ArticlePDF Available

Scour Protection of Underwater Pipelines

Authors:

Abstract and Figures

A detailed literature review on the protection of local scour beneath a submarine pipeline is presented. The review covers two basic parts of countermeasures against the pipeline scour, namely preventing the onset of scour and stimulating the self-burial of a pipeline. The research progress on the methods of the two sections is discussed in detail separately. The methods preventing the onset of scour have been extensively studied, but the understanding on their mechanisms is yet to be improved. The progress in stimulating the self-burial of a pipeline mainly focuses on a spoiler attached to a pipeline, which is investigated comprehensively with both experiments and numerical simulations. Both parts of countermeasures have been applied in some practical engineering projects and the protection effects are generally satisfying.
Content may be subject to copyright.
Scour Protection of Underwater Pipelines
Liquan Xie, Yehui Zhu*
College of Civil Engineering, Tongji University, Shanghai 200092, China
E-mail: yehui_zhu@tongji.edu.cn (Y. Zhu)
Received: 21 October 2018; Accepted: 2 November 2018; Available online: 25 November 2018
Abstract: A detailed literature review on the protection of local scour beneath a submarine pipeline is presented.
The review covers two basic parts of countermeasures against the pipeline scour, namely preventing the onset of
scour and stimulating the self-burial of a pipeline. The research progress on the methods of the two sections is
discussed in detail separately. The methods preventing the onset of scour have been extensively studied, but the
understanding on their mechanisms is yet to be improved. The progress in stimulating the self-burial of a
pipeline mainly focuses on a spoiler attached to a pipeline, which is investigated comprehensively with both
experiments and numerical simulations. Both parts of countermeasures have been applied in some practical
engineering projects and the protection effects are generally satisfying.
Keywords: pipeline scour; scour protection; impermeable plate; geotextile mattress with sloping curtain
(GMSC); spoiler.
1. Introduction
Submarine pipelines are vital to oil and gas transportation from offshore platforms. Oil leakage due to
pipeline failure can trigger both economic and ecological catastrophe. One of the most common causes of the
pipeline failure is the metal fatigue due to vortex induced vibration (VIV). When submarine pipelines are faced
with fierce ocean currents or waves, local scour may appear underneath them. As the scour hole extends along
the pipeline, the span length of the pipeline increases, leading to pipeline spanning. When the free span is long
enough, the pipeline may experience VIV. So the protection of pipelines from scour bears significant importance.
The local scour under the pipelines starts from the onset of scour. The basic mechanism of the onset of scour
below the pipelines was revealed by Chiew[1]. Chiew [1] performed a series of laboratory tests and pointed out
that piping powered by excessive seepage flow is the dominant cause of the scour onset under pipelines in
steady currents. The experimental results showed that the onset of scour starts from the downstream side of the
pipeline and sand particles eject from the bottom of the pipeline. Thus an initial scour hole appears. After the
onset of scour, the scour hole quickly extends along the pipeline powered by the shear stress in the tunnel flow
and the pressure difference on two sides of the pipeline[2]. Sumer et al. [3] conducted a series of flume
experiments on the onset of scour process with an underwater camera and pressure transducers. The results
agreed well with that of Chiew[1]. A criterion formula for the onset of scour in steady current was also proposed.
Zang et al. [4] simulated the onset of scour process numerically. The effects of flow parameters on the pressure
difference coefficient were studied comprehensively, including the Reynolds number, the flow depth, the
pipeline embedment and the thickness of boundary layer. An amplification coefficient was proposed to calculate
the pressure gradient at the downstream side of the pipeline with the averaged pressure gradient along the buried
part of the pipeline.
In general, the countermeasures against pipeline scour can be classified into two parts according to their
principles: (1) preventing the appearance of onset of scour, and (2) accelerating the self-burial of pipelines. In
the following sections, scour preventing measures of these two kinds will be described in detail.
2. Preventing the onset of scour
This part of pipeline scour protection methods mainly aim at reducing the intensity of seepage flow beneath
the pipeline, thus decreasing the possibility of onset of scour. These methods include impermeable plates under
the pipeline, the fiber reinforced mats, concrete mattresses covering the pipeline, the geotextile mattress with
sloping curtain (GMSC), and conventional methods like graded engineering rocks.
