Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
1Underwater Noise Prediction and Development ofSpecial Purpose Ships

YUKI ISHII*1

KAZUHIRO MIYAWAKI*2

KEI SATO*3

SATOSHI TASAKI*1

HIROYUKI MORIMOTO*4 KOICHIRO IKEDA*4

The Mitsubishi Heavy Industries Group has built many state of the art special purpose ships
for many years. To fully demonstrate the performance of underwater investigation systems and tolocalize the escape behavior of marine life, it is important for these special purpose ships to reduce
the underwater radiation noise from the ship itself. Our group developed a method for predicting
underwater radiation noise in a wide frequency range using experimental data and large-scale
analysis technology, and enabled a more silent design corresponding to the cruising speed of thespecial purpose ship. In the underwater noise measurement after ship construction, it wasconfirmed that the prediction method has good accuracy and the special purpose ship has anexcellent quietness with low underwater radiation noise.

|1. Introduction
In Japan, which is surrounded by the sea on all sides, the continuous execution ofcomprehensive oceanographic investigation and research of coastal and inshore water areas, as wellas the utilization of marine resources in the ocean such as minerals and energy sources, areconsidered important issues. It is also important to maintain international contributions throughresearch for unravelling the mechanisms of the global ocean environment and climatic change andfor exploring the origins of the earth and organisms. With such a background, our group has built
many special purpose ships, such as research vessels, investigation ships, exploring ships, training
ships and cable laying ships for government agencies, universities, marine research institutes,
marine resource development agencies, and ocean development companies[1].
To fully demonstrate the performance of underwater investigation systems and to localize theescape behavior of marine life, it is important for these ships to silence the underwater radiationnoise from the ship itself. Regarding the impact on marine life, the International Council for theExploration of the Sea (ICES) proposed the required level of underwater radiation noise from all
ships used in fisheries research in 1995. It is necessary to reach the low level of underwaterradiation noise in a broadband frequency range (1 Hz - 100 kHz) as shown in Figure 1[2].
Our group developed technology to predict underwater radiation noise from ships in abroadband frequency range using experimental data and large-scale analysis techniques andenabled silent design for special purpose ships. This paper describes an outline of these efforts.

*1

Research Manager, Vibration Research Department, Research & Innovation Center*2

Manager, Shimonoseki Designing Department, Marine Engineering Center, Mitsubishi Shipbuilding Co.,Ltd.

*3

Research Manager, Fluid Dynamics Research Department, Research & Innovation Center*4Vibration Research Department, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
2Figure 1 Required level of underwater radiation noise from ships used infisheries research proposed by ICES

|2. Underwater radiation noise prediction method
Designing ships with reduced underwater radiation noise requires the reduction of fluid noiseand machinery noise. This section gives an overview of methods for predicting propeller noise andmachinery noise that greatly contribute to underwater radiation noise from ships, as well as thetypes of countermeasures.

2.1

Propeller noise[3]The most important phenomenon in predicting the propeller noise of special purpose ships isthe propeller tip vortex cavitation as shown in Figure 2. To predict noise caused by propeller tip
vortex cavitation, the following three prediction steps are considered necessary:(1) Prediction of low pressure propeller vortex(2) Prediction of cavitation which appears at propeller tip(3) Prediction of noise caused by cavitation

Figure 2

Cavitation generated around propeller

We established a technique of predicting underwater radiation noise caused by propeller tip
vortex cavitation by combining a theoretical model on tip vortex and cavitation and knowledge
based on experimental results.
As shown in Figure 3, the difference between the predicted value and the measured value onan actual ship is in the range of about ±3dB or less in general, and it was confirmed that thisprediction technique is effective for designing the propeller noise of special purpose ships.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
3Figure 3 Comparison of predicted calculation value and actual ship measurement
value of propeller noise

2.2

Machinery noise

Acoustics and vibration energy generated by the main engine, auxiliary machinery and othernoise creating devices mounted onboard propagate through the air and the water inside and outside
the hull and ship hull structures, and diffuse inside and outside the hull.
Whereas airborne sound diffuses in three-dimension, structure-borne sound propagates
through beams, plates and other structural elements, and therefore the diffusion area is smaller thanthe propagation distance, and all structural elements contribute to the acoustic radiation, so that thearea of radiation becomes wider. Furthermore, in steel plate structures such as ships, since theinternal damping of structural elements is low, the structure-borne sound is more dominant than theairborne sound.
Figure 4 illustrates the mechanism of the generation of underwater radiation noise derived
from the structure-borne sound of a ship. In prediction, the important parameters are the soundsource level, characteristics of propagation paths and acoustic radiation characteristics. Since it isdifficult to theoretically determine the sound source level and the characteristics of the ship
structural material, actual measurement data is usually used. Therefore, for the noise prediction of aship, it is important to estimate the structure-borne sound of the hull structure.
A ship is a structure with complicated propagation paths of vibrations and acoustic energy
and has diverse sources of vibration and noise. In addition, the range of calculation frequencies isvery wide. Therefore each structural element exhibits complex vibration characteristics. Regarding
machinery noise, in consideration of the characteristics of the hull structure through which
structure-borne sound propagates and the characteristics of the underwater frequency range, weapply Statistical Energy Analysis (SEA) for the high-frequency range as a method of analyzing
structure-borne sound in a thin plate structure with complicated high-order vibration characteristics,
and large-scale Finite Element Method (FEM) and Fast-Multipole Boundary Element Method(FMBEM) for the low-frequency range as a method of analyzing response of structure-borne
sound and underwater radiation noise in a narrow band.

