River Crossing Pipeline - Paper

J u r n a l Infrastruktur dan Lingkungan Binaan Infrastructure and Built Environment Vol. IV No. 1, June 2008

Joko Nugroho, Hendra Achiari, and Demi Harlan 89

Preliminary Study on Scouring at Parallel River Crossing Pipelines

Joko Nugroho1), Dhemi Harlan1), and Hendra Achiari2)

ABSTRACT This paper reviews previous research conclusion on submarine single pipeline or river crossing pipeline. Preliminary data was obtained from experiments on single and parallel pipelines. Experiments on counter measure for scour around river crossing pipeline were done as well. The experiments were done by applying time-varied discharge, similar to a hydrograph discharge sequence magnitude. The experiments were done using noncohessive sediment under codition of clear water scour. The results showed that maximum scour for single pipeline were observed for pipeline placed at a distance of half and one time of the pipe diameter from the bed. Protection by placing weir at the downstream of pipeline provide significant result. Key words: scour, river crossing pipeline, river bed, scour protection.

I. INTRODUCTION Pipe line has been used in transporting water, oil and minerals (in the form of slurry). It is often that the pipe line must be lied across the river bed and even lied/burried below the seabed. Due to the dynamic condition of flow and sediment at coastal, ocean and river environment it is possible that the pipe will be exposed to the flow. The exposure of the pipe line to the flow could cause damaged to the pipe, due to the abrassion of the sediment transported by the flow, especially in the high flow condition. It is also possible that along the river there will be more than one pipe line crossing in parallel, hence several pipe lines lied in a close distance crossing the river.

In the beginning of the pipe line placing, the pipe usually placed well below the river bed. In this condition, there is no effect on flow to the pipe and its surrounding, and the pipeline is safe. However, due to the dynamic of the river and sediment transport condition, a degradation of the river bed level will caused the pipe being uncovered and finally exposed to the flow. The protrution of the pipe above the bed level will lead to the formation of vortex shedding with has transversal axis, at the downstream of the pipe. The pressure gradient between the upstream and the downstream of the pipe will cause piping below the pipe. These two mechanism will cause the sediment particles around the pipe being removed and finally the pipe will be unsupported any more by the bed level.

This condition will impose additional force to the pipe, i.e. drag force due to the flow and probably, debris flow.

This paper presents experimental results on scouring at submerged pipe line which placed across a laboratory flume. The experiments aimed to: provide better understanding on scouring mechanism around pipe line subjected to time-varied discharge in single river crossing pipeline and parallel river crossing pipeline, and study the effective counter measure for parallel river crossing pipeline. There are three kind of experiments, i.e. single pipe experiments, experiments on scouring protection, and scouring at parallel pipelines.

This paper aimed to presents the scouring mechanism at river crossing pipeline and the possible countermeasures.

Chiew (1990) studied the mechanism of scouring around submarine pipelines. He concluded that the initiation of piping is caused by the piping mechanism. The piping is due to the difference of pressure at the upstream and downstream of the pipe. The onset of scour will not be occurred for the ratio of flow depth to the pipe diameter less than 3.5 and the pipe is just embedded at the sea bed. For the half burried pipe the piping was not observed as well. He concluded that the maximum scour depth was observed for case that caused by unidirectional flow in clear water scour condition.

1) Research Division of Water Resources Engineering, Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung, Indonesia, e-mail: [email protected]; and [email protected]

2) Research Division of Ocean Engineering, Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung, Indonesia, e-mail: [email protected]


90 Joko Nugroho, Hendra Achiari, and Dhemi Harlan

Chiew (1991) proposed a formulation of scour around submarine pipelines. The parameter involved in his formulation are flow depth ratio, gap ratio and critical shear stress for representative sediment. His formulation needs iteration in finding the maximum scour depth. His prediction is in the range of ± 20% agreement compared to the experimental data.

Moncada-M and Aguiere-Pe (1999) presented the results of experiments on the rivercrossing pipelines placed on top of river bed and at several distance from the bed. They concluded that Froude was negligible in the scour process but it ismportant in defining the parameter in the definition of the scour process.

It is clear that scour around submarine pipeline and river crossing pipeline has been gone through a detailed studies and researches, however there is not much study in the river crossing pipelines which is placed in parallel.

II. EXPERIMENT SETUP Experiments were conducted at Water Recources Laboratory, Faculty of Civil and Environmental Engineering, Bandung Institute of Technology (ITB). Equipment and material used in the study are: a. Glass walled open channel of 50 cm wide, and 7 m

long. b. Water circulation system c. Pipe of 38.1 mm diameter d. Sand, d50 = 0.6 mm

The flow was measured by a Rehbock weir placed at the upstream of the flume. The setup of the experiments is presented in Figure 1 and Figure 2. The determined parameters are flow depth (Y1 and Y2), pipe position relatif to the initial sediment bed level (e). The observed parameter is scour depth and scour length.

