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The study of multiphase flow characteristics of hydrate slurry was crucial for controlling the risks associated with hydrate slurry transport in deep-water oil and gas operations. Currently, multiphase flow research on the hydrate slurry flow system focuses on two-phase (solid-liquid) flow studies in single short straight pipes or elbows. This paper established a geometric model based on an experimental loop of high-pressure natural gas hydrate slurry. The Euler model contained n momentum equations and continuity equations for each term, and the Euler model can couple the pressure term with the surface exchange coefficients. Therefore, Euler model was suitable to deal with complex fluid motion problems. Such as bubble flow problem, floating problem, particle suspension problem and fluidized bed problem. In this paper, the hydrate particles in the grout were set as regular round spheres and do not decompose, so the Euler model was suitable for the simulation of the gas-liquid-solid three-phase flow of hydrate grout in the loop. The Euler model was used to simulate the three-phase (gas-liquid-solid) expansion in the loop, focusing on the influence of hydrate slurry inlet flow rate, inlet hydrate volume fraction, liquid phase viscosity and pipe diameter on slurry flow characteristics. The results are as follows:1) Based on orthogonal experiments, pipe diameter has the most significant impact on the pressure drop of the entire pipe section, while liquid phase viscosity has the greatest impact on the average aggregation particle diameter of hydrate on the cross section.2) The distribution of hydrate phase is related to the direction of the loop. Under the influence of gravity, hydrates are primarily dispersed in the liquid phase, with a high concentration in the solid-liquid mixing zone, tend towards the gas phase zone. Hydrate particles are symmetrically distributed in the vertical cross-section, with smaller particle diameters in the solid-liquid mixing zone compared to the gas phase and liquid phase zone 3). In the elbow section, centrifugal force and density difference cause hydrate particles to aggregate towards the lower pipe wall on the outside of the elbow. This results in the disappearance of the liquid phase zone, higher hydrate concentration on the outside of the elbow, and smaller hydrate particle diameter in the solid-liquid mixing zone compared to the gas phase zone.4). Throughout entire loop, the viscosity of the slurry in the solid-liquid mixing zone was the highest, and the volume fraction of hydrate particles was symmetrical both vertically and horizontally. To validate the simulation results, high-pressure multiphase flow hydrate generation experiments were conducted in a 6 MPa hydrate slurry experimental loop. The stable flow pressure drop value of the slurry was compared to the simulation results. The relative error between the simulated and experimental pressure drops is less than 15%, falling within an acceptable range and matching the experimental results. This proved that the multiphase simulation technique of hydrate slurry gas-liquid-solid in the loop can effectively guide the safe transportation of hydrate slurry under actual working conditions. • Establishing a geometric model based on an experimental loop of high-pressure natural gas hydrate slurry. • The Euler model effectively simulate multiphase flow both before hydrate formation and during hydrate flow. • The distribution of hydrate phase in multiphase flow simulation is closely related to the pipeline profile. • To enhance transmission efficiency, the flow rate can be appropriately increased to avoid dynamic loss and the transmission pipe diameter can be enlarged. • Flow velocity and pipe diameter exhibit similar degrees of influence, indicating that the average aggregate particle size on the cross-section can be effectively regulated by adjusting the liquid phase viscosity to improve the flow ability of hydrate slurry. [ABSTRACT FROM AUTHOR]

Cylindrical shells with corrugation are widely used in deep sea, underground pressure structures, and other fields. It is mainly subjected to uniform and concentrated external pressure. The current research mainly includes circumferentially corrugated shells and longitudinally corrugated shells. These two kinds of corrugations improve the strength of the cylindrical shell but also have some problems such as stress concentration, buckling extension, and so on. To solve these problems, a shell with composite corrugation is proposed and it has the structural characteristics of both circumferential and longitudinal corrugations. In this paper, the numerical analysis method is used to explore the stability of the composite corrugated shell under uniform and concentrated pressures. It is analyzed by Donnell's large deflection equation. In addition, it can reduce the area of damage caused by buckling. The excellent mechanical properties of the composite corrugated shell are proven. The influence of the number of corrugations and "diameter to thickness ratio" on the buckling performance is studied by using the control variate method. It is compared with a cylindrical shell without corrugation with equal volume and weight. The concentrated pressure tests of composite corrugated shells and shells without corrugation are finished. The results show that the curve of "radial compression-reaction load" obtained by the experiment is consistent with the simulation. The ultimate load of the composite corrugated shell is 34.96% higher than the shell without corrugation. The composite corrugation can effectively enhance the compressive strength and stiffness of the cylindrical shell. Composite corrugation can make more material of the shell to be involved in resisting external deformation. This method of increasing material utilization is effective. • The advantages of the composite corrugated shell are verified from mechanical theory, finite element analysis and test. • The influence of local curvature on the buckling performance of cylindrical shell is verified. • Donnell's large deflection equation has been applied and innovated in this paper. [ABSTRACT FROM AUTHOR]

