This paper presents the results of an experimental investigation of a wall-normal pressure gradient in a turbulent open-channel flow with a smooth bottom (wall). Neutrally buoyant particles 1 mm in size were used in the experiments. Particle trajectories were photographed in the side view, and the particle accelerations in the vertical direction were determined from the particle trajectory data. The acceleration data were used to determine the wall-normal pressure gradient, implementing the vertical component of the equation of motion of the particle. The conditionally averaged wall-normal pressure gradient, upward directed or downward directed, is plotted as a function of depth. It was found that the latter was radically different from the wall-normal mean-pressure gradient, obtained from the Reynolds equation. The conditionally averaged pressure gradient, upward or downward, reaches very large values near the wall in the range O(5)

Local scour around vibrating monopile foundations has been studied using flume experiments. Multiple empirical equations for equilibrium scour depth prediction based on experimental data have been proposed, while variability in vibration amplitude measurements has produced isolated equations applicable only to specific scenarios, hindering cross-study comparisons. Establishing a unified predictive equation within flume experimental contexts is therefore essential. This study proposes a standardized protocol recommending bed-level vibration displacement measurements in flume experiment, identified as the optimal parameter reflecting vibration intensity and its influence on surrounding bed deformation. This practice eliminates discrepancies from non-standardized measurement locations and resolves incompatibility arising from varied measurement heights. A unified equation incorporating the vibration intensity, flow intensity and KC number has been established through mathematical analysis and experimental regression. Validation against published datasets confirms robust applicability and accuracy for predicting equilibrium scour depth around vibrating monopiles under both current and combined current-wave conditions. This work resolves inconsistencies among formulas derived under differing experimental conditions and provides both methodological recommendations for future studies and a reference framework for engineering applications using field data.

Pipelines buried in the seabed have been used as one of the basic infrastructures in the oil and gas industry for many years. However, these infrastructures may face challenging hydro-geotechnical engineering problems, such as soil liquefaction, under cyclic loads caused by earthquakes or waves, especially when buried in loose and fine seabed soil. In this study, wave-induced residual liquefaction around a buried cylindrical structure (pipelines) in a backfilled trench is studied by means of a numerical model developed on the OpenFOAM (foam-extend 4.1) platform. First, the model is validated against experimental data from flume tests. Then, the model is used to parametrically study the effect of backfilled trench on liquefaction susceptibility of the seabed around the pipelines. The results, among others, show that the risk of wave-induced liquefaction around cylindrical structures increases significantly if loosely backfilled and low-permeability soil is used. The findings indicate that, for the backfill soil, a permeability greater than O(10-5) m/s, combined with a medium-dense to dense state, significantly enhances resistance to wave-induced liquefaction around the pipeline.

The design challenges of offshore wind are multifaceted, comprising techno-socio-enviro-economic aspects. Among the technological design challenges, seabed dynamics and seabed response to wave-induced and structural loading are of significant importance. While a number of studies are focused on the morphodynamics and scour around, for instance, monopolies, much less research has been dedicated to the liquefaction around marine structures and offshore wind installations in particular. Novel ocean structures such as floating offshore wind installations require complex anchoring systems for station keeping; thus, seabed liquefaction needs to be considered during the design phase. This study, together with the companion paper Part I, aims to provide unique insights into seabed liquefaction around a gravity-based, tension leg platform-type floating offshore wind system. To that end, experiments in the large wave-current flume, GWK+, at the Coastal Research Centre, Hannover, Germany, were performed at a very large length scale of 1:15.56. The results show, for the first time, the severity of compound wave-induced and structural loading. Small wave heights in the order of 0.56 m (model scale) trigger seabed liquefaction and lead to significant structural displacement in the order of meters (model scale). In addition, a discussion on model effects and uncertainties provides suggestions for future improvements of the experimental setup.

Many different subsea structures are used in the offshore industry, particularly for floating offshore wind farms, such as gravity anchors, tensioners, clump weights, and chains. Exposure of a subsea structure to currents and/or waves leads to changes in flow around the structure, forming flow contraction at the sides of the structure and secondary flow patterns such as horseshoe vortices, lee-wake vortices, and counter-rotating vortices (Sumer and Fredsøe, 2002). Often placed on erodible beds under current and/or wave action, scouring around these structures are experienced. The present paper presents the early results of an experimental study aiming to investigate the aforementioned scour and sinking processes around cuboid blocks (cubes, rectangular prisms and horizontal cylinders) under the action of waves, currents, and waves combined with currents.

