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Hydrographic Survey, pipelay and other offshore survey activities

Hydrographic Survey - Geotechnical Sampling

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 otwer, CPT and box corer
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, Mediterranean, December 2009
CPT rig, Mediterranean, December 2009.

CPT rig being deployed
CPT rig being deployed, Shtokman, Barent's Sea, October 2010

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.

Box corer
Box corer

Box core sample, Mediterranean, December 2009
Box core sample, Mediterranean, December 2009

Drop Corer
Drop corer, deployed over the stern of the vessel using an A-frame.

Core sample opened for inspection
Core sample opened for inspection
Acoustic Coring
An alternative to taking physical samples is acoustic coring.

Acoustic Corer being deployed
Acoustic Corer being deployed. Two MST beacons can be seen mounted on the top of the corer.

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