The nation’s marine aggregate needs are satisfied by a fleet of 27 purpose-built marine aggregate dredging vessels, operating around the clock, 360 days-a-year. The ships are predominantly registered in the UK and have a replacement cost of between £25 and £50 million each.
At the heart of the dredging process are powerful electric pumps which, on large vessels, are capable of drawing up to 2,600 tonnes of sand and gravel an hour from water depths of up to 60m.
Two types of dredging technique are employed:
- Static dredging involves a vessel anchoring over a deposit and is effective in working thick, localised reserves
- Trailer dredging requires the dredger to trail its pipe along the seabed at speeds of up to 1.5 knots, and is ideal for working more evenly distributed deposits.
In some cases, the vessel will retain all the sediment dredged as an “as dredged” cargo. On other occasions, vessels may process the dredged sediment using a technique termed ‘screening’. This allows the ratio of sand to gravel retained onboard to be modified from the in-situ composition of the resource on the seabed. Where a cargo with a higher gravel content is required, the dredged material passes over a mesh screen located in a screening tower or box, before entering the cargo hopper. A proportion of the water and finer sediment falls through the screens and is returned to sea, while the coarser sediment is retained. This process can also be reversed to load sand-only cargoes.
While the environmental implications have to be carefully considered, screening allows more marginal resources to be worked efficiently. This extends their lifetime, thereby reducing the need for new dredging areas. Screening also enables the industry to deliver cargoes to the specification required by the construction industry.
Once the aggregate dredgers reach the wharf they are able to self-discharge a dry cargo using a variety of techniques, including bucket wheels, scrapers and wire-hoisted grabs.
Click on the /images below to see how the dredging and discharge processes
work and how a dredger delivers its load for coastal protection projects.