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The prediction of object embedment into the seabed is important for the deployment and efficacy of submerged sensors and sensor networks, as well as for other applications such as unexploded ordnance risk assessment, subaquatic search and rescue efforts, or anchors and moorings. The embedment of objects into the seabed during placement depends on the strength parameters of the seabed sediment. However, a detailed characterization of seabed strength can be complex and costly since quality seabed samples and/or in situ testing are required. This study evaluates the importance of a detailed geotechnical seabed sediment characterization over seabed classification from geoacoustic surveying or accepting unknown seabed conditions for the prediction of seabed embedment of cylindrical objects. Monte Carlo simulation was applied to model the uncertainty of sediment strength parameters. The results suggest that prior knowledge of the general sediment type (e.g., fine-grained versus coarse-grained as determined from side scan sonar imaging) significantly improves the precision of the embedment predictions over unknown conditions, reducing the coefficient of variation (CV) in prediction of percent embedment (PE) by about 50 % to 65 %. Performing a detailed geotechnical testing of sediment strength represented a further improvement by 15 % to 35 % over the knowledge of sediment type. Both the mean and standard deviation of PE in fine-grained sediments are at least about 3 times the values for coarse-grained sediments indicating a wider distribution of PE with deeper embedment for cohesive fine-grained soils. The embedment behavior is a function of the method of deployment (free fall with a certain speed versus placement with no speed) in stiff cohesive sediments but is less relevant in soft sediments since PE values are always greater than 100 % (i.e., full embedment). Similarly, in cohesionless sediments, PE is always less than 50 %. [ABSTRACT FROM AUTHOR]

• Instantaneous liquefaction within a non-cohesive seabed under progressive waves is analytically investigated. • Explicit solutions for the liquefied soil depth are derived to reflect the phase-lag effect. • Due to the phase-lag effect, the maximum liquefied soil depth would no longer appear exactly under the wave trough. • Instantaneous liquefaction potential is predominantly related to a combined parameter for waves and soils. Instantaneous liquefaction can be induced by the upward seepage force while waves propagating over a porous seabed. In this study, the instantaneous liquefaction within a non-cohesive seabed under progressive waves is analytically investigated and verified with the existing offshore observations. To characterize the instantaneous liquefaction potential, the vertical gradient of excess pore-pressure is employed, and the explicit solutions for the liquefied soil depth are then derived to reflect the phase-lag effect. Wave scale effect is further examined by comparative analysis of the liquefaction potential between laboratory and field conditions. It is indicated that, due to the phase-lag effect, the maximum liquefaction potential and the maximum liquefied soil depth no longer appear exactly under the wave trough; nevertheless, such phase difference relative to the wave trough wouldn't exceed π /4. Neglecting the phase-lag effects would considerably underestimate the liquefaction potential as well as the liquefaction depth. Moreover, both the liquefaction potential and the corresponding liquefied soil depth are predominantly related to the non-dimensional parameter I c for characterizing the combined influence of wave and seabed properties. [ABSTRACT FROM AUTHOR]

Gassy clay deposits are widely distributed in marine sediments. Clarifying the influence patterns of the trapped gas phase on the mechanical properties of gassy clay is of significant importance. Establishing an in-situ strength parameter and consolidation coefficient inversion method based on CPTu is crucial for gassy clay characterization. In this study, gassy clay was prepared using the zeolite method. The variations of strength and consolidation parameters of gassy clay concerning gas content were obtained based on laboratory triaxial and one-dimensional tests. It was observed that the trapped gas phase enhances the undrained shear strength and hinders drainage. Specifically, gassy clay with a gas content of 3.5 % exhibited an 18 % increase in undrained shear strength compared to saturated clay and a 50 % reduction in consolidation coefficient. In addition, laboratory calibration chamber tests were conducted to investigate the cone penetration test (CPTu) in gassy clay with varying gas contents and penetration rates. The drainage effect during the CPTu penetration process in gassy clay was discussed. The reasons behind the variations in static cone penetration parameters under undrained conditions for normally consolidated gassy clay were analyzed by combining the results of triaxial tests and one-dimensional consolidation experiments. A proposed formula for the cone factor N kt was also provided under undrained penetration conditions. The accuracy of conventional methods for estimating the consolidation coefficient of gassy clay was verified, the basic properties of the gassy clay used in the indoor experiment are similar to those of the seabed gassy silt at the project site, and the environmental conditions are similar to those at locations with low initial pore pressure, such as mudflat or shallow sea bed, thereby offering a reference for in-situ testing of strength parameters and consolidation coefficients in gassy clay seabed. [ABSTRACT FROM AUTHOR]

• Experimental measurements in the wind tunnel on the drag and lift force of special unexploded ordnance (UXO). • Numerical simulations CFD of the drag and lift force of special UXO's. • Numerical simulations BEM of the added mass of special UXO's. • Polynomials for the drag, lift and added mass coefficients as a function of the Reynolds number and the burial depth. Unexploded ordnance (UXO) and Discarded Military Munitions (DMMs) are found in many coastal regions. These objects are constantly exposed to environmental influences such as saltwater, which causes corrosion, and waves and currents as well as mobile seabed sediments. The basic assumption to decide if an object is mobilised by waves and currents or not is to compute the critical loads that are needed to move it from its stable position on the sediment bed. The loads on the object are induced by hydrodynamic and hydrostatic forces as well as by friction, gravitation, and inertia. In this study, the hydrodynamic forces were calculated by analytical approaches and numerical simulations (CFD). These forces have been validated for the current-induced mobilisation by wind tunnel experiments. Finally, the dependency of the dimensionless coefficients for lift, drag and added mass of the Reynolds number and the burial depth is transferred into a resulting equation. [ABSTRACT FROM AUTHOR]

Pipelines are frequently embedded in layered sloping liquefiable seabed. This study proposed a dynamic effective stress numerical model for buried pipelines in the clay-over-sand sloping seabed under wave loading. The u-p formulation of Biot's theory was used to describe the porous saturated seabed. The elastoplasticity of clay was determined by Mohr-Coulomb yield criterion. The liquefaction characteristics of sands was modeled by a bounding surface plasticity constitutive model. The proposed model was validated through a wave flume model test and can capture the influence of pipelines on the excess pore water pressure (EPWP) in the liquefiable seabed. The pipeline-soil interaction can significantly increase the EPWP ratio at the bottom of the pipeline under wave action. The rate of increase in the EPWP ratio at the bottom of pipelines increased as the thickness (h c) of upper clay layer increased. Increasing h c caused a significant increase in the settlement of pipelines in the flat seabed and the horizontal displacement of pipelines in the sloping seabed. The study also highlights the significance of considering the slope angle, the relative density of sands, and the specific gravity of pipelines. [ABSTRACT FROM AUTHOR]

