COLUMN | Fishing for metal [The Bow Wave]

I’ve written previously about what can be done with the OSVs that appear to be clogging up dock space and anchorages from Scotland to Singapore.

If the boats available have sufficient bollard pull, perhaps they could be used for collecting polymetallic nodules from the sea floor.

Polymetallic nodules are found in most oceans with the greatest concentrations found on the open seabed planes of the Eastern Pacific in an area known as the Clarion-Clipperton Zone, in depths of 4,000 to 5,000 metres.

What makes the nodules so interesting to geologists like myself is the high metal content that they contain with manganese (27 to 30 per cent), nickel (1.25 to 1.5 per cent), copper (1.0 to 1.4 per cent) and cobalt (0.2 to 0.25 per cent) as well as iron, silicon, aluminium, calcium, magnesium, potassium, titanium and barium.

The nodules themselves have formed over millions of years, with slight changes in ocean chemistry causing dissolved metal to precipitate onto the nodule “seed”, much like the crystal growth experiment most students do in high school.

After eons, large swathes of the sea floor are covered with these nodules that range in size from marbles to bowling balls, with surface textures that can be smooth or knobbly like a cauliflower. Most importantly, the nodules sit on the surface of the seabed, with no digging required to access them.

Attempts at mining have been made previously with trials conducted in the 1970s collecting 1,500 tonnes of nodules. Low metal prices and high equipment expense put a hold on the project.

Being somewhat interested in underwater mining I’m often amazed at the “bigger is better” attitude that seems to have pervaded the industry. I think that this is often driven by land based mining companies approaching offshore oil engineering firms to develop the mining techniques rather than trying to develop something internally.

The problem I have with this approach is that the offshore engineering firms are used to the mega budgets of the oil industry rather than the more modest ones of the mining guys.

The systems so far proposed all follow the basic idea of a massive FPSO-style mothership with 30+MW of power generating capability and large pump systems. The pump system is connected to an oil industry-style riser with the idea being that the minerals collected on the sea floor will be introduced into a slurry that can be pumped to the surface for dewatering. The mineral rich nodules on the seabed are collected by robotic crawlers that feed them into the pump system.

Aside from the phenomenal amounts of fuel needed to power the system, I can’t help but be sceptical of the fact that the entire operation is reliant upon a very complex system, which – if even a single point fails – will see the operation cease for several days whilst continuing to accrue significant costs.

Unlike oil companies that typically have multiple producing wells to supply cash flow in the event that one is damaged, these mining ships will be “one horse” operations that have no outside source of income.

So, getting back to nodules and unemployed OSVs, why can’t we take a leaf out of the fishing industry, specifically the scallop fishermen.

Would it be possible to tow an upsized scallop dredge, at a depth of up to 5,000 metres, behind a modified AHTS?
Assuming that the bollard pulls are sufficient to drag a decent sized dredge, the next issue is the towing cable. A metal cable will be out of the question as at the depths envisaged, a metal cable will have a maximum load of only a few tonnes.

Would a neutral buoyancy fibre rope work? The advantage of a fibre rope over a metal cable is that at any point on the metal cable, it is carrying not only the primary load, but also all the cable beneath it. A neutral buoyancy rope has an effective weight in water of zero, and so the rope is only carrying the payload.

The aft decks of AHTSs are spacious enough to accommodate spare drums of rope in the event that a breakage occurs. If the operation involved a fleet of vessels, one of them could have an ROV installed that could be sent down to retrieve lost dredges by attaching a new line. The vessel that had lost the dredge would have a spare dredge and would be able to attach to a new line and continue work with only a small interruption.

Once a dredge is full of nodules and returned to the surface the minerals need to be stored somewhere. One solution would be to open up the deck for access to product tanks. The work involved would be quite serious and, would probably exceed the cost of purchasing the vessel if it was sufficiently dated.

Another solution could be to offload to barges that are anchored around the area. Alternatively, the dredge could be attached to air-bags and simply towed along at surface level whilst another dredge is sent to the sea floor and so on until a bulker comes along to take the product.

A single bulker could stop by a number of vessels or barges taking the nodules and also providing a food and/or crew resupply to the boats.

So, when compared to a US$500+ million mining ship that takes several years to build, a fleet of 10 second hand AHTSs and a bulker could be bought for a fraction of the price, and enter service almost immediately with full redundancy assured.

Who’s with me?

Any comments, or perhaps you’ve come across something interesting? Feel free to contact me at

Andrew Baird

Experienced geologist and seabed mining entrepreneur, Andrew reviews cutting edge technology from around the world across a wide spectrum of industries, and considers their potential applications in the work boat world.