Validation of underwater radiated noise predictions for a merchant vessel using full-scale measurements

Frans Hendrik LAFEBER1; Thomas LLOYD2; Johan BOSSCHERS3 1, 2, 3 Maritime Research Institute Netherlands (MARIN), The Netherlands

ABSTRACT Underwater radiated noise (URN) was previously primarily of interest in connection with the

signature of naval vessels. Recently it has become increasingly relevant for other vessel types, such as cruise and merchant ships, due to a growing concern that marine life is affected by rising anthropogenic noise levels in the oceans. Shipping is a main contributor to those noise levels, with the cavitating propeller being the dominant noise source. Marine mammals and fish use sound to communicate and to sense their environment and this requires low background noise levels. The URN of a cavitating propeller can be predicted before the ship is built by means of computations and model-scale tests. High-quality validation material is needed for the validation of computational models and model test procedures, which include the scaling of the noise levels.

Within the CRS framework (Cooperative Research Ships, http://www.crships.org/) a full-scale measurement campaign was carried out on a single-screw cargo vessel by DNV-GL, DAMEN and MARIN. Model tests for several conditions have been carried out in MARIN’s Depressurized Wave Basin. During these tests, URN was measured and cavitation patterns were observed using high-speed video cameras. Within the CRS a semi-empirical model has also been developed, which predicts the broadband pressure fluctuations and URN of cavitating tip vortices on marine propellers.

This paper discusses a validation study of the computational and model test procedures for determining URN due to cavitating propellers, using the full-scale data of the aforementioned ship. Some aspects of the analysis and scaling procedures are discussed. Results are shown for three pitch settings. The agreement between the results of the various methods is very good; mostly within 3 dB. The change in noise levels between the three tested conditions is well captured. Keywords: underwater radiated noise, propeller, cavitation. I-INCE classification: 73, 76

1. INTRODUCTION Many marine animals use sound to sense and interact with their environment: for communication,

navigation, finding prey and even for self defence. If the background noise levels are too high, masking or other potentially adverse effects on marine life will occur. Shipping is a major contributor to the background noise levels. The world’s oceans are becoming increasingly noisy. The number of ships in the world's commercial fleet has roughly tripled between the 1948 and 2008 (see Hildebrand (1)), which equates to about 5 dB. On the other hand, anthropogenic noise in the oceans has been increasing by 3.3 dB per decade (Frisk (2)), or about 20 dB since the 1950s. In other words, ships are not only more numerous but individual vessels have become more noisy due to increased size, speed and delivered power. This leads to more cavitation, the main source of underwater radiated noise (URN).

Therefore, shipping noise has received increasing regulatory attention. Class rules include noise limits for fishery and seismic research vessels because self noise can influence the operability of such vessels. Nowadays there also are URN-related class notations for other ship types, such as DNV-GL Silent-E, BV URN (NR 614) and RINA Dolphin, while ABS has also announced URN notations. These cover a range of vessel types such as pleasure yachts, cruise ships and merchant vessels and aim to address environmental impact. These notations show that the URN of such a vessel has been controlled 1 f.h.lafeber@marin.nl 2 t.lloyd@marin.nl 3 j.bosschers@marin.nl

to reduce the impact on marine wildlife. Furthermore the European Union has included URN in methodological standards regarding good environmental status and IMO has published MEPC Circ. 833 “Guidelines For The Reduction Of Underwater Noise From Commercial Shipping To Address Adverse Impacts On Marine Life”. It is expected that in the near future local regulations concerning URN will come into force to protect wildlife in particular sensitive areas. Presently the focus is on monitoring of URN. An example thereof is the ECHO (Enhancing Cetacean Habitat and Observation) program4, managed by the Vancouver Fraser Port Authority (VFPA). Within this program, URN in the Georgia Strait is being continuously monitored. In a bid to encourage the industry to build and use quieter ships, VFPA is offering a discount on harbour rates5 for ships with a quiet vessel class notation.

In order to check whether a ship complies with the noise requirements, the URN has to be measured on full scale. However, at that stage it is difficult to make modifications to the ship if the requirements are not met. It is therefore desirable to have reliable methods to predict the ship’s URN during the design stage, when it still possible to make changes to the design. These predictions can be made by means of computational procedures or model-scale tests. To validate these procedures, a validation study has been carried out for a single-screw merchant vessel. Full-scale data of the URN of this vessel is compared to the results of a computational procedure and of a model test campaign.

