Cavitation and noise performance of offshore thrusters

A,RCHEF

LIPS TECHNICAL REPORT 1003-7510

CAVITATION AND NOISE PERFORMANCE OF OFFSHORE THRUSTERS by

C.C. Schneiders

and

C. Pronk Prepared for the

OFFSHORE SOUTH EAST ASIA CONFERENCE AND EXHIBITION S? NGAPORE

17-20 February 1976

Technkche Hogeschoal

ABSTRACT

The large powers required for dynamic station keeping

of offshore drilling platforms have to be transmitted into the highest possible thrust, exerted at the location and in the direction required.

At the same time, thruster generated noise should interfere with the acoustic position sensoring system to an acceptabel extent only.

This paper gives experimentally obtained data on noise generated by controllable pitch propeller

thrusters. It is shown that these noise data are

generally applicable to various configurations of the same thruster type.

It is also shown how main design parameters

simultaneously affect thruster performance and noise.

I NTRODUCT ON

Dynamic positioning of offshore drilling platforms requires a thruster system to counter the

environmental conditions.

It _{is assumed that the forces and moments, acting}

on the object to be dynamically stationed, are known for the DP-design condition. Preliminary decisions must be made with respect to the number of

thrusters, their location and thrust as well as the

direction(s) in which the thrust is to operate.

This means that the decision over the adoption of an X-Y recta-linear system with fixed-in-position

thrusters each working in two directions only, a Vector system with azimuthing thrust units, or a combination

of the two, has been made. In Figure 1, two thruster

configurations are shown. The thrusters in a configuration can be of different type both hydrodynamically and mechanically.

Hydrodynamically, four types of thrusters can be

without predominant thrust direction, each with fixed pitch or controllable pitch propeller.

Thruster types reviewed in this paper are ducted controllable pitch propellers (CPP).

Thrusters can be driven through a straight gearbox or through a right-angle drive. They can be flxed-in-position, retractable or of the azimuthing type. Ninety degrees trainable nozzle thrusters can be made

operational both ¡n the longitudinal (X) and the

athward ship (Y) direction. tn Figure 2, these

mechanical types are reviewed.

Table i indicates which thruster configurations

are practical ¡n various types of offshore vessels.

The technical feasability of dynamically positioned vessels is decided by the possibility to achieve a pre-determined thrust at a sufficiently low power. Moreover, when an acoustic position sensoring system is used, propeller generated noise must be reduced to a level acceptable for this system. Operational

economics depend to a large extent on fuel

consumption ¡n part-load condition and on costs of preventive maintenance.

Obviously, for vessels using an inertia position sensoring system, rather than an acoustic one,

propeller noise considerations play a role only with respect to crew comfort.

There is little published information on noise

generated by ducted CPP. The reason ¡s that systematic

tests are merely carried out on model scale and model

tests can only be carried out in a depressurized tank of sufficiently large dimensions, see reference [i].

In this paper, experimental data obtained in this way,

are given. It ¡s shown that these noise data are

generally applicable to various configurations of the

same thruster type. It ¡s also shown how main design

parameters simultaneously affect thruster performance and noise.

DESIGN REQUIREMENTS

There are four important design requirements that determine the feasability of thrusters for dynamic station keeping systems.

The technical feasability is decided by thruster efficiency and by the generated underwater noise

level. Operational economics mostly depend on part-load performance and costs of preventive maintenance. Thruster efficiency ¡n the DP-design condition

should be better than T/SHP = 11.0 whereas a value

of 1L+.0 should be aimed at ¡n 5Q0/ part-load

condition. The noise level should preferably not

exceed a value of 125 dB re i p Pa, 1 Hz, i m, ¡n

the specific frequency range. Mean time between failure should be better than 750 000 hours with reasonable preventive maintenance prescribed and accepted.

