For more
about hydrographic survey operation click here.
For pictures of platforms
and other offshore installations click here.
An example of part of the development of one field, Tyrihans, click here. For more pictures of
offshore survey and construction vessels see here.
For an online surveyor this kind of work does not usually present any
challenges - merely getting the vessel into the correct position
and taking position fixes at the sample location using
transponders mounted on the rigs. Occasionally there is more involved -
usually the determination of sinkage and / or slope gradients from
altimeters on the rig.
Communication with geotechnical equipment whilst it is subsea varies
from non-exisitent through acoustic modems and umbilicals.
An
integral part of any seabed survey is often a geotechnical sampling
campaign. Samples may include vibrocores (VC), Cone penetrometer (CPT),
box corer, gravity corer and, in some cases, drilling. The information
and samples collected during these tests is used to determine the
properties of the seabed. These data may then be used to decide
whether the seabed can support a given structure, whether or not a
cable or pipe can be trenched, design of piles and much more. Sampling
locations are often picked from Sub Bottom Profiler or Side
Scan Sonar records at
points of interest. These points may be picked at interesting features
(in order to "ground truth" interpretation) or at the sites
of planned installations.
Geobay
equipped with drill tower, CPT rig and box corer, Kirkenes, September
2010
During cable and pipeline
route surveys it is often practical to mobilise the
geotechnical spread at the same time as the rest of the survey spread.
There are a number of advantages to doing this and it is popular to do
this when designing cable or pipe routes "on the fly".
Cone
Penetrometer
A cone is pushed into the seabed and the force (pressure) required to
do this is measured. No sample as such is recovered.
The depth to which the cone will be pushed is usually between five and
15 metres. Depending on the type of CPT rig this may affect the size of
the rig and the ease of deployment.
CPT rig being deployed, Shtokman, Barent's Sea, October 2010
Vibrocore
Sometimes an independent rig and sometimes part of a combined vibrocore
/ CPT rig. A sample tube is pushed (vibrated) into the seabed and
recovered. Once on deck the sample is cut up into sections
(usually 1m lengths) and stored. Various tests may be performed on the
samples - basic tests ane often conducted onboard and laboratory
testing may be done ashore.
Gravity Corer, Box corer and similar
There are various types of pipelay vessel - some operate in "DP" using
thrusters and propellors to keep themselves in position. Others use
anchors - and during such a pipelay there can be an amazing number of
vessels involved. The barge itself, anchor handling vessels, pipe
carriers (to re-supply the barge) and lay support vessels.