Journal of Civil Engineering and Construction 2018;7(4):171-177
https://doi.org/10.32732/jcec.2018.7.4.171
171
2.1. Impermeable plate under a pipeline
Chiew [1] pointed out that the scour onset can be eradicated by placing an impermeable plate on the upstream
side of a pipeline in steady currents. In this occasion, the interaction between the seepage flow and mainstream
flow is terminated in the area covered by the plate. The length of seepage path below the pipeline is extended to
that of the plate, and the hydraulic gradient can be greatly decreased. The critical lengths of the plate in two
different flow depth conditions were also proposed. Zhang et al. [5] further investigated the protection effects of
a rigid impermeable plate on a pipeline in currents with numerical simulation. The calculated results coincide
well with the experiment results of Chiew [1]. The results showed that the unidirectional plate (see Fig. 1(a)) and
the bidirectional plate (see Fig. 1(b)) with the same length have similar protection effects. The effects of flow
depth and non-dimensional flow velocity on the critical length of the protection plate were also analyzed in
detail. When the flow depth is over 4 times of the pipe diameter, the critical length of the plate is almost
independent from the flow depth.
Fig. 1 Impermeable plates beneath the pipeline[5].
Yang et al. [6] conducted a series of experiments on a flexible plate protecting a pipeline in currents. The
results demonstrated a critical pressure difference on two sides of a pipeline for the onset of scour. A critical
length of the rubber plate was identified. The scour does not occur when the length of the protecting plate is
over this critical value. An empirical formula for the critical plate length was proposed based on the results of
critical pressure difference. The calculated critical plate length meets well with the experiment results.
2.2 Fiber reinforced mat
Fiber reinforced mats are also termed as artificial grass or bionic grass. A fiber reinforced mat is composed of
a polymer textile mat, a bunch of vertical polymer “grass” and short piles under the mat for installation on the
seabed (see Fig. 2). Fiber reinforced mats can reduce the flow velocity near the bed and stimulate the deposition
of sediment. When fiber reinforced mats are installed on two sides of a pipeline, the impact of flow and waves
on the pipeline can be reduced. The flow velocity over the pipeline can be decreased and the seepage flow under
the pipeline is less intense, thus descending the hydraulic gradient in the sediment. So the pipeline can be
protected. Furthermore, the fiber reinforced mats can stimulate sediment deposition by slowing down the near
bottom flow carrying a high concentration of bed load, which is also helpful to protect the pipeline from scour
172
L. Xie et al.
Journal of Civil Engineering and Construction 2018;7(4):171-177
onset. The fiber reinforced mats can be applied in various flow conditions, and they need little maintenance
work after deployment.
Fig. 2. Fiber reinforced mat.
Yang et al. [7] designed a series of experiments in a slope flume, and the protection effects of two different
fiber reinforced mats were compared with bare slope flow. When the slope is equipped with artificial grass, the
sediment erosion volume is significantly decreased to 2% - 5% of that on the bare slope, and the flow rate on the
slope and the time used to reach a balanced profile drop remarkably as well.
Zhao et al. [8] studied the protection effects of fiber reinforced mats on a pipeline in waves. The experimental
results showed that the fiber reinforced mats successfully eliminates the scour under the pipeline on a sandy bed.
Sand dunes appear on both sides of the pipeline. The results also showed that when the fiber reinforced mat is
installed on the top of the pipeline only, the scour is not prevented.
Fiber reinforced mats have been applied in practical scour control projects during the past decade [9]. Jiang
and Chen [10] carried out a field survey on the protected pipeline sections which were threatened by pipeline
spanning. They found that the length of the free span and the scour depth reduce remarkably after the fiber
reinforced mats are installed for all 11 protection pipe sections. However, they also reported that about 50% of
the mats installed 5 to 6 years before were missing.
2.3 Concrete mattress covering a pipeline
Concrete mattresses covering a pipeline are originally designed to protect the pipeline from the damage of
falling objects, like anchors [11]. In some occasions, they are also used to improve the stability of the pipeline.