CAVITATION AND NOISE

Cavitation is a well-known phenomenon that generates noise and sometimes leads to erosion. Thrusters may show the following types of cavitation: tip vortex, sheet cavitation at the suction side beginning at the tip, cloud cavitation originating from the sheet cavity and mid chord bubble cavitation. Normally, pressure side cavitation is not present because of

Propeller generated noise is observed both inboard and outboard. Inboard noise is of interest with respect to all types of vessels since it causes

discomfort to passengers and crew. In this paper,

only propeller generated outboard noise is dealt

with and simply referred to as noise. This noise used

to be of interest to navies only for well-known

reasons. Recently, a wide interest was taken by those concerned with acoustic systems for dynamic station keeping of offshore drilling platforms ¡n view of the technical feasability of such systems.

Noise or sound ¡s a disturbance, that propagates through an elastic material at a speed characteristic for that medium. The disturbance is caused by a

collapsing cavity, producing (sound) pressure waves ¡n the surrounding water. Because of the number of

cavities and their complicated shapes, a number of sound pressure waves of various frequencies is generated. Sound pressures encountered ¡n practice

range from l0 to N/rn2. To avoid large

exponents in the numbers involved, logarithmic scales are used. Sound pressure levels are commonly

used to describe a sound. The decibel (dB) is a unit

of sound pressure (squared) level, in short notation:

2

L = 10 log 2_. - 20 log

2_

p (i)

= reference rms sound pressure, io_6 N/rn2

for underwater sound. See reference [2].

Noise Spectrum.

The only possibility to describe the pressure

where: L = sound pressure level in decibel (dB)

P _{= root mean square of sound pressure in}

field, resulting from a number of sound pressure

waves of many frequencies, is by means of a

spectrum, reference [3], ¡n which for each of

the components of the pressure waves the sound

pressure level is given. In Figure 3, an example

of a thruster spectrum is shown. Typical for such a spectrum is that the sound pressure level distribution over the frequency range of the components is rather constant ¡n the frequency

range of interest. This means that there is no

specific frequency band where the noise generated by the propeller is low so that this band should be used for the acoustic system. Such systems have

acertaindesign signal-to-noise ratio. This ratio

is to be increased as much as possible. On the

other hand, cavitation is to be avoided completely or reduced as much as possible. Recently, attemps have been made to correlate certain types of cavitation with their resulting contribution in the noise spectrum, see reference [i].

Since presently no method is available to obtain a

specific spectrum by controlling the type and extent

of cavitation, it is common to try and reduce the

overall cavitation pattern as much as possible.

Model Test Results.

In order to investigate the possibilities to reduce

the overall cavitation pattern and noise level, a

series of tests was started. Three blade designs were tested ¡n the depressurized towing tank of the

Netherlands Ship Model Basin (NSMB). All propellers were working in a slightly modified nozzle NSMB 37 at constant RPM. The standard or reference blade

design is shown in Figure L

Test conditions:

- pod with right-angle drive was located upstream, so wake conditions were unfavourable,

- noise was measured at the suction side of the propeller, where the higher levels occur,

- propeller model shaft speed was considerably higher than follows from Froude's law, this being a good compromize between all model rules involved.

Full scale predictions, obtained from the model tests with the standard blade design, are shown ¡n Figure 5.

Cavitation patterns are given in Figure 6 for 220 RPM and 1/3, 2/3 and 3/3 of design power respectively. Comparing Figures 5 and 6 gives an idea about the relation between size of sheet cavity and noise level.

Impact of Blade Design.

Next to the lack of tools to compute the influence of thruster design on noise levels, the effectiveness of

design changes on noise levels plays a role. In this

respect, thrusters with and without predominant thrust direction must be distinguished.

For symmetrical thrusters, only blade thickness, blade contour and nozzle parameters can be used as

variables. From the modeltests mentioned above, it

appears that even rigorous variations ¡n these

parameters, such as those described in reference [Li], did not change the noise level with more than 2 or 3 decibels for those cases where already a relatively high noise level was present. Changing thickness and blade contour can be reasonbly effective on the threshold of cavitation inception.

For non-symmetrical thrusters, in addition to the above, both radial pitch distribution and camber can be varied with the proviso that a desired relation between ahead and astern performance restricts the range of this variation.

Given the fact that the actual noise level is rather insensitive to changes ¡n the above mentioned

secondary parameters, the only remaining way to reduce thruster noise is to vary the main parameters. This of course leads directly to a trade-off with thrust

efficiency.

