TSHD (Trailer Suction Hopper Dredgers)
Trailer Suction Hopper Dredgers were originally designed to operate within the civil engineering industry to extract sand and fine gravel for the aggregates industry. The units have since evolved to clearance of mud etc from harbours and channels and also are utilised to back fill trenches for the pipe laying industry. These are now becoming large sea going vessels of substantial draft and have the ability to dredge sand and gravel from a considerable depth, to approximately 100m for the largest vessels.
TSHD vessels cannot operate unless the weather and sea state is calm and slight. In order to commence dredging, the vessel slows down to under 3 knots and suction pipe(s) are lowered onto the sea bed. The spoil pumps transfer the spoil and water to the hold or hopper, where the excess water is drained off and the spoil retained in the vessel's holds. When the hold or hopper is full, the vessel sails to where discharge is required.
See below for a video of Boskalis Trailing Suction Hopper Dredger 'Queen of the Netherlands' in action.
The Trailing Suction Hopper Dredger unloads its cargo in one of the following manners -
- Pumping the sand/spoil through a pipeline, when the dump site is at some distance.
- Unloading the sand and spoil through the hopper bottom doors, when in deeper waters.
- 'Rainbowing' through a Snout on the bow, which jets water and spoil ashore or to reclaim groins. This pump ashore method is similar to a fixed fire nozzle on the ship's bow, but instead of a water column it creates a water and spoil column. Once the material is ashore, the weight of the spoil retains it there and the water runs off back into the sea.
- A discharge pipe can be connected to the bow Snout to allow discharging of the cargo to up to 5,000m ashore. The shore facility must be able to allow the water to run off.
- The cargo can also be discharged through the suction pipe, to allow back filling of ploughed trenches for oil, gas pipe lines, and cables that are required to be buried. Dredgers which are able to carry out this task are all 'DP' (Dynamic Positioning) vessels.
See below for a 3D video visualisation of Trailing Suction Hopper Dredger 'Jan de Nul' in action in the Panama Canal.