Assessment of shallow water random wave-induced scour at the trunk section of breakwaters using deep water wind and wave conditions

This article provides a simple analytical method giving estimates of random wave-induced scour at the trunk section of vertical-wall and rubble-mound breakwaters in shallow water from deep water wind and wave conditions. Results are exemplified by using a Pierson-Moskowitz model wave spectrum for deep water wind waves with the mean wind speed at the 10 m elevation above the sea surface as the parameter. The significant value of the scour depth within a sea state of random waves is provided and an example typical for field conditions is given. The method should serve as a useful tool for assessing shallow water random wave-induced scour based on input from deep water wind and wave conditions.acceptedVersio

Note on estimating bed shear stress caused by breaking random waves

This note presents a method of how the bed shear stress caused by breaking random waves on slopes can be estimated. This is obtained by adopting the Sumer et al. (2013) bed shear stress formula due to spilling and plunging breaking waves on hydraulically smooth slopes combined with the Myrhaug and Fouques (2012) joint distribution of surf similarity parameter and wave height for individual random waves in deep water. The conditional mean value of the maxima of mean bed shear stress during wave runup given wave height in deep water is provided including an example for spilling and plunging breaking random waves corresponding to typical field conditions. Another example compares the present results with one case from Thornton and Guza (1983) estimating the wave energy dissipation caused by bed shear stress beneath breaking random waves.publishedVersio

Time scale for scour beneath pipelines due to long-crested and short-crested nonlinear random waves plus current

This article derives the time scale of pipeline scour caused by 2D (long-crested) and 3D (short-crested) nonlinear irregular waves and current for wave-dominant flow. The motivation is to provide a simple engineering tool suitable to use when assessing the time scale of equilibrium pipeline scour for these flow conditions. The method assumes the random wave process to be stationary and narrow banded adopting a distribution of the wave crest height representing 2D and 3D nonlinear irregular waves and a time scale formula for regular waves plus current. The presented results cover a range of random waves plus current flow conditions for which the method is valid. Results for typical field conditions are also presented. A possible application of the outcome of this study is that, e.g., consulting engineers can use it as part of assessing the on-bottom stability of seabed pipelines.publishedVersio

On-Bottom Stability of Umbilicals and Power Cables for Offshore Wind Applications

With the increase in offshore wind farms, the demands for umbilicals and power cables have increased. The on-bottom stability of umbilicals and power cables under the combined wave and current loading is the most challenging design issue, due to their light weight and the complex fluid–cable–soil interaction. In the present study, the methodology for dynamic lateral stability analysis is first discussed; and the reliable hydrodynamic load model and cable–soil interaction model based on large experimental test data are described in detail. The requirement of the submerged weight of a cable w s ws to obtain on-bottom stability is investigated for three types of soil (clay, sand and rock), using the finite element program PONDUS, and the results are w s,rock >w s,clay >w s,sand ws,rock>ws,clay>ws,sand under the same load conditions. Several different aspects related to optimization design of the on-bottom stability are explored and addressed. There is a significant benefit for the on-bottom stability analysis to consider the reduction factors, due to penetration for clay and sand soil. The on-bottom stability is very sensitive to the relative initial embedment z 0 /D z0/D for clay and sand soil, due to the small diameter of the cables, and therefore, reliable prediction of initial embedment is required. In the energy-based cable–soil interaction model, the friction coefficient μ μ and the development of penetration affect each other and the total effect of friction force F f Ff and passive resistance F r Fr is complicated. The effect of the friction coefficient μ μ on the on-bottom stability is different from engineering judgement based on the Coulomb friction model. The undrained shear strength of clay is an important parameter for the on-bottom stability of umbilicals and cables. The higher the undrained shear strength of the clay, the larger the lateral displacement. Meanwhile, the submerged weight of sand has a minor effect on the lateral displacement of cables. The method used in the present study significantly improves the reliability of the on-bottom stability analysis of umbilicals and power cables for offshore wind applicationpublishedVersio

Comparative study of five commonly used gravity type fish cages under pure current conditions

Gravity type fish cages have been commonly used in marine aquaculture for years. However, only limited research efforts have been made to assess the influences of different design parameters on the structural responses of gravity type fish cages. The present study first develops and validates an open-source numerical library under the toolbox Code_Aster for the structural analyses of fish cages. Then, the newly developed library is employed to conduct time-domain simulations to investigate the effects of the main design parameters on the cultivation volumes and drag forces. In this paper, five circumferences of the floating collar, five depths of the net bag, five weights and nine current velocities are considered in the parametric study of five commonly used fish cages. Moreover, regression functions are proposed based on the large number of numerical results to provide accurate predictions for the most concerning aspects in the design process for fish cages. Based on the parametric study, recommendations for selecting fish cage types and practical guides for cage construction are given. This study should be of value to structural designers as well as researchers wishing to optimise cage design.publishedVersio

Numerical simulations of flow around two tandem wall-mounted structures at high Reynolds numbers

In the present study, the flow around two-dimensional (2D) square and trapezoidal wall- mounted structures in tandem on a horizontal flat wall has been simulated using two-dimensional Reynolds-averaged Navier-Stokes (RANS) equations combined with the standard SST turbulence model. The Reynolds number (Re) based on the free stream velocity and the height of the structures is set to be 1.19 × 105. The effects of the gap ratio between the two structures as well as the slope angles of the two sides of the trapezoidal structures on the hydrodynamic quantities and the flow around the two structures have been investigated and discussed.publishedVersio

