1. Vibration control of ship
Vibration & Noise R&D
Ship and Ocean R&D Institute

2. Vibration measurement Ship vibration control Conclusion
Contents
Introduction
Vibration criteria
Initial prediction
Vibration analysis
Global vibration analysis
Local vibration analysis
Vibration measurement
Ship vibration control
Conclusion

3. Introduction
Process of ship vibration control to avoid excessive vibration
Select propeller blades, main engine cylinder and M/C
Calculated natural frequency and vibration response
Measured vibration response

4. Criteria for human beings
Vibration criteria
Criteria for human beings
Ensuring comfort and well-being.
The international standard ISO 6954(2000)[1]
Evaluation of vibration w.r.t habitability on passenger and merchant ships
Structural vibration
the low risk of fatigue cracks
VIBRATION CLASS(2011)
Equipment
protect the machinery from the excessive vibration
internal sources(machinery itself) and the external sources(from other machinery)
Building Specifications
International standards
class recommendation
Vibration criteria

5. Vibration criteria for human being
Overall frequency-weighted r.m.s. values
Frequency range : 1 Hz to 80 Hz
Area classification
A Passenger cabins
B Crew accommodation
C Working areas
Acceleration
(mm/s2)
Velocity
(mm/s)
Velocity (mm/s)
Values above which adverse comments are probable
143 4
214 6
286 8
Values below which adverse comments are not probable
71.5 2
107 3
NOTE : The zone between upper and lower values reflects the shipboard vibration environment commonly experienced and accepted.

6. Criteria for the structural vibration r.m.s
vibration r.m.s. values
4 Hz to 200 Hz
low risk for fatigue cracks
REF : DNV VIBRATION CLASS(2011)

7. Criteria for the machinery vibration

8. To check the possibility of resonance
Initial prediction
To check the possibility of resonance
Hull girder natural frequency and 2nd order moment of main engine
To decide installation of Moment compensator
Wheel house natural frequency and main excitation source
To select No. of Propeller blade and Main Engine cylinder

9. Initial prediction 2 node 3 node 4 node
Natural frequencies of hull girder vertical 2 node mode (ref. Jung and Todd)
2 node
3 node
4 node
N2v, L, B and D are the natural frequency of vertical 2 node, length(m), breadth(m) and depth(m)
High order natural frequency

10. Initial prediction Mode shape Compensation force Lf La
Fm : compensation force
Fa : Aft force due to 2nd order moment of main engine
M : 2nd order moment of main engine
La : distance between main engine aft and nodal point
Lf : distance between main engine forward and nodal point
L : length of main engine
Compensation force Lf La

11. Mode shape of super structure
Initial prediction
Natural frequencies of Super structure(ref. JH Park )
Mode shape of super structure
(Hz)
H:Height(m) of Super structure
L:length(m)
B: breadth(m)
K:Stiffness of foundation (N/m)
M: Mass (ton)
α, β, γ : Coefficient

12. Vibration Analysis Global vibration analysis
To investigate the overall vibration characteristics of whole ship
Resonance check
Free vibration analysis of hull girder including deckhouse, aft body and engine room
Response check
Forced vibration analysis to calculate the vibration responses of selected points representing overall vibration behaviors
Excitation force
Propeller fluctuation pressure
Engine H and X moment
Three dimensional finite element model
Mode shapes of a ship
Calculated vibration response at deck house top

13. Vibration Analysis Local vibration analysis To avoid the resonance
Design target frequency
10% higher than a excitation frequency
Excitation frequency of fluctuation pressure on hull surface due to propeller
Main engine external extraction forces
Analysis area
aft body, engine room and deckhouse area
modification works
increase of a plate thickness, stiffener size and or girder size based on a concerned mode shape
Areas for local vibration evaluation
Ex)
Propeller 4 blade
M/E 6 cylinder
NCR: 60, MCR: 70
10.3Hz
Design target freq.
MCR +10%
4.4 Hz
NCR -10%
Life boat davit
Heli-deck
Thruster motor

14. Vibration measurement
To confirm accordance with the criteria
global vibration measurement
to check global vibration characteristics of whole ship
to measure vibration level with increase of RPM by using fixed monitoring system
Local vibration measurement
To check vibration in cabin w.r.t habitability
To check vibration levels of local structures wrt structure failure.
To check vibration level of machinerys and outfittings in the deckhouse, engine room and aft body in viewpoint of machinery malfunction by using portable analyzer
Typical measurement positions of global vibration
Ref: ISO :2008[5]

15. Vibration measurement
To confirm a measured vibration level below the limit
Waterfall plot
to find out the natural frequency of hull structure
vibration level is distinguishably high at around the natural frequency
Peak plot
The slice of waterfall along the each excitation force
the vibration level due to each excitation source
Waterfall plot
Peak plot

16. Vibration control measure
Moment compensator
Application areas
Excessive vibration
same magnitude
anti-phase
The most effective and widely used method to reduce the vibration is to reduce the excitation force
Vibration cancellation
Resultant vibration
compensated by the control force having same magnitude and opposite phase to the existing force.
Vibration control

17. Vibration control measure
Top bracing
If source of vibration is main engine, then we can consider top bracing with on-off control
The main purpose of top bracing for main engine is to reduce engine vibration.
The top bracing is installed on the side of engine.

18. Vibration control measure
the vibration can be controlled by using the on-off control of hydraulic top bracing
The vibration level of deckhouse can be keep under the limit by controlling top bracing active(on) below 89 rpm(blue line) and top bracing inactive(off) above 89 rpm(blue line)
Vibration reduction by using on-off control at deckhouse
T/B on
T/B off
Vibration limit
Velocity (mm/s)
RPM of Main Engine

19. Vibration control measure
Modification of structure
The simplest and most common way to reduce a vibration
increasing the stiffness or supporting the structure
the vibration behavior of hull structure shall be figured out by measuring vibration considering the phase of supporting structure
Deformed shape
Mode shape
Modification
Vibration reduction

20. Conclusion
In this paper, the process of ship vibration control is explained to avoid excessive vibration from design stage to sea trial.
Vibration criteria have to be set up based on the Building Specifications.
Initial design stage : predominant excitation sources are selected by using the data of experienced ships such as numbers of main engine cylinders and propeller blades
Detail design stage : global vibration and local vibration analysis are carried out by using the Finite Element Analysis
Measurements are carried out during the sea trial to assess the vibration performance of ship.
If they are not satisfied the criteria, appropriate methods for vibration control are applied.