The Art of Dredging

Dredging and shipping

 

This is the first in a series of three articles on STPM:

STPM for dummies

Accuracy of STPM

STPM troubleshooting

 Both last articles will be published within a few days.

What is "STPM" ?

 "STPM" is an acronym for "Suction Tube Position Monotor".

 

 

 

 

 

 

 

 

STPM is a system that displays the position of a hopperdredger's suction pipe(s), most importantly of the draghead depth and position in relation to the ship's hull.

 

 A few specialised companies make these systems: IHC Systems, Damen Dredging, Seatools, amongst others.

 

The system works with sensors placed on the suction pipe.

 

The vertical sensors are electro-mechanical; and mounted on lower- and upperpipe.

Inside the sensorbox is a vertically suspended weight on a shaft, this shaft is connected to a synchro-meter; the position of the weight a measure for the angle of the pipe with the horizontal.

 

Horizontal sensors work much the same, placed on the hinge parts of a suction pipe.

The depth of the suction inlet is measured with a pressure sensor, close to the suction inlet.

With some goniometric hanky-panky, the position of the draghead from the ship's reference axis can be calculated. Since the position of the suction inlet to the GPS-antenna is fixed, the x, y position of the draghead, referenced to a geo-based grid is known.

If tidal height is mixed in, position is know in three coordinates, x, y, z in a geographi

STPM-systems are often interfaced with (or integrated with) dredge computers,positioning systems, and DP-computers, and are able to display suction pipes referenced to the underwater world.

 

Accuracy

Imagine you have a dredge project; deepening an access channel by a few metres, 300 meter wide, 5 km long. ...Not a large project by today's standards ...

Suppose your STPM is wrong by 30 cm (a common situation), say in reality your draghead is deeper than indicated by your STPM.

You will have to dredge an additional 450.000 m3, which will probably take more than a week with a medium dredger.

Vertical accuracy is dead important.

Horizontal accuracy of an STPM system may equally be important, especially in offshore jobs like trenching, or backfilling for a pipeline.

During a project offshore Sachalin, 2004, Gerardus Mercator was equipped with a unique device; to catch large boulders, clearing up the route of a pipeline from large boulders.

Boulder were located by multibeam scan, and this data fed into the dredge computer. "Gerardus Mercator" was able to catch any stone found; due togood horizontal accuracy of the whole system.


More on STPM accuracy in the next article of this serie.

 

 

 

Marc Van de Velde

august 2011

 

 

 

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Manu's scripts

- a sailor's fifth column