Efficient hull designs from Rolls-Royce save fuel and cut emissions

January, 2008

The hullforms of offshore supply vessels are influenced by many factors that have to be balanced if the vessel is to give above average performance.

One area in which Rolls-Royce supply vessels excel is in reducing the amount of fuel burnt per deadweight tonne of cargo transported at a given speed. This is a very good way of assessing the transport efficiency of a load-carrying ship, because measures that reduce the amount of fuel used at a given speed to transport a given amount of cargo not only represent fuel cost savings but are key elements in reducing emissions of CO2 and other gases.

It is inevitable in the process of converting the energy in fuel to movement of supplies that energy losses will occur. A medium speed diesel engine may have an thermal efficiency of 40-45 per cent. The power output has to be transmitted to the propulsors, and here there is an energy loss of a few per cent in the case of mechanical transmission and up to 10-15 per cent in a diesel electric system. Although all of the systems, and the propellers themselves, are under constant development and show efficiency improvements year by year, it is in the hull itself that the greatest gains can be made.

Caption: The amount of fuel consumed in carrying one deadweight tonne one nautical mile at a given speed is a good way of assessing PSV transport efficiency. New generation UT-series vessels are much more efficient than conventional vessels.

The resistance experienced by a hull moving through water has a number of components. Naval architects have spent several hundred years improving the understanding of these components. Part of the resistance is caused by friction, and depends on the wetted surface area and the smoothness of the hull shell.

A major part of the vessel’s resistance is the result of wave-making, which is dependent on several factors, including shape and the Froude number, or relationship between speed and length. Then there is eddy drag and the resistance of propeller shafts, rudders, A-brackets and other appendages. Real offshore vessels operate in waves from different directions apart from the head-seas condition, and these cause additional resistance, hindering the vessel’s progress. The designer’s job is to minimise the total of these components for the operating profile of the vessel.

The UT-Design team at Rolls-Royce has focused on reducing hull resistance, bringing together years of experience from offshore service vessels, the latest computerised design tools, verification using large-scale models in test tanks, and input from the joint Rolls-Royce and university research covering performance in a seaway. This work has paid off, and in the latest generation of UT hulls resistance has been reduced by up to 30 per cent compared with average industry designs, and offshore supply vessels with these new hullforms can maintain their efficiency over a wide range of speeds, loads and sea conditions.

Caption: This efficient UT 776 E diesel electric supply vessel is typical of the latest generation of Rolls-Royce designs.

Clearly, offshore supply vessels must be able to survive extreme weather conditions, but their daily work typically takes place in less severe conditions. Offshore vessels need to combine stability and reduced motions with low resistance whilst in transit in order to be effective tools.
Tough North Sea conditions were the cradle of UT-Designs. While always addressing the need for seaworthiness, UT-Designs are tailored to an owner’s requirements, for example powerful anchorhandlers for work in large significant wave heights, or vessels optimised for the more benign weather conditions general in some other parts of the world.

Logistics

Once an offshore vessel is delivered, the charterer can strongly influence its actual emissions by the way in which it is required to operate. Over-specifying supply boats can cost money – and increase emissions – because the vessel spends too much time away from its design point.
Charterers with supply vessels on long-term contracts can help to cut CO2 and other emissions by optimising their logistics – for example, working supply boats at somewhere near full deadweight as much of the time as possible, and specifying a service speed close to reality. Some are very good at this, others less so.

Another factor is the time spent in dynamic positioning (DP)mode at a platform. Rolls-Royce designs, with their optimum thruster location and an efficient DP controller, minimise the power required. Even so, holding position against wind, waves and current takes energy, and with it the need to use fuel, and emissions-conscious charterers can help by cutting the time spent handling supplies at a rig as much as possible.

For further information and high resolution images, please contact

Arnfinn Ingjerd, Vice President, Communications - Marine
Rolls-Royce plc
E-mail: arnfinn.ingjerd@rolls-royce.com
Phone: +47 70 01 42 17
Fax: +47 70 01 40 05