Zhang et al. [12] investigated the flow pattern adjacent to the surface of a concrete mattress, revealing a low
velocity zone in the lateral margin between two blocks and a high velocity zone near the surface of the upstream
concrete block. They designed a series of flume experiments on the protection effects of concrete mattress. The
result showed that the concrete mattress can protect the pipeline under it from local scour. They also found that
scour holes appear near the edge of the concrete mattress.
2.4 Geotextile mattress with sloping curtain (GMSC)
A geotextile mattress with sloping curtain (GMSC, see Fig. 3) is a novel countermeasure against scour and
erosion on seabed, river bed and river banks [13]. A GMSC is constituted of a geotextile mattress and a sloping
curtain sewn together. The geotextile mattress is composed of a string of geotextile tubes, which are filled with
local sediments like sand or gravel, or artificial materials like cement or concrete. The bottom edge of the
sloping curtain is sewn on the middle of the mattress, and the top edge of the curtain is attached to a floating
tube, which is filled with light materials. When the GMSC is placed in still water, the buoyancy of the floating
tube pulls the curtain straight up. When the GMSC is affected by steady currents, the flow will push the curtain
forward, and the curtain leans to the downstream side. The curtain is thus termed as sloping curtain. Some sand-
pass openings are set close to the bottom of sloping curtain, providing a pass for bed load. Belts on the mattress
and the curtain are designed to improve the stability of the device.
Xie et al. [14] researched the protection effects of a GMSC in currents in a live-bed flume. They found that
the GMSC can successfully prevent the bed nearby from scour and a sand dune formed immediately
downstream to the GMSC (see Fig. 4). They proposed the basic protection mechanism of a GMSC (see Fig. 5).
When a GMSC is deployed in steady currents, the approaching flow is separated into two sections: an upward
flow and a downward flow. These two parts collide on the leeside of the GMSC and creates two vortex systems:
the top vortex system and the bottom vortex system. The top vortex system stays close to the top edge of the
173
L. Xie et al.
Journal of Civil Engineering and Construction 2018;7(4):171-177
curtain and can hardly affect the bed. The bottom vortex system provides a long low velocity area on the leeside
of the GMSC. The bed in the low velocity area can be effectively protected from erosion.
Fig. 3. Geotextile mattress with sloping curtain.
Fig. 4. Bed profile variation near a GMSC in currents.
Fig. 5. Sketch of flow structure around GMSC.
Xie et al. [15] verified the protection effects of a GMSC on a partially buried pipeline with a visualization
experiment in steady currents. An optimal distance between the GMSC and the pipeline was proposed, where
the flow velocity approaching the pipeline approximately reduces to zero and the seepage beneath the pipeline
generally stops. They pointed out that the seepage hydraulic gradient decreases remarkably as the GMSC gets
closer to the pipeline and the seepage flow reverses when the GMSC is close enough to the pipeline.
174
L. Xie et al.
Journal of Civil Engineering and Construction 2018;7(4):171-177
3. Accelerating the pipeline self-burial
Burying the pipeline under the seabed is an effective way of protecting the pipeline. When a pipeline is below
the surface of seabed, it is protected from the threat of local scour due to currents, waves and accidental damage
from anchors. The conventional method of burying a pipeline beneath the seabed surface is digging a trench for
the pipeline before or after installation. After that, the trench is artificially refilled. Sometimes, the trench for the
pipeline can be also refilled automatically by sediment deposition. However, digging a trench on the seabed
requires specially designed machines and the cost may sometimes be very considerable. Hulsbergen [16] found
that a pipeline can bury itself under the surface of the seabed in specific flow conditions and the burial depth can
reach up to 3 times of the pipeline diameter. This process is termed as self-burial. To accelerate this process,
some attachments are installed on the surface of the pipeline to change its cross section profile. Two of the most
widely studied attachments are the spoiler (see Fig. 6) and the flexible plate in waves. In practical application,
spoilers have already been used on the pipeline across Hangzhou Bay in China where tidal currents are fierce
[17].
Fig. 6. Spoiler installed on the top of a pipeline.
