Higher speed, DP, hull size broaden semi's ability to transport vessels

Oct. 1, 2000
Catamaran can move 30,000 ton structures

Heavyweight platforms for major projects in the southern Atlantic could soon be shipped across from distant yards via semisubmersible heavy-lift vessels curr-ently under construction in China. Cosco Guangzhou is due to complete work on two DP-2 carriers in October 2001, a contract value of $35 million. The two carriers will join the fleet of the newly formed Cosco Shipping Co., which is being marketed exclusively by Rotterdam-based NMA Maritime & Offshore Contractors. - Side view of the new Cosco semisubmersible transporter.

Construction of the vessels was intentionally delayed by a year to coincide with the oil price recovery and the anticipated surge in deepwater developments, particularly off West Africa. Increasing transocean shipping of oil and gas structures could generate a steady workload for the vessels almost immediately. Negotiations are underway for two contracts in 2002.

The vessels, to be named Dynamic Lifter and Dynamic Carrier, will each have an open-deck space of 126 meters by 32.2 meters, expandable to 40 meters width, capable of transporting cargoes of up to 16,500 tons. But they could also be configured in a catamaran arrangement to jointly transport topsides weighing up to 30,000 tons, according to NMA Director Peter Hansen.

Alternatively, the vessels can be adapted for pipe or cable laying on long-term assignments by fitting a moonpool between the web frame spacings and installing the laying equipment on deck.

The vessels were designed by NMA, in association with Dutch naval architects Vuyk Engineering, based on extensive feedback from operators, and drilling, fabrication, and engineering contractors such as Kv

Faster transit speeds

Prior to embarking on this project, NMA already managed two multi-purpose, semisubmersible transport vessels, but their transit speed was limited to a maximum of nine knots. The new vessels will be able to travel at 15 knots, while fully laden. The owners claim this speed is substantially fraster than main competitors. NMA states that a 1.0-1.5 knot increase in transit speed can cut journey times by several days, thereby improving project delivery times.

Cosco claims that unlike competitors, the two new vessels incorporate Class 2 dynamic positioning, in this case the new SSP podded diesel-electric propulsion units jointly developed by Siemens and Schottel. "The SSP propulsion enables our vessels to maintain heading even at zero speed," Hansen says. "That's important for protecting a cargo in adverse weather conditions and with the inherent redundancy of Class 2 adding to the safety." Model tests at the Marin basin in Wageningen have proven that the vessels can function with just one SSP unit and one of the two 11.5 ton bow thrusters operational.

The advantages of DP increase for topside floatovers in remoter locations such as Angola. "However, even for simple straightforward transports in DP mode," says an NMA spokesman, "we can maneuver under the heavy lift vessel's crane without the need for assisting tugs or a complicated mooring spread."

The vessels have also been designed to upgrade comfortably to Class 3 DP, including the addition of a retractable forward podded azimuth thruster, should the industry demand it, without the need for a structural overhaul.

Tests in the Marin tank showed that the vessels can travel at 15 knots when carrying their maximum deadweight of 18,000 tons, with a load line draft of 7.5 meters. At 6.5 meters draft, safe travel at 16-18 knots was shown to be feasible.

Removable stern

The main engines are situated in the fore part of the Cosco vessels generating electricity to drive the stern thrusters. All exhausts are also in the fore part. This means that the buoyancy casings at the stern (which provide stability when the vessels are submerging) can be removed to leave the cargo deck open from three sides. Consignments can then be loaded on or off at the sides or the stern via skidding or roll-on.

The removable stern casings make the vessels ideally suited for float-over installation of topsides, NMA claims. Other competing vessels, however, are configured with single screw, direct drive diesel engines based at the stern, which requires the exhaust to run through one of the two casings. That casing has to be cut away and reinstated before and after loading operations, with attendant costs and valuable time lost.

The maximum water level above the submerged deck is 9 meters. There are 44 independent ballast tanks arranged in three levels, which can be filled or emptied at a rate of up to 8,000 cu meters/hour. An air compressor system pumps in pressurized air to evacuate ballast water during de-ballasting. Fast ballasting speeds are achieved by pre de-pressurizing tanks, and with the large diameter inlet valves that have been designed for the vessel, a high capacity of water ballast intake is provided.

Hull shape

To improve motion characteristics, the Cosco vessel's hulls have been designed to slant inwards, along the lines of a yacht, as opposed to the conventional box-type shape. Double-bottom tanks and upper wing tanks allow the vertical center of gravity and therefore stability to be adjusted, depending on the load being carried. Roll motions must be allowed for in the case, for example, of a jackup with its legs sticking 400 ft into the air. These motions can be accommodated through use of seafastenings or internal braces.

A spud-can recess is built into the main deck to accommodate jackups with protruding spud-can tips. Additionally, sponsors can be attached to the vessel, increasing the beam to 40 meters, which allows for higher cargo vertical centers of gravity while still complying with stability requirements.

Permanent accommodation for 20 charter personnel is available. Additional accommodation for a further 50 or more can easily be retrofitted (pipelay welders).

Noble Denton Europe has produced three example transport manuals for NMA, one each for the transport and floatover installation of a topside, a jackup rig transport and for fully erected container crane transports.

The topside manual addresses the transport of a 14,000 tons (55 meters by 42 meters by 13 meters) topside from a Far East yard to offshore Angola. It was assumed that the topside could be skidded on and installed by the floatover method.

Hypothetical cargo

Due to the cargo's high weight and corresponding high vertical center of gravity, primarily caused by the support grillage, additional sponsons were required to provide adequate transport stability. The sponsors also helped reduce the gap between the jacket and the vessel, thus reducing any impact loads during the topside installation.

The jackup that was analyzed was a standard Le-Tourneau 116-C class. It was assumed that the rig would be floated on and off the vessel, and that it would have a leg length of 400 ft and a weight of 10,500 tons.

The transports analyzed, covered a range of typical cargoes, transportation, loading and offloading scenarios, the main purpose to highlight the Cosco vessels' versatility. The manuals contain details on the vessels, assumed voyages, loading conditions, stability, route, environmental data, motions, accelerations, seafastening, and marine operations.

During the derivation of the design seastate, the fast 15 knot transit speed of the new vessels was used to reduce voyage duration. Noble Denton was able to calculate, based on hindcast data from the 10-year return seastate, a 10 journey seastate on the reduced exposure. This reduction is justified by the vessels' ability to avoid bad weather and reduce exposure to the worst section of the route. The lower seastate reduces the vessel's motions and accelerations, and therefore also the seafastening requirements.