Armor Layer Research Articles

Local Scour of Armor Layer Processes around the Circular Pier in Non-Uniform Gravel Bed

Flume experiments have been carried out under clear water scour conditions to analyze the maximum equilibrium scour depth and scour processes in armored streambeds. A total of 85 experiments have been carried out using different diameters of circular piers and non-uniform gravels. A graphical approach for dimensionless scour depth in equilibrium condition around the circular pier in armored streambeds has been developed. As per this curve, the maximum dimensionless scour depth variation with dimensionless armor particle size depends on the densimetric particle Froude number (Frd50), and the decreasing rate of dimensionless scour depth decreases with the value of Frd50.

Impact of armour layer on the depth of scour hole around side-by-side bridge piers under ice-covered flow condition

Abstract In the present study, experiments were conducted in a large-scale flume to investigate the issue of local scour around side-by-side bridge piers under both ice-covered and open flow conditions. Three non-uniform sediments were used in this experimental study. Analysis of armour layer in the scour holes around bridge piers was performed to inspect the grain size distribution curves and to study the impact of armour layer on scour depth. Assessments of grain size of deposition ridges at the downstream side of bridge piers have been conducted. Based on data collected in 108 experiments, the independent variables associated with maximum scour depth were assessed. Results indicate that the densi-metric Froude number was the most influential parameter on the maximum scour depth. With the increase in grain size of the armour layer, ice cover roughness and the densimetric Froude number, the maximum scour depth around bridge piers increases correspondingly. Equations have been developed to determine the maximum scour depth around bridge piers under both open flow and ice covered conditions.

Parameter sensitivity analysis on the buckling failure modes of tensile armor layers of flexible pipe

Unbonded flexible pipes have become widely employed in marine environments with the development of the offshore oil and gas exploration industry. Such pipes are subject to complex loads, which may lead to significant failure. Tensile armor layers are key components of these flexible pipes, and radial buckling failure may occur under high compressive or other combined loads. In this paper, a simplified numerical model of the flexible pipe is developed using the finite element (FE) method, and the main failure modes of the tensile armor layers are simulated. This simplified model is an improvement over those employed in previous studies on tensile armor failure. Several parameters that have important effects on pipe stiffness are investigated. The results show that an increase in the winding angle of the tensile armor wires as well as the damage to the outer sheath of the flexible pipe will decrease the compressive stiffness significantly. The presence of friction between adjacent layers will increase the stiffness to a small extent while the external hydrostatic pressure enhances its axial stiffness greatly and improve the stability of the pipe. These results may be used as a reference in the development and engineering applications of flexible pipes.

Bending tests and cross-sectional analyses of multilayered flexible pipe models

Simplified models of multilayered unbonded flexible pipes for offshore applications were manufactured and put to a series of bending tests where biaxial bending strains of tensile armor wires were directly measured. Focusing on the effects of constraint on the tensile armor wires by outer layers on the bending stiffness of the pipe and the bending stresses of tensile armor wires, four types of pipe models with different specifications of holding bandages, which hold the tensile armor layers from the outside, were prepared. Cross-sectional analyses based on four kinds of methods were also carried out and comparatively examined with the above bending test results, and the applicability of each method was evaluated. As a result, it was found that the normal and binormal bending stresses of the tensile armor wires in the slip region remarkably change depending on the specification of the holding bandages, suggesting that the degree of constraint on the tensile armor wires by the outer layers should be taken into consideration in the cross-sectional analyses.

Stability of Rubble Mound Breakwaters—A Study of the Notional Permeability Factor, Based on Physical Model Tests

The Van der Meer formulae for quarry rock armor stability are commonly used in breakwater design. The formulae describe the stability as a function of the wave characteristics, number of waves, front slope angle and rock material properties. The latter includes a so-called notional permeability factor characterizing the permeability of the structure. Based on armor stability model tests with three armor layer compositions, Van der Meer determined three values of the notional permeability. Based on numerical model results he added for a typical layer composition one more value. Based on physical model tests, the present paper provides notional permeability factors for seven layer compositions of which two correspond to the compositions tested by Van der Meer. The results of these two layer compositions are within the scatter of the results by Van der Meer. To help determination of the notional permeability for non-tested layer compositions, a simple empirical formula is presented.