Seabed liquefaction, a phenomenon induced by dynamic loading, poses a significant risk to the structural integrity of marine installations such as pipelines, breakwaters, and offshore platforms. Accurate numerical modeling of this complex process is essential for ensuring the safety and longevity of such structures. This study introduces an advanced OpenFOAM (foam-extend 4.1)-based numerical model that-for the first time-holistically simulates seabed liquefaction and compaction processes. Building upon Biot's poroelasticity theory for nonliquefied regions and the drift-flux model for liquefied regions, the new model offers a comprehensive hydro-geotechnical representation of seabed response to wave-induced loading, covering the full spectrum of the liquefaction process in a unified framework. Through validation against experimental data, the model demonstrates high accuracy in capturing the intricate dynamics of seabed soil subjected to varying wave periods and heights. In addition to its successful simulation of the onset of residual liquefaction, the model excels in addressing the accumulation of pore pressure and soil behavior after liquefaction takes place. The numerical model results consistently achieve a R 2 score greater than 0.9 compared to experimental pore pressure measurements, indicating its high accuracy in predicting seabed responses. The model's performance signifies its potential as a valuable tool for the coastal and offshore engineering community. This research provides a robust toolset for the research on and an understanding of seabed liquefaction and its implications for marine structures.

Submarine pipelines have been one of the most essential marine infrastructures for the oil and gas industry for many decades. Likewise, with the advancement of offshore wind energy technology, offshore cables for power transmission have become crucially important assets, since the numbers and lengths of inter-array and grid connection cables have been increasing constantly. The diameter of an offshore cable for power transmission is generally O(0.05–0.4)m, whereas the diameters of oil and gas pipelines are O(0.2–1.5)m. Although the sizes of offshore cables are relatively small compared to those of submarine pipelines, general design guidelines for stability of submarine pipelines have been mostly adopted by the offshore wind industry for offshore cables, given the geometric resemblance of these structures. This paper presents the results of a numerical modelling study in which the liquefaction potential around a pipeline/cable buried in seabed soil is investigated. For this purpose, a numerical model developed for modelling wave-induced liquefaction around marine structures under the NuLIMAS (Numerical Modelling of Liquefaction Around Marine Structures) Project is utilized.

This paper presents the results of a numerical study on the retention time in the Golden Horn, a natural estuary that connects with the Bosphorus Strait at the point where the Bosphorus meets the Sea of Marmara. The numerical model is a three-dimensional model, incorporated with the large eddy simulation model for the horizontal turbulence closure and a modified mixing-length model for the vertical turbulence closure. The retention time was based on the so-called volume-averaged concentration, and the latter quantity was calculated for an initially uniformly distributed instantaneous volume source by solving the turbulent diffusion equation. The retention time was calculated for two cases, namely, (1) for the entire volume of the Golden Horn, and (2) for only the shallow-depth zone of the estuary, extending 4,300 m interior from the Kagithane Creek. In both cases, four different scenarios were tested in terms of the head difference between the Black Sea and the Marmara Sea, Delta h, the main parameter that governs the flow in the Golden Horn Estuary: (1) the case of mean flow with Delta h = 33 cm; (2) that of mean flow but with inflow from Kagithane and Alibeykoyu Creeks present; (3) that of extreme flow, with Delta h = 50.0 cm; and (4) that of another extreme flow, with Delta h = 0cm. It was found that the retention time of the Golden Horn Estuary appears to be O(3) days, irrespective of the scenarios studied, with the exception that it is O(2) days in the scenario where the inflows from the Kagithane and Alibeykoyu Creeks are also present. (Here, O is the order of magnitude.) Regarding the retention time of the shallow-depth zone alone of the Golden Horn, it was found that this latter time is increased by a factor of 1.5-4, being in the range of O(4)-O(12) days. This is with the exception of the scenario where the inflows from the Kagithane and Alibeykoyu Creeks are also present, in which case the retention time is reduced to only O(1) day. Overall, the self-cleaning of the Golden Horn occurs over a time scale of O(3) days, on average, with no inflow from outside pollutants, while this time scale is reduced to O(2) days when there exists, on average, a constant inflow from the Kagithane and Alibeykoyu Creeks. The results have been interpreted in terms of dispersion caused by the variation of the velocity in the transverse direction combined with the turbulent diffusion in the same direction.