Avoiding in-situ skidding of tracked deep-sea mining vehicles on soft sediment and reducing disturbance to soft sediments are the basic requirements for ensuring the regular operation of seabed mining. The in-situ skidding problem of the tracked deep-sea mining vehicle can be explained by the block shear failure phenomenon of deep-sea soft sediment. We analyzed the force state and elastic-plastic state of soft sediment disrupted by a crawling grouser and established the block failure model. The elastic-plastic failure range calculations obtained in the model is used to select the spacing between adjacent grousers to maximize the thrust provided by the grouser. Due to the irreversibility of plastic failure, the range of plastic zone failure is used to measure the extent of the sediment disturbance by the grouser. By analyzing the influence of multiple parameters on the failure range of the plastic zone, we provide direction for finding ways to reduce sediment disturbance. It is found that among these parameters, the grouser's width and height, which represent the shape parameters of the grouser, have the most critical effect on the plastic failure range of the sediment. But when the width exceeds 0.3 m, and the height exceeds 0.2 m, the benefit of reducing the failure range by increasing the grouser's width and height will be minimal. [ABSTRACT FROM AUTHOR]

This study extends the seismic dynamics of two-phase/three-phase poroelastic sediments in coupled ocean-sediment-substrate systems to gas hydrate-bearing sediments. The ocean sediment floor is described as a finite-thickness porous medium with two solids and a compressible viscous fluid (porous skeleton, gas hydrate, and fluid), which sits on an elastic seabed. In the coupled system of seawater, gas hydrate-bearing sediment layer and seabed, analytical solutions for the P wave reflection and transmission coefficients are derived. Two key factors are derived: displacement amplitude and magnification factor. The relationship among two key factors with incident angle, frequency, sediment thickness, cementation parameter, contact parameter, temperature and saturation in the sedimentary layer containing gas hydrate is analyzed. The research shows that when the P wave is vertically incident (θ ip = 0°), the incident frequency has a greater influence on the vertical displacement amplitude, and when the P wave is grazing incident (θ ip = 90°), the influence is not observed. The displacement of the sedimentary layer fluctuates in different degrees with the increase of the layer thickness. When the temperature increases, the saturation is higher, that is, the hydrate content is lower, and the influence of the contact parameters gradually disappears. [ABSTRACT FROM AUTHOR]

• The interlayer dissipates the largest energy for SPs lying on the flexible seabed. • The light-high speed falling object causes greater dent depth in the scenario of flexible seabed. • The outer pipe experienced the maximum strain and obviously strain concentration. • The dent depth and strain prediction formulas are developed for sandwich pipe. Sandwich pipe (SP) is regarded as an ideal deep water oil and gas transportation solution and has attached more and more attention attributed to its good structural resistance and thermal insulation performance. The local dent damage of SP induced by the falling objects impact might result in the leakage of oil and gas resources. This work investigates the dent behavior of sandwich pipes subjected to falling object impact through finite element(FE) numerical simulation by considering the effect of seabed flexibility. A three-dimensional FE model was established and validated to accurately capture the structural behavior of SP lying on the seabed. The sensitive parameters studies were performed to assess the influence of falling object mass, kinetic energy and buried depth on the dent characteristics. The results show that the flexible seabed foundation can effectively reduce the local dent damage degree of sandwich pipes and most of the kinetic energy is dissipated by the annular layer so as to provide protect for the steel pipes. Besides, the outer pipe, annular layer and inner pipe show different dent depth and strain levels as the kinetic energy of falling objects changes. The degree to which the speed and mass of falling objects affect the characteristics of depressions depends on the magnitude of the kinetic energy of the falling object. Based on the parameters analyses, a group of empirical formulas are established to provide prediction of dent depth and dent strain for sandwich pipes. [ABSTRACT FROM AUTHOR]

The pipe-soil interaction behaviors are crucial factors for the J-lay pipeline design, which are mainly investigated on the horizontal seabed in previous studies. This paper develops an innovative method for the static and dynamic analyses of pipe-soil interactions considering the J-lay pipeline on a sloping seabed. This approach simulates the motion processes of the pipeline particles and solves the internal forces of the pipeline elements by the vector mechanics and finite element principle. The position coordinates formulations of pipeline particles are derived to calculate the pipeline locations on the sloping seabed. The non-linear hysteretic soil model and the modified Coulomb friction model are employed to simulate pipe-soil interactions in the vertical seabed plane as well as the axial and lateral directions. Moreover, a large-scale pipe-laying experimental test system is established to carry out the model experiments of a J-lay pipeline on different seabed conditions. The static and dynamic pipe-soil interaction responses are then analyzed by the VFIFE method for a 6-inch gas pipeline J-laying on the negative sloping seabed. The influence of seabed slope type and angle and soil characteristics is further explored on the pipe-soil interaction behaviors, which provide significant evidence for the design of pipe-lay on the sloping seabed. [ABSTRACT FROM AUTHOR]

• Wave supported fluid mud on the seabed can be formed in energetic regime (R e Δ > 450), which was observed in yellow river delta under winder storm wave-related oscillatory flows. • Although the flow is in the energetic regime with high R e Δ from field observations, the turbulence is found to be suppressed and the total velocity spectral flux E(f) is decreased by approximately 14% to 22% due to the formation of the wave-supported fluid mud layer. • Lab measurements further show that once the fluid mud layer forms, stronger flow with u w >45 cm/s (R i b < 0.2) can disturb the interfacial wave of the fluid mud and the flow above it, which cause strong entrainment of the fluid mud layer and sediment mixing into the water column. Data from 5 monthly field observations of the modern Yellow River delta demonstrate the formation of the fluid mud layer (FML) above the seabed under continuous storms and the sediment resuspension and vertical mixing over the FML. It was observed that the storm wave-related oscillatory flows during winter were mainly in energetic regime (400 < R e Δ < 1200). The high concentration sediment is trapped in a thin layer stably stratified above the seabed due to the formation of the wave-supported fluid mud (WSFM) layer. Above the WSFM, the total velocity spectral flux E (f) at z =0.7 m is decreased by approximately 14%∼22%, indicating a significantly decreased sediment carrying capacity of the flow. To explore detailed flow and sediment dynamics within the WSFM, a series of experiments in a wide wave flume with WSFM generated were carried out. The velocity and sediment concentration profiles measured over the water depth suggested that the sediment was confined in a thin FML even in high R e Δ regimes (R e Δ >480), and the behavior of FML can transition between a laminar to turbulent with varying wave orbital velocity and R i b. Laminar fluid mud dominates when u w <45 cm/s and R e Δ < 518 , resulting in exceptionally high viscous dissipation of wave energy. This can explain the observed significant decrease of E (f) during the onset of storm wave recession. Once the fluid mud layer forms, stronger flow with u w >45 cm/s (R i b < 0.2) can disturb the interfacial wave of the fluid mud and the flow above it, which cause strong entrainment of the fluid mud layer and sediment mixing into the water column. [ABSTRACT FROM AUTHOR]

