Answer EKG Question 25

Q25. With respect to Energy efficient running of ships;

(a) Sketch and explain the optimization of propeller hull interface flow devices and improvement of propulsion efficiency.

(b) sketch and explain the optimization of Auxiliary machinery using VFDs.

Answer:

(a) Optimization of propeller hull interface flow devices and improvement of propeller efficiency:-

Adoption of EEDI (energy efficiency design index) was mandated by IMO for all vessels contacted since 2013. IMO also resolved to reduce the noise generated by ship to 3dB within 10years and 10dB within 30year to conversion of marine mammals and fish.

Goal is to improve the propulsion efficiency and to protect the marine environment.

Propulsion efficiency of ship and cavitation:-

Only 70% of power of ship propeller is used for ship propulsion and rest disappears due to friction, heat loss and vortex etc.

In order to improve the propulsion efficiency of the ship it is necessary to minimize the loss by improving the design technique of the propeller and developing the high efficiency propeller appendages.

Cavitation: - as the velocity of the fluid increases the pressure near the surface of the object touching the fluid is lowered, causing the fluid to vaporize, it then result in physical change such as creation if empty space in water.  Various type of strong cavitation occur in the propeller. This is the main cause of noise and vibration of the ship loss of propulsion efficiency and propeller & rudder erosion.

Technologies for enhancement of propulsion efficiency and noise reduction:-

The existing ship propeller has a round cup shape cover, this shape contrary to expectations create strong vortex flow and forms a strong hub vortex cavitation. In an attempt to inhibit the formation of the vortex, new cap design called K-CAP is developed. It can improve the propulsion efficiency while efficiently inhibits the formation of hub vortex cavitation. 

Technologies for enhancement of propulsion efficiency and noise reduction.

K-CAP & FIN:- it uses the combination of K-CAP and simple shaped plate, the fin attached helps to absorb the rotational energy behind the propeller and removes the propeller hub vortex cavitation, while improving the propulsion efficiency, it reduces propeller broad band noise and prevent water erosion by completely eliminating hub vortex cavitation with an increased efficiency of the propeller. 

Vortex generator:-  it is a structure built in front of propeller which controls the flow of fluid into the propeller to improve the propulsion efficiency and reduces the noise and vibration of the ship.

Conventional energy saving devices (ESD): - they are installed in the form of the fin or duct close to the propeller. With such a structure the propulsion efficiency can be improved by the pre-swirl effect. But the possibilities of increased cavitation and erosion in there with such an arrangements. They also form a large structure and difficult in maintenance also they shows structural problems.  

(b) Optimization of Auxiliary machinery using VFDs.

production and distribution of high voltage is been proven economical over long period of services. the high voltage also facilitates to install an electric propulsion system on the ship. electric propulsion system have advantages over diesel propulsion.

    The system generates a significant amount of power, excess power utilized by supplying it to cargo pumps, fire pumps and other important auxiliary machinery. The space required for installation of electrical propulsion machinery is very less and compact as compared to conventional system. There is no direct connection of propeller shaft and prime mover, and hence transmission of severe stresses such as torsional and vibration is restricted. There is more flexibility in installation of machinery.  It provides improved maneuverability and high redundancy. Increased payload through flexible location of machinery components. Environmental benefits from lower fuel consumption and emissions. High performance in harsh ice conditions due to maximum torque at zero speed. Reduces life cycle cost by less fuel consumption and maintenance cost. Minimal standstill time for maintenance and service. Vessels with potential trim problems, such as stern Wheeler, where machinery needs to be located forward to avoid trim problems. Better comfort due to reduced vibration and noise. Much better dynamic response from zero to maximum propelling speed compared to other propulsion systems. Less reversing time compared to other propulsion systems. Availability of maximum torque across the entire speed range at the propeller. Reduced space requirements in the shaft system. Design and engineering of the propeller are independent of the drive. Flexibility in the choice of diesel engine speed.

 System consists of an AC generator which produces fixed voltage at fixed frequency. But for the purpose of slow streaming and maneuvering the fixed output of the generator needed to be lowered frequency and adjusted voltage. Speed of the synchronous motor used for the propulsion shall be controlled with this adjustment. There are many methods to achieve this. Some methods require to convert the AC to DC then DC to AC with a new controlled frequency.

One method known as cycloconvertor which do not have an intermediate DC form. This method of controlling frequency relies on the ability of converter to accept current from switch board at constant frequency and voltage but to pass this current to the AC motor at a reduced frequency and with voltage adjusted. The fixed-frequency supply from the a.c. generators is applied simultaneously to the three pairs of Graetz thyristor bridges of the cycloconverter. The upper and lower bridges of each pair are arranged to operate alternately so that a number of triggering occur in the top set of thyristors-followed by an equal number from the bottom set, to deliver an output with a lower frequency. The two bridges for each phase are required to supply both the positive and negative half-cycles.

The triggering of the thyristors is continually changed relative to the three-phase supply so that output can be tailored to provide the exact frequency and amplitude of voltage required. Frequency is variable from 0 to 60 Hz.