The orbital motion of water particles under a progressive wave in shallow waters becomes a straight line parallel to the bottom, i.e., oscillatory motion, at the seabed. A new time-dependent boundary layer develops over the seabed for each half-cycle of this motion. The turbulent oscillatory wave boundary layer is of great importance in many engineering applications, especially in coastal engineering. Even though both laminar and turbulent regimes have been considered in oscillatory boundary layers, of particular interest is the transitional regime. The laminar-to-turbulent transition first occurs in the form of tiny turbulent patches close to the wall, called turbulent spots, just before the near-bed flow reversal. These coherent structures are arrowhead-shaped isolated areas where the flow bursts with intense oscillations, in an otherwise laminar boundary-layer flow (Sumer and Fuhrman, 2020). Single or multiple spikes in the bed shear stress signal reaching up to 3 or 4 times the magnitude of the bed shear stress is a good indicator of a turbulent spot (Carstensen et al., 2010). Although much experimental and numerical research has been conducted (e.g., Carstensen et al., 2010; Jensen et al., 1989), there are still many unanswered questions regarding the transition. This study aims to address these questions using the DNS method, which has become very popular in turbulence-related problems (e.g., Mazzuoli et al., 2011; Xiong et al., 2020), by focusing on observing turbulent spots and locating their birthplace, concurrently with the bed shear stress under the spot structure. The present study is being conducted in close collaboration with Professor Liang Cheng and Drs. Chengwang Xiong and Chengjiao Ren of the University of Western Australia. The study is only in the early stages, and some early results will be presented at the meeting.

This paper presents the results of a numerical study on a two-layer flow system in the Bosphorus, the strait connecting the Black Sea and the Marmara Sea. The numerical model is a three-dimensional (3D) model incorporated with the large Eddy simulation model for the horizontal turbulence closure and a modified mixing-length model for the vertical turbulence closure. The model, tested and validated against field data and other numerical models, was used to study hydrodynamic processes associated with the two-layer flow system in the Bosphorus. It was found from the 3D model calculations that the length-averaged flow rates are 18.55 x 103 m3/s for the upper layer and 6.55 x 103 m3/s for the lower layer for the mean flow conditions, corresponding to the mean head difference between the Black Sea and the Marmara Sea of 33 cm, in good agreement with the field observations. It was also found that, for the mean flow conditions, a net amount of transfer of water of 2.9 x 103 m3/s from the lower layer into the upper layer occurs, and this is due to mixing between the two layers, also in good agreement with the field observations. It was further found that the present model captures the location of the density interface between the layers and its variation with the longitudinal distance quite well. Given the time variation of the head difference between the Black Sea and the Marmara Sea over a time span of a year, the time variations of the flow rates in the upper and lower layers, calculated through the present model, compared favorably well with the field data. The present model solution indicated that the flow in the lower layer essentially stops when the head difference between the Black Sea and the Marmara Sea approaches approximately 40 cm, and, by contrast, the upper layer, likewise, stops when the head difference approaches approximately 10 cm. However, the model indicated that, for truly stagnant water in the upper layer, the head difference needs to approach nil. Additionally, hydrodynamics of the water body in the Marmara Sea adjacent to the Marmara Sea end of the Bosphorus and that in the Black Sea adjacent to the Black Sea end of the Bosphorus were unveiled by the implementation of the present 3D model.

Residual liquefaction, a significant issue in marine engineering, results from accumulated pore-water pressure in the seabed due to cyclic shear stresses, which compromises soil stability. This study aims to investigate residual liquefaction around gravity-based marine structures by means of a 2D numerical model. The model employs a two-step procedure: First, the stresses in the soil domain are determined via solving Biot equations, and subsequently the generation and diffusion of accumulated pore pressure in the soil is simulated by means of a pressure diffusion equation with a source term. The model was first validated against analytical solution for pore pressure buildup in the seabed under progressive waves, and against experimental data for residual liquefaction around a buried submarine pipeline. The results showed that the model can satisfactorily capture pore pressure buildup and residual liquefaction in the seabed around structures. Once validated, the model was utilized to model the pore-water pressure buildup and residual liquefaction potential around a caisson breakwater under the action of standing waves and the wave-induced rocking motion of the caisson, separately and in combination. Spatial distribution of liquefaction potential was determined in the seabed soil around the caisson with and without a bedding layer on the seabed. The model results revealed the critical role of the bedding layer in reducing liquefaction susceptibility under standing waves and rocking motion, and highlighted that the rocking motion alone poses a significant risk of inducing residual liquefaction in the seabed around the caisson.

