Rock drag head
Normally, dredging of hard rock is for Cutter Suction Dredgers (CSD) as they can generate higher normal and cutting forces than a TSHD and create their own - relatively - smooth cutting face. Nevertheless there are economical and operational limits that are reached sooner by CSD's than TSHD's:
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workability in swell is significantly higher for TSHD's. Some jobs in exposed areas are even impossible for a CSD's
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small layerthickness: for small layerthicknesses, the amount of overcutting by CSD will be very important whereas this will be lower for a TSHD
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high mobilisation cost for a CSD compared to a TSHD
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when distance from dredging area to disposal area is long
With this in mind a hard rock draghead for TSHD was developed as it would generate an important economical and operational advantage as well as making some impossible projects feasible.
Design
To be able to dredge hard rock with a TSHD, the problems of low downforce and the roughness of the bottom need to be solved. A low downforce means low penetration forces and low production rate. A rough bottom means very high peak values of pulling force on the draghead and dredging equipment resulting in structural damage.
This has been solved by using the milling principle. As for the standard dredging teeth, point attack picks are fitted on the rock head by a bolt-on unique adaptor system. This gives the possibility to swap between different types of picks and replace worn or damaged adaptors easily.