Poster PO-14

PO-14.1

VIBRATION ANALYSIS OF 140,000M3 MOSS TYPE LNG CARRIER FOR THE SNØHVIT PROJECT

Taiji Maeda Section Chief, Structural Design Section

Jun Inuki Manager, Structural Design Section

Akihiko Fujii Deputy General Manager, Ship Hydrodynamic Section

Basic Design Department Mitsui Engineering and Shipbuilding Co., Ltd.

Tokyo, Japan (http://www.mes.co.jp)

Koyu Kimura Manager, Ship Performance Department

R&D Subdivision I, R&D Division Akishima Laboratories (Mitsui Zosen) Inc.

Akishima City, Tokyo, Japan (http://www.mes.co.jp/Akiken/index-j.html)

ABSTRACT

It is an important factor for the safe operation of LNG carriers, especially operated in harsh environment, to have a highly comfort accommodation space, so that the ship’s crew can take sufficient rest to concentrate in their work. For this purpose, minimizing vibration level is an essential factor. The subject LNG carrier is planned to be operated in harsh environment such as North Sea and North Atlantic Sea, and it is required by the Owner to obtain DNV notation COMF-V(1), which is the highest grade of comfort class with regard to noise and vibration.

In order to satisfy the severe vibration requirement, extensive study was carried out during the design stage in view of both hydrodynamics and structure.

In case of the steam turbine ship, the main source of vibration is propeller excitation forces. Therefore, it is important to select the most suitable number of blades of the propeller in view of vibration. At the first design stage, 3 types of propellers with 4, 5 and 6 blades, and the suitable superstructure corresponding to each blade were pre-designed and compared, in view of propeller excitation forces, vibration response and also the propulsive efficiency.

After the optimum combination of the number of propeller and the superstructure were decided, the more elaborate design of hull form and propeller were carried out, and it was confirmed by model test and calculation that propeller excitation forces were low enough.

Then, full ship finite element vibration analysis was carried out by correctly modeling the ship’s entire structure, cargo tank structure. As a result, it was confirmed that the vibration levels of accommodation area were within the allowable limit.

Finally, the results of the measurements of fluctuating pressure, vibration responses during sea trial are also presented, and show good agreement with predicted values.

Poster PO-14

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INTRODUCTION

It is an important factor for the safe operation of LNG carriers, especially operated in harsh environment, to have a highly comfort accommodation space, so that the ship’s crew can take sufficient rest to concentrate in their work. For this purpose, minimizing vibration level is an essential factor. The subject LNG carrier is planned to be operated in harsh environment such as North Sea and North Atlantic Sea, and it is required by the Owner to obtain DNV notation COMF-V(1), which is the highest grade of comfort class with regard to noise and vibration.

In order to satisfy the high level vibration criteria, extensive study was carried out in the design stage by the good collaboration of hydrodynamic design engineers and structural design engineers. This paper presents the procedure and the results of the study, together with the comparison of the analysis and the measurements during the sea trial.

OUTLINE OF THE VESSEL

The vessel is a Moss type LNG carrier with 4 spherical cargo tanks. Principal particulars of the vessel are as below.

• Length, overall Loa 289.50 m • Length P.P Lpp 277.00 m • Breadth(Mld.) Bmld. 48.40 m • Depth(Mld.) Dmld. 26.50 m • Design Draft (Mld.) d 11.30 m • Cargo Tank Capacity (98.5%full) 140,000 m3 • Maximum Continuous Rating (MCR) 27,000 kW x 81.0 rpm • Normal Service Rating (NSR) 24,300 kW x 78.2 rpm

VIBRATION CRITERIA

For 50% MCR to MCR, the lower value of ISO-6954:2000(E) shall be applied for overall frequency-weighted r.m.s values.

For normal service rating (NSR), the criteria of DNV COMF-V(1) as below shall be applied for the peak amplitude for single frequency component.

Table 1 Vibration Criteria of DNV COMF-V(1)

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FLOW OF VIBRATION ANALYSIS

Flow chart of vibration analysis applied to the vessel is shown in Figure1.

Figure 1 Flow Chart of Vibration Analysis of the LNG Carrier

For this vessel, the above procedure was applied both in the early design stage when the number of propeller blade was selected, and the detail design stage when the final design of the propeller and the hull structure were designed.

SELECTION OF THE NUMBER OF BLADE

In case of the steam turbine ship, the main source of vibration is propeller excitation forces. Therefore, it is important to select the most suitable number of blades of the propeller in view of vibration.