Figure 4 Generation mechanism of underwater radiation noise caused bystructure-borne sound of ship

(1)

High-frequency range (about 100 Hz or more)
To systematically analyze underwater radiation noise from ships, we developed the SEA
House Code and applied it to the design of reducing shipboard noise and underwater radiation

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
4

noise for actual ships. This section describes the application of this prediction system tounderwater radiated noise prediction in the high-frequency range for machinery noise.
The power flow equation for multiple subsystems of SEA is expressed by:
=,(1)

=+

= diag ∙,= diag ∙=,,,=,=(4, 5),= diag ∙, ⋯,(2, 3),,(6, 7)=,, ⋯,,(8, 9)

where, Ni, ηi, ηij, Woi, and Ei are the number of vibration modes, internal loss factor, couplingloss factor, input power, and subsystem energy, respectively, and the subscripts i and j representsubsystems. dij is 1 when the power flow is from subsystem i to j, -1 when the power flow isfrom subsystem j to i, and 0 when there is no power transfer between subsystems i and j. Bysolving equation (1) for all subsystems, the energy of each subsystem can be obtained. To solve
equation (1), the SEA parameters of the structural system need to be given. Figure 5 presents
the flowchart of structure-borne noise analysis using the SEA method. Each parameter isderived from theoretical and actual measurement data[4].
The underwater noise radiated from the hull bottom plating is obtained by dividing theplating into subsystems and adding sound pressure pi from individual subsystem i. Therefore,sound pressure p at the receiving point distant by++from the sound source isexpressed by:==

12(10)d++,where, ρc, σi, si, and <vi2> are the acoustic impedance in water, and the acoustic radiation
efficiency, area and spatial mean square velocity of individual subsystem i. In our system, thevibration level <vi2> of each subsystem of the hull bottom plating is obtained by the SEA
method, and the sound pressure level at a certain point is estimated by equation (10).

Figure 5(2)SEA analysis flowchart

Low-frequency range (about 100 Hz or less)
For the low-frequency range, it is important to predict and evaluate the response
amplitude and phase of each vibration mode at the excitation frequency of the mounted
equipment in a narrow frequency band, therefore large-scale FEM (Figure 6: hundreds ofMitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
5

thousands elements of model scales including virtual fluid mass elements) is used to predict
structure-borne noise through the hull structure. The large-scale calculation of the entire ship isrealized by our large-scale computers and the following techniques are used to shorten thecalculation time:- Mesh size tuning of fluid and structure in consideration of wavelength and interaction- Vibration mode synthesis method in consideration of the influence of fluid- Partial structure synthesis method
Regarding underwater radiation noise, large-scale calculation of ship size was difficult
using conventional BEM (Boundary Element Method), therefore a large-scale analysis ofseveral million elements was realized by applying FMBEM combined with the fast multipole
method. The FMBEM model shown in Figure 7 is a mirror image model that takes intoaccount the effects of water surface reflection.
These methods enable detailed prediction of structure-borne sound and underwaterradiation noise in the low-frequency range.

Figure 6 Example of large-scale FEM modelof special purpose ship

2.3

Figure 7 Example of FMBEM model of
special purpose ship

Countermeasure design against underwater radiation noise

In the design stage, propeller shapes that control the propeller cavitation, support frames andvibration isolation supports that take into account the excitation frequencies of the apparatus
mounted, the damping materials application area in hull structures that considers structure-borne
sound, etc., were designed properly to reduce underwater radiation noise using these prediction
methods.

|3. Measurement of underwater radiation noise
To check the underwater radiation noise from the ship, underwater sound at the far field wasmeasured using a measurement boat. Figure 8 depicts a schematic diagram of the measurement
carried out in the deep sea where acoustic reflection from the seabed can be ignored according toISO17208-1:2016. During this measurement, the special purpose ship to be measured sailed away
and returned near the measurement boat and also the distance between the two, both equipped witha Differential Global Positioning System (DGPS), was being measured every moment. Sincehydrophones may generate noise depending on the sea conditions, measures against noise were also
taken for hydrophone cables, etc.

Figure 8 Outline of underwater radiation noise measurement of special purpose shipMitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
6|4. Results
Figure 9 compares the predicted calculation value and the actual measured value ofunderwater radiation noise spectrum of a special purpose ship. The two spectral patterns are ingood agreement, indicating that this prediction method is valid. Our group develops and builds
ships with excellent quietness through various measures at the design stage using this prediction
technology, responding to the required underwater radiation noise level of each special purpose
ship, for example, underwater radiation noise satisfying the ICES proposal level.

Figure 9 Comparison of predicted calculation value and actual
ship measured value of underwater radiation noise|5. Conclusion
This paper introduced a method for predicting underwater radiation noise from shippropellers and machinery, and confirmed that the predicted underwater radiation noise spectral
pattern is in good agreement with the actual ship measurement results to verify the validity of theseprediction technologies.
Using these prediction technologies, our group has been designing propeller shapes that
control the propeller cavitation, support frames and vibration isolation supports that take into
account the excitation frequencies of the apparatus mounted the damping materials application area
in hull structures that considers structure-borne sound, etc. to reduce underwater radiation noise of
special purpose ships. We will continue to improve the quietness of ships.

References
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(2)
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