Flat Seabed Research Articles

A DMM-EMM-RSM hybrid technique on two-dimensional frequency-domain hydroelasticity of floating structures over variable bathymetry

This paper proposes a hybrid technique combining the discrete modules method (DMM) (Lu et al., 2016), the eigenfunction matching method (EMM) (Tsai et al., 2011) and a further extended Rankine source method (RSM) to treat the two-dimensional hydroelastic problem of floating structures over variable bathymetry. In the hybrid technique, DMM is employed to discrete a floating structure into a set of rigid modules connected by elastic beams with equivalent deformation conditions of the structure; the incident wave potential over variable bathymetry without the presence of floating structures is obtained by EMM; a Rankine source method developed by Feng et al. (2017) is further extended to gain the diffracted and radiated wave potentials with the discreted multi-rigid-modules over variable bathymetry. A coupled model for the hydroelasticity of floating structures over variable bathymetry can be established by substituting the external wave loads exerted by the obtained incident, diffracted and radiated potentials into the multi-rigid-body dynamic equation by DMM in the frequency-domain. The convergence of the hybrid technique is validated through a good agreement against published data (Riyansyah et al., 2010) for hydroelastic responses of a two-dimensional floating structure over a flat seabed. Effects of sloping seabeds on hydroelastic responses of the floating structure are preliminarily investigated by the present hybrid technique.

2D residual-based LES of flow around a pipeline close to a flat seabed

To extend the application of the variational multiscale method to ocean engineering, 2D residual-based large eddy simulations of flows around a circular cylinder close to a flat plane have been considered in the present study. A generalized-α method is applied to solving variational forms of the Navier–Stokes equations. Flows around a spanned marine pipeline at a subcritical Reynolds number 1.31×104 have been computed. The hydrodynamic forces are compared to the published experimental data by a PIV and numerical results by a RANS and a classical LES. Although the magnitude of the predicted hydrodynamic forces are over-predicted compared to the experimental data, they are in good agreement with those obtained by the classical LES. The effect of the gap distance between the cylinder and the seabed on the hydrodynamic forces are analyzed. It is found that the drag coefficients and the root-mean-square of the lift coefficients decrease as the gap distance decreases. The flow characteristics around the spanned pipeline are examined. It is demonstrated that the proximity of the cylinder to the seabed severely affects the wake flow of the cylinder, and that the vortex shedding is suppressed as G∕D<0.3, which is in agreement with the experimental and other numerical results. The pressure distributions along the seabed appear to coincide with the experimental data. It is significant that the present method results in more reliable shear stresses (friction velocities) along the seabed than the RANS, which is very important for the research of the sediment transportation.

Experimental study on the formation and migration of submarine sandwaves under wave-only and combined wave-current conditions

Based on the assumption that the crest lines of sandwaves are perpendicular to the main flow direction, the formation and migration of submarine sandwaves are simulated in a 2 D flume under wave-only and combined wave-current conditions. The effects of various flow parameters, including the wave height and current velocity, on the characteristic dimensions and migration rates of sandwaves are discussed. Measured morphological data indicate that the seabed is composed of both large-scale sandwaves and small-scale sand ripples, and sandwaves are dominant in shaping the seabed form. Starting from a flat seabed, the height of sandwaves increases gradually and tends to become dynamically stable, while the wave length of sandwaves reaches a constant value at the initial stage. Under wave-only conditions, the characteristic wave heights and wave lengths of sandwaves increase with increasing incident wave height. The relative locations of the sandwave peak and trough are almost constant, indicating that sandwaves scarcely migrate under wave-only conditions. Under combined wave-current conditions, the characteristic heights and lengths of sandwaves also increase with increasing current velocity. The migration direction of sandwaves coincides with the direction of the residual current, and the migration rate increases with increasing current velocity. Finally, the characteristic dimensions of sand ripples are also analyzed, revealing that the Reynolds number (based on the particle size) is dominant in determining the size of a sand ripple. Both the characteristic wave lengths and the characteristic wave heights of sand ripples decrease with increasing Reynolds numbers.

