
seabed to either a pre-set depth or to refusal due to limitation in pushing
force, capacity of load sensor(s) or other factors, which is called seabed
mode, or from the bottom of the borehole to either a pre-set depth or to refusal
due to limitation in pushing force, capacity of load sensor(s) or other factors,
which is called drilling mode.
It is very important that the rig does not interact with the seabed soil in such
a way that the result of the in-situ test is influenced by its presence. Ideally, the
footprint and weight of the seabed rig should be such that the topmost soil
where the in-situ test is carried out is not disturbed or influenced by stresses
from the seabed rig.
The effect of the seabed rig on the in-situ test results may be reduced by
careful consideration of whether
the contact area (footprint) is ring- or rectangle/square-shaped, with an open
space where the in-situ tool is pushed into the seabed
●
skirts are used on the periphery of the rig to transfer forces to stiffer soil
●
the weight of the rig is balanced so that it is no larger than what is required
to provide sufficient reaction force.
●
In order to be able to evaluate any effects of the rig on the in-situ measure-
ments, any penetration into the sea bottom should be monitored, using a TV
camera mounted on the rig.
Use of a remotely operated vehicle (ROV) is a possible alternative for per-
forming shallow testing, depending on soil type and required penetration depth.
The drilling of the borehole should be carried out in such a way that the dis-
turbance to the soil below the drill bit is minimized. In order to avoid any dis-
turbed zone below the drill bit, the in-situ test tool should penetrate at least 1 m
if soil strength and density allow. The disturbed zone can be assessed by con-
tinuous CPT/CPTU (see next section) penetrated to approximately 3 m below
the drill bit.
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