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Saturday, 13 February 2010 |
 Floating docks have been around for many years, serving ship repair yards by acting as a platform capable of lifting ships out of the water simply by pumping out their tanks to gain buoyancy. Acting on the same principle, the submersible barge was produced; a large, flat- bottomed structure
fitted with many tanks which could be flooded down to enable other craft
or heavy lifts to be floated into position.
Then the barge would be
pumped out to regain its buoyancy and lift the cargo right out of the
water.
This was seen as a major development in the shipment of heavy
“indivisible” loads, making it possible to lift weights that no crane or
derrick could ever have lifted. Dredging plant, damaged ships, offshore
equipment and floating installations could be moved around the world in
safety, without the need to disassemble them for shipment.
From the submersible barge which would be moved around by powerful tugs,
it was but a short step to outfitting such a craft with its own
propulsion machinery, and building on a bow to protect the load while
under way. The “dockship” had arrived. It was also possible, rather than
offering merely a flat deck for the load, to provide further weather
protection by the addition of side caissons.
Some dockships have been constructed from new , while others have been
converted from tankers or bulkers, effectively removing a section of the
ship from the weather deck downwards to a massively reinforced deck
upon which cargo will rest. They have been particularly successful in
penetrating the offshore industry, moving semi-submersible drilling rigs
and other enormous structures around the world safely and more speedily
than the alternative of towing. Dredging equipment, itself very heavy
and often awkward, is another major customer of this heavy lift sector.
The fact that such sea transport is available also makes it possible for
construction of rigs, platforms and other similar equipment to be split
between construction sites, often in different countries, with
steelwork being undertaken in one and fitting out in another. This is a
very specialist business, as these ships have to accommodate loads which
are often bigger than the ship itself, with each voyage having to be
carefully planned to optimise stability and minimise stresses. Great
care must be taken with the securing of these massive loads on deck, and
the ballasting and deballasting operation.
One specialist sector employing dockships deals with the transport of
yachts, which tend to be moved seasonally between the Mediterranean and
Caribbean. The ship floods down, large numbers of yachts are floated
over the deck and “grounded” as the ship rises out of the water.
Source: BIMCO Seascapes
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