Figure 1 Experimental setup

Figure 2 Pipeline is represented by a cylinder placed accross the glasswalled flume

e

D

Y1 Y2

s

L



The discharge can be calculated using Rehbock formula (Chadwick and Morfet, 1994):

( ) 2/31

1 0012.0083.0602.0232 +

+= hb

P

hgQ

s

m3/s

h1 = flow depth at the upstream of the weir, measured above the crest.

Ps = height of crest b = width of the crest g = gravitational acceleration.

The above equation is valid for: 30 mm < h1 < 750 mm, b > 300 mm, Ps > 100 mm, and h1 < Ps. 1. Sediment Properties

The grainsize of the sand material used in the experiment is shown in Figure 3. The sand used in the experiments has d50 of 0.6mm.

Figure 3 Grainsize of the sediment

2. Scenarios of the experiment The experiments were grouped into three stages, i.e: a. Basic experiments b. Pipe protection structure model experiments c. Scouring around parallel river crossing pipe

For each experiments the flow were applied for 50 minutes with a change in every 10 minutes time interval. This is to immitate a sequence of a hydrograph which has rising and reccession limb features.The applied discharge to the system is in the range of 3 to 8 l/s, and the range of the upstream flow velocity is in the range of 15 to 27 cm/s. The range of Froude number of

the flow is in the range of 0.11 – 0.17, hence the flow is subcritical. The scouring observed will be in the type of clear water scour as the mean flow velocity applied at the upstream is below the critical velocity of sediment movement. The critical velocity of the sediment movement in this experiment is in the range of 24 to 27 cm/s. III. RESULTS OF THE EXPERIMENTS 3.1 Basic Experiments The basic experiments consists of three experiments i.e., for e/D = 0, 0.5 and 1.0. The data is presented in Table 1 to Table 3. The relative scour depths and relative scour length are presented in Figure 4 and Figure 5.

Table 1 Scour depth and flow condition of basic experiment for

e/D = 0 Time interval

x 10 min Y1 cm

S cm

L cm

V1 cm/s

Fr -

1 3.29 2.0 11.9 16.8 0.11 2 4.30 2.8 14.0 17.7 0.12

3 5.42 2.9 15.0 27.9 0.18

4 3.94 2.9 16.0 19.4 0.13

5 3.13 2.9 16.0 17.7 0.12

Note: V: velocity, Fr: Froude Number

Table 2 Scour depth and flow condition of basic experiment for

e/D = 0.5 Time interval

x 10 min Y1 cm

S cm

L cm

V1 cm/s

Fr -

1 3.55 0.5 10.0 15.6 0.10 2 4.28 1.4 13.0 17.8 0.12

3 5.90 2.8 22.0 25.7 0.16

4 4.85 2.8 22.0 15.7 0.10

5 3.34 2.8 22.0 16.6 0.11

Table 3

Scour depth and flow condition of basic experiment for e/D = 1.0

Time interval x 10 min

Y1 cm

S cm

L cm

V1 cm/s

Fr -

1 3.45 0.0 0.00 16.1 0.11 2 3.95 0.0 0.00 19.3 0.13

3 5.59 0.8 16.0 27.1 0.17

4 3.97 0.8 15.0 19.2 0.13

5 3.30 0.8 15.0 16.8 0.11


92 Joko Nugroho, Hendra Achiari, and Dhemi Harlan

Figure 4

Graph of S/D vs time interval for scenario 1

Figure 5 Graph of L/D vs time interval for scenario 1

The maximum scour achieved after the maximum flow velocity applied. The depth was maximum when the pipe is placed right above the bed level. The scour of depth was minimum for pipe placed at a distance of 1 pipe diameter.

For the pipe half burried at the bed, the initial scour was occurred at the bed downstream of the pipe. The sand removal then propagated towards upstream, which finally provide the way below the pipe for a portion of the flow to pass. The shear stress caused by the flow passing under the pipe will remove the sand particles until the shear stress magnitude is less than the critical shear stress for related bed material. For the pipe which placed at a distance from the bed the scour is mainly due to the increasing of the shear stress caused by pipe obstruction.

3.2 Pipe Protection Structure Model Experiments Pipeline will be safe if it is unexposed to the flow. However it is likely that a pipe will be exposed due to flow and river bed dynamic. Degradation of river bed due to to general scour will affect along the river reach, hence local scour countermeasure will be less effective. From the scour mechanism observation it can be seen that the pipe which has a distance to the bed the flow effect is prominent. Hence the scour could be avoided by changing the flow condition around the pipe. A scour counter measure structure which could promote a back water profile will help to increase flow depth, slow down the flow velocity, and promote sedimentation and eventually there will be no scouring.

