Ships’ generators have a critical function to perform. They provide the electrical power for a wide variety of appliances and uses on a vessel, from the cargo cranes to the galley ovens. It is essential that the generators can meet the power demand even when the demand is at its highest. Demand is typically highest during cargo and ballasting operations or during mooring operations when the bow thrusters and winches are in use and under load. There are two obvious outcomes when the generators cannot supply the demand. One is that the generators trip on overload, leading to a potential black out situation. The other is the pre-emptive action by the crew to reduce the power demand of one system to allow for the increase in another. An example of the latter is a container ship carrying a number of refrigerated (or reefer) containers. Whilst on board and under the care of the vessel, the reefer containers are connected to the ship’s electrical power supply. There have recently been occasions where some, if not all, of the reefer containers have been disconnected from the ship’s supply prior to arrival at a port in order to allow for enough electrical capacity to run the mooring equipment and the bow thruster. The reefer boxes remain off-power until after the vessel is secured alongside and the electrical supply is reconnected. Although being switched off for a short period of time should not cause damage to most refrigerated cargoes, this step should be unnecessary. Some cargoes are very temperature sensitive, such as medical/ pharmaceutical material, and if damaged the losses can be costly.
There are two factors that influence this reduced capacity to cope.
Adding Non-design Demand!
A shipowner may wish to increase the number of reefer boxes that a container ship can carry. This might look like a simple modification that requires the installation of some additional reefer sockets on deck and perhaps upgrading some junction boxes and electrical breakers, but it must also be considered if the ship’s generators can comfortably meet the increased demand. When deciding on modifications to the vessel and its systems, the impact of the increased electrical demand must be borne in mind – this especially applies to system redundancy. If the greater demand for power means that all of the ship’s generators need to be run all of the time, then there will be little flexibility if one generator fails or requires maintenance.
Reduced Capacity of the Generators!
A more common occurrence affecting the ability to meet the electrical demand is when one or more of the generators are out of service or they are no longer able to run at their design capacity. In both cases, the root cause is generally related to poor standards of maintenance. Generators can and do fail in service. The vast majority of generator sets at sea comprise of a diesel engine, often referred to as an auxiliary engine, directly coupled to an alternator. The alternator is quite robust with fewer moving parts and is a lot less likely to fail compared with its diesel prime mover. The same level of care has to be given to these engines as to the main propulsion engine with operation and maintenance.
Look After Your Generator!
The chances of a generator failing can be greatly reduced if it is properly maintained. Repairs and maintenance should be in accordance with the manufacturer’s instructions, carried out by competent personnel and use licensed parts. The impact of an engine breakdown on passage can be mitigated by having adequate spare parts on board. This allows the crew to quickly rectify the problem. The performance, and therefore the capacity of a generator can reduce slightly over time. Engine components wear, turbochargers get dirty, compression drops and combustion deteriorates. This leads to a reduced power output. However, these can be held at reasonable levels if a good maintenance regime is in place and followed. The engine’s services can affect reliability and performance. The fuel should be treated so water content and other contaminants such as cat fines are as low as possible. The lubricating oil should be purified and filtered and regularly tested. The cooling water system should be clear and properly treated. Regular testing, measuring and recording of the engine’s power output can help identify problems early. There are a number of engine diagnostic testing systems available that trace the power cycle of each cylinder. Older engines may draw power cards, or allow an engineer to measure peak pressures. In all of these methods, a drop in performance can be identified and promptly acted on. Ensuring that there is always sufficient electrical power to cope with peak demand is vital to the safe and efficient running of a vessel. Always look after your generators.
A diesel generator
Generator capacity on container ships
Source: North P&I Club – Signals