The effect of the propeller jet on pile groups

China Ocean Eng., Vol. 28, No. 1, pp. 17 – 30 © 2013 Chinese Ocean Engineering Society and Springer-Verlang Berlin Heidelberg DOI 10.1007/s13344-013-0080-2, ISSN 0890-5487

The Effect of the Propeller Jet on Pile Groups

Kubilay Cihan1

Department of Civil Engineering, Yildiz Technical University,

Esenler-Istanbul 34210, Turkey

(Received 26 December 2012; received revised form 22 February 2013; accepted 6 May 2013)

ABSTRACT

The objective of this study is to determine the effect of jet propeller on the damage of berthing structures combined

of armoured slope with pile groups. For this purpose, scour measurements were performed for four types berthing

structures, which were armoured slope with tandem arrangements of piles for two and three piles and with side by side

arrangements of piles for two and three piles. The effect of gap between piles on damage was investigated. The damage

level induced by propeller jet between piles was determined. The gaps were 1, 2, 3, and 4 times the pile diameter. Three

different values of Rpm (690, 820, and 950) were chosen for the tests. The diameter of circular piles is 40 mm. The slope

ratio was 1/3 and the diameter of propeller was 10 cm.

Key words: erosion; propeller jet; armoured slope; pile groups

1. Introduction

The erosion of the sea bottom in front of a berth structure and of the filling under an open berth

structure will generally be due to the wave actions at the upper part of the filling and from propeller

current from the main ship propellers and/or the bow and stern thrusters at the lower part of the filling

and of the sea bottom (Thoresen, 2003).

Pile groups are widely used in the field to support bridges and marine structures. Clearly scoring

at the piling is of importance in connection with the stability of the structure since extensive scour may

reduce the stability of the structures and leads to its failure (Sumer, 1992). The propeller jet causes

substantially serious scour at quay structures and nearshore structures. Although this type of scour is

analogous to erosion at the bridge piers, scouring in marine structures are more complex than in steady

current flows. In literature, there are a lot of studies on scour at a bridge pier. Heidarpour et al. (2010)

investigated the effect of using collars on reduction of local scour around bridge pier groups. Ezzeldin

et al. (2006) presented an experimental study on local scour around pile groups subjected to current.

And also Sumer and Fredso (2001) studied experimentally on scour around a pile under combined

waves and current. Raudkivi and Ettema (1983) carried out tests on clindrical piers of different

diameters.

Propeller-induced scour problems have been investigated experimentally by use of physical

models such as Blaauw and van de Kaa (1978), Bergh and Cederwall (1981), Verhey (1983), van

Veldhoven (2001) and Schokking (2002).

Chin et al. (1996) investigated the erosion mechanism and the parameters affecting local scour

1 Corresponding author. E-mail: [email protected]

Kubilay Cihan /China Ocean Eng., 28(1), 2014, 17 − 30

18

around a vertical pile under propeller jet flows. They found that the maximum equilibrium scour depth

is highly dependent on densimetric Froude number (Fr). Yuksel et al. (2005) investigated local scour

around pile and pile groups on a sand bed caused by submerged round jet flow. Chin et al. (1996) and

Yuksel et al. (2005) used submerged round water jet to simplify the propeller jet in their experiments.

Schokking et al. (2002) studied on bowtruster-induced damage on a slope experimentally. They

considered ducted and non-ducted propeller and 1/3 slope ratio in their study and found that the

damage on a slope occurs at the lowest part of the slope.

The erosion protection of the slope under an open piled berth structure will depend upon the angle

of the slope, the coarseness of the materials in the front of the filling, the danger of erosion from wave

action at the upper part of the filling, the danger of erosion from propeller current from the main ship

propellers and the ship bow and stern thrusters at the lower part of the filling (PIANC, 1997). Despite

all of the studies on this problem, no literature could be found on the effect of propeller jet on the scour

at berthing structures with pile groups.

The objective of this study is to determine the effect of jet propeller on the damage of berthing

structures combined armoured slope with pile group. For this purpose, scour measurements were

performed for four type berthing structures, which were armoured slope with tandem arrangements of

piles for two and three piles and with side by side arrangements of piles for two and three piles. The

effect of gap between piles on damage was investigated. The gaps were 1, 2, 3, and 4 times the pile

diameter. Three different values of Rpm (690, 820, and 950) were chosen for tests.