This paper presents the results of a numerical study on the flow through the Golden Horn, a natural estuary that connects with the Bosphorus Strait at the point where the Bosphorus meets the Sea of Marmara. The numerical model is a three-dimensional model, incorporated with the large eddy simulation model for the horizontal turbulence closure, and a modified mixing-length model for the vertical turbulence closure. It was found that, when viewed from above, there exists a clockwise vortex that forms at the entrance of the Golden Horn, driven by the north-to-south flow in the upper layer of the Bosphorus, penetrating O(1) km inside the Golden Horn. This vortex, with a north-south extent of O(5) km, extends to the north as far as Dolmabahce. The three-dimensional (3D) calculations also reveal the presence of a smaller vortex [with an extent of O(0.5) km] forming to the west of the former vortex, rotating in the counterclockwise direction. The aforementioned vortex system, comprising the clockwise and the counterclockwise vortices, penetrates O(2) km inside the Golden Horn from the entrance of the Golden Horn. The velocities at the periphery of the major vortex can be as high as O(0.5) m/s and those of the secondary one, O(0.1) m/s. This vortex system is maintained down to a depth of z = -15 to - 20 m. When moved toward the bottom, the aforementioned flow pattern changes at this depth to a pattern consisting of a counterclockwise major vortex and an accompanying clockwise minor vortex, driven by the south-to-north flow in the lower layer of the Bosphorus. The plan view extents of these latter vortices are O(1) and O(0.5) km, respectively. The depth where the flow pattern changes coincides precisely with the depth where the two-layer flow in the Bosphorus (at the entrance of the Golden Horn) changes direction, an important result, linking the mechanism of the flow in the Golden Horn directly to the flow in the Bosphorus. It was inferred that the previously mentioned vortices play a key role for the water exchange between the Bosphorus and the Golden Horn Estuary. The present 3D calculations showed that the effect of the upstream creeks (Kagithane and Alibeykoy) is felt only at the surface waters but not below, in the top O(1) m depth, a layer even shallower than the half depth of the water in the shallow-depth zone (within 4 km from the entrance) of the Golden Horn.

The understanding of the wave-structure-soil interaction around marine structures is important in order to drive design decisions towards lean, yet resilient marine infrastructure. In particular, in the light of the increasing use of the marine environment for renewable energy generation, analysis of the wave-structure-soil interaction becomes increasingly relevant. While most experimental studies on wave-structure-soil interaction are conducted at small to medium scale, this paper presents the initial results of an experimental test campaign on the wave-structure-soil interaction of a floating offshore wind turbine at large scale. The data of the period averaged pore pressure reveal a significant buildup of pore pressure beyond the onset of liquefaction, which can be attributed to the additional structural loading when comparing results to similar, isolated wave-soil interaction cases in the absence of a structure.

Wave-induced liquefaction results in significant seabed dynamics and can lead to a complete loss of the bearing capacity of the soil. With the increasing use of marine space for energy harnessing, geotechnical challenges move into focus during the development and planning of such installations. To date, a lack of comprehensive modelling tools for wave-induced liquefaction around marine structures has been observed. This paper documents the efforts in the framework of the NuLIMAS project to provide such a modelling tool. In particular, this paper presents a numerical implementation of the hydrogeotechnical processes together with a detailed overview of the calibration and validation strategy employing small- and large-scale experimental data, respectively.

The study established the water quality modelling of the Bosphorus system, based on hydrodynamic data as well as the results of the water quality survey carried out in the last five years. The model revealed significant decrease in the magnitude of pollutant loads in the upper layer at the exit into The Marmara Sea providing numerical proof that no pollutant transport would take place from sewage discharges to the upper layer. A similar modelling approach was implemented at the Bosphorus/Marmara interface, a significant hotspot as it included two major deep marine outfalls. The results asserted that the entire sewage flow would enter the lower flow in The Bosphorus through the interface without an appreciable mixing with the upper flow. This way, the study provided a significant scientific support for the sustainable management of marine discharges in this area, since they have no physical interference with The Marmara Sea.