• A new underwater rammed stone column (RSC) is proposed to improve soft marine clay. • A series of model tests are conducted to evaluate the bearing capacity of RSC. • Bearing capacities of RSCs are better than those of traditional stone columns (TSCs). • The load transfer mechanism and failure mode of RSCs and TSCs are revealed. An underwater rammed stone column (RSC) is proposed in this paper to improve the bearing capacity of the coastal marine clay. The main idea of this technology is to drill a hole into the seabed from the sea surface with protection from a large steel pipe and then use a rammer to compact the aggregates into the seabed to form the RSC composite foundation. A series of model tests were conducted to validate the installation method and evaluate the bearing capacities of RSC. The test results showed that the bearing capacities of RSCs are much better than those of the traditional stone columns (TSCs) with the ultimate bearing capacity of maximum improving 77.6% for the full-length columns and 65.3% for the floating columns. The effective length of the RSCs was 6 times column diameter and most of the load is born by the top sections of RSCs with a length of 3 times column diameter. The bulging failure occurs in TSC and RSC with the maximum bulging position at approximately 1.5 times column diameter from the column head. [ABSTRACT FROM AUTHOR]

• The high cycle accumulation model is used to calculate the accumulation strain of seabed sands due to cyclic loading on OWTs for the first time. • Increased risk of foundation failure of OWTs in inclined submarine slopes. • Swaying and tilting of OWT monopiles significantly reduces submarine slope stability. • Uni-directional lateral loads on OWT monopiles destabilise the slope more than multi-directional ones. Long-term cyclic loading on offshore wind monopiles will lead to a decrease in the mechanical properties of the seabed near the monopile's foundation, therefore, causing instability of the submarine slope and the monopile foundation. To capture the accumulation strain caused by high cycle numbers and plastic deformation of seabed sands, the high cycle accumulation (HCA) model is complied with Mohr-Coulomb constitutive models. The shear strength reduction (SSR) method is used to obtain the safety factor of submarine slopes. The coupled numerical model simulates a 6 MW offshore monopile foundation with a single periodic lateral load applied on the monopile at the sea level, which represents concentrated wind and wave loads in storm conditions. The results show that the plastic zones begin to form at the monopile foundation, then extends towards the slope toe and slope top, and eventually across the entire slope, leading to slope instability. In this process, the safety factor of the slope is significantly reduced with each stage of the process. It was found that both the sharp changes of load amplitude and large average load also decreases the safety factors. Moreover, at the beginning of the monopile installation, the safety factor will increase by nearly 10%, but it will drop by nearly 30% due to long-term periodic lateral loads. Therefore, these have important implications for the installation of monopile foundations for offshore wind turbines (OWTs) since the slopes we consider are within the range of slopes at planned sites for OWTs. An increase in monopile diameter and embedded depth can enhance the overall slope stability. However, an increase in embedded depth could potentially instigate more extensive seabed landslides despite enhancing overall stability. [ABSTRACT FROM AUTHOR]

To simulate the wave-induced response of coupled pore fluids and a solid skeleton in shallow water, a set of solutions with different formulations (fully dynamic, partly dynamic, and quasi-static) corresponding to each soil behavior assumption is presented. To deal with Jacobian elliptic functions involved in the cnoidal theory, a Fourier series approximation is adopted for expanding the boundary conditions on the seabed surface. The parametric study indicates the significant effect of nonlinearity for shallow water wave, which also enhances the effect of soil characteristics. The investigation of the applicability of reduced formulations reveals the necessity of a partly or even fully dynamic formulation for the wave-induced seabed response problem in shallow water, especially for thickened seabed. The analysis of liquefaction in the seabed indicates that the maximum depth of liquefaction is shallower, and the width of liquefaction is broader under cnoidal wave loading. The present analytical model can provide more reasonable result for the wave-induced seabed response in the range of shallow water wave. [ABSTRACT FROM AUTHOR]

The responses of a monopile offshore wind turbine subjected to irregular wave loads are investigated numerically and experimentally, considering a range of sea states. An extensive experimental campaign was carried out on a fully flexible model, representative of a 5 MW offshore wind turbine, at 1:40 scale. An assessment of the experimental results for the response amplitude operator for regular waves and the 90th percentile seabed bending moment in long-crested irregular waves is carried out using two models (analytical and numerical) for uncertainty propagation, suggesting that bias errors in the model properties and in the wave elevation contribute the most to the total uncertainty. The experimental results are also compared to a numerical model using beam elements and Morison-type wave loads with second order wave kinematics. The numerical model does not capture all of the responses within the level of uncertainty of the experiments, and possible reasons for the discrepancies are discussed. [ABSTRACT FROM AUTHOR]

Dynamically installed anchors (DIAs) are considered to be one of the most efficient mooring solutions for taut mooring systems. During their installation, DIAs are released from a certain height above the seabed and reach high velocities (up to 35 m/s) before impacting the seabed. However, a potential issue with DIA installation is that the anchor may not remain completely vertical during descent, resulting in the anchor impacting the seabed at an oblique angle and a significant reduction in holding capacity. To address this issue, this paper proposes a new numerical model for DIAs with attached mooring lines, which can consider the forces acting on anchors and mooring lines. A total of 176 numerical simulations were conducted under different initial configurations of mooring lines, and 128 simulations were performed, taking into account the influence of the horizontal motion of Anchor Handling Vessels (AHV). By comparing the velocity, embedment depth and tilting angle, possible combinations of the mooring line configurations in situ were back-analyzed. The simulations results show that the presence of the mooring line significantly reduces the final penetration depth of the anchor, and that the proposed installation model accurately simulates the change process of velocity and displacement during installation. Additionally, when DIAs impact the seabed at an inclined state, the tilting angle will increase due to soil resistance on the anchor tip. Finally, the initial state of the DIA at release is influenced by the horizontal motion of the AHV, which can influence the tilting mechanism of the DIA. [ABSTRACT FROM AUTHOR]