3.1 Spoiler
A spoiler is a thin plate attached to the surface of a pipeline, usually on the top. The height of the spoiler
usually ranges from 15% to 25% of the pipeline diameter. When waves and currents affect a pipeline with a
spoiler, the flow is blocked by the spoiler. The flow rate and velocity through the scour hole can increase due to
the enhanced blockage effect, thus increasing the scour rate under the pipeline. When the scour hole extends
long enough, the pipeline may sag into and touch the bottom of the scour hole. After that the scour hole can be
refilled due to sediment deposition. The blockage effect of the spoiler may trigger secondary scour holes on both
sides of a pipeline thereafter, and pipeline can sag into the scour hole again. When the scour hole is refilled
again, the self-burial process is completed.
Chiew [18] investigated the scour depth under a pipeline with and without a spoiler. Detailed flume
experiments were designed to reveal the effect of the spoiler height and the location of the spoiler on the scour
depth. The results showed that the scour rate and the scour depth under the pipeline rise remarkably after a
spoiler is installed. The results also indicated that the scour depth reaches a maximum value when the tip of the
spoiler touches the bed on the upstream side. This was attributed to the flow separation at the tip of the spoiler.
Chiew [19] further analyzed the effects of a spoiler on a pipeline in waves. The width and the depth of the scour
hole increases remarkably when a spoiler is installed. The rise in the width of the scour hole was attributed to the
intensified lee wake vortex due to the increase of blockage area of the pipeline. The rise in the scour depth was
reckoned to be associated to the strengthened tunnel flow through the scour hole. A secondary scour hole was
observed adjacent to the tip of the spoiler in some occasions. Cheng and Chew [20] established a finite
difference model solving the Navier-Stocks equations and analyzed the influence of spoiler on the
hydrodynamic forces on the pipeline, the vortex shedding frequency and the velocity profile in the scour hole.
The results showed that the spoiler on the pipeline can increase pipeline drag force and the bed shear stress.
Alam and Cheng [21] conducted a series of numerical simulation with a Lattice Boltzmann model. The model
was proved to be capable of revealing the development of the scour hole. The scour process under a pipeline
with a spoiler was studied. The effects of the location and the length of the spoiler on the scour process were
analyzed. Yang et al. [22] proposed two formulae with theoretical analysis for the scour depth under a pipeline
with and without a spoiler, respectively. The formulae were validated with a series of flume experiments and can
reflect the scour process correctly. The approaching flow Reynolds number and the height of the spoiler was
found to affect the scour depth remarkably.
Oner [23] studied the flow field around a pipeline with a spoiler in steady currents with PIV (Particle Image
Velocimetry) method. The effects of the spoiler were compared in different Reynolds numbers. The results
175
L. Xie et al.
Journal of Civil Engineering and Construction 2018;7(4):171-177
showed that the length of flow separation zone is extended after a spoiler is installed, but there were no
significant changes in the flow rate through the scour hole and the bed shear stress.
Han [24] studied a flexible spoiler attached on a pipeline at two different locations: on the top of a pipeline
and on the bottom. The pressure distribution around the pipeline and the scour process were observed and
measured. The flexible spoiler installed on the top of a pipeline behaves like common rigid spoilers, and can
increase the scour depth and the drag force on the pipeline. On the contrary, the flexible spoiler attached on the
bottom of a pipeline can reduce the scour depth remarkably and seize the scour process in some occasions.
Zhu et al. [25] simulated the scour process adjacent to a pipeline with a spoiler. The flow field was modeled
with SST k ω turbulence model and the variation of the bed surface profile is described by an Euler-Euler two-
phase model. The simulation showed that the bed surface and the flow field are very sensitive to the height of
the spoiler and the gap between the pipeline and the bed.
Oner [26] simulated the flow pattern around a pipeline with or without a spoiler in steady currents. The
results of three different turbulence models and five different meshes of various densities were compared and
validated with previous experimental results. It was reckoned that the standard k ω model with the finest mesh
reflects the flow field most reasonably. The results showed that the installation of a spoiler on the top of the
pipeline can trigger a rise in the drag force on the pipeline and a drop in the lift force.
