Tarmac’ Marine business owns and operates four purpose-built aggregate dredgers, the City of London and Westminster and the City of Chichester and Cardiff. They carry crews of between nine and 12, including a ship's master, deck officers, chief engineer and subordinates, able seamen and a cook. All crew members must have a variety of merchant navy qualifications. The ships regularly navigate in busy seaways and rivers and, as far as is possible, engines and machinery are maintained while the ships are operating at sea.
These ships can dredge in over 40m of water and mainly operate in the North Sea and English Channel, supplying large wharves on the River Thames and in Southampton. Typical cycle times from wharf to extraction area and back are between 24 and 36 hours, with dredging taking between three and six hours to load a cargo.
The City of Chichester and City of Cardiff are almost identical in design These ships can dredge in over 30m of water and are designed to meet the particular constraints of small wharves with shallow tidally restricted waters. Both ships typically operate on a 13 hour cycle time between the wharf, the extraction area and back, discharging their cargo at high tide. Loading of a full cargo takes between one and a half anf and three hours.
All four ships self discharge their cargoes in around two to four hours, using grabs and drag-scrapers with ship to shore conveyors to rapidly offload the cargo on to stockpiles on the wharf, ready for processing and sale.
The ships must comply with international shipping legislation, for pollution and environmental control, under the Merchant Shipping Act. The Company’s ships also operate to auditable standards set by the Maritime and Coastguard Agency (MCA) and go through regular structural surveys and dry dockings to ensure their continued seaworthiness.