Dredging areas are held under a commercial agreement (known as a Production Agreement) issued by The Crown Estate, who own the seabed out to 12 nautical miles and the mineral rights to the limit of UK territorial waters. Crown Estate Production Agreements are issued subject to the granting of a Marine Licence obtained by the Company from the Government following a rigorous environmental assessment process.
Two types of dredging technique are employed:
- Static dredging is where the vessel anchors over a deposit and is effective in working thick, localised reserves.
- Trailer dredging, which requires the dredger to trail its pipe along the seabed at speeds of up to 1.5 knots, is ideal for working more evenly distributed deposits. Dredging is undertaken to a high degree of accuracy with reference to high-resolution seismic data and seabed core samples. The dredger’s position and tracks are displayed in real-time together with geological and licence boundaries on the ship's navigation computer to ensure that only the best quality resources are dredged.
The vessel's activity is constantly recorded by a Crown Estate Electronic Monitoring System linked to the ships navigation system and dredge gear sensors. This records when and where the ship is dredging to ensure compliance with licence conditions
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.
British Marine Aggregate Producers Association: