Scour Protection Research Articles

Assessment of foundation design for offshore monopiles unprotected against scour

When designing offshore monopiles without scour protection, the stiffness of the foundation will vary with time due to the dependency of the scour depth on current and sea conditions. Currently, design regulations of organizations such as Det Norske Veritas (DNV) and the International Organization for Standardization (ISO) recommend the use of the most extreme local scour depth as the design scour depth. This is a conservative approach, because the scour depth depends on the sea conditions and because the equilibrium scour depth is low during moderate to extreme wave loading. In this paper the effect of using expected scour depths when designing for the ultimate limit state and the fatigue limit state is illustrated by means of a desk study.

Scour process around monopiles during various phases of sea storms

ABSTRACT Umeda, S., 2013. Scour process around monopiles during various phases of sea storms An understanding of scour processes during the passage of sea storms is particularly important for the design of structures and the development of effective scour protection works. This study investigates the scour process at a slender monopile during various phases of a storm to give a better understanding of scour and backfilling mechanisms. Laboratory experiments were carried out to simulate the scouring effect at a pile by a storm consisting of a succession of seven phases of irregular waves. Detailed examinations were conducted to explore variations in the scour depth and bed topography. It turns out that scour patterns resulting from irregular waves can be characterized using the Keulegan-Carpenter number and the Shields parameter (KC1/3, θ1/3), which require significant near-bottom velocities. The scour process during a heavy storm consists of four main phases: (1) no bed deformation, (2) accretion, (3) ero...

Dimensionless wave height parameter for preliminary design of scour protection in offshore wind farms

Matutano, C., Negro, V., López-Gutiérrez, J.S., Esteban, M.D. and del Campo, J.M, 2013. Dimensionless wave height parameter for preliminary design of scour protection in offshore wind farmsThe scour phenomenon jeopardizes the operating capacity of offshore structures since it compromises their stability. The study of this phenomenon in the field of offshore wind farms began few years ago. Considering that these structures are subjected to waves, currents and tides.This article describes the results of an investigation aimed at the analysis methods used in the design of the protections against scour phenomenon on offshore wind farms in transitional waters, using medium and large diameter monopile type deep foundations.In order to do this, a thorough review of those premises was performed, from one of the first experiences in 1991 (Denmark), to the recently implemented and proposed in the UK, defining its main characteristics (acting maritime climate, the correlation between wave height and period associated with a given recurrence) and the type of protection used (weight average, nominal diameter in the case of breakwater type protection, width and behavior).Taking into consideration the experience with offshore wind farms in current operation, the design data analysis, the basis of climate and the soil-structure interaction, the dimensionless wave height parameter (H0), the erosion extension (Lext), and the quantification of maximum scour depth (Smax) have been calibrated for a preliminary design of protections around a large diameter pile, obtaining initial values for H0 in the vicinity of 6 and 15, and values for the extension of protection design between L/4 and L/2.

EVOLUTION OF LOCAL SCOUR AROUND A COLLARED MONOPILE THROUGH TIDAL CYCLES

This paper presents the results of an experiment designed to assess the time-development of scour around an offshore wind turbine collared monopile over a number of tidal cycles. One collar shape and location was investigated. The scour developed more slowly and the scour depth was shallower than for the case of a smooth monopile throughout the majority of the first half-cycle. This difference reduced quite rapidly during the second half-cycle and the scour depth at the end of two tidal cycles was essentially the same as for the smooth monopile. The time development of the scour was compared with results from existing empirical models for the time-development of scour under unidirectional flow. As expected, these models give a much smoother evolution of scour and different scour rates than those measured. Time variation in scour depth was better reproduced with a simplified approach for prediction of the time-varying development of scour. This also highlighted a problem with estimation of the time scale for the development of the equilibrium scour depth. Further investigations are needed before this alternative scour protection is completely rejected.

PHYSICAL MODELING STUDY ON SCOUR AND SCOUR COUNTERMEASURE FOR SEA-CROSSING BRIDGE PIERS

There are significant engineering challenges in placing the sea-crossing bridge piers in more or les shallow coastal waters because, as well as having to withstand storm-force waves and tidal currents, the bridge piers have to remain stable on a sea bed that may be continuously changing. One of the major challenges faced by designers is how to predict, and prevent, scour of the seabed sediments around the sea-crossing bridge piers. This paper describes a physical model study on scour and scour countermeasure for sea-crossing bridge piers. A 1:49 scale movable bed model tests were carried out in the Near-shore Wave Basin (NSWB, 27m x 19m) at the Tainan Hydraulics Laboratory, National Chang Kung University (NCKU), Tainan, Taiwan, with the sea-crossing bridge piers in the test area. Three series of physical model tests were performed in this study. The aim of the first test series is to investigate the maximum scour depth for the initial construction of bridge piers with only one pier model. Then the second test series were conducted to investigate the greatest magnitude of local scour and potential scour area in the sand bed around the sea-crossing bridge group piers with no scour protection. Based on the analysis from the former NSWB experimental results, the suitable scour countermeasure for sea-crossing bridge piers would be proposed and validated its function of preventing scour in the third test series.

INVESTIGATIONS ON SCOUR DEVELOPMENT AROUND A GRAVITY FOUNDATION FOR OFFSHORE WIND TURBINES

Due to insufficient insights about the scour development around the STRABAG gravity foundation, the Franzius-Institute conducted small- and large-scale physical model tests in wave flumes on a scale of 1:50 and 1:17 in order to investigate the wave-induced scouring phenomena around the foundation and to design a scour protection system. The tests on scour development without a scour protection system show that the main areas which are vulnerable to scour are the contact areas of the foundation. Furthermore, the experiments show that a scour protection system is necessary for the given and investigated wave boundary conditions; the performance of the selected protection system using geotextile sand containers is verified. Additional numerical simulations indicate an amplification of the resulting flow around the foundation under combined loads (waves and current), but without significant change of the flow pattern

Levee scour from overtopping storm waves and scour counter measures

Laboratory wavetank experiments were performed on the landside levee scour induced by floodwall overtopping. The objectives of this study were to elucidate the landside levee scour characteristics and evaluate the effectiveness of selected scour counter measures. Model levees, both with and without a scour protection measure, were constructed. Four scour protection armoring were tested: riprap, coreloc, riprap with an underlayer, and coreloc with an underlayer. The levee soil was composed of a sand-clay mixture. The geometric scour characteristics of interest driven by their practical importance were maximum scour depth and scour length. Both the maximum scour depth and the scour length exhibited an exponential trend in approaching equilibrium values, with the scour length reaching equilibrium sooner. Parameterizations for predicting the values of the maximum scour depth and length while approaching the equilibrium and at the equilibrium state were developed. Though the riprap armoring cover proved more effective than the coreloc armoring cover, the presence of an armoring underlayer was a greater impetus of the overall effectiveness of both protection measure types. Therefore, it is recommended that the placement and size distribution of the underlayer be adjusted in the field accordingly for the greatest disaster prevention and economic advantage.

Flow and bed shear stresses in scour protections around a pile in a current

Transport of bed sediment inside and beneath the scour protection may cause deformation and sinking of the scour protection for pile foundations. This may reduce the stability of the mono pile and change the natural frequency of the dynamic response of an offshore wind turbine installed on it in an unfavourable manner. Using physical models and 3D computational fluid dynamic (CFD) numerical simulations, the velocity and bed shear stresses are investigated in complex scour protections around mono piles in steady current. In the physical model the scour protections consisted of an upper cover layer with uniformly distributed coarse stones and a lower filter layer with finer stones. For the numerical simulations, the Flow-3D software was used. The scour protection layers were simulated with different numerical approaches, namely regularly arranged spheres, porous media, or their combinations (hybrid models). Numerical simulations with one or four layers of cover stones without filter layer were first computed. Three additional simulations were then made for a scour protection with a cover layer and a single filter layer. Finally, a simulation of a full scale foundation and scour protection was made with porous media approach.Based on the physical and numerical results, a method to determine the critical stones size to prevent motion of the base sediment is established and compared to a full scale case with sinking of scour protection (Horns Rev I Offshore Wind Farm, Denmark). It is also found that the CFD simulations are capable of calculating the flow velocities when the scour protection is represented by regular arranged spheres, while the turbulence in general is underestimated. The velocity can also be calculated using porous media flow approach, but the accuracy is not as good as for spheres. The deviation is more severe for more complex scour protections. In general, computational models provide valuable information for the prediction and design of scour protections for offshore wind farms.

Investigation of Apron Setting on the Main Deep Trough of River Bed by 2D Unsteady Flow Simulation

This study uses SMS-2D unsteady flow model to simulate apron setting and discuss its scour protection due to bow wave around the bridge pier on the main deep trough of 24-pier ChihShang Bridge and 3-pier Taitung Bridge in 100 years return flood peak flow, on the downstream BeiNan River in East-Taiwan as case study. Further, the topic of this study is to evaluate location-oriented scour likely result in accelerated erosion of critical piers by apron setting designed sizes in case of excessive cost, thus, providing reliable information and references for bridge engineers.

Evaluation for Stepped Apron against Erosion around Piers

In order to reduce erosion impact around pier footings, the SMS-2D applied in this study to analysis stepped apron set in critical scour downstream and discuss its scour protection due to bow wave around the bridge pier on the main deep trough of new Taitung Bridge in 100 years return flood peak flow, on the downstream BeiNan River in East-Taiwan as case study. Further, the topic of this study is to compare the different dimensions of aprons for apron deflecting characteristics, making the beach effect and take preventive measures, and in case of excessive cost, suitable sizes and sets for stepped apron are provided for reliable information and references for bridge engineers.

Propeller induced scour around the pile type berth structures

Scour caused by propeller jets around harbour structures results in considerable damage. Scour protection for these structures has been of increasing importance in their design. This paper addresses an experimental investigation into the scour on armoured slopes under berth structures. Three different cases were considered for this study, including an armoured layer with an apron, an armoured layer without an apron and an armoured layer with an apron that included a pile on the slope. Propeller jet velocity and scour measurements were conducted in these experiments. Additionally, armoured layer damages on the slope were defined using a damage level parameter. The critical densimetric Froude Number (Frd,cr) was defined for the start of the damage.

Flood Damage to Rural Buildings Result from Foundation Scour and Scour Protection Strategy

In China, flooding is one of the most common and frequent natural disasters, which can cause life and property losses partly due to heavily damage and collapse of rural buildings. Hence, it is very important to conduct comprehensive study on flood damage to rural buildings. In this paper, the flood damage to rural builds is described and analyzed with emphasis on the foundation scour, which may result in whole building collapse, and scour protection strategy. Damages or collapse caused by foundation scour of rural building can be categorized into two main groups according to the location of rural a building. After summarizing existing scour protection strategies, a new technology is suggested. The contents presented in this paper may be useful references for further study on flood damage to rural building and scour protection engineering practices.

Cumulative impacts of road developments in floodplains

This paper investigates the impact of differing road development strategies in floodplains for increasing levels of road expansion, focusing on hydraulic characteristics of floods such as water level, velocity and inundation duration and extent. Cumulative impacts are presented in term of both a resistant approach where road transport infrastructure has the secondary purpose of flood dykes and, a resilience approach that maintains the floodplain hydraulics through the use of flow through structures. Each method is examined in the context of road development in the Cambodian Mekong floodplain. Results indicate that resistance approaches necessitate higher levels of road structures, designed to higher technical specifications, whilst resilience approaches maintain the hydraulic character of floodplains but require the inclusion of well-designed flow through structures with localized scour protection.

FLOW-3D를 이용한 해상풍력발전기초의 세굴 평가

해상풍력발전기초의 국부세굴을 평가하기 위하여 3차원 수치해석인 FLOW-3D를 이용하여 모노파일과 자켓기초에 대해 해석을 수행하였다. 수치해석 결과에 의하면 모노파일과 자켓기초 레그 주위에서 국부적으로 유속이 증가하는 것으로 나타났으며, 그 후면에서는 후류 및 와류로 인하여 유속의 감소가 나타났다. 자켓기초의 경우에는 모노파일에 비해 단일 레그의 직경이 작고, 자켓레그 사이의 간섭효과 및 자켓구조물의 복잡한 형상으로 인하여 모노파일에 비하여 국부적인 유속 증가가 더 크게 나타났으며 세굴심이 더 깊게 형성되는 것으로 나타났다. 따라서, 해상풍력발전 기초의 세굴 평가 및 세굴방지공 설계시 하부구조물의 형상에 대한 고려가 필요한 것으로 판단된다. In order to evaluate the local scour around offsshore wind foundation, mono pile and jacket foundation were simulated by using FLOW-3D. Numerical analysis results show that local increases of velocity around mono pile and jacket foundation was developed but velocity decreases in backward of pile and leg due to the wake vortex was observed. Local increases of velocity around foundation and scouring of jacket is more significant than that of mono pile, since jacket is the complex structure and has the interference effect with legs. Therefore, in order to evaluate the scour and design the scour protection method, the form and shape of substructures of offshore wind should be considered.

Artificial Reef Effect in relation to Offshore Renewable Energy Conversion: State of the Art

The rapid worldwide growth of offshore renewable energy production will provide marine organisms with new hard substrate for colonization, thus acting as artificial reefs. The artificial reef effect is important when constructing, for example, scour protections since it can generate an enhanced habitat. Specifically, artificial structures can create increased heterogeneity in the area important for species diversity and density. Offshore energy installations also have the positive side effect as they are a sanctuary area for trawled organisms. Higher survival of fish and bigger fish is an expected outcome that can contribute to a spillover to outer areas. One negative side effect is that invasive species can find new habitats in artificial reefs and thus influence the native habitats and their associated environment negatively. Different scour protections in offshore wind farms can create new habitats compensating for habitat loss by offshore energy installations. These created habitats differ from the lost habitat in species composition substantially. A positive reef effect is dependent on the nature and the location of the reef and the characteristics of the native populations. An increase in surface area of scour protections by using specially designed material can also support the reef effect and its productivity.

Design Considerations for Pile Groups Supporting Marine Structures with Respect to Scour

Piles supporting marine structures such as jetties, relieving platforms, quay walls and fixed offshore structures are subjected to lateral loads due to berthing and mooring forces, wind, waves, storm surges and current forces. This paper presents some factors that affect the design of pile groups supporting marine structures founded in cohesionless soils. Some main aspects that should be considered in the pile group design are addressed such as pile batter angle, pile group arrangement, pile spacing, pile slenderness ratio and magnitude of lateral static loading. Numerical analyses were conducted to investigate these design aspects with and without impact of scour. Different scour depths were considered to cover the possible root causes of scour around pile groups such as waves, current and ship propeller jets. The study revealed that scour has greater impact on lateral loading of pile groups compared to its impact on single piles. Pile groups with side-by-side arrangement exposed to scour are more critical than single piles and piles groups with tandem arrangement due to the combined effect of scour and pile-soil-pile interaction. It is also concluded that scour protection is not always required. More attention and considerations should be given to scour protection around piles especially if the piles are closely spaced, arranged side-by-side and if slenderness ratio is less than 12.5.