2. Experimental Set-up

The study was performed in a laboratory flume in Hydraulic and Coastal Engineering Laboratory,

Yıldız Technical University. Both sides of flume are made of glass and it has a propeller system inside.

Propeller rotation was controlled with an automation panel. The experimental set-up is presented in Fig. 1.

Fig. 1. Experimental set-up.

Kubilay Cihan /China Ocean Eng., 28(1), 2014, 17 – 30

19

The propeller characteristics are presented in Table 1. The propeller used in experiment turns

clockwise to produce forward thrust. It is called right-handed. It was located at 19 cm above the bed to

demonstrate the ship propeller above the bed. The model scale is 1/25.

Table 1 Propeller characteristics

Propeller diameter Dp

Blade number N

Pitch ratio P′

Blade area ratio

Thrust coefficient Ct

10 cm 4 1.4 0.4 0.51

In the experiments, four different pile groups with different gap between piles were defined. The

conducted experimental conditions are given in Table 2. in which m is cot , being the slope angle.

Propeller was operated until reaching equilibrium conditions in tests.

Table 2 Experiment conditions

Case 1 2 3 4

m=3 Two piles tandem Three piles tandem Two piles side by sideThree piles side by side

G/D 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 Rpm 690 820 950 690 820 950 690 820 950 690 820 950

The plexiglas cylindrical pile was used in tests, the pile surface was hydraulically smooth, and the

still water depth was 48 cm above the bed which corresponds to the top of the slope.

The eroded area (A) was defined to obtain the slope damage which is the function of the following

variables:

1 0 p n50 p p s, , , , , , , , , , , 0A f U D d D x z g h G , (1)

where U0 is the efflux jet velocity, Dp is the propeller diameter, dn50 is the median gravel size, D is the

pile diameter, xp is the horizontal distance between propeller and slope, zp is the vertical distance

between propeller axis and the bed, is the density of fluid, g is the acceleration of gravity, is the

dynamic viscosity of fluid, s is the density of gravel, h is the water depth, and G is the gap between

two piles. Several researchers, such as Fuehrer and Romisch (1977), Berger et al. (1981), Verhey

(1983) and Hamill (1987) have developed equations to predict the efflux velocity based on the axial

momentum theory. The maximum velocity taken from a time-averaged velocity distribution along the

initial propeller plane was termed the efflux velocity denoted as U0 (Ryan, 2002). In this study, efflux

jet velocity for free propeller jet was calculated with the following equation given by Blaauw et al.

(1978):

0 p t1.60U nD C , (2)

where n is the number of propeller revolutions per second (rps) and Ct is the propeller thrust coefficient.

Values of Rpm and calculated efflux velocity are shown in Table 3. On the other hand, velocity

measurements were performed for propeller’s rotation Rpm of 690. Velocity measurements were

performed with NDV Acoustic Doppler Velocimeter. For the measurements, gravels used in the

experiments were glued up on a plexiglas plate. In Fig. 3, axial, tangential and radial velocities are

presented. Hamill (1987), Stewart (1992), Hashmi (1993) and McGarvey (1996) suggested that the

velocity profile of the efflux plane is a two-peaked-ridge profile. In Fig. 3, it is seen that velocity


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distribution has two-peaked-ridge profile. Measured and calculated efflux velocities according to

Blaauw et al. (1978) are very close. For this reason, efflux velocities are calculated by Eq. (2).

Table 3 Calculated efflux velocity by Blaauw et al. (1978)

Rpm 690 820 950

U0 (m/s) 1.31 1.55 1.80

Fig. 2. Velocity measurements at 0.5Dp.

The velocity at a certain point and time can be written as:

U U u , (3) in which U indicates the average value of U, and u indicates the superimposed fluctuation. The root

mean square method is used to obtain distinct values for the intensity of the fluctuations (a measure for

the standard deviation) 2u . This is divided by mean velocity resulted in the relative turbulence r (Schokking, 2002).