Autonomous Underwater Vehicle (AUV) parameterization is a geometric modification method, and can quickly obtain excellent structure with low vibration and noise performance. Blindness and inefficiency are common outcomes of the lack of an efficient parameter selection approach. But not enough research has been done in this area. In this paper, the finite element (FE) method is used to establish the propeller–shaft–shell double beam system with ball bearings, which is more similar to the real model. The basic theory of the proper orthogonal decomposition (POD) method is introduced in detail, and the original full order model (FOM) is reduced to 4 DOFs. It is found that the accuracy of the reduced order model (ROM) is high enough and the calculation time is greatly reduced. The semi-numerical semi-analytical expression of the ROM system is solved by the harmonic balance and alternating frequency/time domain (HB-AFT) method, whose results are in good agreement with those by the Runge–Kutta method. The sensitivities of the shell responses to the spring stiffness are discussed. The AUV setup is designed to analysis of numerical and experimental results. These results in this paper can provide an advanced theoretical basis for the structural parameter design of the AUV. • The propeller–shaft–shell of the AUV is established by the FE method. • The sensitivity analysis is obtained by the POD method and the HB-AFT method. • The connection stiffness has great impacts on the vibration responses of the shell. • The AUV setup is designed to analyze numerical and experimental results. [ABSTRACT FROM AUTHOR]

This research tackles the challenge of dynamic formation control and collision avoidance in underactuated surface vessels (USVs). Firstly, an affine transformation is employed to implement arbitrary configuration transformations within the multi-USV system. Secondly, by considering both distances and velocities between USVs and obstacles, a novel collision avoidance scheme is proposed based on conventional distance-based artificial potential field methods. This method, building on traditional distance-based artificial potential fields, offers a more precise assessment of collision risks, thereby notably shrinking the potential collision zones. Such refinement leads to reduced energy consumption and minimizes disruption to formation integrity. Thirdly, an anti-perturbation formation maneuver safety control scheme is introduced to achieve a time-varying target formation without collisions, ensuring that local position tracking errors remain bounded. The effectiveness of the proposed approaches is substantiated through comprehensive theoretical analysis and extensive numerical simulations, underscoring their practical applicability. • This paper proposes a practical distributed USV formation framework designed to address challenges posed by model uncertainties and environmental disturbances encountered during environmental surveys. • An anti-perturbation formation maneuver safety controller is proposed to compensate for the lumped disturbances and achieve maneuver control without collision. • The proposed potential field function serves as a dynamic mechanism for tuning repulsive forces, leveraging real-time data on the relative distances and velocities between USVs and obstacles. [ABSTRACT FROM AUTHOR]

