Rolls-Royce wins first contract for innovative product tanker design

Friday, 20 February 2004

NVC-Design, part of Rolls-Royce, has won its first contract for an innovative product tanker design.

The concept, developed over the last three years, combines a tailor-made hull design with a diesel mechanical propulsion system based on twin Ulstein Aquamaster Azipull azimuth thrusters with pulling propellers. This design has been given the type number NVC 604 PT.

The 4,200dwt vessel has been contracted by the Norwegian owner Bergen Tankers AS, will be built by RMK Marine shipyard in Turkey, and is scheduled for delivery in September 2005. Rolls-Royce will deliver the design and the Azipull thrusters, worth about $2 million. Further negotiations may result in additional Rolls-Royce equipment being specified.

The vessel will operate along the Norwegian coast on long term charter to an oil major. Many ports are difficult to access, and good manoeuvring characteristics had high priority through the project phase.

This experienced owner had very specific requirements, but the NVC-Design concept using Azipull propulsion covers a range of product tankers for coastal waters and worldwide trading from 3,000-20,000dwt, the upper limit depending on the required speed and ice class.

Among the many advantages of the concept is an increase in cargo space compared to a conventional single screw product tanker, since the azimuth thruster system allows the engine room bulkhead to be moved further aft. Depending on the size of vessel, the increase in cargo volume is 5-7%. This provides a short payback time on the slightly increased construction cost compared with a basic single screw ship.

By using two independent propulsion lines comprising thruster, shaft generator and main engine, a high level of redundancy is achieved. It also means that maintenance work can be carried out on one engine when the vessel is working cargo in port, since the other unit, assisted by the tunnel bow thruster, is immediately available to take the vessel to safety should an emergency arise at the terminal. Propulsion system redundancy also increases the level of safety in coastal navigation.

In developing the hull form, Rolls-Royce had to ensure that the ship would have good course keeping ability. In the past, hulls with length to beam ratios desirable for low first cost and high earning capacity tended to have poor course stability when propelled by azimuth thrusters, needing large steering corrections resulting in heavier fuel consumption.

Choice of Azipulls for propulsion helps greatly, as these thrusters with their hydrodynamically optimised underwater units provide substantial rudder area. Even so, a major programme of computational design and model testing was undertaken to ensure that the result would be a tanker with course stability well in excess of the IMO zig-zag test requirements.

The hull is double, meeting the new regulations, and incorporates a bulbous bow, plus a short skeg for docking purposes. Azipull propulsors are positioned below headboxes and allow the aft end of the hull to be optimised for minimum resistance and simplified hull construction.

Each independent propulsion system comprises a main engine driving an Azipull thruster equipped with a CP propeller, with a generator interposed in the shaft between engine and thruster. The simple mechanical layout offers flexibility and efficiency. Transmission losses are much lower than a diesel electric drive and the system is less demanding to operate.

When one engine is shut down in port, the other is declutched from its thruster and drives its generator supplying power for the hydraulic deepwell cargo pump powerpacks and the vessel's hotel load.

The design ordered by Bergen Tankers has a cargo hold volume of 5,200m3 split between 10 tanks. These lie within the double hull which has an overall length of fractionally under 90m and a moulded breadth of 16.5m; the summer draft is 6.0m. Cargo heating is by stainless steel coils through which thermal oil is circulated. Each tank has its own cargo pump and the total discharge rate is 1,200m3/hr.

Each main engine develops about 1,400kW at 900rpm, coupled to a 1,000kW generator and an AZP85 Azipull thruster. Allowing for 15% sea margin and hotel load on the shaft generator the service speed at 90% MCR will be 13.3 knots, with a corresponding fuel consumption of about 8 tonnes/24 hrs. A 500kW tunnel bow thruster is electrically driven and the shaft generators are backed up by two 350kW gensets and a small harbour/emergency set.