2r u U . (4)

In Fig. 3, axial, tangential and radial components of turbulence intensity are shown. It is found

that axial, tangential and radial components of turbulence intensities have three peaks. The highest

peak occurs at z/Dp=0.4. The magnitudes of the axial, tangential and radial turbulence intensities at

this point are 0.47, 1.13, and 5.70, respectively.

Fig. 3. Non-dimensional axial, tangential and radial components of turbulence intensity.

The non-dimensional parameters are obtained by dimensional analysis as follows:

2 2

n50 n50 p n50 p n50 n50 d p n50 s, , , , , , , , , 0F Re h d D d x d z d A d Fr D d G D . (5)

Verhey (1983) considered that if Reynolds number of the propeller and Reynolds number of the

flow were larger than 7×104 and 3×103 respectively, the effect of viscosity could be neglected.


21

According to Verhey (1983), Reynolds number of the propeller (Reprop) and Reynolds number of the

flow (Reflow) could be calculated with

p m

prop

nD LRe

;

(6)

0 p

flow

U DRe

, (7)

where is the kinematic viscosity of the fluid, n is the number of revolutions per second, and Lm is the

term depending on the blade area ratio β and number of blades of the propeller N, the propeller

diameter Dp, and the diameter of hub Dh, but hub is not considered in this study. Lm is calculated with

Eq. (6) defined by Blaauw and van de Kaa (1978). 1

hm p

p

π 2 1D

L D ND

. (8)

The Reynolds numbers of the proposed speeds of rotation are compared with Verhey’s (1983)

suggestion to identify the influence of the scale effects due to viscosity. The Reynolds numbers due to

the flow for the propeller exceeds Verhey’s (1983) suggestion. The Reynolds numbers due to the

propeller are slightly lower than 7×104; however, Blaauw and van de Kaa (1978) and Verhey (1983)

proposed that these scale effects were insignificant. The Reynolds number for the jet is larger than

3103 satisfying the criteria for Froudian scaling (Hamill, 2010). In this study, the values of Reynolds

number of the propeller were calculated between 1.3104 and 2.2104. And also the values of

Reynolds number of the flow were calculated between 13104 and 18104. Hence, the effect of

viscosity could be neglected. Additionally, water depth, propeller diameter, propeller distance to the

slope and bed, pile diameter, and gravel median diameter were considered to be constant. Moreover,

non-dimensional parameter, s , was considered inside of the Froude number to consider submerged

flow. As a result, the eroded area is written as follows:

2

n50 2 dS A d f Fr . (9)

Densimetric Froude number Frd is defined as:

d 0 n50 sFr U gd , (10)

where U0 is the jet exit velocity, dn50 is the mean sediment diameter, s is the sediment density, and is

the water density.

Three different Froude numbers were considered between 3.6 and 6.2 in the experiments. Here,

Froude numbers were calculated with median coarse gravel diameter (dcn50).

Slope with coarse gravel and protection layer was composed of fine gravel. The ratio of coarse

gravel to fine gravel was chosen as 2.1 in the experiments. The gravel characteristics are given in

Table 4. Table 4 Characteristics of gravels

dn50 (mm) dn60 (mm) dn90 (mm)

Coarse gravel dcn50=8.62 9.53 12.54

Fine gravel dfn50=4.00 4.50 6.05


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3. Results and Discussion

The mechanism of scouring is very complicated. The basic mechanism of local scouring for pile

is the downflow at the upstream of the pile and horse shoe vortex at the base of pile. Separation of the

flow at the sides of the pile also creates the so-called wake vortices. These wake vortices are not stable

and shed alternately from one side of the pile and to the other (Heidarpour et al., 2010). For non-

cohesive sediments (sand), the main resistance to erosion is provided by the submerged weight of the

sediment influence by the grain size, i.e., gravity forces (Babu et al., 2003). If the drag and lift forces

occurred due to flow exceed the gravitational and frictional forces, the surface is eroded.

In tests, it was observed that when the stones in front of the piles moved down due to downflow,

stones behind the piles along the slope moved firstly in the lateral direction and then they moved down

on the slope due to wake vorticity. Wake vorticity does not have an effect on stones at the upper part of

slope. Therefore, these stones only moved down under propeller jet effect.