Wrapround Multiple Reflectors over the Seafloor in the Gas Hydrate Area: A Possible Indicator of Methane Plume

Seismic wraparound multiples which are produced by the previous shot and trapped between the sea surface and flat seabed in the two-dimensional marine seismic reflection data have the potential to provide information about oceanographic phenomena because their repetition in the water column could enhance the detectability of the disturbance of seismic wave propagation. By focusing on the disturbance of wraparound multiples, we inferred the possible presence of methane gas bubbles emitting from the seafloor over several kilometers within a submarine canyon with a flat seabed. We observed the significant disturbance of wraparound multiples in both pre-stacked gathers and common mid-point stacked sections, which may be caused by methane bubbles. The existence of a bottom simulating reflection, a fault, and Calyptogena colonies provides indirect evidence of methane gas bubble seeps, while the existence of a cold seep is direct evidence of them in the study area. Finally, we conducted preliminary numerical simulations of acoustic wave propagation to investigate the cause of disturbances of wraparound multiples. Our preliminary numerical simulations imply that the disturbance of wraparound multiples might be associated with the internal scattering caused by an aggregation of gas bubble seeps in the water column.

Landform mapping of Okinoshima island and sea bed of Munakata city

Abstract. Munakata city is compiling its city history now. Then, in March 2019, the natural environment part edition of "Umi, Yama, Kawa" is scheduled to be published. I had to do landform mapping of Okinoshima island and sea bed in this project. In this abstract, the geography of Okinoshima, the process of mapping and some of the results are described.The Sacred Island of Okinoshima and Associated Sites in the Munakata Region was registered as a World Heritage in July 2017. Okinoshima island is located about 70 km northwest of Kyushu main island. The island consists of a steep cliff and gentle slope and the area of the plain is small. It consists of shale, quartz porphyry, and talus deposit.Fig.1 shows the landscape of Okinoshima island from the south. The highest peak is Mt. Ichinotake, the rock on the left is Mikadobashira and the rock on the right is Tengu-iwa. There is a holy place, Okitsu-miya of Munakata Taisha at the foot of the island. Residents have unwritten taboos that forbid actions such as removing anything from the island, or revealing anything seen or heard there.Maps of land and sea bed were made by various kinds of geographic information by using GIS. They are ground elevation map, slope map, relief map and 3D image. Fundamental geospatial data of GSI (Geospatial Information Authority of Japan), map information related to isobaths of JHA (Japan Hydrographic Association) and airborne laser scanner data of Munakata city are used as the geographic information.Landform classification map was made by interpretation of each map and field survey. Classified landforms are crest， dissected valley，steep cliff，landslip lobe，talus cone， seaside and man-made land. Farthermore, geologic and topographic lineaments were interpreted.Fig.2 shows the 3D image of Okinoshima from the east. It can be identified that the height of Okinoshima is about 340 m above the flat sea bed. The topography between Okinoshima and Koyajima under the sea surface, is a shallow flat table in the west side and a concave area inclining to the east in the east side. The shape of the set of islands is like a horseshoe-shaped caldera.In this poster presentation, I would like to introduce that we can understand the landform and its formative process of Okinoshima where access is restricted from the analysis and reference of such maps by GIS.

Coupled analysis for response and instability of sloping seabed under wave action

Wave-induced instability of seabed may cause damage to coastal and offshore structures. This issue has been investigated mostly for mildly sloping (<5°) seabed considering uncoupled or one-way coupled response of wave and seabed interaction. However, some of the marine structures are founded on seabed with steeper slopes. In this study, the wave-induced response and instability of sloping seabed are evaluated using a coupled finite element model. The interaction between fluid and porous seabed accounting for the effect of fluid motion on the seabed response, and conversely the effect of seabed response on the fluid motion (but not on the surface wave profile) is considered. The results indicate that the system response (fluid pressure, stresses, etc.) and the extent of instantaneously liquefied zone within the sloping seabed with significant steepness are lesser than those for horizontal seabed. Moreover, for typical sediment and wave characteristics, for the flat seabed, the response obtained from fully coupled analysis is not significantly different from those obtained by uncoupled analysis. For the sloping bed, such difference is slightly greater as compared to that for the flat bed.

Bragg reflection of water waves by multiple floating horizontal flexible membranes with submerged rectangular bars on the seabed

Bragg reflection of water waves by multiple floating horizontal flexible membranes is investigated based on the linear wave theory and the assumption of small membrane response. Under the floating horizontal membranes, periodical submerged rectangular bars are arranged on the flat seabed. The total reflection and transmission coefficients are obtained by using the eigenfunction expansion method and the wide spacing approximation. The calculated coefficients are validated with the results available in the literature, which shows that the present method is applicable. The characteristics of Bragg reflection are systematically investigated by changing various parameters including the height of the rectangular bars, the number, the tension, the spacing, and the length of the flexible membranes. The results can help designing multiple floating horizontal flexible membranes as effective floating breakwaters by taking advantage of Bragg reflection.

Probability of wave slamming and the magnitude of slamming loads on offshore wind turbine foundations

Physical model tests were analyzed with the objective to establish a formulae for the probability of wave slamming on offshore wind turbine foundations and the associated slamming force. Furthermore, the effect of wave slamming is analyzed for a monopile wind turbine foundation using a state-of-the-art aeroelastic model. The laboratory measurements were carried out in Deltares’ Atlantic Basin as part of the joint industry project “wave impacts on fixed turbines”, in short JIP-WiFi. Long crested and irregular waves typical for the North Sea were applied and more than 130 design storms and 500 slamming impacts were analyzed. A simple closed form expression for the probability of wave slamming is presented and a reasonable agreement with the laboratory measurements shown. A criterion for when wave slamming should be included in design computations is formulated based on the sea state steepness. It is observed that wave slamming occurs during tests with a flat seabed, and that seabed features, such as sand waves, significantly increase the probability of wave slamming. Furthermore, a simplified slamming load formulation is proposed based on observations from a subset of the laboratory tests. The magnitude of the slamming load is validated against the remaining part of the laboratory measurements and compared with existing slamming load formulations. The new formulation is in better agreement with the underlying measurements than existing formulations and is of a simpler form which makes it easier to apply. The new slamming load formulation is applied in design computations for a realistic wind turbine with the offshore wind farm Gemini as base case. The computations show that wave slamming, at this specific location, is not governing the structural design. However, wave slamming may introduce high accelerations in the transition piece located in the free surface zone where waves are impacting the structure.

Investigation of spudcan–soil interaction in a sloped seabed using centrifuge model tests

The installation of mobile jack-up platforms on a sloped seabed creates hazardous conditions on the spudcans and legs by inducing detrimental moment and horizontal loads that normally do not occur on a flat seabed. Despite these risks, only a limited number of studies on the effect of a sloped seabed have been reported. In this paper, a series of experiments were performed to investigate the effect of a sloped seabed on the spudcan behaviour on sand using a centrifuge. The scaled spudcan was installed on saturated silty sand at different slope angles (0, 5 and 10°) and the combined vertical, horizontal and moment loads were recorded during installation. The results clearly showed that the horizontal and moment loads increased with slope angles. In addition, the penetration behaviour of the spudcan installed on the sloped seabed was different from that on a flat seabed especially until the full width of the spudcan becomes embedded. However, horizontal and moment loads were attenuated once the full diameter was deeply embedded in the soil.

Calibration of vertical array tilt using snapping shrimp sound.

Snapping shrimp are the dominant biological source of high-frequency (>2 kHz) ambient noise in warm coastal waters. In a recent experiment, the highly impulsive signals produced by shrimp snaps were recorded continually by a large-aperture vertical array (56 m) that was bottom-moored in 100-m deep shallow water. Assuming the array vertical, initial localization of individual snaps based on wavefront curvature along the array indicated that all snaps came from either above or beneath the flat seabed. By constraining all snaps to originate from the seabed, several hundred snaps within a radius of 500 m from the array over a 20-s window were detected successfully and localized in the three-dimensional space of time-of-arrival, range, and array tilt. Since the estimated array tilt for each snap is a projection of the absolute array tilt onto the nominal array-snap plane, the maximal tilt in the range and tilt domain corresponds to the absolute array tilt. Both simulations and data demonstrate that snapping shrimp can be exploited as a source of opportunity for calibration of vertical array tilt.

Numerical Simulation of a Sandy Seabed Response to Water Surface Waves Propagating on Current

An integrated numerical model is developed to study wave and current-induced seabed response and liquefaction in a flat seabed. The velocity-inlet wave-generating method is adopted in the present study and the finite difference method is employed to solve the Reynolds-averaged Navier-Stokes equations with k-ε turbulence closure. The model validation demonstrates the capacity of the present model. The parametrical study reveals that the increase of current velocity tends to elongate the wave trough and alleviate the corresponding suction force on the seabed, leading to a decrease in liquefaction depth, while the width of the liquefaction area is enlarged simultaneously. This goes against previous studies, which ignored fluid viscosity, turbulence and bed friction.

