PLATFORM TRANSPORT Ballast system lessens wave effect on heavy-lift vessel loads

Sea Teal carrying two N'Kossa modules and Cobo piles.

The move strengthens Dockwise's predominance in this sector. The company is itself the result of the 1993 merger of two of the leading players, Wijsmuller Transport and Dock Express Shipping.

The four sisterships - Swan, Swift, Tern and Teal, known as Sea Swan, etc., while under SeaTeam ownership - were built between 1981 and 1984, and can carry up to 25,000 tons of cargo on their spacious 126.8 x 31.6 meter decks.

The vessels have remained busy under their new owner. Three of the four transported offshore industry cargoes - in each case a jackup rig - on their maiden Dockwise voyages: Tern carried the four-legged jackup Interocean III from Suez to Jebel Ali, Swift the slant-leg rig Cliffs Langley from the Gulf of Mexico to Nigeria, and Teal the Aban II rig from Pipavav Bandar to Bombay.

Dagfinn Thorsen, manager of the heavy-lift department at SeaTeam until the sale took effect, was sorry to see the vessels go. They offer significant advantages to offshore customers, he said, not least due to their sophisticated ballasting system involving more than 50 tanks. This can be used to increase the natural period of roll well beyond the wave period - for a wave period of 15 seconds, the ballasting system would be set to give the ship a 40-second roll period. As a result the effect of the wave energy on both the ship and its cargo is greatly reduced.

Depending on the cargo, the system can offer substantial savings to customers. For example, a module transported on a towed barge might well have to be built with significantly more steel to withstand the greater sea forces. According to Torsen, Statoil claimed to have saved 600 tons of steel in the construction of one of the Sleipner A modules which was transported by the Tern from the fabrication yard to the platform assembly site. That not only decreases the fabrication cost of the module, but by reducing the topside weight, also makes for savings in the support structure.

This ballasting system is almost unique to the Swan Class - the only vessels which share it are Dockwise's Mighty Servant and Super Servant ships.

Cargo can be loaded by either skidding or floating it on board - the Swan Class vessels can ballast down so that the decks are submerged to a depth of 7.5 meters to allow cargo, whether items loaded on a barge, or floating units such as jackup or semisubmersible rigs, to be floated over the deck. The ballasting operation during loading is a complex one, Torsen said, with three factors - ship, cargo and tide - to be carefully balanced.

One of the last jobs carried out by SeaTeam was the transport of platform parts for some of Elf's West African developments from the McDermott-ETPM yard in Abu Dhabi. The 3,000-ton Cobo jacket, already loaded on its launch barge, was carried on the Sea Tern, along with the N'Kossa flare boom, while the two N'Kossa topside modules, each weighing 1,500 tons, traveled on the Sea Teal, along with the Cobo piles.

Dockwise has maintained continuity in the operation of the Swan Class vessels by leaving ship management in the hands of Scotland-based Acomarit. The vessels, which retain their Russian crews, have been re-flagged to the Netherlands Antilles registry.

The new owner is keen to stress the bulk liquid carrying capacity of the vessels. Each can load up to 30,000 tons of clean petroleum products, such as naphtha, kerosene and gas oil, or other liquid cargoes such as vegetable oil and molasses. In February the Tern was taking a cargo of molasses from Guatemala to the US.

But it is the offshore market which continues to provide most work for both the Swan and the Mighty Servant class vessels. Early in the year the Swan was transporting the Cliffs la Salle jackup to Qatar, while the Swift was carrying modules for McDermott-ETPM from New Orleans to Bombay, and Mighty Servant 3 was bringing the Neddrill Trigon jackup back to the North Sea from Argentina.

The Mighty Servant 2 was due shortly to mobilize to Norway to transport the hull of the Njord semisubmersible production platform from Verdal to Stord.

Dockwise has also booked a number of repeat orders, including the transport of the Genesis Spar hull from Finland to the Gulf of Mexico on behalf of Aker Rauma Offshore, for which it transported the Neptune Spar over the same route in 1996.