• Proposed a 3-D mooring line dynamics model to consider the mooring line lateral movement in the seabed for the first time. • Revealed two mechanisms of the trench width formation, and proposed a simplified 3-D trench profiles base on the field data. • Proposed a calculation method to predict the potential 3-D trench profiles. Seabed trenches near deep-sea anchors bring risks to the mooring system's safety. The prediction of 3-dimensional (3-D) trench profiles is a key task to reduce the risk. However, the trench width related to the trench wall collapse and lateral movement of the mooring line has not been well studied. This paper first proposes a 3-D mooring line-seabed dynamics model to consider the mooring line lateral movement in the soil. Then, a seabed trenching model to simplify 3-D trench profiles is developed based on the field data. By combining the above models, a novel method to predict 3-D trench profiles around embedded mooring lines is established by considering trench wall collapse and integrating dynamics of mooring line. Based on the verified method, both static and dynamic responses of mooring lines are investigated, then a series of parametric sensitivity studies are carried out. The simulation results reveal that dynamic movement of mooring lines at fairlead may result in remarkable seabed trenches, and lateral motion of mooring lines enlarges the disturbed zone near the mooring line touchdown area. The proposed method in this paper provides a reasonable assessment of the 3-D trench profiles and reduces risks in the operation of mooring systems. [ABSTRACT FROM AUTHOR]

Accurate assessment of pipe-soil interaction under cyclic wave actions is of pronounced importance for the stability analysis of submarine pipelines in sandy seabed. This paper presents a plane-strain numerical study on such a problem using a finite element program DBLEAVES, which incorporates an elasto-plastic soil model that is capable of capturing the cyclic mobility behavior of sandy soils under cyclic loadings. A detailed validation against analytical solution and model test results was provided to demonstrate the robustness of the present numerical model to mimic both pre- and post-liquefaction behavior of sands, before an extensive parametric study was introduced. It was found that the accumulation of excess pore pressure in the vicinity and far field of a pipeline was strongly affected by the existence of it, with an influential range of about two pipe diameters. The influences of wave and seabed properties (e.g. relative densities) on the uplift response of pipelines were then investigated, based on which an explicit model was developed to quantify the degradation effect of waves on the uplift bearing capacity of pipelines against thermally-induced buckling. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Highlights • The influence of submerged berms on the seabed soil response and liquefaction occurrences around breakwaters foundations is investigated. • Wave-structure-seabed interaction has been evaluated by using two numerical tools, employing a one-way coupling algorithm. • A parametric study on the berm configuration has been carried out. • A comparison with straight sloped conventional breakwater is performed. Abstract Berms deployed at the toe of conventional rubble mound breakwaters can be very effective in improving the stability of the armor layer. Indeed, their design is commonly tackled by paying attention to armor elements dimensioning. Past research studies showed how submerged berms can increase the stability of the armor layer if compared to straight sloped conventional breakwaters without a berm. To fill the gap of knowledge related to the interaction between breakwaters with submerged berm, waves and soil, this research aims to evaluate how submerged berms configuration influences the seabed soil response and momentary liquefaction occurrences around and beneath breakwaters foundation, under dynamic wave loading. The effects of submerged berms on the incident waves transformation have been evaluated by means of a phase resolving numerical model for simulating non-hydrostatic, free-surface, rotational flows. The soil response to wave-induced seabed pressures has been evaluated by using an ad-hoc anisotropic poro-elastic soil solver. Once the evaluation of the seabed consolidation state due to the presence of the breakwater has been performed, the dynamic interaction among water waves, soil and structure has been analyzed by using a one-way coupling boundary condition. A parametric study has been carried out by varying the berm configuration (i.e. its height and its length), keeping constant the offshore regular wave condition, the berm and armor layer porosity values, the water depth and the elastic properties of the soil. Results indicate that the presence of submerged berms tends to mitigate the liquefaction probability if compared to straight sloped conventional breakwater without a berm. In addition, it appears that the momentary liquefaction phenomena are more influenced by changing the berm length rather than the berm height. [ABSTRACT FROM AUTHOR]

Highlights • Bragg reflection of water waves by multiple floating horizontal membranes with rectangular bars on seabed is studied. • Reflection and transmission coefficients are obtained using eigenfunction expansion method and wide spacing approximation. • The results are compared with those available in the literature to verify the adopted method. • The effects of the height of the rectangular bars, the number, tension, spacing and length of the membranes are analyzed. Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract Fatigue response of steel catenary risers (SCR) in the touchdown zone (TDZ) is significantly affected by riser-seabed interaction. Non-linear hysteretic riser-seabed interaction models have been recently developed to simulate the SCR cyclic embedment into the seabed. Despite the advancements achieved in the prediction of non-linear hysteretic riser-seabed interaction, several inconsistencies have been recently identified in the nodal performance of some of the popular models. These limitations need to be resolved by proposing new models or improving the existing models. However, it is necessary to evaluate the influence of the identified shortcomings of the existing models on the global performance of the riser. In this paper, the influence of nodal inconsistencies observed in a popular riser-seabed interaction model on the global performance of the riser was comprehensively examined in the TDZ. The riser embedment profile, cyclic contact stress, contact stress envelop, mean shear force, cyclic bending moment, and consequently the cumulative fatigue damage was investigated. The study showed that the soil model overestimates the riser embedment and other global responses. Recommendations were made to overcome the identified shortcomings of the existing models in future developments. [ABSTRACT FROM AUTHOR]

Abstract With the increase in demand and supply gap in the oil and gas industry, new developments of oil and gasinfrastructure are moving into deeper water. This requires design and construction of long high temperature and high pressure pipelines from deep sea to shore. These pipelines are subjected to cyclic expansion during operating cycles. Accumulated axial movement due to repeated thermal cycles may lead to global displacement referred to as 'walking'. Walking rates depend on the restraint associated with seabed friction. In conventional analyses, seabed friction is independent of the rate of thermal loading and expansion but it has been recognised that the sliding resistance between a pipe and the seabed varies with velocity, partly due to drainage effects. In this paper a numerical model is used to explore the effect of velocity-dependent seabed friction. A velocity-dependent friction model is implemented in commercial software ABAQUS and validated via single element and simple (flat seabed) pipeline cases. This model features upper and lower friction limits, with a transition that occurs as an exponential function of velocity. A parametric study is performed using differing rates of heating and cool-down in walking situations driven by seabed slope, SCR end tension and the difference between heat up and cool down rates. The walking behaviour is compared to cases with constant friction and solutions are proposed to express the velocity-dependent response in terms of an equivalent constant friction. These equivalent friction values can then be applied in existing simple solutions or more complex numerical analyses, as a short cut method to account for velocity-dependent friction. [ABSTRACT FROM AUTHOR]