In the present paper we investigate, through numerical analysis, the hydrodynamic behavior of wavebreakers both in static and in floating configuration. The aim is to evaluate and compare the performance of wavebreakers in regular waves in the range of intermediate depth waters. The analysis is performed through evaluation of the waves transmitted downward and reflected back and the dissipative behavior of the wavebreaker. We simulate numerically the fluid dynamic field using the Unsteady Reynolds Averaged Navier Stokes equations (URANS) with the k − ϵ turbulence model, both for the water and the air phases, using the Volume of Fluid (VOF) method to detect the interface. We simulate a numerical wave tank, generating the waves at a lateral boundary of the domain and allowing its own propagation into the domain. First we study the static configuration of the wavebreaker, so it is considered fixed in space. Afterward, we consider the wavebreaker as a rigid body with a Single Degree of Freedom (SDOF) in the vertical direction and we analyze the interaction between the wave system and the structure. With this purpose we use the URANS equations over a dynamic mesh in conjunction with a Fluid–Structure-Interaction (FSI) algorithm, where the mesh displacement is associated to the body's motion through a diffusive Laplace equation; the motion of the solid body is evaluated using the momentum equation of a rigid body subject to hydrodynamic loading. We study two different wavebreakers, the rectangular one and the Π shape one, and evaluate the differences in terms of transmitted, reflected and dissipated energy. First we assess the algorithm of generation and propagation of the regular waves comparing numerical results with analytical data. Afterward, we evaluate the performance of the two wavebreakers in terms of coefficients of transmission, reflection and dissipation and we compare our numerical results with data from the standard Wiegel Theory, 1960 and successive modifications. Finally, we study the performance of the wave system in presence of the floating body. This is done in two steps: we initially validate the results with those of the analytical solution of the governing equation of a SDOF rigid body forced by regular wave trains; successively we calculate the transmission coefficients for a number of waves with different length and height and compare the results with literature empirical formulas. • To study the performance of wavebreakers when exposed to regular waves. • We analyze two wavebreaker shape, namely rectangular and Π -shape. • We compare the static and the floating configuration. • Coupling the interaction between the body motion and the fluid flow (FSI). • Analysis of the transmission, reflection and dissipation coefficients of the wavebreakers. [ABSTRACT FROM AUTHOR]

This paper presents a detailed test procedure, including the test facilities, the design of the ship model and the releasing mechanism of the damaged opening. The aim is to analyze the effects of air compressibility on the flooding process and motion responses of the damaged ship. To assist in the explanation of the test results, necessary numerical simulations are performed. Through carrying out series of model tests, it was found that the limited ventilation level not only delays the flooding process but also affects the dynamic responses of the damaged ship. In addition, the test results verify the assumption of 10% full ventilation publihsed by the International Maritime Organization (IMO) while analyzing the applicability of this assumption to the single flooding compartment. The results also show that ensuring a proper ventilation level in the flooding compartments can mitigate gas-liquid interactions and thus improve the dynamic response of the damaged ship. Based on the research results in this paper, an 8% ventilation ratio can be preliminarily used as a reference standard for similarventilation designs. Finally, the obtained experimental data in this paper can be used as a benchmark for future reliability analysis of the numerical methods. • This paper presents a detailed flooding test procedure, including the test facilities, the design of the ship model and the releasing mechanism of the damaged opening. • Test results verify the assumption of 10% full ventilation in IMO while analyzing the applicability of this assumption to the single flooding compartment. • The experimental data can be used as a benchmark for future reliability analysis of numerical methods. [ABSTRACT FROM AUTHOR]

The overset-grid technique is supposed to be the most theoretically correct and powerful approach to be used in the Computational Fluid Dynamics (CFD) simulations of wind turbines. The results obtained by CFD simulations which leverage the overset-grid technique could be sensitive to many numerical configurations, e.g., grid spacing, time increment, and turbulence models, etc. As a result, it is important to systematically quantify numerical uncertainties in order to obtain trustworthy predictions from these simulations. In this paper, the performance characteristics of the wind turbine tested in the Norwegian University of Science and Technology (NTNU) Blind Test (BT) 1 experiment is analyzed by using the CFD code Finite-Analytic Navier-Stokes (FANS) with overset grids, and a thorough verification study aiming at quantifying the numerical uncertainties in the CFD predictions is performed. First, the CFD approach adopted and the verification method leveraged in the present work are introduced. Next, the wing tunnel tests of the S826 wing section and the NTNU BT1 wind turbine are described in detail. First, as a benchmark, CFD simulations using the FANS code with overset grids are performed targeting a wing section of S826 airfoil before we move to full turbine simulations. Comparison of the predicted surface pressure with the measurement is performed and a good agreement is observed. Then, CFD simulations are performed for the full turbine of BT1. In order to assess the numerical uncertainties relating to the spatial and temporal discretization errors, a numerical database obtained by using six different configurations of grid spacing and time increment is created. In each of the computational grids, the details of the wind turbine's geometry are precisely captured by taking the advantage of overset grids. Unsteady Reynolds. Averaged Navier-Stokes (URANS) simulations with a two-layer k − ϵ turbulence model are performed. The rotation of the rotor is achieved by moving the according overset-grid blocks. Through the utilization of a contemporary verification analysis for the numerical predictions, the numerical uncertainties stem from discretization errors in the computed thrust (C T) and power (C P) coefficients are evaluated. Finally, CFD calculations are conducted for different Tip-Speed Ratios (TSR s), and the CFD results are compared to the experimental data. • CFD simulations of wind turbine were performed using the overset-grid technique. • The numerical uncertainties relating to the spatial and temporal discretization errors were systematically quantified. • The numerical predictions were compared with the experimental data, taking into account the discretization uncertainties. [ABSTRACT FROM AUTHOR]