Detecting and quantifying hydromorphology changes in a chilean river after 50 years of dam operation

This study identifies and characterizes hydromorphological changes along the Rapel River downstream of the first large dam built in Chile (1968). A hydromorphological analysis is carried out to assess changes on the hydrological flow regime, bed sediments, and fluvial morphology along a 19 km river reach. Results classify current global hydrological quality as “Moderate” (according to the Indicator for Hydrological Alteration in RIverS, IAHRIS), however specific indicators within this classification scheme identified quality as “Poor”. The morphological quality decreased from “Very Good” to “Good” (assessed by the Morphological Quality Index, MQI). Changes in the planform were particularly intense during the post dam period when intensive lateral mobility occurred, with the corresponding loss of secondary river branches, and with generation of straighter and regular river sections with presence of an armor layer observed along the entire river reach. Between 1991 and 2015 channel stabilization with less lateral mobility was observed, which thought to be associated with the river new equilibrium trend. River width, sinuosity and braiding index changed at different rates along the studied river reach. Our investigation demonstrates that the Rapel River experienced changes differently than those described in the literature given its lower gradient and hydraulic interaction with the Pacific Ocean.

Finite Element Analysis of Flexible Pipes Under Compression: Influence of the Friction Coefficient

In order to study the compressive behavior of flexible pipes, a nonlinear finite element model was developed. This fully tridimensional model recreates a five-layer flexible pipe with two tensile armor layers, an external polymeric sheath, an orthotropic high strength tape, and a rigid inner nucleus. The friction coefficient is known as a key parameter in determining the instability response of flexible pipes’ tensile armor. Since the featured model includes all nonlinear frictional contacts between the layers, it has been used to conduct several experiments in order to investigate its influence on the response. This article includes a description of the finite element model itself and a case study where the friction between the layers of the pipe is changed. The procedure of this analysis is described here, along with the results.

Mechanical analysis of un-bonded flexible pipe tensile armor under combined loads

Marine un-bonded flexible pipes wrapped with fiber-reinforced polymer are essential in offshore oil and gas exploration because of their high flexibility and reliability. These flexible pipes may be subjected to tensile loads, compressive loads, and torsional loads, causing the armor wires to undergo large displacements that may result in failure. This study develops a nonlinear simplified finite element model to investigate the mechanical behavior of flexible pipe armor wires under various types of load. The stiffness change rule of flexible pipe armor wires is obtained from numerical simulations of the flexible pipe tensile armor layer under combined loads. The results reveal that the flexible pipe stiffness under combined loads varies significantly from that under single loads, and the combination of different load types can either increase or decrease the stiffness. This study provides essential reference data for the further design, fabrication, and application of flexible pipes.

The role of submerged berms on the momentary liquefaction around conventional rubble mound breakwaters

Berms deployed at the toe of conventional rubble mound breakwaters can be very effective in improving the stability of the armor layer. Indeed, their design is commonly tackled by paying attention to armor elements dimensioning. Past research studies showed how submerged berms can increase the stability of the armor layer if compared to straight sloped conventional breakwaters without a berm. To fill the gap of knowledge related to the interaction between breakwaters with submerged berm, waves and soil, this research aims to evaluate how submerged berms configuration influences the seabed soil response and momentary liquefaction occurrences around and beneath breakwaters foundation, under dynamic wave loading. The effects of submerged berms on the incident waves transformation have been evaluated by means of a phase resolving numerical model for simulating non-hydrostatic, free-surface, rotational flows. The soil response to wave-induced seabed pressures has been evaluated by using an ad-hoc anisotropic poro-elastic soil solver. Once the evaluation of the seabed consolidation state due to the presence of the breakwater has been performed, the dynamic interaction among water waves, soil and structure has been analyzed by using a one-way coupling boundary condition. A parametric study has been carried out by varying the berm configuration (i.e. its height and its length), keeping constant the offshore regular wave condition, the berm and armor layer porosity values, the water depth and the elastic properties of the soil. Results indicate that the presence of submerged berms tends to mitigate the liquefaction probability if compared to straight sloped conventional breakwater without a berm. In addition, it appears that the momentary liquefaction phenomena are more influenced by changing the berm length rather than the berm height.

Failure characteristic analysis of tensile armour layer of unbonded flexible riser under axial compression

ABSTRACTUnbonded flexible risers have been applied in the harsh environment for several decades, and structural failure sometimes appears. Owing to the large axial compression by installation or to the combined loads of axial compression and external pressure during operation, radial or lateral failure of tensile armour layers can occur, threatening structural safety. In order to study the mechanism of failure on tensile armour layers of unbonded flexible riser subjected to axial compression, a simplified numerical model, based on ABAQUS/Explicit, with tensile armour layers, structural tape and outer sheath is proposed. The nonlinearity of material is taken into consideration in this model and the validity of this model is verified by a theoretical method. Failure characteristics for helical wire under axial compression are studied in consideration of the nonlinear factors including number of tendons for the tensile armour layer, laying angle of the tensile armour layer, thickness of outer sheath and friction coefficient between layers. In addition, the impact of the external pressure on axial ultimate strength is discussed.

