Raising the bar for dredging
Like massive steel dinosaurs plunging their necks into the water, dredgers gather sediment from the sea- or riverbed, clearing the way for other vessels. Despite their impressive size, these machines have a weak point: their seals.
A dredger’s job is to displace sediment that would otherwise block a shipping lane or entrance to a harbour. To do this, they lower their cutter ladders—which can measure up to 50 meters long and weigh up to 2,000 metric tons—into the sea. The cutter head then digs into the seabed, the pumps kick into action, and water rushes onto the ship. Tons of sediment accumulate in the collection tank.
The dredge pump’s shaft is subjected to enormous circumferential speeds and water pressure that is far greater than on normal ships. And the shaft of the cutter head experiences vibrations that can be so extreme that they cause the entire machine to tremble violently, creaking under the strain.
As each part of the ladder experiences a different force, there is no single solution that works for the whole ladder. Moreover, proper care must be taken to ensure that water contaminated with sediment doesn’t eat right through the sealing rings.
Keep on dredging
A failed seal can have disastrous consequences—particularly if water breaches the ladder’s gear housing. This can cause total failure, which in turn results in huge financial losses. Indeed, dredging companies are paid per cubic meter of dredged material and have to meet strict deadlines. It, therefore, costs a significant sum of money if a pump is out of action—and that’s without taking repairs into account.
Luckily, there are ways to keep pumps running almost entirely uninterrupted whilst extending their lifespan. On the cutter head shaft, the right seal can reduce friction, keep the bearing lubricated, and prevent water from infiltrating the system.
For example, when it comes to protecting the equipment in the machine room, deploying two bulkhead seals is an effective solution—one on the pump shaft and one on the cutter head shaft. This is necessary as the two shafts have completely different diameters: up to 400mm and up to 800mm respectively. The seals absorb all radial and axial movements, and the cutter head shaft is equipped with a plain bearing to dampen vibrations. Furthermore, they are made up of several sealing rings—like all suction dredger seals—and are designed to prevent premature wear.
A watertight solution
Of course, it is not only vital to keep the pump and the cutter head in working order, but also the vessel itself. This is where shaft seals come into play. A stern tube seal has to deliver maximum performance. The sealing rings are lubricated constantly, preventing ageing and wear as well as reducing friction to a level that is gentle on materials.
Modern seals lubricate the stern tube in a way that is both effective and environmentally friendly.
In sandy, coastal waters, grease would be a good lubricant, as it repels dirt particles and keeps sediment out of the seal. However, grease-lubricated seals are typically not as environmentally
friendly as some water- or oil-lubricated stern tube seal solutions. As dredgers operate in coastal areas, which are particularly sensitive and protected by organizations like the EPA, it is especially important to find a reliable, environmentally friendly solution. Modern seals accomplish this by separating seawater and lubricant.
Just like protecting ships, protecting the environment is achieved one small step at a time.
“Seals are a dredger’s Achilles’ heel. But with optimal tribological properties, they can extend a vessel’s lifespan considerably.”
Source: Raising the bar for dredging