3.2 Flexible plate under a pipeline in waves
In the previous section, a flexible plate underneath a pipeline was utilized to prevent the onset of scour in
unidirectional currents [6]. However, a similar device can greatly accelerate the scour rate under the pipeline in
waves. Yang et al. [27] analyzed the scour process around a pipeline with a flexible plate or a spoiler in regular
and irregular waves. They reckoned that the spoiler and the flexible plate under a pipeline can intensify the scour
rate below a pipeline, and proposed an optimal length of the flexible plate for accelerating the scour process,
which is 1.5 times of the pipeline diameter. They pointed out that the spoiler may trigger remarkable variation
on the bed profile on two sides of the pipeline.
4. Conclusions
Researches on pipeline scour protection have made significant progress in the past few decades. In this paper,
the progress on the pipeline scour protection is reviewed in two sections: preventing the onset of scour and
stimulating the pipeline self-burial. The former section includes several different novel methods. These methods
have been extensively studied, but the understanding on their mechanisms is yet to be improved. Some of these
methods have not been applied into practical engineering, due to the cost or difficulties in the installation
process, e.g. the impermeable plate under a pipeline. The progress in the latter section mainly focuses on a
spoiler attached to a pipeline, which is investigated comprehensively with both experiments and numerical
simulations. Spoilers have been applied in many pipeline projects worldwide, and have been widely accepted as
an effective way to protect the pipeline by stimulating the self-burial process. However, some drawbacks of
spoilers have been noticed and are attracting the attention of researchers.
Acknowledgements
This paper was supported by Chinese National Natural Science Foundation Council under Grants 11172213
and 51479137. The corresponding author would like to acknowledge the support from the China Scholarship
Council (Grant No. 201806260166).
5. References
[1] Chiew Y. Mechanics of local scour around submarine pipelines. Journal of Hydraulic Engineering, 1990:
515529. 10.1061/ (ASCE)0733-9429(1990)116:4(515).
[2] Wu Y, Chiew Y. Mechanics of pipeline scour propagation in the spanwise direction. Journal of Waterway,
Port, Costal and Ocean Engineering, 2014: 4014045. 10.1061/(ASCE)WW.1943-5460.0000288.
[3] Sumer B M, Truelsen C, Sichmann T, Fredsøe J. Onset of scour below pipelines and self-burial. Costal
Engineering, 2001, 42(4): 313335.
[4] Zang Z, Cheng L, Zhao M, Liang D, Teng B. A numerical model for onset of scour below offshore pipelines.
Costal Engineering, 2009, 56(4): 458466.
[5] Zhang Z, Shi B, Guo Y, Yang L. Numerical investigation on critical length of impermeable plate below
underwater pipeline under steady current. Science China: Technological Science, 2013, 56(5): 1232-1240.
176
L. Xie et al.
Journal of Civil Engineering and Construction 2018;7(4):171-177
[6] Yang L, Shi B, Guo Y, Zhang L, Zhang J, Han Y. Scour protection of submarine pipelines using rubber
plates underneath the pipes. Ocean Engineering, 2014, 84: 176182.
[7] Yang C, Xiao P, Zhen B, Shen Z, Li L. Effects of vegetation cover on runoff and sediment in field prototype
slope by experimental. Advances in Environmental Science and Engineering. Stafa-Zurich: Trans Tech
Publications Ltd. 2012, 518-523: 4707-4711.
[8] Zhao D, Yu J, Li G, Wang C. An experimental scour-prevention technique for subsea pipeline. Journal of
Harbin Engineering University, 2009, 30(6): 597-601. (in Chinese)
[9] Liu J, Zhang Z. Application of bionic aquatic weed for scour prevention of subsea pipeline. Petroleum
Engineering Construction, 2009, 35(3): 20-22. (in Chinese)
[10] Jiang X, Chen T. Investigation on application of bionic aquatic grass for treating subsea pipeline
suspension and improvement measure. Petroleum Engineering Construction, 2013, 39(5): 15-18. (in Chinese)
[11] Crowhurst A D. Marine pipeline protection with flexible mattress. Costal Engineering, 1982: 2403-2417.
[12] Zhang Z, Ding H, Liu J. Model test on concrete slab interlocking mattress applied in anti-scour protection
of subsea pipeline. The Ocean Engineering, 2015, 33(2): 77-83. (in Chinese)
[13] Xie L, Liu S. Stability of Sand Beds Around Mattress-Curtain Sets. 29th International Conference on
Ocean, Offshore and Arctic Engineering, ASME 2010. American Society of Mechanical Engineers. 2010: 839-
844.
