Imagine the “mouth” of a huge vacuum cleaner connected to the end of each suction pipe of a trailing suction hopper dredger (TSHD). That’s what a draghead is. The draghead collects soil from the bottom, after which it is transported to the hopper of the TSHD. As the first point of contact with the soil, a draghead is of vital importance to the entire dredging process.
Dragheads come in many forms and can be fitted with tools such as water jets, blades and rippers.
In its simplest form, a draghead consists of a fixed part that is connected to the suction pipe. Mounted to the fixed part is a visor that is dragged across the bottom, which can either be fixed or self-adjusting depending on the requirements. A draghead must be able to extract the maximum amount of soil within the shortest possible time and at minimal cost. Based on a number of considerations, including the soil that will be encountered, the vessel size and power, the expert level of the crew and required production rates, the shape and size of the draghead will differ.
Main draghead types and their applications
There are two main working principles in draghead technology: erosion and excavation.
- Erosion dragheads erosion collect the soil by simply sucking up water, taking along the loose particles. These dragheads are especially suited to dredge loose materials, such as coarse and medium fine sand, gravel and silt and they are therefore mostly used in maintenance and aggregates dredging. Their simple design makes them easy to operate and maintain.
- Excavation dragheads scrape the soil of the bottom, mostly by means of one or more rows of cutting teeth. Water jets can be incorporated as a very effective way to loosen and liquefy compact materials. This type of draghead perform well in most soil types, including silt, sand, gravel and can even be used to dredge clay. They are fairly easy to operate and maintain, although they are more prone to wear than erosion dragheads. Debris can get stuck between the cutting teeth causing downtime to clean the draghead.
- Modular designs: in addition to the two main types, there is the option of a draghead with a modular design. These dragheads can be adapted to suit every requirement without much delay.
Size does matter
In dredging with THSDs, the following applies: the wider the visor of the draghead, the more material that can be collected in the same amount of time. In order to achieve the highest possible production rate, the widest possible draghead should be selected, taking into account other operational factors, such as pump power, penetration depth and trailing speed.
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