The Art of Dredging

Dredging and shipping

Rockdumping

 

Some different techniques are used for stonedumping, reflected by specialised vessels.

1. ROCKPLACING WITH A CRANE

This may be the most lowtech way of placing rocks; and it does not needs a dedicated ship. Any coaster with a crane mounted on top will do.  They have limited capabilities, cq. waterdepth, reach, etc...

 

2. SIDE STONEDUMPING

This Demands a dedicated vessel, like the SDV Pompei or Atlantis.

 

Stone are pushed overboard with lateral hydraulic slides. The ship may be either anchored with up to 4 anchors, or dynamically positioned.

These ships find extra employment in their use as platform for all kinds of marine-construction related jobs. They have been used as crane-platform, dive-support vessel, landingships, spray-pontoons, just name it.

The Jan Steen (VOA ACZ) is a hybrid vessel, equipped with both sidedump- and fallpipe capabilities.

 

3. SUBSEA ROCKPLACING

The offshore industry demanded new capabilities, for placing stones in deeper water, more accurately, for protecting the foundation of oilrigs, for covering pipelines, for assisting in subsea constructions. 

Fallpipe ships were developed who could place rock in deeper water with more precision.

Most of these ships are multi-purpose, equiped to place the rock in situ in different ways, but they have a flexible fallpipe with an ROV at the lower end for better positioning, for waterdepths to 1200 m and beyond.

  

 Fallpipe ship Nordnes with ROV deployed.

   

Stones are lowered through the fallpipe at a controlled rate, while the vessel moves along its track under dynamic control. The ROV is controlled from the ship, and can be precisely positioned above the job.

Click on the image here to see an animated overview of the whole process:

Another movie; showing the Tertnes unloading gravel to a sideway chute, covering the foundations of a platform: http://tertnes.nl/Side%20belt%201.MOV. Most of these vessels have more options to unload the stone or gravel.

 

The fallpipe installation is similar to the drill pipe on an oilrig. The whole thing may be mounted on a skid that can be moved sideways overboard (left), or it can be mounted over a moonpool (below). The operating principle is the same for both.

 

 

 

   

  

 

 

 

 

 

 

 

 

 

 

The R.O.V. is controlled from the ship, while the ship is dynamically positioned

 Backfilling a trench, to cover a pipeline:

 Nordnes loading rock in a Norwegian quarry.

 

4. SPECIAL TECHNIQUES

We will not describe here all exotic ways of placing stones subsea. See: http://www.rockdumping.eu/history.html

Just this one:

JDN converted a barge ("Boudeuse", built 2005, 3700 m3) to a stoneplacing vessel.

The ship has a special niche:

  • waterdepth up to 30 metres
  • stones can be precision placed laterally under overhanging parts of a platform
  • it is dynamically positioned

The "Boudeuse" is equipped with a lateral fallpipe. This pipe is doublewalled, and its waterdepth is partly controlled by pumping air between the walls. Lateral angle is controlled by steelwires abd winches.

See the pictures here, which are self-explaining.

    

Marc Van de Velde

 

http://www.rockdumping.eu/index.html

http://www.nordnes.nl/

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