The paper presents the results of 3D coupled cyclic time history numerical analyses of a monopile supporting a 12 MW Offshore Wind Turbine, installed in dense cohesionless soils and subjected to a 600-s load history corresponding to the high phase of a 35-h design storm. The goal of the study is to investigate the governing mechanisms and gauge potential conservatisms or uncertainties in approaches for monopile analysis used in practice. The Ta-Ger constitutive model, implemented in FLAC3D and calibrated against site-specific cyclic tests, is used to model the complex soil response. Emphasis is placed on the effect of drainage conditions, an aspect typically overlooked in practice, although often stated as critical. Analyses show that the drainage of the system can substantially affect the response. In low-permeability soils (e.g., cohesionless soils with low-plasticity fines) widespread liquefaction may occur inducing high rotations above allowable limits. On the contrary, systems that can drain effectively within each cycle, develop moderate excess pore pressures which do not jeopardize performance. Current design procedures are often unable to accurately capture these effects possibly leading to either conservative or unconservative outcomes. Suitably validated advanced numerical analyses can be used as complementary tools to standard methods to assess these uncertainties. • Advanced numerical methodology for cyclic time history analysis of monopiles. • Use of Ta-Ger constitutive model for complex soil response. • Validation against centrifuge tests. • Drainage conditions can have a substantial effect on the response. • Current design methodologies may be unable to capture drainage effects. [ABSTRACT FROM AUTHOR]

This paper demonstrates a series of 1 g model tests and numerical modelling in order to investigate the pushout extraction process of suction anchors in sand. The suction anchor models used in this study have aspect ratios of 1 and 2, and the pumping rate and soil permeability are varied to reveal their influence on the extraction process. It is shown that changes in the pumping rate and soil permeability rarely alter the overpressure during the extraction process but can affect the displacement rate and final extracted length of the suction anchor. The ratio of the final extracted length over the total length of the suction anchor is also found to be different for suction anchors with different aspect ratios in the 1 g model tests. Eventually, a calculation method for this pushout extraction process is proposed and validated to determine the peak overpressure with some basic parameters of the suction anchor and sands. The relationship between the pumping rate and the displacement rate of the suction anchor is also revealed in the calculation method. • Pumping rate and soil permeability barely affect the overpressure during extraction. • The final extracted length is dependent on the pumping rate and anchor geometry. • The initial displacement rate has a linear relationship with the pumping rate. • The overpressure can be obtained with basic parameters using the proposed method. [ABSTRACT FROM AUTHOR]

The utilization of renewable energy is an important way to solve the energy crisis, and wind power is one of the most mature new energy technologies. This paper studies the potential of applying the viscoelastic damper to mitigate vibrations of the OWT tower under wind-wave loadings. The viscoelastic damper directly converts the horizontal deformation of the tower into the shear deformation of the viscoelastic material for energy dissipation, which provides a new idea for suppressing the dynamic response of the OWT. According to OWT structural dynamic characteristics and load conditions, a new high damping wall-type viscoelastic damper (HVED) control is developed. The mechanical properties of HVED are investigated by full scale test. A numerical vibration response analysis of the controlled and uncontrolled 4 MW OWT was carried out in ANSYS. Results indicate that the HVED can significantly reduce the dynamic response of OWT and increase its fatigue life. • A new high damping viscoelastic damper (HVED) is proposed to mitigate the vibration of OWT. • Advantages of HVED are significant energy dissipation, wide-frequency control, and convenient installation. • Full scale test was conducted to verify the mechanical properties of HVED. • Interaction of soil and structure (SSI) was considered in the finite element analysis. [ABSTRACT FROM AUTHOR]