In this experiments two pipes placed parallel, namely upstream and downstream pipe. Two experiments were done by covering the upstream pipe, and one experiment done with the downstream pipe covered. The data for first two experiments is presented in Table 4 and Table 5, respectively. Figure 6 shows relative scour depths at a pipeline placed at the downstream of a weir, the scour depth for e=-D is deeper than for e=-0.5D burried pipe. This is because the scouring is limited by the presence / exposure of the pipe. The scouring is retarded after the pipe is uncovered. The result of placing a cover on the downstream pipe showed that the scour at the upstream pipe is reduced due to the presence of backwater caused by the downstream weir.

Table 4 Scour depth at the downstream pipe and flow

condition when upstream pipe is covered, e=-D.

Time interval x 10 min

Y1 cm

S cm

L cm

V1 cm/s

Fr -

1 4.01 1.0 10.0 0.1 0.22 2 7.51 1.3 17.0 0.1 0.12

3 8.90 3.0 20.8 0.2 0.18

4 7.12 2.8 24.5 0.1 0.13

5 7.02 2.5 24.5 0.1 0.09

Table 5 Scour depth at the downstream pipe and flow

condition, when upstream pipe is covered, e=-0.5D. Time interval

x 10 min Y1 cm

S cm

L cm

V1 cm/s

Fr -

1 7.56 1.0 17.0 0.07 0.08 2 8.00 1.3 42.5 0.10 0.11

3 9.31 1.0 36.0 0.20 0.17

4 7.83 1.0 45.0 0.10 0.11

5 7.40 1.0 46.0 0.10 0.09



Figure 6

Graph of S/D vs time interval for experiment pipe line placed at the downstream of a weir

3.3 Scouring Around Parallel River Crossing Pipes In this experiment three pipe were place in a parallel configuration. This experiment aimed to study the method in preventing scour below the pipe if there are more than one pipe placed parallel. The result, quantitatively, shows that after the pipes exposed, it caused retardation on the flow, and it will cause degradation after the last downstream pipe. IV. EXAMPLE OF FIELD CASE In this paper the authors present a real example of river crossing pipeline more than one pipe or parallel river crossing pipe lines. The map of situation is presented in Figure 7. In this example there are seven pipes, consisting all old and new installation, placed parallel along a distance of 100 m or at interval of 16 m between two pipes. The field condition is likely more complex compared to our laboratory experiments. Pipe line for inland transport usually placed along access road, hence the crossing will be close to the bridge. In this case there is problem of local scouring at the downstream of the pipe and bridge. This is probably due to the presence of the exposed pipe which cause backwater at the upstream and accelerated flow at the downstream part.

As the result, a local degradation is occurred (see downstream part of the last pipe at Figure 7). The lowest bed elevation at the degraded part is up to 5 meters compared to the bed elevation at the last pipe. According to our laboratory experiments, weir construction at the downstream of the last pipe and providing a proper energy dissipater would help the situation. However the backwater effect should be analyzed to provide parameter in designing the elevation

of river embankment. An example of a condition of flow over an exposed river crossing pipeline is presented in Figure 8. V. CONCLUSION AND RECOMMENDATION The results showed that maximum scour for single pipeline were observed for pipeline placed at a distance of half and one time of the pipe diameter from the bed. Protection by placing weir at the downstream of pipeline provide significant result in reducing the scour depth. Acknowledgments

The authors wish to express their sincere appreciation to the Laboratory of Water Resources Engineering, Faculty of Civil and Environmental Engineering in providing the experiments area and apparatus. References Chadwick, A., Morfett J. 1994, Hydraulics in Civil and

Environmental Engineering, 2nd ed., E & FN Spoon.

Moncada-M,A., Aguiere-Pe, J. 1999, “Scour below Pipeline in River Crossings.” J. Hydr. Engrg., ASCE, 125(9), 953-958.

Chiew, Y.M., 1990, ‘‘Mechanics of local scour around submarine pipelines.’’ J. Hydr. Engrg., ASCE, 116(4), 515–529.

Chiew, Y.M., 1991, ‘‘Prediction of maximum scour depth at submarine pipelines’’. J. Hydr. Engrg., ASCE, 117(4), 452–466.


94 Joko Nugroho, Hendra Achiari, and Demi Harlan

Figure 7 Example of placement of river crossing pipe lines in parallel

Figure 8 Flow over an exposed pipeline

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