Highlights • An extra feedback loop is implemented externally to the traditional loop of the dynamic positioning system. This additional loop is responsible for the BOP position control during the reentry operation. • The details for the design of the control system using a linear optimal control are presented. For the control design, we assumed the riser as a linear system with only the first modal shape (pendulum). • The control performance was compared for different current and waves loads. The control performs well for waves, but its performance decreases with current. Abstract During the drilling of ultra-deep-water subsea petroleum wells, a blow-out preventer (BOP), a piece of safety equipment, must be assembled on the wellhead. The BOP is suspended using the drilling riser during the wellhead approach operation, and the riser's top end is connected to the floating platform rig. This article presents a feedback control system for the automatic approach of the BOP to the wellhead. Compared to state-of-the-art controls, ours does not require ancillary thrusters installed alongside the riser nor inclination sensors atop of the drilling riser. Additionally, our proposed control embeds a closed-loop dynamic positioning system, thus retaining the characteristics of the original control system and adding an extra closed-loop. This eases implementation of the BOP approach control to an existing platform. To calculate the optimal gains for the BOP controller, we assume a linear system for the riser, including only the pendulum-shape. The simulation is carried out using nonlinear models for both riser and floating platform. We assume an International Towing Tank Conference standard semi-submersible platform, coupled with a 3000-m free-hanging vertical riser for the time-domain simulation. The results show the BOP tracking to be a step-shaped input signal under current and wave loads. A discussion of the performance of feedback control under different environmental loads is also included. [ABSTRACT FROM AUTHOR]

Highlights • New equation for predicting the maximum seabed shear stress beneath the pipe. • The likelihood of scour occurring increases with current velocity but decreases with pipe diameter. • New equation for predicting the equilibrium scour depth beneath the pipe. • Including the upstream seabed shear stress improved the correlation to experimental data. • The influence of the Reynolds number is small. Abstract When a subsea pipeline is laid on an uneven seabed, certain sections may have an initial elevation with respect to the far-field seabed, e o , and thus potentially affecting the on-bottom stability of the pipeline. This paper focuses on quantifying the effects of the upstream dimensionless seabed shear stress, θ ∞ , and Reynolds number, Re , on: (1) the maximum dimensionless seabed shear stress beneath the pipe, θ max , to be compared to the critical shear stress in order to determine whether scour would occur and progress towards an equilibrium state; and, (2) the dimensionless equilibrium scour depth beneath the pipe, S eq / D. Using a 2-D Reynolds averaged Navier-Stokes (RANS) approach along with the k-ω Shear Stress Transport (SST) turbulence model, a parametric study involving 243 computational fluid dynamics (CFD) simulations was conducted. The simulation results were used to develop a closed-form equation for the prediction of θ max. Subsequently, experimental measurements of S eq / D have been compiled from published literature, to develop a new closed-form equation for the prediction of S eq / D with a high correlation to the experimental data. In summary, we present two closed-form equations for the prediction of θ max and S eq / D for pipelines with an initial e o / D , which are applicable for both clear-water and live-bed conditions. The effects of θ ∞ and Re have been included, albeit Re having a small influence as compared to the other parameters. [ABSTRACT FROM AUTHOR]

Abstract Catenary mooring lines experience liftoff from and grounding on the seabed when undergoing large dynamic motions. Numerical line mooring models account for this interaction using various seabed models and it is known that the action of liftoff and grounding may lead to large dynamic tension fluctuations. These fluctuations may be spurious due to the inability of discretised mooring models to adequately account for the effect of the seabed on the mooring line. In this work, the root cause and conditions that lead to the production of the large dynamic tension fluctuations is determined. The effect of line discretisation and seabed model on the tension fluctuations is investigated using the widely used spring-mattress approach and a modified seabed reaction force model. An in-house mooring code was developed to perform these investigations. For code validation and benchmarking, and to illustrate the existence of the tension fluctuations problem due to nodal grounding in existing mooring line simulation codes, comparisons are made to a commercial software. [ABSTRACT FROM AUTHOR]

Abstract Within the offshore oil and gas industry, the subsea pipelines in riser configuration are the main choice for petroleum transportation till the processing platform. The subsea pipeline is subject to considerable loads ranging from hydrodynamics forces from the sea current to seabed contact forces. Those contact forces are very important in risers design life, since high and concentrated bending moment stresses are observed in the region where the pipeline starts to be supported by the seafloor. The proper representation of the soil behavior in contact with the structure comprises a very important topic of study in subsea pipelines. In this context, a consistent pipeline–seabed interaction model based on Kerr-type foundation is presented. A finite element formulation for the soil, where the stiffness matrix is derived from a variational formulation of a three parameter elastic foundation model, is proposed. The pipeline is modeled as a planar beam undergoing large displacements using a co-rotational approach. The unilateral contact between soil and pipeline is dealt with a step-loading procedure. The proposed model is able to solve problems where the pipeline is initially placed above the foundation surface and subsequent contact with the seafloor is induced by large displacement of the structure. Numerical examples are presented in order to validate the foundation formulation and explore the proposed contact solution between a pipeline and the seabed. A static analysis of a steel catenary riser (SCR) is presented using the proposed soil–structure interaction model. [ABSTRACT FROM AUTHOR]

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]

Model tests aimed at investigating the wave-induced dynamic response and instability of silty clay seabeds around a semi-circular breakwater were conducted in a large wave flume. The pattern of wave forces was measured and analysed. The seabed responses were presented in terms of pore pressure and vertical stress, which were further characterized in terms of their oscillatory and residual values. The wave-induced settlement and instability of the semi-circular breakwater were studied. Test results reveal that the total horizontal wave force was generally greater than the vertical wave force, and a phase difference existed in the total horizontal and vertical wave force patterns. The trends in the oscillatory pore pressure and vertical stress were very similar: they generally developed rapidly in the seabed on the wave side, tended to be larger in the upper silty clay and smaller in the bottom area and increased with increasing wave height. Furthermore, the oscillatory responses mostly reached stable states but were observed to vibrate with cyclic time in the destroyed silty clay. The residual pore pressure and vertical stress mainly developed in the seabed around the breakwater and could be quite different depending on the location. With the exception of the decrease-increase tendency in some seabed areas, the residual pore pressure generally increased with cyclic time beneath the breakwater. The residual vertical stress generally increased with cyclic time beneath the breakwater, except in some regions beneath the toes of the internal rubble bed. The semi-circular breakwater settled substantially into the silty clay seabed, and the settlement was greater on the wave side. This seabed shear failure is attributed to the strength degradation of the silty clay induced by the development of excess pore pressure and the destruction of the soil fabric. [ABSTRACT FROM AUTHOR]

A properly arranged subsea production system reduces costs and contributes to production performance due to favorable hydraulic characteristics and flow assurance. Therefore, the layout design of subsea production systems is very important in offshore field development. The design of these systems mainly includes locating the subsea facilities, determining the subsea topology and identifying the pipe route. Each of these three aspects have been studied, for instance, optimization of the pipe network or identification of the optimal single pipe route. However, the combination of these three aspects has not yet been discussed in detail. This paper presents an integrated optimization model for the layout design of a wellhead-manifold-FPSO system, with the aim of obtaining a minimum total pipe length. There are two key details of this model that distinguish it from other models. The first detail is that the seabed topography and obstacles are taken into consideration. The second detail is that all three abovementioned aspects are considered together in the model to determine the optimal number of manifolds, manifold and riser base positions, pipe network topology and pipe routes. The simulated annealing and Dijkstra algorithms are coupled to solve the model by using a newly proposed process. The application of this method is demonstrated by designing the layout of an oil field with 22 wellheads and one FPSO. The results are compared with the situation that neglects the seabed topography, showing a difference in suggested pipe length. In addition, the pipe route effect on both hydraulic and flow assurance is briefly discussed. The model provides a method to link related issues of interest to the layout design, resulting in a practical subsea layout that can be used to more reliably estimate costs, more accurately describe multiphase flow and help in decision-making for flow assurance. [ABSTRACT FROM AUTHOR]

