Trenching head
With the booming installation of windfarms in the Northern seas, a lot of trenches for cable laying need to be dug into - mostly - hard clayey soils with boulders. Trenches for cables do not need to be wide so the goal is to make them as narrow as possible as this reduces the surface of disturbed soil, the volume to be dredged and the volume to be backfilled over the cable.
Knowing that the soil in the Northern seas is often stiff clay with boulders, dredging in this soil type means a lot of trouble during dredging and backfilling as the head and the hopper will get blocked with clay.
To solve those problems, the trenching head was developed. This was a very challenging project where the head had to combine some crucial elements:
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a very narrow head
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shape of the fixed part that increases the trackkeeping
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cut the soil and sidecast it directly next to the trench, where it will form a ridge that will be used for backfilling of the trench after cable lay
Design
Designing and building a trenching head that will make a very narrow and straight trench in hard clay with boulders and create a ridge with the excavated soil next to the trench was very challenging. The first type of trenching head has proven it was possible. Crucial elements in the design are:
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very narrow draghead design
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design of the visor that makes it possible to directly sidecast the cut soil next to the trench by using the reverse flow mode of the dredgepump
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design of the head that minimizes the risk of blocking with clay or boulders
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shape of the fixed part that increases trackkeeping
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possibility to control side of sidecasting from the bridge
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special teeth to increase cutting depth