The primary focus of the present study is on cavitation noise, which is the dominant source of URN, especially at higher frequencies. Contributions from, for example, machinery noise, are not included in either the model tests or the computational models applied in the current study.

2. FULL-SCALE MEASUREMENTS A large set of full-scale URN measurements on a merchant vessel was carried out by DAMEN,

DNV-GL and MARIN within the CRS BROADBAND2 working group. The 85 m long ship is fitted with a single controllable-pitch propeller. Sea trials were carried out in a fjord on the Norwegian south coast. Various combinations of ship speed, propeller rotation rate and propeller pitch setting were tested.

URN of the vessel was measured by DNV-GL. This was done using a hydrophone mounted in a frame about 20 cm above the base. This setup was lowered to the sea bed as far from the coast as possible. The hydrophone signal was recorded with a sampling frequency of 200 kHz. The locations of the hydrophone and of the vessel during the measurement were determined using GPS.

During the URN measurements, the vessel was free-sailing in a typical loading condition. Measurements were performed following the procedure described in the DNV-GL SILENT class notation. The ship was brought in a steady state about 300 m before the Closest Point of Approach (CPA) after which no changes in the machinery settings were applied and steering was kept to a minimum. URN was measured in beam aspect at an average horizontal distance of 125 m. Two runs were performed for each condition; one portside and one starboard. Shaft power and rotation rate were also measured.

The resulting URN spectra were converted to radiated noise levels (RNL) - the sound level at 1 m assuming spherical spreading loss - using the actual distance between the ship and the receiving hydrophone. The results were then converted to one-third-octave bandwidth and the results of the two runs are then averaged for each condition. A correction of -5 dB was applied to account for the proximity of the hydrophone to the sea bed. Some details of the test conditions reported in this paper are given in Table 1.

Table 1 – Overview of test conditions

Pitch setting Ship speed Vs [knots]

Propeller rotation rate ns [rpm]

Cavitation number σn [-] at shaft

High 10.2 224 2.9 Medium 6.4 181 4.5 Low 3.1 224 2.6

4 http://www.portvancouver.com/echo 5 http://www.portvancouver.com/news-and-media/news/new-incentive-for-cargo-and-cruise-vessels-intended-to-quiet-waters-around-the-port-of-vancouver-for-at-risk-whales/

The cavitation number σn is defined as

, (1)

where is the ambient pressure, the vapour pressure of the water, the water density, g acceleration due to gravity, the shaft immersion, the propeller rotation rate and the propeller diameter.

3. COMPUTATIONAL APPROACH Computational results are provided in the form of estimations from a semi-empirical model for tip

vortex cavitation noise. The Empirical Tip Vortex (ETV) model has been developed by MARIN within the CRS, and can be used to predict both broadband hull pressure pulses and URN. The model uses results from a boundary element method analysis of the propeller in a ship wake field to predict the centre frequency and level of the broadband hump. Empirical parameters are used, which are derived from model-scale and full-scale hull pressure data of twin screw vessels. A general spectral shape for the source level is used, which to some extent can also be tuned. Full details of the model can be found in Bosschers (3).

The RNL results of the full-scale trials were not corrected for the Lloyd’s mirror effect, which is the interference pattern due to the presence of the free surface. The ETV model, however, predicts source levels (SL). That means that the influence of the Lloyd’s mirror effect (Δ ) should be added to the computational SL results to be able to compare them to the RNL results of the full-scale tests:

Δ . (2)

Δ depends on, among other parameters, the relative positions of the source and receiver with respect to the free surface. The formula given by Clay & Medwin (4) was applied, which is fully detailed in Lafeber & Bosschers (5). The test layout of the full-scale trials was used as input, being an average distance at CPA of 125 m and a hydrophone depth of 45 m have been used to compute the Lloyd’s mirror effect on full scale.