It is well-known that the pore water pressure plays an important role in the design of submerged structures since interaction of pore water and soil particles significantly affects surrounding soil behavior. However, the role of pore water within saturated soil was commonly neglected when estimating the blast-induced soil response. A pipeline-seabed interaction (PSI) model is established to simulate blast response of pipelines with consideration of pore water effect. The u-p approximation is incorporated into finite element method (FEM) to study dynamic response of pipelines buried in fully saturated soil subjected to underwater explosion. The Arbitrary Lagrangian-Eulerian (ALE)-Lagrangian algorithm is utilized to solve large deformation in the vicinity of underwater explosion. Test data from previous literature is adopted to validate the proposed model. Then, comparative analysis is carried out between the proposed model and the conventional model that excludes pore pressure. Numerical results from the proposed model are found to be distinctive from those obtained from the conventional model. Blast responses of the pipelines and soil are underestimated generally by the conventional approach. This contrastive analysis emphasizes pore pressure effect in engineering design of submerged pipelines. [ABSTRACT FROM AUTHOR]

Fixed steel platforms rest on the seabed, and uneven seabed settlement can lead to tilting them. Tilting changes the behavior of the platform, and evaluation of these changes is vital for estimating structural safety. This paper uses push-over analysis to investigate six four-leg platforms under different tilting scenarios. Besides modeling in intact condition, the platforms were modeled under four tilting orientations, three angles of tilting, and eight loads applying directions. Results show that tilting in each direction has different consequences, and the failure pattern of the platform plays a notable role in the post-tilting behavior of the platforms. In the most-critical case, 1, 2, and 5 ° tilting decreases the safety factor by 9, 18, and 32 percent, respectively. Also, in the platforms where the leg failure initiates the global failure, the tilting has the least effect on structural safety compared to those with toppling or large displacement failure patterns. [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]

The unmanned sampling and analysis after return of seabed rocks is an important method for deep-sea mineral resources exploration. A novel underwater robot for ultrasonic drilling and sampling tasks of seabed rocks is proposed based on the bionic design method in this paper. The jellyfish-like structure is adopted for the swimming system of the robot, which can move flexibly and stably with low noise, outstanding cruising ability and strong environmental adaptability. The mechanical system and driving control strategy of the robot are designed in this paper. Based on co-simulation method of ADAMS and MATLAB software, the structural parameters of the transmission mechanism of the swimming system are optimized to get larger swing angle under relative less driving power, which will contribute to the improvement of propulsion efficiency. Through the fluid dynamics simulation and analysis in FLUENT software, the propulsion process can be visualized and the propulsion mechanism is revealed. To verify the feasibility of the design, a robot prototype is developed and the experimental platform for the robot's swimming test is built. The underwater experiments on motion characteristics for the robot demonstrates that the maximum propulsive force generated by the swimming system can reach about 16N, and the propulsion speed can reach 0.19m/s. The design and development of the bionic robot for seabed ultrasonic drilling and sampling missions provides new ideas for seabed resource exploration technology. [ABSTRACT FROM AUTHOR]

Wave-induced seabed response around pile foundations affects the seabed stability, thereby threatening the structure safety, and numerous studies have been performed to explore the wave-structure-seabed interaction (WSSI) around mono-pile. However, little attention has been paid to WSSI around pile groups. In the present study, based on an integrated model, WSSI around three-pile group is numerically examined, and the effects of wave obliquity, pile diameter, pile spacing and embedment depth are parametrically explored. In this model, Reynolds-Averaged Navier-Stokes (RANS) equations with k-ε turbulence model and Biot’s quasi-static (QS) model for poro-elastic medium are adopted to govern the wave motion and seabed response, respectively. The results reveal that the effects of wave obliquity and pile diameter are significant, whereas the influences of pile spacing and embedment depth are negligible. [ABSTRACT FROM AUTHOR]

The phenomenon of wave-seabed-pipeline interactions is one of the primary concerns of coastal engineers and researchers, as it could greatly affect the seabed instability and pipeline safety. Numerous researchers have expended great effort in studying wave-seabed-pipeline interactions in the past. However, the majority of them focussed on the wave-induced response around a submarine pipeline, in buried conditions, by numerical models and laboratory experiments. Unlike the previous studies, a series of regular wave experiments and numerical model analyses were conducted to investigate the wave-induced pore pressure in the sandy seabed, around a pipeline with different backfilled depths. The model pipeline with three diameters ( D  = 6 cm, 8 cm, and 10 cm) is buried in three sizes of sand, d 50  = 0.15 mm, d 50  = 0.3 mm, and d 50  = 0.5 mm, with different backfilled depths. The results show that the pore pressure amplitude in the seabed is a minimum with a backfilled depth D , and a maximum with full backfill or a backfilled depth of zero. The pore-pressure amplitude increases as the backfill sand median diameter increases. The effects of the pipeline diameter on pore-water pressure are also analysed and discussed. [ABSTRACT FROM AUTHOR]

A simplified scour model for tripod foundation taking into account both local scour and global scour is proposed in this paper. The model is incorporated into a three-dimensional (3D) finite element model for analysis of a full-scale offshore wind turbine founded on a tripod structure using realistic structural properties. Applicability of the 3D finite element model is validated using full-scale load test data. Four different scour conditions under two wave situations are examined for the ultimate limit state (ULS), serviceability limit state (SLS) and fatigue limit state (FLS). The results show that scour has a minor effect on the natural frequency of the tripod-supported wind turbine but can significantly increase the maximum cross-sectional von Mises stress of piles under the ULS and increase the deflection of piles within nearly 20 m below the original seabed under the SLS. As for the fatigue life of the tripod structure, it can also be reduced by the effect of scour. These findings provide insights which are useful for development of safe and economic design of offshore wind turbines supported by tripod foundations. [ABSTRACT FROM AUTHOR]