Transmission characteristics of convergence zone in deep-sea slope

We analyzed the experimental data from two survey lines in an experiment conducted in the South China Sea. The bottom of one survey line is flat, and that of the other line has a large slope. Comparison indicated significant difference in the convergence zones between the two survey lines. We simulated the experimental phenomenon by using the Bellhop model. Simulation results are consistent with the experimental findings and explained the mechanism underlying the occurrence of the phenomenon. The following conclusions were derived. (1) A strong signal reflection zone, similar to the convergence zone, exists in the near field. (2) The convergence zones in the slope appear earlier and are higher in number. (3) The complexity of the slope (i.e., large fluctuation) easily affects the transmission of the converging sound lines, resulting in significant attenuation of the acoustic signal. (4) When the sound source is deep, the convergence zone widens and the first convergence zone disappears in the signal reflection zone in the case of a flat seabed; Convergence zone in the slope directly disappears.

Wave-induced morphodynamics and sediment transport around a slender vertical cylinder

We study the dynamics of a sandy bed around a slender vertical cylinder forced by progressive, non-linear water waves. The seabed evolves continuously under the effects of the up-welling, down-welling and rolling events induced by vortical coherent structures. In turn, these are closely connected to the shape of the seabed, which is modified by the scouring and/or the deposition of the sand. Starting from a flat seabed, progressive waves induce a rapid and transient modification of the bottom morphology towards a dynamically stable equilibrium state, which is the focus of this work. The dynamical equilibrium state is a function of the wave period and is reached when the seabed morphology is not substantially altered. We describe such a state by an Eulerian in-phase analysis of the sand particle motion, inferred from Lagrangian data collected over a large number of wave passages. This analysis relies on the use of the defocusing digital PIV technique (DDPIV), for the first time applied to the specific flow of interest here. On the basis of the Eulerian analysis, the triggering of the key-events (up- and down-welling, rolling) over the wave phase is captured by identifying, through the Q > 0 criterion, the coherent flow structures responsible for the events. This analysis is coupled with the description of the sediment trajectories, analyzed in a Lagrangian manner and effectively assessing how and where the solid phase is transported during the key-events. Five main mobilization/transport mechanisms have been identified, three during the onshore flow and two during the offshore flow: (i) generation of a coherent structure reminiscent of a horseshoe vortex at the toe, (ii) intense scouring at the top of the flatbed region, (iii) vortex shedding in the wake during direct (onshore) flow, (iv) shear crossflow on the lee-side of the cylinder and (v) large vertical shearing in the flatbed region during the reverse (offshore) flow. At flow reversal, this shearing mechanism impacts on a significant area of sediments in the incoming region of the flow.

An Influence of Trench Formation on Steel Catenary Risers Based on a Hysteretic Nonlinear Seabed Model

A steel catenary riser (SCR) attached to a floating platform at its upper end encounters fluctuations in and near its touchdown zone (TDZ), which causes the interaction with the seabed. Subsea surveys and the analysis of SCR’s indicated that the greatest stress and highest damage occurred near the touchdown point (TDP), where the SCR first touches the seabed. Nowadays, the linear seabed spring is carried out, and it is assumed as a flat seabed. Improved nonlinear hysteretic seabed models have recently been proposed, which simulate the different stiffness in the seabed response in the TDZ. In this study, an advanced hysteretic nonlinear SCR-seabed soil interaction model has been implemented to simulate the exact behavior of the riser in the vicinity of the touchdown zone. This paper focusses on the seabed trench, which develops progressively under the SCR due to repeated contact. Also, different important parameters such as water depth and material of riser have been investigated based on the Caspian Sea environmental conditions. This paper highlights the impact of trenches of different depths on the fatigue performance of riser at TDZ.

LES study of near-seabed tide-induced turbulence in the East China Sea

Near-seabed turbulent properties observed in the East China Sea were investigated using a large eddy simulation (LES) model. Tidal forcing estimated from the observed tidal current is imposed to the LES model assuming a flat seabed. Turbulence stirred by tidal currents near the seabed is simulated and compared to observed turbulence. The observed tide is dominated by the M2 constituent. The energy dissipation rate evolves with a quarter-day period near the seabed, whereas the dissipation rate evolves with a diurnal period in the upper part of the boundary layer. Salinity also oscillates diurnally. Thickness of the boundary layer related to K1 is thicker than that of M2. Orientation of the major axis of M2 in the tidal ellipse does not change downward, but that of K1 rotates counterclockwise. The vertical structure difference of two constituents is attributable to the observation site latitude, which is approximately the critical latitude of K1. All of these features are simulated in the LES model. Particularly, results show that the interference of M2 and K1 induces diurnal variation of the turbulent dissipation rate in the upper part of the boundary layer through the turbulent energy production rate controlled by the vertical shear. A hypothetical simulation without horizontal advection demonstrate that this process can contribute more to the diurnal variation in the upper part of the boundary layer than stratification stability control owing to salinity advection through tidal straining.