This paper presents the results of an experimental investigation of wave-induced liquefaction in the case of multiple wave exposures. The experiments include also standard progressive wave cases as well, for comparison. One kind of sediment was used in the experiments: silt (d50=0.070 mm). Four scenarios were tested with multiple wave-climate exposures, including (1) multiple wave climates separated with “quiet”, no-wave periods, and (2) those with no interruptions between the consecutive wave-climate exposures, resembling a storm situation. It was found that the first strongest wave climate (i.e., the first wave climate, which is strong enough to cause liquefaction) of a multiple wave-climate sequence “secures” the onset of liquefaction, independent of prior wave exposures. It was also found that the wave exposures following the “liquefying” wave (even with stronger wave properties) do not liquefy the soil. The experiments further showed that, after the completion of the liquefaction-compaction cycle, the pore pressure may still build up when the soil is exposed to a new wave climate in the sequence with an even stronger set of wave characteristics (wave height and wave period). However, the accumulated pore pressure will not be large enough to liquefy the soil. Likewise, after the completion of the liquefaction-compaction cycle, or after exposures to not one but a number of waves, again, even with stronger wave characteristics, the pore pressure may not even build up at all. The experiments shed further light on the circumstances in the field under which (1) the seabed soil would eventually become liquefaction resistant (with a very large relative density), and those under which (2) it would remain loose, both of which have been revealed by field surveys. Furthermore, it was found that, in the case when the wave-climate exposures are uninterrupted, the dissipation of the accumulated pore pressure is quite slow when compared with the situation where the wave-climate exposures are interrupted with “quiet”, no-wave periods. The results have been explained in terms of physical processes involved. Also, implications of the results for practice have been discussed in detail.

This paper presents the results of an experimental investigation of wave-induced liquefaction in silt and seashell mixtures. Three kinds of sediments were used in the experiments: silt (d50=0.070 mm), coarse seashell (d50=2.87 mm), and fine seashell (d50=1.46 mm). With these sediments, three kinds of tests were carried out: silt-alone tests (the reference tests), tests with silt and coarse-seashell mixture, and tests with silt and fine-seashell mixture. The experiments showed that the influence of seashell content on wave-induced liquefaction is very significant. The susceptibility of silt to liquefaction is decreased with increasing seashell content. This is up to a certain point beyond which the mixture of silt and seashell becomes liquefaction resistant. For the seashells used in the experiments, this limiting value was found to be approximately SC≈30%, where SC is the shell content by weight. It is argued that this behavior is linked with the elastic modulus of the mixture. The present findings showed that the liquefaction criterion based on the initial mean normal effective stress in pure silt/sand can be extended to silt and seashell mixtures. A chart is proposed for an initial screening check of liquefaction potential. Furthermore, it is recommended that, if there is a liquefaction potential, standard assessment methodology may be implemented to check for liquefaction, provided that the elastic modulus and other properties of the soil and seashell mixture are incorporated into this assessment.

Seabed liquefaction is the phenomenon by which the seabed soil loses its strength and stiffness, due to applied stress, and behaves like a non-Newtonian fluid. Seabed liquefaction can lead to severe failure of marine structures, such as buried pipelines or coastal breakwaters. Numerical modelling of liquefaction can provide valuable insights into the prevailing wave-structure-soil interaction. However, the holistic modelling of seabed liquefaction, including the transition from solid to liquid and back to solid soil, is challenging, due to the prevailing hydrogeotechnical processes. As a step towards the development of such a holistic numerical model of seabed liquefaction, this paper considers the modelling of the soil under a rocking plate as an idealised representation of a caisson breakwater as presented in [1]. The numerical model has recently been presented in [2] and includes equations for the accumulation of pore pressure and a criterion to distinguish between the liquefied and non-liquefied regions. The numerical results are compared to the experimental results taken from [1]. Different numerical model setups are presented to overcome modelling challenges due to occurring punching shear failure.

Submarine pipelines and offshore cables are critical infrastructures for the offshore energy industry. Marine structures like pipelines and offshore cables buried in loose finegrained seabed soil, such as silt or fine sand, maybe under the threat of wave-induced liquefaction. Such failures (flotation or sinking of structures due to liquefaction) have been reported widely in the literature. Observations show that the presence of pipelines/cables can enhance the wave-induced liquefaction susceptibility. This paper presents early results of a numerical modeling study in which the liquefaction potential around a pipeline/cable buried in seabed soil is investigated. A numerical model being developed in the OpenFOAM CFD toolbox under the NuLIMAS (Numerical Modelling of Liquefaction Around Marine Structures) Project is utilized. The results show that the model is capable of simulating the pore pressure buildup around pipelines (or cables), both in near or far field. Additionally, it is shown that different wall boundary conditions of pipelines/cables (i.e. smooth or rough) can be modeled in the numerical model, which is a convenient feature of the model for utilization in practical applications.