Model profile was measured by using electronic limnimeter before and after the tests. The

sensitivity of the limnimeter is 0.02 mm. A 2 cm measurement step was chosen. So, scour and

accumulation zone on slope and upstream of the pile can be determined.

The cumulative variation of the scour and accumulation along the slope for two piles tandem for

different Rpm (690, 820, and 950) are presented in Fig. 4. The gaps between piles are 1, 2, 3, and 4

times the pile diameter. Δz=zf zi is the difference in vertical elevation at a point measured on the

model, zi being the initial elevation and zf being the final elevation which is after the damage under

propeller jet effect.

From the following figures, it is clear that while an eroded area occurs behind the first pile, an

accumulation area occurs between two piles. On the other hand, as the spacing between the piles

increases, deformation on slope with two tandem piles shows different behavior. An eroded area

develops in front of the second pile and accumulated area between two piles increases. It may be said

that the pile group act as a single large pile for small gap values (1 and 2 times propeller diameter). For

large gap values, the interference effect between the individual piles decreases. Additionally, the

existent pile at the upstream of the second pile causes a sheltering effect. This effect reduces incoming

velocity for downstream pile (the second pile). In addition, the reason reducing velocity for

downstream pile is the increasing spacing between piles. In Fig. 4 the cumulative variation of the scour

and accumulation along the slope for two piles tandem are shown for G/D=1 and G/D=4.

Fig. 4. Cumulative variation of the scour and accumulation along the slope for two piles tandem for different Rpm

(690, 820, and 950).


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Armoured materials on slope resist to environmental loading with its weight individually. The

damage of armoured layer was described by the erosion area around still-water level by van der Meer

(1988). This erosion area includes settlement and displacement of armour material. Open berth type

structures consist of piles and armoured slope. Design rules of these slopes are similar with rubble

mound breakwater slopes. During tests, it was observed that deformation of slope occurred due to rock

motion on slope under propeller jet. Because of turbulence characteristics of propeller jet, rocks on

slope may move in each direction. So it is difficult to determine the number of rocks which have net

movement. So, damage parameter (S) was used to the determine effect of propeller jet on pile groups

combined armoured slope. The damage parameter is defined by van der Meer’s (1988) formula. This

damage level is defined by

n50

AS

D , (11)

where S is the damage level, A is the erosion area between piles, and n50D is the nominal diameter of

the rock. According to van der Meer (1988), the level of the start of the damage, S=2–3, is equal to the

definition of “no damage” and “failure” is equivalent to S=12 for a two-diameter-thick armor layer on

a filter layer and for a slope of 1:3. Cumulative damage levels for Case 1 (all piles group) are given

against the densimetric Froude numbers in Fig. 5. Densimetric Froude number was chosen as a non-

dimensional parameter to define limit of damage level because it depends on rock parameter (relative

density) and efflux velocity. Determination of these parameters is easy for designers. In Fig. 5, it can

be seen that the start of damage level exceeds only two test results for G/D=4. But the damage level

increases with the increasing densimetric Froude number. Therefore, the damage levels increase with

the increasing gap between the first and the second piles. For this reason, when an open type berth

structure is designed, the choice of gap value between piles becomes the more important parameter

against the propeller jet effect.

Fig. 5. Damage parameters for two-pile tandem.

The cumulative variation of the scour and accumulation along the slope for three piles tandem for

different Rpm (690, 820, and 950) are presented in Fig. 6 for G/D=1 and G/D=4. From the following

figures, it is observed that the behavior of pile groups located in the smallest spacing (G/D=1) is

different from that of other pile groups. For this pile group, there is a scour zone between the first and


24

the second piles. On the other hand, an accumulation area is developed between the second and the

third piles. For such small gap, while the vortices created around especially the first and the second

piles interact with each other, vortices interaction between the second and the third piles decreases. As

the spacing between the piles increases, an eroded area develops in front of the second and the third

piles and an accumulated area between piles develops. It may be said that the pile group acts as a single

large pile for a small gap value (1 time propeller diameter). For large gap values, the interference effect

between the individual piles decreases.

Fig. 6. Cumulative variation of the scour and accumulation along the slope for two piles tandem for different Rpm

(690, 820, and 950).

