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Abstract The interaction influences of a two dimensional incident wave and current on the hydrodynamic responses of a floating or fixed body in the presence of a flat or sloping seabed are examined. A solution of this wave-current-seabed scenario is determined by considering steady, diffraction, and reflection potential problems individually and through their combinations. All these components are formulated using a boundary element model involving a continuous Rankine source method subject to appropriate boundary conditions. The validity of the mathematical model and accompanying numerical scheme of study are assessed by comparisons with available published data. Numerical results show that either an incoming waves, current or seabed plays a significant role affecting the body hydrodynamic responses for both fixed and freely floating bodies. [ABSTRACT FROM AUTHOR]

• Vertical bearing capacity of mat foundation on horizontal and sloping seabed was studied. • Sloping seabed under the mat foundation exhibits slip failure. • Contribution of pile leg on vertical bearing capacity is limited but significant on horizontal bearing capacity. • Increase of preloading and consolidation degree can improve the vertical bearing capacity of the foundation. Mat foundation is a new type of jack-up wind power installation platform suitable for clay seabed, which avoids the problems of excessive insertion and extraction difficulties of spudcan foundation on soft clay seabed. Evaluating the vertical bearing capacity of the mat foundation on the clay seabed is of great significance for the safe operation of jack-up platform in marine environment. In this research, the vertical bearing characteristics of mat foundation on horizontal and sloping clay seabed were studied through centrifuge tests. Then, numerical simulations were conducted to study the vertical bearing capacity of the full-scale mat foundation on clay seabed, in which the effects of the large mat foundation size, pile leg insertion depth, preloading, consolidation degree of soil, and soil plasticity were analyzed. The results show that the vertical bearing capacity of the mat foundations on the sloping seabed is reduced, and the soil under the mat foundation shows obvious sliding failure. The contribution of the insertion depth of pile legs on the horizontal bearing capacity is more obvious than that of the vertical bearing capacity. The enhancement of the vertical bearing capacity is mainly due to the dissipation of the excess pore pressure and the increase of the mean effective stress of the soil. The effects of preloading, consolidation degree, and soil property on the vertical bearing capacity of the mat foundation are significant. [ABSTRACT FROM AUTHOR]

The dumbbell-shaped cofferdam has a large-scale and complex geometric shape, which may result in a complicated fluid-structure-seabed coupling mechanism and sophisticated seabed response around the structure. To investigate the wave-induced oscillatory seabed response around a dumbbell-shaped cofferdam, this study simulates the wave-cofferdam-seabed interaction process using a numerical model within the OpenFOAM framework. In this study, the finite volume method is applied to both the wave model and seabed model, thus ensuring the computational accuracy of this study. Biot's poro-elastic theory (u-p approximation) is considered as the governing equation for the dynamic response of the porous seabed, and the fluid motions are simulated based on the Reynolds-averaged Navier–Stokes equation. First, the accuracy of the numerical model for simulating wave-induced soil response is validated against existing experiments. Next, the effects of the submerged depth, wave incidence angle, and wave height on the oscillatory pore pressure response are studied in detail. Finally, momentary liquefaction around the dumbbell-shaped cofferdam is examined. The numerical results indicate that the wave-induced oscillatory seabed response around the cofferdam is considerably affected by the wave height but less affected by the wave incident angle. Moreover, the distribution of the wave-induced pore pressure in the vicinity of the dumbbell-shaped cofferdam is complex, and the maximum liquefaction depth around the cofferdam occurs near the steel casing along the centerline of the cofferdam. [ABSTRACT FROM AUTHOR]

This paper investigates the processes involving the generation and evolution of water waves induced by seafloor movements, focusing on the conditions for the occurrence of nonlinear wave and the effects of the nonlinear terms in the basic equations of fluid dynamics. The Reynolds-averaged Navier-Stokes (RANS) equations were incorporated into a k − ε turbulence model and applied to simulate the generation of water waves by treating the seafloor as a moving boundary. The simulation results were analyzed and compared with theoretical and experimental results from the literature. The analysis shows that nonlinear effects are not apparent for either of the following two conditions: a) if both the non-dimensional time scale for the bottom movements and the amplitude of seafloor uplift to water depth ratio are well below unity; b) if the non-dimensional time scale for the bottom movement is well above unity. However, the nonlinearity cannot be neglected when neither of these conditions is satisfied, largely due to the effects of fluid rotation, and to a certain degree, because Reynolds shear stresses are no longer insignificant. The occurrence of the maximum wave rise is determined by the frequency of the bottom movement and its amplitude relative to the water depth. The linear potential flow model may underestimate the maximum wave surface elevation for cases with high frequency or high amplitude seabed movements. [ABSTRACT FROM AUTHOR]

