Offshore support vessels can be divided into the following sub-types, according to their operating functions: platform supply vessel (PSV), anchor handler tug (AHT), anchor handling tug and supply (AHTS), construction and field development vessel, remote operated vehicle (ROV) and dive support vessel, stand-by vessel and various combinations of these.
OSVs are characterised by the fact that they perform multiple tasks. They are often also designed to function in very rough weather conditions.
The speed of the vessels is usually quite low, but the short hull length means the Froude number is relatively high. The bow and bulb should be shaped to work well in heavy seas, not just in smooth conditions.
Many OSVs have a high DP classification with redundant machineries and a high emphasis on manoeuvring performance. The operating profile can have many different operation modes. Long periods are spent in DP and stand-by modes, which can represent 30–40% of the total time. This means that the vessels must be optimised for both transit speeds and various DP conditions.
Most OSVs have either twin shaft lines or twin thrusters as their main propulsion. These are complemented by a number of tunnel thrusters and retractable thrusters to give the desired manoeuvring performance and DP class. A new option is to replace the conventional propulsion solutions with a single propeller on the centre line skeg. This gives both lower resistance and highly improved propulsion efficiency for the propeller behind the skeg. This propeller is used for free running. When better manoeuvring is needed, retractable thrusters are lowered. The steerable thrusters with nozzles provide an excellent DP plot.
AHT vessels often have mechanical propulsion with twin shaft lines with nozzles to give high bollard pull capability. Dieselelectric propulsion is, however, very common today in many other OSV types. The dieselelectric power plant principle provides clear fuel benefits in part load conditions such as stand-by and DP mode. The electric operation allows optimisation of the engine load and the use of variable speed FP propellers for low fuel consumption. It also allows flexible use of the installed power for many different thrusters and other large consumers such as towing winches. The power plant can also be divided into two parts to give good redundancy.
One alternative is to combine the best features of electric and mechanical propulsion. Mechanical propulsion with low transmission losses is used in free running mode. An electric power plant consisting of generating sets and primary PTOs on the main engines power the ship in DP mode with the same benefits as a fully electric vessel.
The electric power plant can be further enhanced with the Low Loss Concept, thereby reducing transformer losses. Applying an intelligent power management system also helps keep down the power demand.