[14] Xie L, Huang W, Yu Y. Experimental study of sediment trapping by geotextile mattress installed with
sloping curtain. Geosynthetics International, 2013, 20(6): 389-395.
[15] Xie L, Zhu Y, Su T C. Scour protection of partially embedded pipelines using sloping curtains. Journal of
Hydraulic Engineering, 2018, in Press. 10.1061/(ASCE)HY.1943-7900.0001571.
[16] Hulsbergen C H. Spoilers for stimulated burial of submarine pipelines. 18th Offshore Technology
Conference. OTC 5339: 441444.
[17] Zhao J, Wang X. CFD Numerical Simulation of the Submarine Pipeline with a Spoiler. Journal of Offshore
Mechanics and Arctic Engineering ASME, 2009, 131(3): 031601.
[18] Chiew Y. Effect of spoilers on scour at submarine pipelines. Journal of Hydraulic Engineering, 1990:
1311-1317. 10.1061/ (ASCE)0733-9429(1992)118:9(1311).
[19] Chiew Y. Effect of spoilers on wave-induced scour at submarine pipelines. Journal of Waterway, Port,
Costal and Ocean Engineering, 1993: 417428. 10.1061/(ASCE)0733-950X(1993)119:4(417).
[20] Cheng L, Chew L W. Modelling of flow around a near-bed pipeline with a spoiler. Ocean Engineering,
2003, 30(13): 1595-1611.
[21] Alam M S, Cheng L. A 2-D Model to Predict Time Development of Scour below Pipelines with Spoiler.
2nd International Symposium on Computational Mechanics and 12th International Conference on the
Enhancement and Promotion of Computational Methods in Engineering and Science. American Institution of
Physics. 2010: 993-998.
[22] Yang L, Shi B, Guo Y, Wen X. Calculation and experiment on scour depth for submarine pipeline with a
spoiler. Ocean Engineering, 2012, 55: 191-198.
[23] Oner A A. The flow around a pipeline with a spoiler. Proceedings of the Institution of Mechanical
Engineers, Part C: Journal of Mechanical Engineering Science, 2010, 224(1): 109-121.
[24] Han Y. Study on the Submarine Pipeline with Flexible Spoilers. Progress in Polymer Processing, Asian
Workshop on Polymer Processing (AWPP2011). Trans Tech Publications Ltd. 2011, 501: 431-435.
[25] Zhu H, Qi X, Lin P, Yang Y. Numerical simulation of flow around a submarine pipe with a spoiler and
current-induced scour beneath the pipe . Applied Ocean Research, 2013, 41(3): 87-100.
[26] Oner A A. Numerical investigation of flow around a pipeline with a spoiler near a rigid bed. Advances in
Mechanical Engineering. 2016, 8(6): 1-13. 10.1177/1687814016651794.
[27] Yang L, Guo Y, Shi B, Kuang C, Xu W, Cao S. Study of Scour around Submarine Pipeline with a Rubber
Plate or Rigid Spoiler in Wave Conditions. Journal of Waterway, Port, Coastal and Ocean Engineering, 2012:
484-490. 10.1061/(ASCE)WW.1943-5460.0000150.
© 2018 by the author(s). This work is licensed under a Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/). Authors retain
copyright of their work, with first publication rights granted to Tech Reviews Ltd.
177
L. Xie et al.
Journal of Civil Engineering and Construction 2018;7(4):171-177
... Longitudinal extension of the scour hole beneath the pipeline is inextricably bound to the pipeline span leading to pipeline spanning. When the extension of free span has sufficient extension in the longitudinal direction, seabed pipelines may experience severe vibration due to the existence of vortex structures (Xie and Zhu [1]). ...
