Giant transports geared for skidding, ballasting

Nov. 1, 1999
Trying to prevent cargo slamming

Oslo-based Offshore Heavy Transport (OHT) claims it is preparing to re-introduce competition into offshore heavy transport. In October, the company was due to take delivery of its first vessel, Black Marlin, which has a contract to launch a new semisubmersible drilling rig and transport it from the Far East to the Gulf of Mexico.

Black Marlin was built by China Shipbuilding Corporation of Taiwan, along with a sister ship, Blue Marlin, which is due for delivery next February. The semisubmersible ships are the largest of their type yet built. The 56,000 dwt capacity is almost 50% greater than any existing vessel.

OHT's ships were designed for the increasing size of cargoes required for the offshore sector, says Dagfinn Thorsen, Vice President of Chartering and Operations. With a free deck length of 178.2 meters, and a free deck area of 7,216 sq meters, they are suited to carrying large semisubmersible rigs, jackups, tension-leg platforms, offshore modules, floating cranes, and spar platform hulls.


A sophisticated ballast system gives great flexibility for achieving optimal sailing conditions for carrying all sorts of cargoes. There are six sets of ballast tanks along the length of the ships, three athwartships and three vertically, providing a total of 54 large tanks in the main cargo area, plus 16 smaller tanks fore and aft. The overall tank number is 70.

New features built into the ballast system include double redundant ballast lines and valves, which make it possible to pump water directly from one tank to any other. This will be particularly useful during skid-on operations to rapidly compensate for large heel moments and tides, says Thorsen.

The speed of ballasting operations is enhanced by four huge ballast pumps with a total capacity of 12,000 tons/hour. Ballasting down is further speeded up by the fact that a large number of the tanks can be flooded by direct lines to the sea, so that the ship can increase its draft by one meter in less than 20 minutes, with the deck still above water. This enables it to operate within short weather windows, particularly for mating operations, thus reducing the critical time periods.

Black Marlin is the largest heavy-lift transport vessel in the world.
Click here to enlarge image

Black Marlin's first job is to launch the fifth-generation semisubmersible drilling rig, Deepwater Nautilus, at the Hyundai yard in South Korea. The 30,000-ton rig, which has been built on land, will be jacked up, put on skid beams and transversely skidded onto the ship in what will be the most demanding operation of its kind yet carried out, Thorsen says. With the skid beams themselves standing three meters high, and the center of gravity of the rig 30 meters in the air, a very delicate ballasting procedure is being worked out to ensure the stability of the ship and its cargo during every inch of movement. The skidding procedure is expected to take about six hours. Preparation of the ship alone will take one week.

The ship will then take the rig out to sea and float it off. After a short period for sea trials and final commissioning, OHT will transport the rig to the US Gulf for the owner, R&B Falcon. Loading is scheduled for early December, and the trip around the Cape of Good Hope is expected to take 50 days. The two contracts are worth some $6 million.

OHT says Black Marlin will have an average cruising speed of 13.5 kn when carrying large cargoes. This compares with 11-12 kn for existing heavy-lift vessels, equivalent to a saving of around one week in sailing time between Korea and the US Gulf.

Cargo slamming

The hull of the OHT ships has been design ed for a sea-going ship, providing favorable motions even in heavy seas. OHT says this represents an improvement over existing vessels, which have more barge-shaped hulls, with a high breadth to depth ratio. The freeboard (distance between the sea surface and the bottom of the cargo) is 3.5-6 meters, higher than any other transportation unit could provide, Thorsen says.

The effect of these features is to minimize the slamming of waves against the hull of the cargo. Slamming, and its potential for causing damage, is a cause of concern for oil company clients. Similarly, gentle motions decrease the strains to which the cargo is exposed during a lengthy voyage, implying reduced expenditure on strengthening the hull of the structure specifically for this purpose.

The cost of OHT's two ships is $94.5 million - $52 million in equity and $42.5 million in debt - which the company is financing. OHT's leading shareholder is Jan-Erik Dyvi AS, a pioneer of heavy-lift transport in the 1980s. Thorsen himself has been in the industry since the early days, serving as manager of Seateam Heavy Lift Department and working for a short time for Dockwise, when it acquired the Seateam fleet in 1996 to become the sole heavy-lift operator.

In addition to offering two state-of-the-art vessels, Thorsen says the competition which OHT will re-introduce to the sector should benefit clients, which have encouraged the company's formation. But the market is also growing and becoming more demanding, particularly with respect to the transport of deepwater floaters.

OHT has the contract for launching a sister rig to Deepwater Nautilus for Hyundai, and also has hopes of winning the contract for a third unit, which is under construction at the yard. Also, it will be competing for the contract to transport Veba's Hanze steel and concrete gravity base substructure from Korea to the North Sea next spring, as well as the topsides which will follow a year later.