Tarmac Marine owns and operates four purpose-built aggregate dredgers, the City of London and Westminster built in 1989/90 and the City of Chichester and Cardiff built in 1997. All four ships were designed by the company with the help of the ship yard and are well suited to the requirements of the business.
They carry crews of between 9 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.
The City of London carries cargoes of up to 4800 tonnes and its sister ship, the City of Westminster, carries cargoes of over 5100 tonnes. These ships can dredge in over 40 m of water and are capable of landing one million tonnes per year each. They 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 3 and 6 hours to load a cargo.
The City of Chichester and City of Cardiff are almost identical in design and carry cargoes of up to 2200 tonnes. These ships can dredge in over 30 m of water and are each capable of landing almost one million tonnes per year. They were 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 1.5 and 3 hours.
All four ships self discharge their cargoes in around 2-4 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.