Experimental Study of Wave-Induced Loading on Breakwater Armour Layers

Experimental Study of Wave-Induced Loading on Breakwater Armour Layers

PREDICTING THE RESHAPING OF TEMPORARILY EXPOSED BUNDS WITH XBEACH-G

During coastal construction works, damages often occur to temporary constructions consisting of fine and often wide graded rock material due to exposure to waves. Examples of vulnerable structures are the bunds that are constructed to serve as the core for a breakwater before placement of the filter and armour layers, or as temporary defense during reclamation works. Such structures – mostly built in bulk – are temporarily exposed to wave conditions, and will reshape depending on the duration of exposure and the prevailing hydraulic conditions. For the exposed breakwater core, it is important to have an idea on how much of the placed material will be moved outside the perimeter, and thus how much material needs to be rehandled before the cover layers can be constructed. For temporary defense works, it is important to know how much material needs to be placed initially in order to offer sufficient protection for a certain period. For both cases, an accurate prediction of the reshaping of such structures is necessary.

EFFECTS OF WATER LEVEL VARIATIONS ON THE STABILITY OF ROCK ARMOURED SLOPES

Physical model tests on the stability of rock armoured slopes have been performed to demonstrate the importance of water level variations during a storm, due to a tide or a storm surge. For the stability of rock armoured slopes also the importance of the sequence of storms at various water levels has been studied. The test results indicate that a smooth sinusoidal water level variation leads to an increase in damage compared to the same wave conditions at a constant water level. Furthermore, a stepwise approach of the sinusoidal water level elevation leads to other results than the approach with a continuous water level variation, whereas the continuous water level variation resembles the peak of a storm or the tidal water level variation better than a stepwise approach. If storms with different water levels attack the armour layer, the damage is generally smaller than if all storms attack the armour layer at the same water level. Furthermore, the results have been discussed based on earlier analyses where the statistics of rock armoured slopes have been addressed and the importance of the length effect has been illustrated using a method to apply results from physical model tests to real structures.

NUMERICAL MODELING OF WAVE INTERACTION WITH A NON-CONVENTIONAL BREAKWATER FOR WAVE ENERGY CONVERSION

The hybrid non-conventional breakwaters are innovative coastal structures, which have as a primary function the coastal and harbours protection, but with the important benefit of electricity production, due to their combination with Wave Energy Converters. The most recent example of a non-conventional breakwater is called OBREC, standing for Overtopping Breakwater for Energy Conversion (Vicinanza et al., 2014). The device consists of a traditional rubble mound breakwater, in which the seaward armour layer in the upper part is replaced with a frontal sloping ramp and a reservoir. The structure is designed to capture and gather the water that overtops the crest ramp. The potential energy of the water stored in the reservoir is converted into kinetic energy and then into electrical energy by flowing through low head hydraulic turbines coupled with generators, exploiting the different water levels between the reservoir and the sea level.

ROCK ARMOR DAMAGE IN DEPTH-LIMITED BREAKING WAVE CONDITIONS

The armor layer of a mound breakwaters is usually designed with a formula derived from physical tests in non-breaking wave conditions; however, most rubble mound breakwaters are placed in the wave breaking zone where the highest waves break before reaching the structure. The hydraulic stability formulas developed for rock-armored breakwaters in non-breaking conditions are not completely valid to characterize the hydraulic stability of these structures under depth-limited wave attack. In this study, five series of 2D physical tests were carried out on a bottom slope m=1/50 to analyze the hydraulic stability of double-layer rock armored breakwaters in depth-limited breaking wave conditions. Measurements taken by 12 wave gauges placed along the wave flume were compared with estimations of Hm0, H2% and H1/10 obtained from numerical model SwanOne. The significant wave height, Hm0, estimated or measured at a distance 3hs from the toe of the structure was the best characteristic wave to relate armor damage with stability number. The six-power relationship between dimensionless armor damage and stability number, found in this study, explained more than 94% of the variance in the damage observations. This relationship is valid for conventional non-overtopping double-layer rock-armored breakwaters on bottom slope m=1/50 and depth-limited breaking wave conditions.

HYDRAULIC STABILITY AND OVERTOPPING PERFORMANCE OF A NEW TYPE OF REGULAR PLACED ARMOR UNIT

The XblocPlus is a new type of interlocking single layer armour units that is placed with uniform orientation. This is novel and different from all other single layer, interlocking armouring systems. The hydraulic stability of the XblocPlus breakwater armour unit was tested in 2D and 3D hydraulic model tests. Wave overtopping tests were performed to determine the roughness coefficients of the EurOtop overtopping formula for the XblocPlus. Model tests on a rubble mound breakwater with XblocPlus armour included 2D tests with a 1:30 seabed slope and with 1:2 and 3:4 breakwater slopes and 3D model tests with a flat seabed and with a 3:4 breakwater slope. Wave heights up to 150% of the design wave height were tested in the 2D tests and up to 200% with wave directions 0° to 60° in the 3D tests. No armour unit displacements were observed in 2D tests with 1:2 slope. In the 2D tests with 3:4 slope one armour unit was displaced when the wave height reached 159% of the design wave height. No damage to the XblocPlus armour layer was observed in the 3D tests. A roughness coefficient of 0.45 was deduced from overtopping tests with wave heights of 60% to 100% of the design wave height. The model test results indicate little or no influence of wave steepness on XblocPlus stability and no adverse influence of wave obliquity while the seabed slope in front of the breakwater may have some impact on the XblocPlus armour layer stability.