Numerical and Experimental Investigation of Nonlinear Wave-Current Propagation over a Submerged Breakwater

AbstractThe nonlinear wave-current propagation over a submerged rectangular breakwater on a flat seabed was investigated both experimentally and numerically. Experiments were conducted in a two-dim...

Shore protection by oblique seabed bars

Shore protection by small seabed bars was once considered possible because seafloor undulations strongly reflect surface waves of twice the wavelength by the so-called Bragg resonance mechanism. The idea, however, proved ‘unreliable’ when it was realized that a patch of longshore seabed bars adjacent to a reflective shore could result in larger waves at the shoreline than for the case of a flat seabed. Here we propose to revamp the Bragg resonance mechanism as a means of coastal protection by considering oblique seabed bars that divert, rather than reflect, shore-normal incident waves to the shore-parallel direction. We show, via multiple-scale analysis supported by direct numerical simulations, that the creation of a large protected wake near the shoreline requires a bi-chromatic patch to deflect the incident waves to the shore-parallel direction. With two superposed sets of oblique seabed bars, the incident wave energy becomes efficiently deflected far to the sides, leaving a wake of decreased wave activity downstream of the patch. We demonstrate that the shore protection efficiency provided by this novel arrangement is not affected by reflection of leaked waves at the shoreline, and that it is relatively robust against small frequency detuning.

Large Eddy Simulation of Flows Associated with Offshore Oil and Gas Pipeline

Fluid structure interaction (FSI) applications are of wide range from offshore fixed and floating structures to offshore pipelines. Reynolds Averaged Navier Stoke (RANS) solution has limitation for unsteady and turbulent flow modelling. A possible approach is Large Eddy Simulation (LES) and it is applied to flows past a circular cylinder located far above, near and on a flat seabed. The Reynolds number considered is based on the real situation off Malaysia Coast and is sub-critical around 105. Hydrodynamic quantities in terms of mean pressure are predicted and vortex shedding mechanism is evaluated. The results are validated by comparing the simulation and experimental previous studies.

Numerical simulation of the flow around a subsea pipeline with different protection methods

Hydrodynamic stress induced by marine currents subject subsea pipelines to failure vulnerability. Therefore, several methods have been established to protect such pipelines from hydrodynamic forces. The objective of this work is to investigate the performance of two different protection methods using computational fluid dynamics. A second-order accurate upwind finite volume computational fluid dynamics model was used to simulate isotropic turbulent flow around a subsea pipeline located on flat seabed. A comparison between four turbulence models revealed that both Menter’s shear stress transport k[Formula: see text] and the standard k[Formula: see text] models yield the best agreement with experimental measurements. Pipeline trenching and double-barrier protection methods were simulated with different geometrical characteristics. A comparison between those two methods was conducted and discussed. It is found that at small aspect ratios, the double-barrier method prevails over trenching in terms of its ability to isolate the pipe from the main current. While at large aspect ratio, trenching provides near-zero pressure coefficient along the pipe wall, which demonstrate its prevalence in protecting the pipeline.

Large Eddy Simulations of flow around a circular cylinder close to a flat seabed

Large Eddy Simulations (LES) with Smagorinsky subgrid scale model are used to study the turbulent flow and wake dynamics behind a circular cylinder close to a horizontal, plane wall at subcritical Reynolds number. The focus is on investigating the details of the flow around the cylinder placed at different distances from the wall and immersed in boundary layers of various thicknesses.The Reynolds number is Re = 13,100. The simulations with gap to diameter ratios (G/D) of 0.2, 0.6 and 1 are carried out in order to investigate the changes of the flow field and the vortex shedding due to the presence of the plane wall. The influence of the incoming boundary layer profile is investigated through simulations with logarithmic boundary layer inlet profile of thickness 0.48D and 1.6D, and compared with a uniform inlet flow profile. 2D and 3D simulations are performed for Re ranging from 100 to 13,100, to explore the importance of the flow three-dimensionality. The flow field in the cylinder wake, as well as the mean flow values, the spectral analysis and the pressure distribution on the cylinder surface are computed to study the flow physics due to the influences of the wall.