Article
Full-text available
Subsea pipelines carry oil or natural gas over long distances of the seabed, but fluid leakage due to a failure of the pipeline can culminate in huge environmental disasters. Scouring process may take place beneath pipelines due to current and/or wave action, causing pipeline suspension and leading to the risk of pipeline failure. The resulting morphological variations of the seabed propagate not only below and normally to the pipeline but also along the pipeline itself. Therefore, 3D scouring patterns need to be considered. Mainly based on the experimental works at laboratory scale by Cheng and coworkers, in this study, Artificial Intelligent (AI) techniques are employed to present new equations for predicting three dimensional current- and wave-induced scour rates around subsea pipelines. These equations are given in terms of key dimensionless parameters, among which are the Shields’ parameter, the Keulegan–Carpenter number, relative embedment depth, and wave/current angle of attach. Using various statistical benchmarks, the efficiency of AI-models-based regression equations is assessed. The proposed predictive models perform much better than the existing empirical equations from literature. Even more interestingly, they exhibit a clear physical consistence and allow for highlighting the relative importance of the key dimensionless variables governing the scouring patterns.
Article
Full-text available
A geotextile mattress with sloping curtain (GMSC) was introduced to protect underwater pipelines from scour in steady currents. GMSC is a newly developed countermeasure against river and estuarine scour. To better understand the beneficial effects of GMSC on seepage flow under pipelines, dye was injected to determine the hydraulic gradients across the pipeline in a series of verification tests. Hydraulic gradients across the pipeline with and without a GMSC were quantified for different curtain angles, curtain heights of the GMSC, and distances between the pipeline and the GMSC, and installing GMSCs with or without sand-pass openings, which are openings to allow bottom flow with a high concentration of sediment to pass. The results of the experiments show that a GMSC is capable of protecting a pipeline against scour in a steady current by effectively reducing the hydraulic gradient in the bed soil below the pipeline. The hydraulic gradient across the pipeline decreases nonlinearly with the increase of curtain height and angle of the GMSC. The effect of sand-pass openings on the hydraulic gradient across the pipeline is not significant. The hydraulic gradient decreases nonlinearly with decreasing distance between the pipeline and the GMSC. When the distance decreases below a critical point, the seepage flow under the pipeline reverses. The GMSC may provide an optimum protection effect on the pipeline when the distance between the pipeline and the GMSC is about 6 times the obstacle height of the GMSC (Hc sin a) by reducing the flow velocity approaching the pipeline close to zero.
Article
Full-text available
Geosynthetic structures created for channel erosion protection offer environmental friendly benefits and have demonstrably lower construction and lifetime costs than similar hard structures. A geotextile mattress with a sloping curtain (GMSC) offers an alternative countermeasure against channel erosion. In the present study, experiments were conducted to investigate the working mechanism and effectiveness of GMSCs which were installed on movable beds in a rectangular flume. The bathymetry of the plastic sand beds was measured before and after the tests. The results showed that the presence of the GMSC led to sediment deposition and dune formation at both upstream and downstream edges of the GMSC structure. This will prevent bottom erosion near the structure and increase its stability against flow-induced sediment scour, so that the erodible beds will be protected.
Conference Paper
Full-text available
We propose a new countermeasure to protect sand beds from erosion, called mattress-curtain sets. Physical model tests in a rectangular flume with suspended sediments were carried out to investigate stability of the sandbed around a mattress-curtain set, and the working mechanism of a mattress-curtain set is discussed. Moreover, based on the experimental results and simulations with software FLUENT, the interactions between sand beds and near bed flow were investigated when a mattress-curtain set was installed on the sand beds. The results show that the stability of sand beds is closely controlled by the seepage under the fabricated mattress, and is affected by liquefaction, excess pore pressure buildups in the sand beds when subjected to changing flow or wave current.
Article
Sea floor scouring is the main threat to the safety of operational subsea pipelines. Fiber reinforced fixed mats were proposed as a kind of imitation vegetation that provides protection, based on scour mechanisms affecting subsea pipelines and the resulting fundamental principles of scour-prevention. An experiment was conducted with an existing pipeline project. It was shown that the suggested approach can effectively prevent the problem of scour induced suspended spans, providing a reference for the design, construction and protection of subsea pipelines.
Article
Development of the mattress technique for protection of pipelines is discussed generally, and applications of the gabion construction method are described. Sand mastic asphalt has been used increasingly in gabion and mattress studies; sand mastic grouted gabion mattresses (Sarmac) were used for inshore and river works, but have been adapted for use in offshore environments. The use of such a mattress for a gas pipeline from Algeria to Italy is discussed. The placing of the mattress and its properties are discussed. (A.J.)
