DISCOVER Magazine #7

Dredging for aggregates. A tough business

Published in category: Dredging & Marine Contracting
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In the world of dredging, there are few sectors as challenging as aggregates. While sand is highly abrasive, gravel is even more of a challenge, being dense, irregular in shape and unforgiving when it comes into contact with decks, hulls and equipment. Bringing it up from the seabed, depositing it in the hold and then off-loading it once back at the terminal takes a toll on a vessel, yet they can ill-afford time off work. Aggregates are a low-margin product, and the dredgers that do it need to work almost constantly to make a return.

Tarmac Marine is a subsidiary of Tarmac, the UK’s largest supplier of building materials with a history that dates back over 150 years. With over 120 quarries in its portfolio producing every conceivable type of aggregate it is not surprising that, even within the company, not everyone is aware that it also operates four trailing suction hopper dredgers.

“The City of London and the City of Westminster are sister ships,” explains Keith Marshman, Marine Fleet Manager for Tarmac. “They were delivered in 1989 and 1990 respectively. 100 metres in length and with a deadweight tonnage of 5,989 they are capable of dredging down to 40 metres. They operate in the North Sea off the coast of England and land their cargoes within Tarmac’s network of Thames wharves to supply London and its surrounding region “The City of Chichester and the City of Cardiff are very similar to each other in size and design. 72 metres by 15 metres, they can each carry 1,418m³ of material and were built in 1997. The City of Chichester operates in the English Channel, serving the south coast of England, while the City of Cardiff extracts sand on the UK west coast and delivers it to wharves in the Bristol Channel and in the North West.

All the gravel is used mainly in the production of concrete and over 90% of the total aggregate that the four vessels together extract each year is used internally within the group.

Keeping the show on the road

Given the work they do, the ships take quite a battering with both the equipment and the vessels themselves having a very high wear rate. A lot of repairs are required year-on-year, mainly to the cargo housing equipment, but also elsewhere. With the ships needing to be operational almost continuously it is essential that the repairs not only take up as little time as possible, but also that they be of the highest quality so as to avoid any unplanned downtime.

“Tarmac has had a relationship with Damen Shiprepair Amsterdam (DSAm) that stretches back 20 years or more,” continues Keith. “In recent times, currency factors meant that Tarmac had to source its repair works closer to home but, in August 2018, the City of London returned to DSAm. There she underwent a wide range of repairs. As well as general steel works, her dredging equipment had new seals, piper elbows, screening towers, a new discharge bucket and wear plates, along with many more minor repairs and servicing.

“It was a very quick turnaround, just nine days! We normally allow two weeks for these annual overhauls, so this was a great result. We are always very confident with Damen Shiprepair Amsterdam and once again they delivered on Damen’s philosophy of always doing their best to keep to their quoted timescale for a project. “We considered sending the City of Westminster to DSAm as well,” he continued, “but unfortunately they didn’t have any capacity for our particular window, and we wouldn’t use another Damen yard at short notice as we have an in-house safety audit process that we have to undertake for partner yards and there simply wasn’t time. However, we hope to send her and the others there in the future, as long as the numbers add up. For the City of Cardiff, it is a long trip to make, but in the past the whole fleet used to dock in Amsterdam, so it could happen again. We know that we can always rely on DSAm for workmanship, safety and fast delivery times.”

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