At the very early design stage, 3 type of propellers with 4, 5 and 6 blades, and the suitable superstructure corresponding to each blade number were pre-designed and compared, in view of propeller excitation forces, vibration response and also the propulsive efficiency.

Wake Distribution

Initial Structural Drawings

3-D FEM model for Vibration Analysis

Free Vibration Analysis

Blade Frequency Natural Frequency

Design Alteration

Forced Vibration Analysis

Resonance?

Vibration Response OK?Criteria of Vibration level - ISO 6954:2000(E) - DNV COMF-V(1)

Final Drawings

-Surface Force -Bearing Force

No

Yes

Yes

No

Propeller Excitation Force Calculation

M/E revolutions Number of Blades

Hull Form

Tank Test Propeller Design

Cavitation Test

Pressure Fluctuation

Start

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Comparison of Propeller Excitation Forces

Table 2 and Figure 2 show the principal particulars and shapes of pre-designed 4, 5, 6 blade propellers. Propeller diameter was determined by considering the restriction of propeller-hull clearance. Other particulars were determined systematically in accordance with the same design concept.

Table 2 Principal Particulars of 3 Types of Propeller with 4, 5 and 6 blades

Number of Blades 4 5 6

Diameter(m) 9.40 9.10 9.10

Expanded area ratio 0.650 0.670 0.660

Pitch ratio 0.902 0.871 0.843

Skew angle(degree) 37.5 30 25

a) 4 blades b) 5 blades c) 6 blades

Figure 2 Comparison of 3 Types of Propellers with 4, 5 and 6 blades

Figure 3 shows the comparison of the propeller excitation forces, and Figure 4 shows the coordinate system of exciting forces. The pressure fluctuation was estimated by the experimental formula obtained from our database. Bearing forces were estimated by our propeller performance program of Unsteady VLM (Vortex Lattice Method). It was observed that surface force became lower from 4 to 6 blades, and the bearing forces of 5 and 6 blade propellers were lower than 4 blade propeller.

F sx F sy F sz F x F y F z M y M z

4B L

5B L

6B L

Figure 3 Comparison of Propeller Excitation Forces of 4, 5, 6 Blade Propellers

(ton) or

(ton-m)

20

10

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FxFy

Fz

Mx My

Mz

θ

a) Surface Force b) Bearing Force

Figure 4 Coordinate System of Propeller Excitation Forces

Preliminary Vibration Analysis

At the very early design stage, full ship FEM model for preliminary vibration analysis as shown in Figure 5 was prepared based on the preliminary rough drawings developed for the analysis, in order to investigate the natural frequency of the vessel’s hull and superstructure.

Figure 5 Preliminary Vibration Model at Early Design Stage

In the original design, accommodation deck house and engine casing were separated. In order to examine whether the natural frequency of the super structure could be well above the blade frequency of 4 blade propeller, the model with bracket connection between accommodation deck house and engine casing was also prepared.

ConnectedNot Connected

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Comparison of Possibility of Vibration Resonance

Figure 6 schematically shows the relation between blade frequency from 50% MCR to MCR and natural frequencies of longitudinal and horizontal vibration of the superstructure.

Figure 6 Comparison of the relations between blade frequency and the natural

frequency of the super structure for 4, 5, 6 blade propellers

In case of 4 blade propeller, it was found that the natural frequency of longitudinal vibration of superstructure was near the blade frequency in the normal operation range. If the accommodation deck house and the engine casing are connected by bracket, the natural frequency of longitudinal vibration becomes somewhat higher than the blade frequency at MCR. However, it was considered that the margin to avoid resonance was not sufficient taking account of the accuracy of the preliminary vibration analysis.

In case of 5 blade propeller, it was found that it was difficult to avoid resonance in the normal operation range.

In case of 6 blade propeller, it was found that the natural frequency of longitudinal vibration was well below the normal operation range, and that of horizontal vibration was sufficiently higher than normal operation range.

In view of speed performance, it was found that comparative performances could be obtained for 4, 5 and 6 blade propeller, when optimum design for each propeller was applied.

<6 blade>

4 5 6 7 8 9 10 11(Hz)

LongitudinalVibration

HorizontalVibration

50%M C R M C RNSR

<5 blade>

4 5 6 7 8 9 10 11(Hz)

LongitudinalVibration

HorizontalVibration

50%M C R M C RNSR

<4 blade>

4 5 6 7 8 9 10 11(Hz)

LongitudinalVibration

HorizontalVibration

50%M C R M C RNSR

LongitudinalVibration(Accom m odationand E/C asing areconnected)

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Therefore, it was determined that the 6 blade propeller was the most suitable in view of vibration, both for avoiding resonance, smaller excitation forces and sufficient speed performance.