Numerical investigations of flow around subsea covers at high Reynolds numbers

Two-dimensional (2D) numerical simulations of flow over wall-mounted rectangular and trapezoidal ribs subjected to a turbulent boundary layer flow with the normalized boundary layer thickness of δ/D = 0.73,1.96,2.52 (D is the height of the ribs) have been carried out by using the Reynolds-averaged Navier-Stokes (RANS) equations combined with the k – ω SST (Shear Stress Transport) turbulence model. The angles of the two side slopes of trapezoidal rib varies from 0° to 60°. The Reynolds number based on the free-stream velocity U∞ and D are 1 × 106 and 2 × 106. The results obtained from the present numerical simulations are in good agreement with the published experimental data. Furthermore, the effects of the angle of the two side slopes of the trapezoidal ribs, the Reynolds number and the boundary layer thickness on the hydrodynamic quantities are discussed.publishedVersio

An evaluation of different RANS turbulence models for simulating breaking waves past a vertical cylinder

The purpose of the present study is to evaluate the performance of different turbulence models for predicting the interaction between breaking waves and a vertical cylinder based on the volume of fluid (VOF) method. Six different models are investigated in the present study, i.e., no turbulence model, the k − ω SST turbulence model, the buoyancymodified k − ω SST turbulence model, the stabilized k − ω SST turbulence model, the modified stabilized k − ωSST turbulence model and the realizable k − ε turbulence model. The vertical cylinder is installed at the edge of a 1:10 slope on the bottom of the numerical wave tank. The numerical simulations are conducted by solving the unsteady ReynoldsAveraged Navier-Stokes (RANS) equations using waves2Foam (a solver based on the open-source Computational Fluid Dynamic (CFD) software OpenFOAM). The present numerical results of the surface elevations and the breaking wave forces are compared with published experimental data. The kinetic characteristics beneath the free surface including averaged velocity, turbulent kinetic energy and turbulent kinematic viscosity are also investigated. It is observed that the stabilized k − ω SST turbulence (λ2 = 0.05, αβs = 1.36) and the buoyancy-modified k − ωSST turbulence model (αβs = 1.176) effectively reduce the turbulent kinetic energy before wave breaking, but the predicted breaking wave forces on the cylinder are smaller than that of the experimental data. The k − ω SST turbulence model shows good agreement with the experimental data in terms of the free surface elevation and the breaking wave force, but it overpredicts the turbulent kinetic energy. The realizable k − ε turbulence model does not give good predictions of both the free surface elevation and the breaking wave force as compared to the published experimental data.publishedVersio

On the three-dimensional effects of the water entry of wedges

Three-dimensional (3D) effects on the total and spanwise slamming coefficients and the pressure reductions acting on the wedge surface during water entry are modeled in a unified manner. First, the water entry of wedges with a constant speed is numerically studied using the finite volume method (FVM) combined with the volume of fluid (VOF) method. The wedge is assumed to enter water with high speeds such that the compressibility, viscosity, gravity and surface tension effects of the fluid can be neglected. The numerical method is validated against an experimental measurement of a freefall water entry of a wedge with a deadrise angle of 30°. Then, the water entry of 3D wedges with deadrise angles of 30, 35, 40 and 45° and beam-span ratios between the width and length varying from 0 to 1 are simulated. The total and spanwise slamming coefficients and the reduction of the pressure distribution are analyzed. The total slamming coefficient is derived with a 3D effect coefficient using a dimensional analysis in the slamming stage and its expression for the transition stage is proposed based on a two-dimensional (2D) transition stage model of the water entry. The spanwise slamming coefficients on the different spanwise sections are modeled with the pressure coefficient distribution of a supercavitating flow around a 2D flat plate. The reductions of the pressure acting on the wedge surface are found to be approximately constant along the wetted length for all spanwise sections, which can be derived using the expressions of the total and spanwise slamming coefficients. Finally, using the proposed model, the predictions of the abovementioned variables are in good agreement with the numerical simulation results in the slamming and transition stages during the water entry of the wedges with different deadrise angles and beam-span ratios.publishedVersio

Development of a coupling algorithm for fluid-structure interaction analysis of submerged aquaculture nets

A coupling algorithm between two open-source numerical toolboxes, i.e., OpenFOAM and Code_Aster, is implemented for fluid-structure interaction analysis of submerged nets. This algorithm is developed to handle the wake effects of thin, flexible and highly permeable structures with complex geometries. Compared to previous approaches, the present algorithm simplifies the procedures of the model preparation by removing additional data-fitting processes for porous coefficients, and improves the accuracy of structural responses by employing a fluid solver to calculate the flow field and a superior Screen model to calculate the hydrodynamic forces. The coupling algorithm is comprehensively described and validated with published experiments for both fixed and flexible nets. Different solidities, inflow angles, incoming velocities and dimensions of nets are also considered. The comparisons of flow velocity in the wake, deformation of flexible nets and drag force on the full-scale fish cage show that the numerical results obtained from the present coupling algorithm are in good agreement with published experimental data.publishedVersio