Vibration control of ship Vibration Noise RD Ship

Vibration control of ship Vibration & Noise R&D Ship and Ocean R&D Institute 2016. 10. 27

Contents 1. Introduction 2. Vibration criteria 3. Initial prediction 4. Vibration analysis 1. Global vibration analysis 2. Local vibration analysis 5. Vibration measurement 6. Ship vibration control 7. Conclusion DSME R&D Institute 중앙연구

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 DSME R&D Institute 중앙연구

Vibration criteria • Criteria for human beings Building Specifications International standards class recommendation Vibration criteria – Ensuring comfort and wellbeing. – 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 – VIBRATION CLASS(2011) – internal sources(machinery itself) and the external sources(from other machinery) DSME R&D Institute 중앙연구

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 Velocity (mm/s 2) (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 3 143 4 NOTE : The zone between upper and lower values reflects the shipboard vibration environment commonly experienced and accepted. DSME R&D Institute 중앙연구

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) DSME R&D Institute 중앙연구

Criteria for the machinery vibration DSME R&D Institute 중앙연구

Initial prediction • To check the possibility of resonance • Hull girder natural frequency and 2 nd 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 DSME R&D Institute 중앙연구

Initial prediction Natural frequencies of hull girder vertical 2 node mode (ref. Jung and Todd) N 2 v, L, B and D are the natural frequency of vertical 2 node, length(m), breadth(m) and depth(m) • 2 node • 3 node • 4 node High order natural frequency DSME R&D Institute 중앙연구

Initial prediction Mode shape Fm : compensation force Fa : Aft force due to 2 nd order moment of main engine M : 2 nd 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 DSME R&D Institute 중앙연구

Initial prediction Natural frequencies of Super structure(ref. JH Park ) (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 DSME R&D Institute 중앙연구 • Mode shape of super structure

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 – • Three dimensional finite element model Forced vibration analysis to calculate the vibration responses of selected points representing overall vibration behaviors Mode shapes of a ship Excitation force – – Propeller fluctuation pressure Engine H and X moment Calculated vibration response at deck house top DSME R&D Institute 중앙연구

Vibration Analysis • Local vibration analysis • To avoid the resonance • Design target frequency – – – • Ex) Propeller 4 blade M/E 6 cylinder NCR: 60, MCR: 70 10% higher than a excitation frequency Excitation frequency of fluctuation pressure on hull surface due to propeller Main engine external extraction forces Analysis area – • Areas for local vibration evaluation aft body, engine room and deckhouse area 10. 3 Hz Design target freq. modification works – increase of a plate thickness, stiffener size and or girder size based on a concerned mode shape 4. 4 Hz NCR -10% Life boat davit DSME R&D Institute Heli-deck 중앙연구 Thruster motor MCR +10%

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 DSME R&D Institute 중앙연구 Typical measurement positions of global vibration Ref: ISO 20283 -2: 2008[5]

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 DSME R&D Institute 중앙연구 • Peak plot

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

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. DSME R&D Institute 중앙연구

Vibration control measure the vibration can be controlled by using the onoff 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 Velocity (mm/s) • Vibration limit RPM of Main Engine DSME R&D Institute 중앙연구

Vibration control measure • Modification of structure • The simplest and most common way to reduce a vibration Deformed shape • 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 Mode shape Modification Vibration reduction DSME R&D Institute 중앙연구

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. DSME R&D Institute 중앙연구