Cumulative damage levels for Case 2 (all piles group) are given in Fig. 7. The damage level

increases with the increasing densimetric Froude number. Therefore, the damage levels increase with

the increasing gap between piles. According to Fig. 7, for G/D=4, damage levels almost reach the

failure damage condition. Under lights of this result, the gap value between piles should be chosen

smaller than 4D to prevent erosion on slope.

Fig. 7. Damage parameters for three-pile tandem.

Cumulative variation profiles along the transversal line in front of piles and behind the piles

(G/D=1 and G/D=4) for Case 3 are presented in Figs. 8a–8d for different Rpm respectively. For lower

gap values (G/D=1 and G/D=2), eroded areas usually develop at both upstream and downstream of the

piles. These eroded areas increase with the increasing Rpm for all gaps. When the distance between

propeller axis and pile’s axis rises (G/D=3 and G/D=4), flow interaction between two piles decreases.

Therefore, eroded areas develop at upstream of piles due to effect of propeller jet. But the size of

eroded area for G/D=4 is smaller than that of other side-by-side arrangements. On the other hand, an


25

accumulated area occurs locally at the axis of propeller due to sliding of rock on slope. It can be

observed that the flow interaction between two piles is the lowest for G/D=4 in proportion to other

arrangements.

Fig. 8. Cumulative variation profiles along the transversal line in front of piles and behind piles.

Cumulative variation profiles along the transversal line in front of piles and behind piles (G/D=1

and G/D=3) for Case 4 are presented in Figs. 9a–9d for different Rpm respectively. For this side-by-side

arrangement, eroded areas develop at both upstream and downstream of the piles. These eroded areas

increase with the increasing Rpm. For all side-by-side arrangements, eroded areas developing at both

upstream and downstream of the piles are not symmetric due to using right-handed propeller in the

experiments. The biggest erosion occurs in front of the pile which is placed along the propeller axis.

For this reason, rock size in front of these piles should be increased to reduce erosion. Additionally, for

a high gap value (G/D=3), accumulation areas occur at upstream of side piles and their sizes increase

with the increasing Rpm.

Fig. 9. Cumulative variation profiles along the transversal line in front of piles and behind piles.


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The cumulative variations of the scour along the protection layer in front of the pile with different

values of Rpm for Case 1 and Case 2 were also measured and typical measurement is shown in Fig. 10a.

It is determined that gaps between piles do not have an important effect on scouring in front of the pile

(on protection layer). On the apron, generally the propeller jet mechanism is active and eroded area

increases with the increasing Rpm. Therefore, in a short distance at the upstream of the pile, the scour

depth is smaller than the scour upstream of protection layer where it is closer to the propeller. In this

zone, pile mechanism is active and downflow due to the pile obstruction acts on the protection material.

But the pile obstruction causes the decrease on the jet energy. For this reason, eroded area upstream of

the pile is smaller.

Regression analysis is used to deal with the experimental results and to finally obtain equations

and relations relating scour parameters to the flow parameters and the geometric properties of the piles.

Fig. 10. Cumulative variations of the scour along the protection layer in front of the pile for Case 2.

4. Development Equations

Relative scour depths z/Dn50, relative distances from propeller axis xp/Dp, relative distances

between piles G/D, densimetric Froude number Frd, and damage parameter S defined by van der Meer

(1988) are used to develop equations which represent scours on protection layer upstream of the pile

and between piles. For this purpose, four different cases are taken into account to develop equations.

Pile group tandem: Regression analysis is shown that pile number or the gap between piles does

not have any effect on scour on protection layer downstream of the first pile. For this reason, average

scour depth at all measurement points between maximum scour points the pile mechanism is active

and the first pile is used in regression analysis. It is seen that the location of maximum scour depth for

all cases occurs at 4.2–4.4 times the relative distance from propeller axis. For this reason, relative scour

depths before this point are not taken into account in regression analysis. In Fig. 11, the locations of

scour used in regression analysis are shown.

Fig. 12 shows the predicted from Eq. (12) and the observed relative scour depth together. It is

seen that the predicted and observed relative scour depths are compatible. 1.998

p 2.74

d

fn50 p

0.3xz

Frd D

, (12)

Correlation coefficient R=0.925.

