Double DOP solution for trencher

On a cable burying project on the west coast of Denmark, two DOP submersible dredge pumps type DOP250 are being used. Two subsea high voltage cables from the Thor offshore wind farm – Denmark’s largest offshore wind location to date –  reach land on a beach. In the surf zone the cables are to be buried to approx. 3 m below the sand bed. A special trencher was used to carry out the precision dredging required.

Intertidal trencher with DOP pumps

Jan De Nul, a major maritime construction company, created a tandem of special trenchers for the job. The challenge is that the equipment has to work in an intertidal nearshore zone at varying water depths up to 5 m.

The trencher is a Starfish Hitachi 890ZX excavator with jet skid. They look like the tracks of a standard excavator with a raised pedestal for the control cabin and the drives. This set-up has been specifically designed for these challenging circumstances.

Jet sword

To actually bury the high voltage cables a few metres below the surf zone, a trench is to be created in the loose sand of the seabed.  This trench is made by a jet sword, which is handled by the Starfish excavator. This jet sword blows the sand away using  a constant flow of pressurised water supplied by the DOP pumps.

The two DOP250 submersible dredge pumps have been fitted at the side of the Starfish. The reason for choosing dredge pumps is obvious: the surrounding waters contain a high percentage of abrasive sand, and the DOP pumps can withstand this wear and tear. The hydraulically operated slurry pumps work in series, supplying the jet sword with sea water at approx. 10 bar pressure.

Thor offshore windfarm project

The construction of the windfarm started back in 2021, and is now well on its way. The 72 offshore wind turbines with a capacity of more than 1 GW will be fully operational in 2027, when the wind farm will produce green energy for over a million Danish households.