With the offshore oil exploration moving farther away from the coast the designers of offshore support vessels (OSV) are being faced with new challenges, as those vessels must now be capable of efficiently sail farther away at high speeds and also remain in dynamic positioning (DP) for longer periods in harsher seas.
Historically, the propulsion system is optimized to one of these conditions - free sailing and DP - while the performance in the remaining condition being consequential. We then have OSVs being fitted with either one of the two main propulsion systems widely known in the market: either mechanical or diesel-electric propulsion, the former being optimal for free sailing or traction (bollard-pull) and the latter being the preferred choice for vessels that must remain in dynamic positioning for extended periods of time. However, the new far away offshore fields have complicated this fixed scenario as vessels fitted with mechanical propulsion are now in DP condition for longer periods while diesel-electric vessels are required to steam at high speeds for distances that get longer every day. In both cases, their propulsion systems are operating far from their optimal point during extended periods of time, which causes excessive fuel consumption and increases the wear and tear of the equipment. The latter can be a major problem in regions where repair shipyards and dry docks are in short supply.
The so-called " hybrid propulsion system" is still somewhat unknown and has relatively few applications on the commercial maritime transportation when compared to the most utilized propulsion systems - the already mentioned mechanical and diesel-electric systems. Also known as Diesel-Electric-Mechanic (or by the acronym DEM) this system can be shown to be a fairly good alternative for vessels with an operational profile which alternates between high and low propulsive power demand, for vessels which need very high redundancy in the propulsion system but would not benefit from a fully diesel-electric propulsion, and for vessels that may have more than one operational profile.
This paper has the purpose of introducing the hybrid system, as well as describing its main benefits for the offshore designer and operator.
The search for the most efficient propulsion system for a given vessel at a given mission is not an easy task. The problem becomes more difficult when the vessel in question has two (or more) very different operational modes, and when the option for higher efficiency in one of the modes will necessarily cause a lower efficiency on the other.
The proposal of the most efficient propulsion configuration is a task usually given to the designer, who then in its turn selects the configuration that is more adequate to the dominating operational profile, while the remaining condition will be dealt with less efficiently. This is the kind of situation from which there is no escape, for the design of a ship is always balanced between conditions and compromises.
This compromise can be clearly seen on the design of a modern Platform Supply Vessel (or PSV). The use of the diesel-electric solution introduces losses mainly related to the multiple energy conversion process due to the power transmission (mechanical energy to electrical then mechanical again), and these total losses amount to approximately 10% of the generated power. It means that, whenever the vessel is steaming at service speed, 10% of the generated energy is being wasted on energy conversions. But this energy expense is seen as an acceptable side effect of a vessel which uses its main propulsion system for dynamic positioning, which must have a very high degree of redundancy. The lower steaming efficiency is a side effect.