Two-pile tandem: Propeller turning direction in tests is from left to right. So scour on the left side

of the model are more than that on the right side. Therefore, scour depths occurring under different jet

flows and different gap distances at point A are considered for regression (Fig. 13). Eq. (13) is


27

obtained with the regression analysis and the correlation coefficient is R=0.922.

Fig. 11. Location of scour at upstream of piles used in Fig. 12. Predicted and observed relative scour depth. regression analysis for pile group tandem.

Fig. 13. Observed point for two piles side-by-side. Fig. 14. Observed points for three piles side by side.

0.277

2.72

d

fn50

0.035z G

Frd D

. (13)

Three-pile tandem: In this case, scour depths occurring under different jet flow and different gap

distances (G=2D, 3D, and 4D) at points A and B are considered for regression (Fig. 14). Eqs. (14) and

(15) are obtained with regression analysis for points B and A and correlation coefficients are R=0.927

and R=0.966, respectively. 1.03

4.29

d

fn50

0.00074z G

Frd D

; (14)

1.567

3.42

d

fn50

0.0026z G

Frd D

, (15)

Damage parameter between piles on slope: damage parameters for three-pile tandem are

determined by use of van der Meer’s formula. It can be found that Eq. (16) can represent fairly damage

parameters between piles on slope (R=0.948). 0.87

3.42

d0.07G

S FrD

. (16)

5. Conclusions

The study was performed in a laboratory flume in Hydraulic and Coastal Engineering Laboratory,

Yıldız Technical University. The objective of this study is to determine the effect of jet propeller on

the damage of berthing structures combined armoured slope with pile group. For this purpose, scour

measurements were performed for four type berthing structures, which were armoured slope with


28

tandem arrangements of piles for two and three piles and with side by side arrangements of piles for

two and three piles. The effect of gap between piles on damage was investigated. Gaps were 1, 2, 3,

and 4 times the pile diameter. Three different values of Rpm (690, 820, and 950) were chosen for tests.

The conclusions of this study are as follows.

(1) For Case 1, while the distance between the piles decreases, the accumulation area between

piles decreases. It is observed that the pile group acts as a single large pile for small gap values (1 and

2 times propeller diameter). For large gap values, the interference effect between the individual piles

decreases.

(2) Damage parameter S is used to determine the effect of propeller jet on pile groups placed

armoured slope. The damage level increases with the increasing densimetric Froude number. Therefore,

the damage levels increase with the increasing gap between the first and the second piles.

(3) For Case 2, as the spacing between the piles increases, an eroded area develops in front of the

second and the third piles and an accumulated area between piles develops. It is observed that the pile

group acts as a single large pile for small gap values (one time propeller diameter). For large gap

values, the interference effect between the individual piles decreases.

(4) For Case 3, an eroded area develops in front of piles generally. But erosion depth decreases

with the increasing spacing between piles. On the other hand, accumulated areas occur due to increase

of spacing. When the distance between propeller axis and pile’s axis increases, the flow interactions

between piles decrease.

(5) It is observed that the behavior of Case 4 is similar to Case 3.

(6) Gap values between piles do not have an important effect on the erosion depth in front of the

pile (on protection layer). On the apron, generally the propeller jet mechanism is active and the eroded

area increases with the increasing Rpm. Therefore, in a short distance at the upstream of the pile, the

scour depth is smaller than that upstream of the protection layer where it is closer to the propeller.

(7) Gap value between piles should be chosen smaller than 4D to prevent erosion on slope. If the

damage level reaches failure condition, rock on slopes begin to wash-out due to propeller current

rapidly and the maintenance of the slope will be difficult and expensive.

(8) For side-by-side arrangements, the biggest erosion occurs in front of the pile which is placed

along the propeller axis. For this reason, rock size in front of these piles should increase to reduce

erosion.

(9) Empirical equations are developed for scour occurring under different conditions in regression

analysis. These could help in predicting the relative scour depth at different locations in pile groups.

On the other hand, erosion-induced propeller effect on slopes depends on a lot of parameters. The

effect of other parameters (height of propeller from bottom, diameter of propeller, diameter of pile, etc.)

should be investigated to understand erosion mechanism exactly.

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The effect of the propeller jet on pile groups

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