This paper investigates the stability and nonlinear dynamics of a multi-field coupling system comprising gas, liquid, and a functionally graded porous plate reinforced with graphene platelets (GPLR-SFGP) in the local immersion state. The multi-field coupling nonlinear dynamic system model of the GPLR-SFGP plate, considering the ideal fluid hypothesis and nonlinear plate theory, is established. The state-space approach is utilized to analyze the natural frequencies of the partially submerged plate with respect to the porosity and GPL weight percentage. The plate's critical divergence plane is also graphed, and an evaluation is conducted regarding how these parameters affect the stability of the system. For the nonlinear dynamic analysis, the amplitude-frequency resonance curve of the system is provided. The impacts of the immersion depth, the fluid velocity and design parameters on the nonlinear response of the system are explored. Derived from theoretical and numerical analysis, an optimal design principle for the GPLR-SFGP plate in a gas-liquid-solid coupling environment is proposed. • Considered the particular circumstance of the plate in the local immersion state. • Established the nonlinear dynamic model of the graphene reinforced plates with various pore types. • The instability curves and surfaces of the system are drawn in the local immersion state. • The amplitude-frequency resonance response curve of the system is drawn. [ABSTRACT FROM AUTHOR]

At present, the numerical analysis of the monopile foundation of an offshore wind turbine subject to complex environmental loads is a major challenge in engineering design. Based on the concept of the macroelement model, this paper proposes a straightforward, speedy, and accurate numerical tool. Firstly, the numerical limit analysis method is employed to investigate the three-dimensional failure envelope surface of a large-diameter monopile in sand, and the analytical formula of the three-dimensional failure envelope surface is obtained. Then, using the three-dimensional failure envelope and combining the basic principles of the hypoplastic theory, a hypoplastic macro-element of a large diameter monopile foundation in sand is proposed. The performance of the macroelement model is calibrated by comparing it to centrifuge test results of a small-diameter pile and three-dimensional finite element results of a large-diameter monopile under cyclic loading. Finally, the macroelement model is used to analyze the performance of the OWT large-diameter monopile foundation under wind and wave loads. The results indicate that the macro-element model can precisely reproduce the performance of large-diameter monopile foundations under cyclic loading, and can effectively and accurately solve the complex pile-soil interaction problem of OWT large diameter monopiles under cyclic loading. • The failure surface of a large-diameter monopile foundation in sand is established. • A macroelement model of a large-diameter monopile foundation in sand is established. • The behavior of a large-diameter monopile foundation is studied using a macroelement model. [ABSTRACT FROM AUTHOR]

For the new anti-roll device of the Fluid Momentum Wheel (FMW), the driving pump is necessary to generate the circular circumferential fluid flow in the FMW. In order to evaluate the influence of the pump on the fluid characteristics of the FMW, this paper proposes experimental method based on the Particle Image Velocimetry (PIV) technique and numerical method based on Computational Fluid Dynamics (CFD) theory to predict the characteristics of fluid flow, which have been successfully validated through mutual comparison of the two methods. On this basis, the method can be applied in two FMW models with the driving pump or pressure source to comparatively analyze the specific swirl flow caused by the rotating blades of the pump. Moreover, its effects on the phenomena of velocity stratification in circumferential direction and secondary flow in cross-section are further discussed. Then, the study evaluates the characteristics of precession moments of the FMW with a driving pump along different rotation axis. Finally, concluding remarks and future perspectives are presented. • Experimental method of PIV and numerical method of CFD are presented to study FMW with driving pump. • Effects of swirling characteristic by driving pump on stratification distribution and secondary flow are analyzed. • Low-frequency and high-frequency moments of FMW corresponding to pressure gradient and swirl flow are studied. [ABSTRACT FROM AUTHOR]