Empirical design of scour protections around monopile foundations. Part 2: Dynamic approach

When building offshore wind turbines, a static scour protection is typically designed, allowing no movement of top layer stones under design conditions, leading to an often conservative design. Furthermore, little design guidelines exist for a scour protection around a monopile foundation under combined wave and current loading.In part 1 (De Vos et al., 2011), preceding this paper, a static design guideline for a scour protection around a monopile is suggested, based on a combined wave and current flow field. By allowing some movement of the top layer stones of the scour protection, a more economical design is obtained. This paper describes the derivation of a dynamic design formula to calculate the required stone size for a scour protection around a monopile foundation in a combined wave and current climate. The formula is based on the results of an experimental model study, described in this paper. The formula gives an expected damage level to the scour protection, based on the wave orbital velocity, wave period, steady current velocity, water depth, relative stone density and stone size.When applying the formula for a typical situation in the North Sea, a reduction of 20% to 80% of the required stone size is obtained, compared to the static design approach, described in part 1.

Sinking of armour layer around a cylinder exposed to a current

The flow processes in a scour protection around a monopile in steady current are described in relation to transport of sediment in the scour protection based on physical model tests. The scour protection consisted of uniformly distributed coarse stones without filter layer. Transport of sediment in the scour protection may cause sinking of the scour protection. This may reduce the stability of the monopile and change for instance the natural frequency of the dynamic response of an offshore wind turbine in an unfavourable manner. The most important flow process with regard to transport of sediment and sinking of the scour protection is found to be the horseshoe vortex. It is found that a larger pile diameter relative to the size of the protection stones will cause a larger sinking and that two layers of stones will decrease the sinking relative to one layer of stones with the same size.

Evaluating scour at marine gravity foundations

This paper analyses scour at offshore foundations, with special reference to gravity base foundations which are used in a range of applications including oil and gas production, wind farm foundations and subsea infrastructure protection, such as concrete covers to pipelines. Two published empirical methods available for evaluating scour are reviewed and compared with laboratory and field data. The methods that are presented are reasonable for first-order assessment although they do not represent the inclusion of skirts. In a vigorous scour environment the foundation is liable to be undermined and scour protection will be required. A flow chart for a scour management plan is proposed to integrate scour considerations into the life cycle management of the gravity base foundations and some improvements that can be made to the predictive capabilities are outlined.

Coherent structure dynamics and sediment erosion mechanisms around an in‐stream rectangular cylinder at low and moderate angles of attack

Local scour around elongated in‐stream structures (e.g., high–aspect ratio rectangular bridge piers) is mainly driven by the interactions between the erodible bed and the large‐scale coherent structures generated by the presence of the flow obstruction. The present investigation uses eddy‐resolving numerical simulations to study the mean flow and turbulence structure around a high–aspect ratio rectangular cylinder placed in a flat bed channel. Simulations are conducted for three angles of attack (α = 0°, 15°, and 30°) at a channel Reynolds number of 2.4 × 105. This paper focuses on the dynamics of the large‐scale coherent structures forming around the rectangular cylinder and their role in controlling sediment entrainment at conditions corresponding to the start of the scour process. Simulation results show that most of the sediment is entrained from the bed by the eddies shed inside the separated shear layers (SSLs), by the legs of the necklace vortices, and by the strongly accelerated flow on the outer side of the SSLs. For α = 0° and 15°, the horseshoe vortex (HV) system plays a relatively minor role in the entrainment of sediment in front of the cylinder, and the passage of the wake vortices (rollers) results in a small amplification of the bed friction velocity. In contrast, for α = 30°, the unsteady dynamics of the main necklace vortices part of the HV system and of the roller vortices results in a significant amplification of the instantaneous bed friction velocity. The mean flux of sediment entrained from the bed calculated on the basis of the mean flow field is found to underestimate by 2–3 times the same quantity when estimated more correctly on the basis of the instantaneous flow fields. The primary reason for this underestimation is sediment entrainment in regions where the mean flow bed friction velocity is smaller than the critical value for entrainment. Quantitative information on the extent of the regions of high values of the bed friction velocity and its standard deviation and on the sediment entrainment flux as a function of the angle of attack is essential to guide scour protection measures around the in‐stream noncircular obstructions.