4. EXPERIMENTAL APPROACH

4.1 Depressurized Wave Basin

Model tests involving cavitation on ship propellers have been performed at MARIN for many years in the Depressurized Wave Basin (DWB), see Figure 1. The basin is 240 m long, 18 m wide and 8 m deep. The air pressure in the facility can be lowered to 30 mbar for performing model tests with cavitating propellers. Model speed, propeller rotation rate and basin pressure are prescribed to obtain model scale test conditions that best represent the full scale condition. The presence of the free surface not only allows free trimming and sinkage of the model, but also implies that the pressure release boundary condition is satisfied.

Figure 1 – MARIN’s Depressurized Wave Basin with normal towing carriage

The background noise has to be low enough to avoid interference with the cavitation noise measurements. The main sources of background noise are the towing carriage and the propeller drive train. Therefore, a lightweight silent towing carriage (Figure 2) fitted with polyurethane wheels is used for the noise measurements. To reduce the noise from the propeller drive train, solid shafts and special bearings are fitted to the ship model.

Figure 2 – Silent towing carriage

URN is measured by means of a hydrophone fitted to a mast positioned in the middle of the basin at

a depth of 1.45 m. The ship model sails over the hydrophone, giving a model-scale version of a noise range. The data window length is limited to about 2 m up- and downstream (total window length 4 m) due to the reverberation radius, see Lafeber et al. (6). Since most ship models range from 12 m down to 6 m in length, the data window is of the order 0.3 – 0.7 times the ship length. It is noted that only propeller cavitation noise is considered. More details on the test facility and the equipment can be found in Lafeber et al. (7) and Lafeber et al. (8).

4.2 Model preparation

A wooden ship model was constructed to a scale of about 1:12, as shown in Figure 3. The model was about 7 m long. Figure 4 shows the rudder and propeller model.

Figure 3 – Ship model

Figure 4 – Rudder and propeller model

Open water and propulsion tests were conducted for the applicable speed range for each of the three pitch settings. These showed a good agreement with the full-scale results for the speed-power curve and corresponding propeller rotation rate. The propulsion tests also deliver input for determining the test conditions for the noise measurements.

During the cavitation tests, the propellers were painted to avoid light reflections, increase contrast and improve visibility of the cavitation. In order to minimise the viscous scale effect on cavitation inception, roughness strips of 60 μm carborundum grains were applied to the leading edge of the propeller blades as a means of simulating transition to turbulence, as shown in Figure 5.

Figure 5 – Close-up of roughness on propeller leading edge

The nuclei content of the water should be sufficient to avoid problems with respect to inception of cavitation (9). Electrolysis is used to generate small hydrogen and oxygen bubbles, which act as nuclei. This is done by applying an electrical current to small conductive strips glued to the hull just upstream of the propeller. The strength of the electric current depends on the length of the strips, the basin pressure and the ship model speed.

4.3 Test conditions

Model tests are typically performed by enforcing cavitation number similarity. Adopting the subscripts m and s to denote model and ship scale values, we obtain cavitation number similarity when

1 , (3)

with / the geometrical scale factor. The equation becomes independent of hm if √ , which corresponds to Froude number identity of the propeller rotation rate. Based on the

Froude number / , with the ship length, the equivalent condition for ship speed

becomes /√ . Accounting for the difference in water density and vapour pressure at model and full scale, the required basin atmospheric pressure can thus be calculated. The actual model speed is taken a little larger than the Froude scaled value to compensate for viscous scale effects on the ship wake such that the average propeller loading, i.e. propeller thrust, at model scale is equivalent to full scale. The propeller thrust is presented non-dimensionally by the thrust coefficient

. (4)

For the high and medium pitch settings, a correction to the model speed for the wake scale effect of +9% was estimated from the propulsion tests in order to achieve the correct thrust. This is not needed for the lowest pitch setting.

Other deviations from cavitation or Froude number identity may be necessary due to the presence of viscous (Reynolds number) scale effects on cavitation inception. Such deviations can be performed by:

varying the cavitation number by changing the basin pressure, while keeping constant and Reynolds number;

varying the Reynolds number by changing propeller rotation rate and model speed, with and held constant;

choosing a combination of these two.

In non-Froude cases, the model scale cavitation number will only be correct at one depth, which is chosen based on the cavitation location. This is often taken with the blade in the 12 o’clock position and between 70% and 100% of the propeller radius.