Design chart.

For a particular hydrodynamic thruster configuration, the cavitation pattern ¡s only depending on the

loading, which is represented by its value of torque

coefficient K and the cavitationnumber G

Q n

Consequently, the noise level is depending on the

same parameters. The effect of changing main

parameters can be derived directly from the effect of

K and e on the noise level. The dependance of

Q n

cavitation on KQ and e is normally given in a

cavitation inception diagram, see reference [5 For a constant RPM thruster in bollard condition, such diagram ¡s made for the complete range of pitch values. Since there is a unique relation between KQ and pitch, the cavitation inception

diagram is based on KQ. Lines of constant noise level in a given frequency range can also be plotted in a

KQ - e diagram. However, sound pressures cannot be

given in absolute values because, amongst other parameters, the static pressure which is already

included in e, also has an influence on the absolute

value of the pressure fluctuations. In addition, the

absolute value of the pressure fluctuations causes a frequency shift when scaling. However, these differences in frequency are relatively small while

in the spectrum the sound pressure level is only

slightly depending on the frequency itself. In

conclusion, the influence of frequency on sound pressure levels can be neglected for the purpose of developing a design chart. Using a non-dimensional

number concerning the sound pressure level radiated from bubbles, see reference [6], the

dimensionless sound pressure level can be given as:

C p p

p D -pCnD

p+gh

Model test results were made dimensionless as described above and plotted in Figure 7. The curve of cavitation inception in this figure is the

inception of suction side sheet cavitation; the inception of tip vortex is at about the same K0 value. For this type of thruster the inception line

is only determined by bucket width of two-dimensional sections and by inhomogenity of the incoming flow. The two-dimensional buckets determine the slope while

the velocity fluctuations in incoming flow cause a shift of the inception curve to lower KQ values.

For non-symmetrical thrusters, the position of the inception line can be influenced by reduction of tip load for tip vortex inception delay and by proper cambers for inception delay of sheet cavitation.

EFF C I ENCY

In the DP-design condition, thrusters should have a very good efficiency in view of the power to be

installed. However, thruster loading in de prevailing

type of weather, is far lower than l000/. lt is even

below 5O over long periods of time.

In fair weather conditions, power management will prevent that more power generation sets and thrusters are running than strictly necessary. Proper programming of the computer can contribute to reduction of overall power absorbtion. The result ¡s an appreciably lower fuel consumption. Adequate thruster design can contribute to this

effect.

The quality of a thruster ¡s normally judged by the

thrust-power relation ¡n bollard condition. This relationship is determined by two parameters, the

thruster load and the merit coefficient as follows:

2/3 T lT2 = 26.7 SHP 712 14D (3) where

D gives a direct comparison of thrust

efficiency for different thruster geometries.

Results of measurements on performance of thrusters

have been analyzed in reference

_[71.

shown in Table 2 and it should be noted that n0 remains relatively constant over a wide range of pitch ratios. The following conclusions were drawn.

In the DP-design condition, the maximum value for the

torque coefficient K is 0.055 for reasons of

efficiency and mechanical pitch angle capability. In 50/3 part-load condition, the K0 value for the same thruster should not be lower than 0.025, because otherwise the efficiency will drop below acceptable values in view of overall fuel economy. Using

D as given ¡n Table 2, Formula (3) is

graphically represented ¡n Figure 8 a/c, from which various alternatives can be read.

NOISE AND EFFICIENCY

Model test data on noise were analized and a design chart was presented, see Figure 7. This information can also be plotted ¡n Figure 8, thereby enabling the DP-system designer to obtain an idea about power required and noise generated ¡n an early stage.

Assuming a certain shaft submergence, lines of constant sound pressure at one diameter distance from the

propeller can be constructed. The definition of C,

Formula (2), leads to the following relation between C p

and propeller tip speed: V. = constant

¡C2.

From Figure 7, the KQ value can be read for each Ve.. With the definition of KQ the relative SHP per unit

disc area can be found.