Article
The characteristics as well as the correlation between runoff and erosion was studied under 3 kinds of cover conditions of bare-slope, artificial grass slope and ecological-restoration slope with 20°slope gradient by runoff scouring intensities of 9l/min. The results showed that: (1) Sediment yield which sourced from artificial grass slope and ecological-restoration slope was separately decreased by 95% and 98% than bare slope, and the runoff reduced by 20% and 50% than bare slope respectively; The infiltration rates of the two kinds of grass slope were higher than the bare slope by 38.32% to 51.10% and 114%; (2) Sediment and runoff process showed that the stable state appeared respectively at 26min, 18min and 6min under bare slope, artificial grass slope and ecological-restoration slope, the amount of runoff was bare slope> artificial grass slope > ecological-restoration slope. (3) For bare slope and artificial grass slope, the relationship between sediment and runoff, sediment and the drag coefficient which both showed a negative correlation, but the correlation which from the ecological-restoration slope wasn't obvious, so, further studies should carry out to promote runoff-sediment relations on ecological-restoration slope.
Article
This study proposes a new approach in which an impermeable plate is placed under the pipeline to prevent the local scour around the pipeline. In order to understand the performance of this approach, the finite volume method (FVM) and volume of fluid (VOF) method are adopted to simulate the flow field around the pipeline. The pressure distribution along the sandy bed surface is obtained by considering the variation of water surface. Furthermore, the effects of water depth, unidirectional and bidirectional impermeable plates on pressure difference are discussed. The seepage flow field of sandy bed near underwater pipeline is numerically simulated using the laminar and porous media model. On this basis, the effect of the impermeable plate length on hydraulic gradient is investigated and the critical length of impermeable plate is obtained. The simulated results show that when the water depth is smaller than 5.00D (D is the diameter of pipeline), the effect of the water depth on the pressure difference is remarkable. The pressure differences between two endpoints of both the unidirectional and bidirectional plates decrease with the increase of the plate length. The variations of the pressure differences for both the unidirectional and bidirectional plates are similar. With the increase of plate length, the hydraulic gradient decreases and the piping at the seepage exit is avoided effectively as long as it reaches a certain length. Such a critical length of the plate decreases with the increase of the water depth. When water depth is larger than 4.00D, the effect of the water depth on the critical length is small. For the same water depth, the critical length of impermeable plate increases with the increase of the dimensionless flow parameter. Numerical simulation results are in good agreement with the available experimental measurements.
Article
A numerical study is performed on the flow field around a submarine pipe equipped with a spoiler in a rectangular configuration and the variation of seabed surface caused by current-induced scouring beneath the pipe. The study is aimed to investigate the effects of spoiler height and gap between pipe and seabed on flow characteristics and seabed scouring. Euler–Euler two-phase flow model is employed to capture the flow characteristics of seawater and seabed sandy particles based on two-dimensional Reynolds-Averaged Navier–Stokes (RANS) equations. And shear stress transport (SST) k–ω turbulence model is applied to model the turbulence. Simulations are carried out for relative spoiler heights of S/D = 0, 0.15, 0.25 and 0.35 and gap ratios (G/D) ranging from 0.2 to 0.5 with an interval of 0.1. The Reynolds number ranges from 0 to 5.076 × 104 in the computational domain according to the logarithmic velocity profile used in simulations. The numerical results show that both flow field around submarine pipe and seabed scouring are sensitive to the relative spoiler height and gap ratio. Increasing the spoiler height or decreasing the gap ratio can accelerate the self-burial process for submarine pipe equipped with a spoiler.
Article
Seabed scouring presents severe safety issues for subsea pipelines. It is crucial to be able to predict scour depths for various flow conditions. In this paper, the effects of flow velocity and spoiler on scour depth are investigated using laboratory experiments. It is found that both the fluid Reynolds number and the spoiler height have strong effects on scour depth. Mechanistic formulas for scour depth are proposed and validated using laboratory test data. The formulas consider the fluid velocity profile beneath the pipeline, which greatly affects scour depth. The formulas produced good match with tests data.