The evaluation of potential liquefaction is an important part in the design of marine structures and offshore installations. However, the liquefaction phenomenon of porous seabed under the action of strong earthquake is traditionally been ignored. This paper aims to explore the momentary liquefaction mechanism of porous seabed through the newly analytical solutions of seabed response induced by vertical seismic excitation. Based on the boundary conditions at the surface and bottom of the seabed, the induced displacements and pore pressure in the sediment are rigorously derived as a function of seawater depth, seabed parameters and seismic characteristics of bedrock. A criterion of earthquake liquefaction in the seabed is developed, employing the concept of induced excess pore pressure. The representative cohesionless marine soils with different properties are selected in the parametric analysis. The results show that the liquefaction of seabed could be influenced by seawater parameters, seabed parameters and earthquake ground motion parameters. The significant finding is that current understanding that the vertical motion effect on soil liquefaction is negligible may not always hold true. [ABSTRACT FROM AUTHOR]

With the increasing development and utilization of offshore oil and gas resources, global buckling failures of pipelines subjected to high temperatures are becoming increasingly important. Due to human factors or the condition of an uneven seabed, local bends are produced in the process of pipeline manufacturing and laying. That bend which called geometric initial imperfection could trigger the global buckling of a submarine pipeline. The analytical solutions of high-order lateral buckling for a pipeline with a single arch symmetric and a two-arch anti-symmetric initial imperfection are derived in this paper. The analytical results of the high-order modes are then compared with those of the low-order modes. The modified Riks method is applied to simulate global buckling of the pipeline with imperfection under high temperature and high pressure. The numerical simulation results are compared with the analytical ones, and the effects of initial imperfection, the restraint of the foundation soil to the pipeline, the geometry of the pipeline on global buckling are analyzed. [ABSTRACT FROM AUTHOR]

This paper presents the basic physics required to predict the performance of a down-cutting operation of a complete mechanical system including underwater tracked vehicle (UTV) and ladder trencher (LT) in a subsea application. The bar forces on a complete LT are assessed, by considering the forces generated by individual cutting tools when they are mounted on an LT. The thrust or traction requirements for the overall system (UTV and LT) are then estimated, and the required tractive thrust and normal reaction forces of the overall system can be calculated. In order to analyze the weight and balance requirements of the UTV, the moment of the LT caused by the cutting system is studied. The energy and power needed to drive the LT are then estimated and considered in relation to the tool characteristics. To demonstrate the application of various equations to practical problems of machine design or performance analysis, a number of numerical simulations are performed using the derived approach. This simulation provides important insight into the behavior of the trenching machine that can be used to design a trenching machine for the required working conditions. For this, the parameter study and sensitivity analysis are conducted to determine the variations of the operational and geometric parameters, and to observe how the cutter tools and material condition affect the forces, moment, weight, and power components. [ABSTRACT FROM AUTHOR]

The interaction between a spudcan and an existing footprint is one of the major concerns during jack-up rig installation. The influence of spudcan-footprint interaction has recently been well addressed by a number of researchers. A lack of investigation exists in mitigating spudcan-footprint interaction issues. In the field, stomping and successive repositioning is conventionally used in installing a rig adjacent to an existing footprint. Water jetting and perforation drilling are also sometimes suggested. This paper reports a measure for easing spudcan-footprint interaction issues, with the efficiency of a spudcan with 4 slots tested through model tests carried out at 1 g on the laboratory floor. The soil conditions tested simulate soft to moderate seabed strength profiles close to the mudline, varying the undrained shear strength. The most critical reinstallation locations of 0.5D and 1D (D = spudcan diameter) and existing footprint depths of 0.33D and 0.66D were investigated. By comparing with a conventional spudcan, the spudcan with slots reduced the induced maximum moment, horizontal force, and horizontal sliding distance by up to 80%, 40%, and 98% respectively. Critically, no additional operations, such as stomping/repositioning, perforation drilling, water jetting, are required to be performed offshore. [ABSTRACT FROM AUTHOR]

Offshore floating facilities are fixed by anchoring systems embedded in seabed soils through chains or ropes. The chain inverse catenary profile embedded in soils influences both the anchor failure mechanism and the anchor holding capacity. The chain mobilizes varying soil normal and tangential resistances during motion, hence it is with difficulty to depict the chain profile. The present work proposed a modified method to estimate the chain inverse catenary profile with high accuracy based on the chain equations and the chain yield envelope. A testing arrangement with three load cells and two MEMS (Micro-electromechanical systems) accelerometers included was designed in model tests. By model tests, the loading combinations of the soil tangential and normal resistances on the chain were obtained and the yield envelopes for both chain and rope were determined. In addition, supplemental model tests were performed to validate the modified method proposed in this study, and the testing results indicated that the estimated chain inverse catenary profile was in good agreement with the actual one. Moreover, the testing arrangement is beneficial in investigating the chain-soil-anchor interaction. [ABSTRACT FROM AUTHOR]

Taut mooring systems have become prospective alternatives for the station keeping of offshore floating facilities in deep water. The associated embedded anchors cause a part of the mooring line to be buried in the seabed − the inverse catenary − which introduces a requirement to predict the load and uplift angle at the padeye, where the chain is connected to the anchor. The padeye load and angle depend on the shape and tension profile of the inverse catenary, which must be assessed in the mooring system design. The dynamic interaction between the embedded chain and the soil in the inverse catenary is not usually considered in this analysis. Instead, the inverse catenary is assessed statically, albeit potentially using cyclically-degraded soil strength parameters. The present study employs the lumped mass method to simulate the dynamic response of mooring lines under different imposed oscillations at the fairlead, where the chain is connected to the floating facility. A new chain-soil interaction model, which includes hysteresis effects associated with irrecoverable relative chain-soil displacement is calibrated by experimental results. Simulations of cyclic vessel motion are then performed, and the resulting chain-seabed interaction is observed. During constant-amplitude vessel motion cycles, the load angle at the padeye significantly decreases due to progressive ratcheting or ‘shakedown’ of the inverse catenary from the initial static profile towards a straighter profile. This effect is due to the hysteretic soil response and creates a less onerous loading condition for the anchor which may be beneficial, but is conventionally overlooked in design. At the end of the present study, an elastic bound method is proposed to estimate the profile of the inverse catenary after shakedown. A parametric study illustrates the performance of this simple method for predicting the steady state condition. [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]

The cyclic movement of a riser at the touchdown zone (TDZ), which involves complicated pipe–soil interaction, is critical to its long-term safety. Most previous studies have focused on the pipe–soil interaction of the riser in the vertical plane at the TDZ. Therefore, we conducted laboratory tests to investigate the pipe–soil interaction during lateral cyclic pipe movements and the influence of seabed evolution around the TDZ on the following vertical cyclic pipe movements. At the TDZ, the bell mouth shape of the clay bed and ladle-like shape of the pipe were observed during lateral cycling. The movement trajectory of the pipe as well as the gradual penetration process was recorded, which may be mainly attributed to the lateral ploughing and soil softening around the pipe. The test dynamic embedment factor (ratio of the dynamic embedment and static embedment) was 1.7–2.7. The enlargement of the TDZ during cycling was measured, which was mainly related to the movement of the trench surface point. The increase in the undrained strength of the soil at the TDZ after dynamic tests proves that a crust of soil was formed beneath the pipe owing to consolidation during cyclic pipe–soil interaction. [ABSTRACT FROM AUTHOR]