DESIGN OF HULL FORM

Hull form of the vessel was carefully designed to obtain more uniform wake distribution with smooth flow near the top position of propeller, so as to minimize pressure fluctuation and bearing forces. Hull form design was carried out using CFD, taking the improvement of the propulsive performance into account [1]. Figure 7 shows the example of hull surface and wake distribution calculated by CFD.

Figure 7 Distribution of Hull Surface Pressure and Wake at Propeller Plane (Seen from below and aft)

PROPELLER DESIGN AND EXCITATION FORCE CALCULATION

High efficiency propeller with low exciting forces was designed by our propeller design system using VLM as shown in Figure 8 [2]. The efficiency and cavitation performance of propeller was confirmed by the model test.

Design conditionDesign condition

Design conditionDesign condition

Design conditionDesign condition

Prototype PropellerPrototype Propeller

Propeller shape m odificationPropeller shape m odification Perform ance estim ationPerform ance estim ation

M odel Test : POT & Cavitation TestM odel Test : POT & Cavitation Test

Date BaseDate Base

Final PropellerFinal Propeller

Im provem ent using VLMIm provem ent using VLM

Exciting ForceExciting Force

Propeller & HullPropeller & Hull

YES

YESNO

NO

Diam eter, Expanded A reaPitch , Cam berSkew , RakeBlade thickness , etc

Speed, RevolutionCavitationExciting forcesBlade strength , etc

Figure 8 Propeller Design Procedure

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The cavitation test was carried out in the cavitation tunnel in our laboratory and only a small cavitation was observed, as shown in the Photo.1.

Photo. 1 Apparatus of Cavitaion Test and Observed Cavitation

Figure 9 shows the distribution of pressure fluctuation on hull surface estimated from results of cavitation test. The surface forces were predicted by integrating these pressure fluctuations.

Figure 9 Distribution of Pressure Fluctuation on Hull surface

Propulsion performance test (Photo.2) also was performed in our laboratory and confirmed that the final hull form and propeller satisfied the contract speed.

Photo. 2 View of Model Test in Akishima Laboratory

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VIBRATION ANALYSIS IN THE DETAIL DESIGN STAGE

In the detail design stage, more elaborate FEM model for the final vibration analysis as shown in Figure 10 was prepared based on the fully developed structural drawings, in order to confirm that the predicted vibration levels are below the allowable limit.

Figure 10 FEM Model for Final Vibration Analysis

Both sides of the hull structures including spherical cargo tanks were modeled. The used program is MSC/NASTRAN.

In order to make accurate estimation of the vibration response for propeller excitation forces, structures around stern tube, propeller shaft, thrust blocks were carefully modeled.

As representative load conditions, both normal ballast condition and full load condition were examined.

Free Vibration Analysis

At first, free vibration analysis was carried out to investigate the natural frequency of hull and the superstructure.

Typical modes of hull girder vibration in normal ballast condition are shown in Figure 11.

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Figure 11 Natural Mode of Hull Girder Vibration

Typical modes of the longitudinal and transverse vibration of the superstructure are shown in Figure 12.

Figure 12 Natural Mode of Superstructure

As a result, it was confirmed that estimated natural frequencies of the superstructure were outside the normal operation range as expected in the early design stage.

2-node vertical vibration mode 0.67Hz (Full Load)

0.74Hz (Normal Ballast)

1-node torsional vibration mode0.92Hz (Full Load)

0.99Hz (Normal Ballast)

2-node horizontal vibration mode 1.18Hz (Full Load)

1.35Hz (Normal Ballast)

3-node vertical vibration mode1.32Hz (Full Load)

1.45Hz (Normal Ballast)

4-node vertical vibration mode 1.90Hz (Full Load)

2.18Hz (Normal Ballast)

Longitudinal Vibration 5.03Hz (Full Load)

5.17Hz (Normal Ballast)

Torsional/Athwartship Vibration9.24Hz (Full Load)

9.49Hz (Normal Ballast)

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Forced Vibration Analysis

In order to examine the vibration level of accommodation area, forced vibration analysis using the calculated propeller excitation forces was carried out. The 1st and 2nd order propeller excitation forces, which correspond to 6th and 12th order of the shaft revolution were considered with phase angle of each component of excitation forces.Figure 13 shows the typical results of calculated peak vibration velocity for 1st order propeller excitation forces at navigation bridge deck in normal ballast condition.

