The Art of Dredging

Dredging and shipping

Subsea trenching

Cutters and trailer hopper dredgers are occasionally employed in subsea trenching, and burying cables and pipes with soil.  This slice of dredging business roots mainly in the offshore oil- and gas industry.

However: subsea trenching and burial of pipes and cables is also handled by highly specialised equipment, operated from DP-operated "motherships". This equipment uses dredging techniques -such as jetting and cuttering- to excavate soil.  

 The process of pipe- or cablelaying, in three steps:

  • Trenching: cutting a trench or ditch in the seabed
  • Laying the cable or pipe
  • Backfilling the trench, and burying the cable or pipe

Sometimes the process is reversed: a cable (or pipe) is laid bare on the seabed, and jetted down in the soil afterwards (postlowering).

Or cablelying and lowering into the soil can be done in one simultaneous process.

 

The most simple design of a post-lowering skid: this contraption is towed along a prelaid pipe or cable.

The "sword" in front is lowered around the cable, laid out on the seabed. The sword is equipped with high pressure water jets, able to liquify the soil around the cable and bury it.

The skid is lowered from a vessel, equipped with an A-frame or a crane, often swell-compensated.

 The skid -shown left- is quite primitive, as it has no own propulsion, but is rather towed by the (DP-controlled) mothership.

 

 

 Types of motherships: with A-frame at the stern (Maersk Advancer) or with swell compensated crane (Volantis); both DP-equipped.

    

 

 

 

 

 

 

 

 Beyond the abilities of above mentioned apparatus is a whole range of equipment with various types of auto-propulsions, control, and means of excavating soil to trench or bury a pipeline or cable.

Seen right: with full tracks, and swords, with jetwater.

 

 

 

Depending geotechnical conditions, jetwater may not be enough to handle soil.

Comes in cutter technology.

 These cutters are smaller than anything we know in convential cutter suction dredgers. But they have excellent  postional accuracy.

    

Conventional dredgers deployed for subsea trenching tend to have lower accuracy, especially in the horizontal plane. This lower accuracy is compensated by their massive dredging capacity.  

 

 

  The RT-1 from CTC-Marine Projects (UK) is the largest apparatus currently in the subsea contractors market.

It measures 16*13*7,5 meters, weights 160 tons, has a total installed power of 2,35 MW.

CTC claims that this device can operate in waterdephts of 500 metres (against an outer pressure of 50 bar, that is) and it should be able to dredge clay up to 40MPa.

All this equipment is extremely specialised, contrary to ordinary cutterdredgers ans tariling hopper dredgers.

Within their specialisation they are able to handle jobs currently beyond the possibilities of ordinary dredgers in terms of waterdepth, and accuracy. 

 

Marc Van de Velde

 

 

 

 

 

 

 

 

 

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Manu's scripts

- a sailor's fifth column