For each test condition mentioned in Table 1, two tests were carried out: one at the correct cavitation number and thus with cavitating propeller and one with an increased cavitation number (by increasing basing pressure) to avoid the occurrence of cavitation. This latter measurement is considered the background noise measurement since only cavitation noise is of interest. The background noise includes contributions from the non-cavitating propeller, the propeller drive train, the towing carriage and external noise sources.

4.4 Analysis and scaling

The measured data are processed and scaled in order to compare the spectra to the full-scale data, which consist of one-third-octave bandwidth RNL. A symmetric window of total length 4 m was used in order to avoid influence of acoustic reflections from the tank walls. With this small total window the variation in the distance between the noise source and the receiver is very large because of the small depth of the receiving hydrophone. This varying distance is accounted for by windowing the data into 15 segments with 50% overlap, which are subsequently processed separately before the levels are averaged.

The noise power spectrum is obtained by means of a Fast Fourier Transform (FFT) and converted to sound pressure level (SPL) using a reference pressure of 1 μPa and a constant 1 Hz bandwidth. Each segment is then corrected to RNL assuming spherical spreading loss

20 log , (5)

where r is the source-receiver distance, taken from the centre of each window segment and r0 = 1 m. The Lloyd’s mirror correction model (see Section 3) is used based on the layout of the model test

setup. Radiated noise levels can thus be corrected to source levels as

Δ . (6)

Full-scale source levels are obtained, following de Bruijn and ten Wolde (10) by adding the difference between the mean square pressures at full and model scale, that is

10 log′

′ , (7)

with

′

′

σ ,

σ ,

.ρ

ρ, (8)

which is valid for a constant bandwidth. Ignoring small differences in the model- and full-scale values for the fluid density and sound speed, this reduces to

p′

p′nn

σ ,

σ ,

.

λ (9)

for distance-corrected noise levels. The frequency scales as

,

,. (10)

The full-scale source levels are then converted to one-third-octave bandwidth. Subsequently, a background noise correction is applied: for a difference in level of more than 10 dB, no adjustment is made; for a difference between 3 and 10 dB, the background noise is subtracted from the cavitating noise; and for less than 3 dB the cavitating noise measurement is discarded due to insufficient signal-to-noise ratio. The background-corrected source levels are then averaged over all segments.

As mentioned in Section 3, the measured full-scale results have not been corrected for the Lloyd’s mirror effect and are thus given as radiated noise levels. In order to consistently compare the results, the scaled-up source level results from the model tests need to be transformed to full-scale radiated noise levels by including the Lloyd’s mirror effect for the full-scale test layout and conditions. This is done in the same way as for the computational results.

5. RESULTS The results for the three test conditions detailed in Table 1 are presented, from the highest to the

lowest pitch setting.

5.1 Highest pitch setting The cavitation observations (Figure 6 (right)) show some sheet cavitation when the propeller blade

is in the 12 o’clock position. This sheet cavitation rolls up into a tip vortex when the propeller blade leaves the wake peak. The noise levels of the full-scale tests, see Figure 6 (left), are dominated by cavitation noise.

Figure 6 – High pitch setting: comparison between model test, full-scale and computational RNL results (left)

and model-scale cavitation observation (right) The computational results agree very well with the full-scale measurements. The predicted hump

coincides with that from the full-scale measurements. The ratio between high frequency noise levels and hump level has been adjusted to match the measured values at high frequency. Only between 200 Hz and 500 Hz is a notable difference seen; this is introduced by the addition of the Lloyd’s mirror effect.

The scaled-up RNL results of the model test show a reasonable agreement with the full-scale data. For high frequencies (>5 kHz) the slope of the model test results is too steep. There are two dips in the model test results (at 250 Hz and 500 Hz). These are introduced by the Lloyd’s mirror correction. It appears that the applied Lloyd’s mirror correction is not smooth enough although the general level of the correction appears to be correct.

5.2 Medium pitch setting

At the medium pitch setting both the ship speed and propeller rotation rate are lower than for the high pitch condition. Only a small cavitating tip vortex is visible around the 12 o’clock position, see the indicated region in Figure 7 (right). The reduced cavitation extent results in lower noise levels than the high pitch setting. In the full-scale measurements, a contribution from the machinery noise is seen as the peak between 60 and 70 Hz in Figure 7 (left); this is no longer masked by cavitation noise.