Sea-crossing bridges face complex site environments compared to onshore bridges, with the marine environment significantly influencing seismic responses. Despite this, current seismic design for these bridges relies on onshore earthquake records. Therefore, investigating the impact of seawater layers and site conditions on sea-crossing bridge seismic performance is crucial. This paper investigates the influence of characteristics of offshore ground motion, site conditions, and hydrodynamic effect on the seismic performance of piers. Initially, based on the validated finite element model, 5 offshore and onshore strong ground motion records from K-NET were selected for assessing the seismic performance of piers. Subsequently, the effects of the water depth and site conditions on the seismic responses of piers were investigated. Finally, the effects of the section size, section shape, and boundary conditions on the piers were investigated. The result shows that the seismic response of piers under offshore ground motion exceeds that under onshore ground motion. In addition, the seismic response of the pier increases with greater water depth, while they exhibit a slight decrease with increasing soil depth. Notably, the larger the section size, result in higher the hydrodynamic pressure, and square piers experience greater hydrodynamic pressure compared to circular piers. • The seismic performance of piers subjected to offshore ground motion is studied. • Finite element models considering seawater layer and site conditions are developed. • The seawater layer significantly amplifies the seismic response of piers. • The soil layer has a constraint effect on the response of piers. • The hydrodynamic effects of underwater piers are explored. [ABSTRACT FROM AUTHOR]

Pile tip damage has become of particular concern for the offshore wind industry due to the extensive use of large-diameter, thin-walled piles for monopiles and jacket structures. The potential for damage has increased in recent developments due to the occurrence of embedded boulders within the seabed sediments (e.g. glacial tills) or partially weathered soft rocks (e.g. limestones, chalk). To assess the risk of pile tip damage, this paper presents a simple schematic approach to estimate: i) whether pile tip damage might occur following impact with embedded boulders; and ii) what soil strength (e.g. as reflected in the cone tip resistance q c) might initiate progressive distortion of an initial minor dent (extrusion buckling). The methodology was developed based on results of centrifuge model tests and large displacement finite element analysis, encompassing: i) undamaged thin-walled piles installed through embedded layers of different sized boulders; and ii) initially 'pre-dented' piles driven through homogeneous sand triggering progressive distortion. Numerical analysis was used to develop predictive guidelines for conditions that might cause significant pile tip damage. The guidelines were validated by comparison with results from model tests and also through well documented field cases where extrusion buckling occurred. The paper provides guidance for choosing the critical parameters involved in the calculation processes to enable direct application in design, allowing assessment of whether more detailed investigation and analysis should be undertaken. [ABSTRACT FROM AUTHOR]

The paper studies the effect of soil strength and stiffness degradation on the undrained cyclic performance of offshore foundations in low-plasticity cohesive soil using 3D finite element modelling. Cyclic triaxial tests on reconstituted kaolin are conducted at the ETH Zurich laboratory, providing insights into key parameters affecting the degradation process. A simplified soil constitutive model accounting for cyclic degradation is developed and encoded in Abaqus via a user subroutine. The model is calibrated against experimental results and validated with published centrifuge model tests of monopiles under cyclic lateral loading. It is subsequently used to evaluate the performance of suction caisson foundations with different aspect ratios (L/D = 0.5 and 2) under short-term cyclic and seismic loading. Due to its ductile resistance mechanism, the L/D = 0.5 caisson exhibits superior performance under vertical cyclic loading in fast-degrading soil. Under inclined cyclic loading, the slower degradation rate of the L/D = 2 caisson governs response, reversing the trend. Under seismic shaking, the degradation-induced resistance imbalance amplifies the irrecoverable settlements produced by kinematic shearing at the caisson sidewalls. For the fast-degrading soil examined, degradation is shown to increase settlements by up to 50%. • Cyclic strength and stiffness degradation of cohesive soils is explored via undrained triaxial tests on kaolin samples. • A simplified total stress-based constitutive model is developed for capturing such effects. • The model is validated against published centrifuge tests of a monopile subjected to lateral cyclic loading. • The undrained behaviour of caissons under cyclic and seismic loading is assessed, considering cyclic degradation effects. • For the case of fast-degrading soil examined, soil degradation may increase foundation settlements by up to 50%. [ABSTRACT FROM AUTHOR]