Figure 13 Vibration Response at Navigation Bridge Deck (Normal Ballast)

The calculated vibration levels were compared with the criteria of ISO 6954:2000(E) and DNV notation COMF-V(1), and were found to be below these limits.

Shaft Revolution [ r.p.m ]

0

1

2

3

4

5

6

50 55 60 65 70 75 80 85

NC-L

NP-L

NS-L

WP-L

WS-L

DNV CONF-V(1)

Workplaces

Normal Ballast

Nav.bridge

(Longitudinal)

Shaft Revolution [ r.p.m ]

0

1

2

3

4

5

6

50 55 60 65 70 75 80 85

NC-T

NP-T

NS-T

WP-T

WS-T

DNV CONF-V(1)

Workplaces

Normal Ballast

Nav.bridge

(Transverse)

Shaft Revolution [ r.p.m ]

0

1

2

3

4

5

6

50 55 60 65 70 75 80 85

NC-V

NP-V

NS-V

WP-V

WS-V

DNV CONF-V(1)

Workplaces

Normal Ballast

Nav.bridge

(Vertical)

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RESULTS OF MEASUREMENTS AT SEA TRIAL

In order to confirm the accuracy of the analysis in the design stage, and to confirm that the vibration levels in accommodation area are within the allowable limit, following measurements were carried out during the sea trial.

Photo. 3 LNG Carrier “Arctic Discoverer” at the sea trial

Observation of Cavitation and Measurement of Pressure Fluctuation

Cavitation observation windows made of acryl and small pressure transducers were installed above the propeller as shown in Photo.4 & 5. The cavitation patterns of propeller was observed with use of VTR through these windows, and pressure fluctuations on hull surfaces were measured in accordance with vibration test during the sea trial. Observed cavitation patterns were similar to the model test results, as shown in Photo.1 and 5.

Photo. 4 Arrangement of Cavitation observation windows

Cavitaion Observation Windows

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Photo. 5 The Result of Cavitaion observation

Figure 14 shows the measurement results of pressure fluctuation. These results show very good agreement with the estimation and model test results.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

CAL MODEL SHIP

Pre

ssur

e Fl

uctu

atio

n [k

Pa]

B.F.

2ND

3RD

4TH

Figure 14 Comparison of Pressure Fluctuation on Hull Surface

Measurement of Vibration Response

During the sea trial, vibration responses at various locations of accommodation were measured.

Vibration levels in various locations of the superstructure at 50%MCR, 70%MCR, NSR, MCR were evaluated by overall frequency-weighted r.m.s values according to the lower value of ISO 6954:2000(E) [3], and the criteria was satisfied at all locations.

Vibration levels at NSR(90%MCR) were evaluated by peak amplitude for single frequency according to the criteria of DNV COMF-V(1), and the criteria was satisfied at all locations.

Figure 15 shows the typical results of measuring point at the navigation bridge deck in comparison with the calculation results. It is seen that the measurement results are in good agreement with the calculation results in general.

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Figure 15 The Results of Vibration Measurement

*) Shaft revolution during sea trial was somewhat higher than the design revolution because of the revolution margin.

CONCLUSION

In this report, the procedure and the results of the vibration analysis of the LNG carrier during the design stage were described. The key factors for the successful results are as follows.

• Thorough investigations from the early design stage to the detail design stage were made.

• Good collaboration of hydrodynamic design engineers and structural design engineers was established.

• Improved analysis methods based on the accumulated experiences for previous LNG carriers were applied.

As a result, it was confirmed by the measurements during the sea trial that the vibration levels were sufficiently lower than the specified criteria, and also in good agreement with the results of the analysis.

REFERENCES CITED

[1] Fujii A., et al. (2005) Application of hull form design system using CFD. Proceedings of the International Conference on Marine Research and Transportation, ICMRT05, Italy

[2] Kimura K., et al. (2006) Development of Low-Excitation and High-Efficiency Propeller Design System, Mitsui Zosen Technical Review, No.189, pp.25-30

[3] Toyama, Y., et. al, (2003) Analysis and Evaluation of Vibrations in Superstructure of Ships Based on New ISO6954, Proceedings of The Thirteenth(2003) International Offshore and Polar Engineering Conference, USA