The computational method overpredicts the hump below 100 Hz, see Figure 7 (left). Above this frequency, the agreement between the ETV model and full-scale test results is good. The model test results show a very good agreement with the full-scale test results; the results are within 3 dB for most of the frequency range. The dip around 250 Hz is again caused by the Lloyd’s mirror correction. At the frequencies below 100 Hz, the signal-to-noise ratio was not sufficient due to the smaller contribution of cavitation noise.

102 103 104

frequency [Hz]

10 dB

model testfull scaleETV model

Figure 7 – Medium pitch setting: comparison between model test, full-scale and computational RNL results

(left) and model-scale cavitation observation (right)

5.3 Lowest pitch setting For the lowest pitch setting only pressure side cavitation is expected. In order to check the

dependency on the Reynolds number, the model test has been carried out at two combinations of , and , (as discussed in Section 4.3). The conditions are such that both the cavitation number

at the tip of the propeller in the 12 o’clock position and are correct. The cavitation patterns at two propeller rotation rates (224 rpm and 280 rpm on full scale) are shown in Figure 8. In the left-hand picture, two small interacting cavitating vortices are seen aft of the blade. At the higher rotation rate (right-hand picture) the vortices are larger and are already visible at the leading edge of the propeller blade.

Figure 8 – Low pitch setting: model test cavitation pattern at two different Reynolds numbers (left:

Froude-scaled, 224 rpm / right: 280 rpm)

The noise levels from the full-scale tests (Figure 9) are significantly higher than those from the high and medium pitch settings. This is caused by the unstable pressure side cavitation. The noise levels are dominated by cavitation noise.

102 103 104

frequency [Hz]

10 dB

model testfull scaleETV model

Figure 9 – Low pitch setting: influence of Reynolds number on the noise levels from the model tests and

comparison to the full-scale results Note that for these conditions, no computations have been performed since pressure side cavitation

has not yet been implemented into the ETV model. The difference in cavitation patterns in the model tests leads to relatively small changes in the noise

levels. The results at the highest rotation rate (and thus Reynolds number) are marginally higher than those of the Froude-scaled rotation rate. The high rotation rate also gives a small frequency shift. These effects bring it more in line with the full-scale results. The agreement with the full-scale results is generally very good. However, as for the other cases, the Lloyd’s mirror correction introduces some dips in the results of the model tests.

6. CONCLUSIONS A validation study of URN prediction methods has been carried out for the cavitating propeller of a

single-screw merchant vessel. The URN has been predicted by means of a computational procedure and model-scale experiments and compared to full-scale measurements. The model test data were corrected for the varying distance between the noise source (the cavitating propeller) and the receiving hydrophone. Furthermore, a correction for the Lloyd’s mirror effect has been applied. Thereafter, the noise levels were converted to full-scale values in one-third-octave bandwidth. The final step was to check the signal-to-noise ratio between the noise of the cavitating propeller and the background noise. Since the focus was on the noise levels of cavitating propellers, the noise levels of tests with non-cavitating propellers were used as background noise levels.

Validation was performed for three operating conditions, each with a different propeller pitch setting. Results from both the computations and model tests show a good agreement with the full-scale results. The results are often within 3 dB of the full-scale data. For cases with pressure side cavitation, applying a non-Froude-scaled condition with higher propeller rotation rate, while maintaining thrust and cavitation number identity, helps to reduce viscous scale effects, leading to an improved agreement with the full-scale trial data. The current correction method for the Lloyd’s mirror effect introduces small dips in the resulting spectra, which are not present in the full-scale results. Despite this, the overall level of the correction appears to the correct. The application of other correction formulations should be studied further.

ACKNOWLEDGEMENTS We thank the CRS, and especially DAMEN, DNV-GL and Caterpillar Propulsion, for providing the

full-scale data and ship and propeller geometries. The model-scale measurements and data-analysis were partly funded from the TKI-allowance of the Dutch Ministry of Economic Affairs.

102 103 104

frequency [Hz]

10 dB

model test (224 rpm)model test (280 rpm)full scale

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