As an example, sound pressure levels of 115, 120 and 125 dB were calculated and plotted ¡n fig. 8f, assuming 10m

shaft submergence. It can be seen that sound pressure

levels are mainly depending on SHP per unit disc area and that a variation of about 10 dB can be found within the practical range of thruster

loadings. The effect of deviations from a 10m shaft submergence ¡s shown in Figure 9.

RELIABILITY

''Reliable'' should not be used as a word giving some general idea about the quality of equipment.

Objectives and requirements should be specified, redundancy built ¡n and preventive maintenance

accepted and carried out. Under such conditions, mean time between failure should be better than 750.000

hours, see reference [8] and [9].

CONCLUS I ONS

- Main design parameters, thruster loading and

cavitation number, have a major influence on sound pressure levels.

- Blade design only affects generated noise in the

range of torque coefficients where sound pressure levels are low already.

- _{Thrusters designed within today's practical range}

of main parameters may show a difference in generated sound pressure levels of not more than

10 dB.

AC KNOWLEDGEMENT

We wish to thank the Netherlands Shipping Union (NSU Scheepvaart) for their valuable contribution to the research into propeller generated noise.

REFERENCES

Oosterveld, M.W.C. and van Oossanen, P.: "Recent Results of Tests in the NSMB

Depressurized Towing Tank'', First Ship Technology and Research (STAR) Symposium, Washington D.C.,

August

_{26-29, 1975.}

Urick, Robert J.: Principles of Underwater Sound

for Engineers, Mc Graw-Hill Book Co., Inc.,

New York

_(1967).

Stuurman, A.M. : ''Fundamental Aspects of the

Effect of Propeller Cavitation on the Radiated Noise'', paper presented at the Symposium on ''Nigh Powered Propulsion of Large Ships'',

Wageningen, December 10-13, 197L+.

L4 _{Brown, Neal A. and Norton, John A.: "Thruster}

Design for Acoustic Positioning Systems'',

Society of-Naval Architects and Marine Engineers,

San Diego Section,

_197k.

Van Gunsteren, L.A. and Pronk, C.: ''Propeller Design Concepts', International Shipbuilding

Progress, No.

_{227, 1973.}

de Bruijn, A. and ten Wolde, T.: "Measurement and Prediction of Sound Inboard and Outboard of Ships as Generated by Cavitating

Propellers'', paper presented at the Symposium on ''High Powered Propulsion of Large Ships'',

Wageningen, December 10-13, 197k.

Schneiders, C.C. and Pronk, C.: "Performance

of Thrusters'', paper OTC _{2230 presented at the}

1975 Offshore Technology Conference, Houston,

Drenth, B.W.: 'Reliability of Controllable Pitch Propellers'', International Shipbuilding Progress, No. 220, 1972.

Hageman, L.A.S. and Schneiders, C.C.: "Propeller Design Consideration for Dynamically Positioned Vessels'', paper ¡n preparation.

LIST OF SYMBOLS

C -= dynamic pressure coefficient

C = velocity of sound in water [m/sec]

D = propeller diameter [ml

e = vapour pressure [N/rn2]

g = acceleration of gravity [m/sec2]

h = shaft submergence [mi

KQ = torque coefficient

L = sound pressure level in decibel (dB)

n = rotational speed [sec]

P = propeller pitch at 0.7 Radius [ml

p = rms sound pressure [N/rn2]

Po = static pressure [N/rn2]

= reference rms sound pressure [N/rn2]

SHP = shaft horsepower [metric HP]

T = propeller thrust [kgf]

V.

x = distance to propeller [rn]

no = merit coefficient

p = specific density [kg/rn3]

= cavitation number n

TABLE 1, THRUSTER CONFIGURATIONS

X = Longitudinal direction Y Athward ship direction V = Azimuthing direction Type of Vessel

Hydrodynarnic Type Mechanical Type

Non-ducted nozzle tunnel symmetric predominant

direction straight-drive right-angle drive fixed-in-position retractable azimuthing Drillship i 2 3 ¿4 5 6 Semi-submersible 7 8 Spar type 9 10