The airlift pump has the advantages of simple structure, strong reliability, convenient maintenance and low cost in the fields of deep seawater upwelling and seabed mining. This paper focuses on numerical analyses of steady air–water–solid three-phase flow as well as water–solid two-phase flow in the airlift pump based on one-dimensional multi-fluid model. The depth distributions of 8 physical quantities such as volumetric fractions and axial velocities of all phases, air density and pressure are calculated by solving the governing equations or by integrating the vector form of nonhomogeneous ordinary differential equations for three-phase flow interval and two-phase one, respectively. Upon successful verification of the present numerical model through a comparison with antecedent experimental results in case of a vertical pipe with length of 7.86 m, the model is extended to the case of seabed mining in water depth of 2000 m. The effects of the submerged depth of air–liquid–solid mixer, the diameter of solid particle and its density are considered. • The steady air–water–solid three-phase flow as well as water–solid two-phase flow in the airlift pump are considered by using one-dimensional multi-fluid model. • The depth distributions of 8 physical quantities such as volumetric fractions and axial velocities of all phases, air density and pressure are calculated by solving the governing equations for three-phase flow interval and two-phase one, respectively. • The model is extended to the case of seabed mining with water depth of 2000 m. • The effects of the submerged depth of air–liquid–solid mixer, the diameter of solid particle and its density are considered. [ABSTRACT FROM AUTHOR]

Ice loading is the main controlling design load for offshore platforms in Bohai Bay. The ice-induced vibration of offshore platform can be monitored every winter. Both the freezing and drifting sea ice can damage an offshore platform causing structural damage, discomfort for workers, gas leakage and other problems. Two platform designs were studied aiming to reduce ice-induced vibration. One design is the offshore platform with concrete filled double-skin legs (JOP-CFDSL). The other design is the self-centering offshore platform with concrete filled double-skin legs (SRJOP-CFDSL). Both platforms were tested in laboratory experiments, and the data were used to prepare numerical simulations. The results showed that the ice-induced vibration of the two novel jacket platforms are all reduced under push loads, and the SRJOP-CFDSL have better control effect. The replacement of jacket offshore platform's steel circle hollow section legs with concrete filled double-skin legs can effectively improve the structural stiffness. And prestressed tendons were found to improve the platform's performance even further, helping it to reset after lateral impact. It should be noted that the prestressed tendon as an important bearing member, must do some anti-icing protection. Thus, the influence of ice force on the prestressed tendon isn't considered in this paper. • CFDST legs can effectively improve platform's stiffness. • JOP-CFDSL and SRJOP-CFDSL have good ability to resist ice-induced vibration. • SRJOP-CFDSL has great self-centering performance. • SHS improve the corrosion resistance of JOP-CFDSL and SRJOP-CFDSL. [ABSTRACT FROM AUTHOR]

Existing studies mainly focus on the bearing capacity of monopiles, with less emphasis on reliability analysis, which is an important aspect of the performance of offshore wind turbine (OWT) foundations. This paper investigates the bearing capacity and reliability of traditional and hybrid monopiles using deterministic and probabilistic analyses. The three-dimensional finite element model (FEM) considering the structure-soil interaction is established and its accuracy is verified through comparisons with existing research findings. The responses of monopiles under wind and wave loads are compared in detail, including the rotation angle at the mudline, structural stress, and soil displacement. The environmental design parameters are obtained based on the 30-year annual extreme data, and the correlation between these environmental parameters is considered to obtain a large amount of random environmental parameter data. Then, the back propagation neural network (BPNN) and the response surface method (RSM) are combined to construct the failure response surface. Research shows that the reliability index of the plated monopile does not improve significantly, while the skirted-monopiles perform much better than the traditional monopile. The proposed FEM-BPNN-RSM coupled methodology provides a more comprehensive understanding of the monopile bearing capacity, and can be used for the future monopile structure design of OWTs. • Hybrid monopile configurations, incorporating additional structures, are proposed to enhance the bearing performance. • Bearing performance and reliability of traditional and hybrid monopiles are numerically investigated. • Three-dimensional finite element models considering structure-soil interaction are used to calculate monopile responses. • The FEM-BPNN-RSM coupled method is proposed to conduct reliability analysis. • Modified monopiles exhibit improved reliability compared to traditional monopiles. [ABSTRACT FROM AUTHOR]