Stability analysis of a non-conventional breakwater for wave energy conversion

This paper presents the stability analysis of a non-conventional breakwater cross-section integrating an overtopping wave energy converter, named OBREC. The device consists of a traditional rubble-mound breakwater in which the upper part of the armour layer is replaced by a smooth ramp and a reservoir.The analysis of the structure is carried out by combining model scale experiments and numerical simulations based on the Volume-Averaged Reynolds Averaged Navier-Stokes (VARANS) equations.The numerical analysis is used to complete and extend the results of the physical model test campaign, providing a deeper understanding of the pressure distribution and resultant force behaviour in locations where laboratory measurements were difficult to obtain or not available. Results show that the maximum vertical and horizontal total forces on the device are not simultaneous. At the time instant of the maximum total horizontal force, the vertical force is zero or directed downward, due to the significant positive contribution of the force acting on the sloping ramp. Additional numerical simulations show the influence of the submerged ramp length on the forces acting on the structure using the global and local stability analysis. The presence of the shaft contributes positively to the global stability against the sliding failure mode by reducing the uplift force exerted on the horizontal base. Moreover, the stability analysis shows that the critical conditions for the global failure modes occur at different time instants.

Tension-bending analysis of flexible pipe by a repeated unit cell finite element model

Unbonded flexible pipes consist of multiple subcomponents which interact through frictional contact. A full 3-D finite element analysis of unbonded flexible pipes is computationally expensive, and a more efficient approach for practical engineering purposes is required. This work presents a repeated unit cell (RUC) finite element model for analyzing flexible pipes subjected to combined constant tension and curvature. Periodic boundary conditions reduce the model size by taking advantage of the structural and loading periodicities, and by assuming uniform wire behavior in the armor layer. The RUC model is suitable for resolving the local tensile armor stress distribution and the global pipe response. A flexible pipe is studied with the RUC model for various tension-bending load configurations and the results have been compared with existing analytical models for validation. The study showed strong correlation between the RUC model and the analytical models, with some difference in the wire bending stresses. This difference can to some degree be explained by the difference in the wire kinematics assumptions. It is found that the proposed RUC model is a robust and computationally efficient approach for analyzing flexible pipes.

Cubipod® Armor Design in Depth-Limited Regular Wave-Breaking Conditions

Armor stability formulas for mound breakwaters are commonly based on 2D small-scale physical tests conducted in non-overtopping and non-breaking conditions. However, most of the breakwaters built around the world are located in breaking or partially-breaking wave conditions, where they must withstand design storms having some percentage of large waves breaking before they reach the structure. In these cases, the design formulas for non-breaking wave conditions are not fully valid. This paper describes the specific 2D physical model tests carried out to analyze the trunk hydraulic stability of single- and double-layer Cubipod® armors in depth-limited regular wave breaking and non-overtopping conditions with horizontal foreshore (m = 0) and armor slope (α) with cotα = 1.5. An experimental methodology was established to ensure that 100 waves attacked the armor layer with the most damaging combination of wave height (H) and wave period (T) for the given water depth (hs). Finally, for a given water depth, empirical formulas were obtained to estimate the Cubipod® size which made the armor stable regardless of the deep-water wave storm.

Hydraulic Stability of the Armor Layer of Overtopped Breakwaters

Mound breakwaters with significant overtopping rates in depth-limited conditions are common in practice due to social concern about the visual impact of coastal structures and sea level rise due to climatic change. For overtopped mound breakwaters, the highest waves pass over the crest producing armor damage, not only to the front slope, but also to the crest and the rear slope. To guarantee the breakwater stability, it is necessary to limit the armor damage in the three parts of the structure: Front slope, crest, and rear slope. This paper describes the hydraulic stability of the armor layer of medium and low-crested structures in wave breaking conditions. Small-scale physical model tests were carried out with different relative crest freeboards and three armor units: Rocks, cubes, and Cubipods. The armor damage progression in the front slope, crest, and rear slope was analyzed using the Virtual Net method to consider the heterogeneous packing and porosity evolution along the armor slope. A comparison is provided between the hydraulic stability of the different armors and their relationship with the measured overtopping volumes.