Two- and three-dimensional finite element analyses are performed to identify the optimal internal skirt spacing for the maximum undrained capacity of subsea skirted mudmats. Fully three-dimensional loading (vertical, biaxial horizontal, biaxial moment and torsion) is considered for subsea mudmats with skirt embedment ranging from 5% to 20% of the foundation breadth in soil with a range of linearly increasing strength with depth. The results have identified the governing case for determining the optimal skirt spacing for mudmats subjected to fully three-dimensional loading. It is also shown that optimal skirt spacing for rectangular or square mudmats can be determined in plane strain conditions using the equivalent foundation embedment ratio. The number of internal skirts required to ensure soil plug rigidity under fully three-dimensional loading is presented as a function of skirt embedment ratio, soil heterogeneity index and vertical load mobilisation. Results also indicate that effects of skirt roughness become negligible as foundation embedment ratio increases in terms of determining the optimal skirt spacing. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a well-developed numerical model based on the immersed boundary (IB) method is used to study oscillatory flows over a bed with large-amplitude ripples in a systematic manner. The work shows that the complex flow over the rippled bed can be numerically dealt with in Cartesian coordinate by the IB method and that the IB method is able to provide main features of the flows near the ripples. An accurate simulation of vortices generation as a result of flow separation at the rippled bed is obtained. It is found that the oscillatory flows start to separate during the flow deceleration when the Keulegan–Carpenter (KC) number is small. The steady streaming for various ripple steepness is simulated and the criterion for separating the single and double structure streaming is also discussed. Moreover, a new type of steady streaming which consists of a pair of embedded recirculations in the vicinity of the ripple trough is obtained for relatively steep ripples in this work. The numerical results, including the steady streaming in particular, may be helpful to improve the understanding of the sediment transport and the seabed evolution with natural ripples under sea waves. [Copyright &y& Elsevier]

Abstract: An immersed boundary (IB) method has been used to simulate oscillating flows over a bed-mounted circular cylinder at Reynolds number . Two Keulegan–Carpenter (KC) numbers are considered in the study. For the smaller number of 0.99, there is no boundary layer separation and hence the flow is relatively simple. The acting forces on the cylinder can be evaluated by the potential theory. For the larger KC number of 6.6, the flow is very complicated due to the interactions between the shed vortices and the cylinder. The mechanisms of the boundary layer separation, vortex shedding and motion are explored by carefully calculating the transient vorticity field near the cylinder. In particular, the predicted vortex pair movement towards the bed is consistent with other researchers’ experimental flow visualization and is well explained by this numerical investigation. In addition, possible local scouring near the cylinder is suggested by the calculation of the bed shear stress and is proved by laboratory experiments. The acting force on the cylinder is also explained in relation to the patterns of the flow field. [ABSTRACT FROM AUTHOR]

Abstract: If point absorbers are employed in the extensive exploitation of the offshore wave energy resource, they should be deployed in arrays, the distance between the elements in the array being possibly tens of meters. In such cases, it may be more convenient that the array is spread moored to the sea bottom through only some of its elements, located in the periphery, while the other array elements are prevented from drifting and colliding with each other by connections to adjacent elements. An array of identical floating point absorbers located at the grid points of an equilateral triangular grid is considered in the paper. A spread set of slack-mooring lines connect the peripheric floaters to the bottom. A weight is located at the centre of each triangle whose function is to pull the three floaters towards each other and keep the inter-body mooring lines under tension. The power take-off system (PTO) is a linear damper activated by the buoy heaving motion. The whole system–buoys, moorings and power take-off systems–is assumed linear, so that a frequency domain analysis may be employed. Hydrodynamic interference between the oscillating buoys in array is accounted for. Equations are presented for a set of three identical point absorbers. This is then extended to more complex equilateral triangular grid arrays. Results from numerical simulations, with regular and irregular waves, are presented for the motions and power absorption of hemispherical converters in arrays of three and seven elements and for different mooring parameters and wave incidence angles. Comparisons are given with the unmoored and independently-moored buoy situations. [Copyright &y& Elsevier]

• The influence of seabed obstacles on the supercavitating flow is studied. The study shows that the cavity around the projectile is primarily affected by three key factors: the size and slope of the obstacle and the distance between the projectile and obstacles. • At the projectile arrival stage, the smaller the slope of the obstacle edge, the lesser the damage it will cause to the cavity. However, at the projectile leaving stage, the small slope will hinder the regeneration of the cavity. • As the distance increases, the impact of obstacles on cavitation gradually decreases. When h/D ≥ 4, the obstacles have no effect on the cavity. In this paper, several shape obstacle models are built to observe the characteristics of the cavity around the axisymmetric projectile near different obstacles. Numerical simulations using a detached eddy simulation approach, the Schnerr–Sauer cavitation model, and the olume of fluid method are performed to present details and mechanisms at different distances between the projectile and obstacles (0.5 ≤ h/D ≤ 4). The shape and length of the supercavity are in good agreement with the experimental observation, which guarantees the accuracy of the numerical methods. The study shows that the cavity around the projectile is primarily affected by three key factors: the size and slope of the obstacle and the distance between the projectile and obstacles. At the same distance, the larger the size of the obstacle, the greater the impact on the cavity. At the projectile arrival stage, the smaller the slope of the obstacle edge, the lesser the damage it will cause to the cavity. However, at the projectile leaving stage, the small slope will hinder the regeneration of the cavity. As the distance increases, the impact of obstacles on cavitation gradually decreases. [ABSTRACT FROM AUTHOR]