The evaluation of seabed response under wave loading is important for prediction of stability of foundations of offshore structures. In this study, a stochastic finite element model which integrates the Karhunen-Loève expansion random field simulation and finite element modeling of wave-induced seabed response is established. The wave-induced oscillatory response in a spatially random heterogeneous porous seabed considering cross-correlated multiple soil properties is investigated. The effects of multiple spatial random soil properties, correlation length and the trend function (the relation of the mean value versus depth) on oscillatory pore water pressure and momentary liquefaction are discussed. The stochastic analyses show that the uncertainty bounds of oscillatory pore water pressure are wider for the case with multiple spatially random soil properties compared with those with the single random soil property. The mean pore water pressure of the stochastic analysis is greater than the one obtained by the deterministic analysis. Therefore, the average momentary liquefaction zone in the stochastic analysis is shallower than the deterministic one. The median of momentary liquefaction depth generally decreases with the increase of vertical correlation length. When the slope of the trend function increases, the uncertainty of pore water pressure is greatly reduced at deeper depth of the seabed. Without considering the trend of soil properties, the wave-induced momentary liquefaction potential may be underestimated. [ABSTRACT FROM AUTHOR]

The asymmetric three-dimensional radiation pattern and resultant elastodynamic response of stress waves in a model comprising a compressible water column overlying a transversely isotropic seabed in which a time-harmonic source acts is theoretically investigated. The use of potential functions, the Hankel transform, and a Fourier series expansion are adopted to deal with the equations of motion for both media. Closed-form integral expressions are developed for the potentials and the stress/displacement components. The expressions and introduced procedure are sufficiently flexible to incorporate various types of source loads. To evaluate the field quantities, the residue method and a robust integration scheme are utilized to handle the poles and branch points within the integrand. Any possible number of dispersive propagation modes are taken into account in the integral evaluation. The deduced velocity dispersion curves depict the characteristics of the various modes. They also indicate the existing singular points (poles) for a specific dimensionless frequency and the surface wave type associated with each pole. Numerical results are presented for the hydrodynamic pressure and displacement in the liquid layer and stress and displacement components in the solid seabed due to distributed and concentrated source excitations. The formulation and the numerical scheme are valid for calculating the wavefield anywhere within the model including both far- and near-field effects. The sensitivity of the results to different parameters is also analyzed. Both analytical and numerical comparisons with existing solutions for simpler cases are made to confirm the validity of the results. The results are especially useful in seismic hazard assessment of submarine earthquakes, landslides, and tsunamis. They can also be extended to deal with the fluid-solid-structure interaction problems. [ABSTRACT FROM AUTHOR]

• 1,First comprehensive experimental data for the wave-current-induced pore pressure along the surface of the pipeline and beneath the pipeline. • 2,Effect of both the wave and the current characteristics on the distribution of the pore pressure around the pipeline are discussed in detail. • 3,Liquefaction risk around the pipeline is tested and investigated for both the exposed and completely embedded pipelines. Wave and current coexist widely in the oceans. Ever-changing wave characteristics, such as height and length, have a significant influence on the dynamic response within the seabed soil due to the presence of currents. Some aspects of wave, seabed and pipeline interactions have been widely investigated over past years, incorporating both experimental and numerical approaches. However, previous studies on seabed dynamic response around pipelines have mainly focused on cases with wave loading. In this experimental laboratory study, the current loading and diverse backfill conditions which commonly occur far offshore are discussed in depth. It is found that when subjected to the combined wave and current loads, the response of the seabed around a pipeline is more complex than that subjected to pure wave loading as current velocity and propagation direction would significantly change the wave characteristics as well as seabed response. Most notably when comparing with the pure wave loading, the pore pressure amplitude and its attenuation rate within the seabed increases when current and wave travel in the same direction. This could increase the possibility of liquefaction around the pipeline. When current travels in opposing direction to the wave, the effect of current on the decrease of pore pressure and its attenuation rate is more significant than that of the co-current actions. This means that the counter-current will reduce the probability of soil liquefaction. [ABSTRACT FROM AUTHOR]

• The exact artificial boundary conditions are used to simulate the wave radiation effects of the far-field domain. • The stiffness matrix method for the multi-layer soil medium is extended to evaluate the seismic response of seawater-sediment-bedrock system. • A parametric analysis is conducted to investigate the effects of site properties on the peak response of the sediment layer. In this study, a numerical model for the seawater-sediment-bedrock system is proposed to evaluate the seismic responses of ocean sites under obliquely incident P-SV wsaves. First, the stiffness matrix method for the soil medium with an arbitrary angle input of seismic waves is extended to evaluate the seismic responses of the seawater-sediment-bedrock system. The wave radiation effects of the far-field domain are fully considered by the exact artificial boundary conditions. Second, the proposed model is verified by degenerated models and a numerical model developed by the authors, which are inapplicable to arbitrary-angle inputs of seismic waves. Third, the effects of site properties, including the porosity and permeability of the seabed, incident angle and depth of seawater, on the peak response of the sediment layer under obliquely incident P and SV waves are analysed. The results are conducive to understanding the seismic responses of ocean sites. [ABSTRACT FROM AUTHOR]

Steel catenary riser (SCR) offers a cost-effective solution to deep water deployments. Hanging from a floater, an SCR is usually subjected to large tensions at hang-off location and large bending moments at touchdown zone (TDZ) which leads to fatigue damage. Field observations showed that a trench gradually formed in seabed with the depth might reach to five times of SCR's diameter. However, flat seabed was often assumed for the modeling of SCRs to avoid the time-consumingly dynamic simulation of trench development. Few studies have been conducted to evaluate the influence of trench on SCR fatigue performance, and led to contradictory conclusions, i.e., some studies suggested that fatigue damages of SCRs at TDZ were reduced, while, others proved that they were increased, if considering the presence of trench. This contradiction may be explained by factors including inappropriate trench model and different sea states assumed by different researchers. An iterative procedure initially developed by Wang and Low and further improved in the present work was used to estimate the position and the length of a trench and, then, an improved empirical formulation was generated to predefine the trench model for the finite element analysis of SCR. Additionally, dynamic simulations were conducted to study the influence of trench on fatigue performance of SCRs encountering directional waves, currents and floater offset. Results showed that presence of trench always reduced fatigue damages of SCR at TDZ compared with the flat seabed. [ABSTRACT FROM AUTHOR]