Pipe laying ship Il

12 Pipe laying 13 barge 14 Mono hull 15 utility vessel 16 Double hull 17 utility vessel 18 19 Mining vessel 20 21 22 X X X X X Y X XV X XV V XV XV XV X XV X XV X XV V X Y XV Y Y Y Y Y Y Y Y Y V Y Y Y Y V Y XV XV V Y V Y Y Y Y Y Y Y X X X X X X V XV X X X X X X X XV V X X XV X X X X X XV X X X X X X X X X X X Y Y Y Y XV XV V V XV Y Y Y Y Y Y Y V VV Y Y V XV X XV X XV X X XV XV XV XV XV XV XV X XV X Y XV X X Y Y Y Y Y V V V V

Thruster type average range

Ducted propeller ¡n

predominant thrust direction 3.65 3.60-3.75

In opposite direction 1.70 1.60-1.80

Ducted propeller without

predominant thrust direction 3.15 O,3Ç 3.10-3.25

Non-ducted propeller in

predominant thrust direction 2.90 o, 33 2. 85-2. 95

Non-ducted propeller with

LIST OF ILLUSTRATIONS

Figure 1 , Thruster Configurations

Figure 2 , Mechanical Types

Figure 3 , Spectrum, Typical for Ducted Thruster

Figure 14 _{, Standard Thruster Blade Design}

Figure 5 , Noise Test Results with Standard Thruster

Figure 6 , Cavitation Patterns.

Figure 7 , Dynamic Pressure Coefficient as a Function

of Torque Coefficient and Cavitation Number

Figure 8 , Relation between Main Parameters of

Thrusters

Figure 9 , Effect of Shaft Submergence on Sound

VECTOR SYSTEM

FIGURE

FIGURE

2, Mechanical Types

a) TUNNEL THRUSTER

b) RETRACTABLE TUNNEL

e) FIXED POSITION

THRUSTER

C) RETRACTABLE NOZZLE

d)X/Y NOZZLE THRUSTER

f) AZIMUTHING THRUSTER

130

120

110

100

dBre lPa,1Hz,lm1

9

14

18

22

28

F

EN CV

nR

RADIUS

M..THICKNESS

PITCH

EXPANDED OUTLINE- PROJECTED

OUTLINE

10.0 21.8 34.0 48.0 62.8 79.1.

110.13 164.1.

FIGURE

14, Standard Thruster Blade Design

214.4 214.1. BLADE SECTIONS

PROPELLER DATA

EXPANDED BLADE AREA RATIO

.481.7 DELIVERED HORSEPOWER 1975 H.P PROPELLER REVOLUTIONS 220 R.P.M. 1. 1382.0 .9 121.3.8 .8 1105.6 .7 967.1. .6 029.2 .5 691.0 .1. 552.8 .35 /.83.7 .29 397.5 98.0 0. 540

90

NOISE 130

LEVEL

110

FIGURE

5,

Noise Test Results with Standard Thruster

[dBre 4LPablHzlmj

/v

I

120

33.3

66.7

100

POWER

/

POWER

= 33

POWER =67

R.P. M.=220

R.P. M.=220

6,

C7

_b

_t

4

3

.010

.020

Cp:.5

.030

.040

TORQUECOEFFICIENT K0

(b)

N

o

o

D-I

I

DUCTED PREDOMINANT DIRECTION

'

DUCTED SYMMETRICAL

1

NON-DUCTED SYMMETRICAL\

NON-DUCTED PREDOMINANT OIRECTION\

f10

125 dB

120dB

115 dB

\\

IiIt_ j

j

IIiIiWAWI1

JI,,

_{IiIW: A_4}

11111 'N 11

111111' AW

,IIi,,M_ r

RIIIIIV__4

IIt_

IItMAW41I

(e)

(f)

35

300

500

DIAMETER (M)

HORSEPOWER PER UNIT AREA C HP/M2

()

Cd)

125

NOISE

LEVEL

¶

120

115

110

dB re 1LPa,1Hz,1D1

30 rn/s

= .04

10

20

30

FIGURE 9, Effect of Shaft Submergence on Sound

SHAFT SUBMERGENCE