In the context of numerical self-propulsion predictions, the literature on high-speed marine vehicles (HSMV) is still relatively scarce. Actually, extensive numerical analyses are rarely considered for these applications because of the not-so-stringent design constraints and contract requirements concerning these crafts and also thanks to the advantages of series production. On the contrary, with the growth of the market requests, in particular, in the direction of Greenhouse Gas Reduction, the exact knowledge of the vessel working point, in terms of speed and engine configuration, pushes the researchers to investigate all the factors which deeply define the propulsion condition. In the present paper, two existing vessels have been numerically studied using a robust and industrially affordable procedure based on some simplified viscous Computational Fluid Dynamic approaches to predict the self-propulsion working points. The obtained results have been then compared with available sea trials showing a very high agreement even if some uncertainties in both the experimental measurements and the CFD results still exist. The impact of these uncertainties has been further investigated to assess the reliability of the self-propulsion prediction, showing that the maximum speed can be obtained with an uncertainty of about 1 knot and 10% of the corresponding engine load. A more robust prediction can be obtained by including the uncertainty assessment in the propulsion analyses. The validity of the proposed analyses is further supported because the numerical uncertainty falls inside the uncertainty obtained from the available full-scale measurements, so defining a more robust prediction of the vessel propulsion point for design purposes. • Self-propulsion prediction of high-speed marine vehicles. • Test cases are two industrial planning hulls. • Uncertainty analysis of the self-propulsion point using a Montecarlo approach. [ABSTRACT FROM AUTHOR]

It is worth deliberating on torpedo anchors, a development that emerged in the offshore exploration field in recent times. The demand for ocean renewable energy exploration and this interest required developing a new sort of mooring and anchoring systems for offshore platforms. Though a vast type of anchors has been developed, the literature lacks a critical review of torpedo anchors. This critique summarizes a state-of-the-art discussion on torpedo anchors. It aims to review the developments of deep-water torpedo anchors that occurred during the last few decades. The level of growth achieved in this field is significant, economical, and reliable, making it simple and easing the anchor installation operation. This paper recapped the detailed survey of theoretical, experimental (laboratory and field-test) and numerical analysis and design of torpedo anchors and reviewed their performance. And a complete discussion of the torpedo anchor's primary factors (pullout capacity and embedment depth) influencing characteristic features is explained. In addition, it describes the soil failure process of the anchors during penetration and pullout of the anchor. A wide range of features for the application of torpedo anchors is readily enabled. In conclusion, the current challenges encountered in this field are mentioned. • This critique summarizes a state-of-the-art discussion on the torpedo anchors and aims to review the recent developments. • This paper recapped the theoretical, experimental and numerical method of analysis and design of torpedo anchors. • This paper also reviewed various characteristic features influencing torpedo anchor's performance. • A wide range of features for the application of torpedo anchors are readily enabled from this study. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical and numerical analysis based on a model of mangrove forests incorporating stems and roots to determine the nearshore wave angle and wave height for one-dimensional (1-D) and two-dimensional (2-D) problems. The shallow water equations are solved using the two-step Lax-Wendroff method. The resulting wave height and wave direction vectors are displayed and analyzed to assess the impact of mangrove effects on various bathymetries, including constant, sloping, and parabolic profiles. This procedure is also applied to actual bathymetric data obtained from the Hatiya channel to analyze the influence of mangrove forests and shoreline geometry on the amplitude and wave directionality of nearshore waves. The obtained numerical results demonstrate that both coastal geometry and vegetation exhibit non-linear characteristics in wave behavior. Furthermore, these factors significantly contribute to the directionality and attenuation of nearshore waves. • Mangrove models studied for nearshore wave angle and height via theoretical and numerical methods. • Wave vectors explore mangrove impact on various bathymetries: constant, sloping, parabolic, real. • Numerical results show nonlinear wave behavior due to coastal geometry and vegetation. [ABSTRACT FROM AUTHOR]

The offshore wind industry is a growing business. New and alternative installation methods which have the potential to reduce the cost of developing offshore wind farms are urgently needed. In this paper, an innovative concept that is tentatively designed for median or large water-depth sites is studied. The conventional jacked-up installation vessel is replaced by the small waterplane area twin-hull vessel (SWATH) equipped with the dynamic position (DP) system, and the heavy lifting is replaced by the low-height lifting crane with lifting wires for lowering the assembled OWT to the floating foundation. The global dynamic responses of the installation system under environmental loads are investigated by the fully coupled nonlinear numerical models, and the corresponding strength capacity for the lifting crane is validated. Several sea states are selected to study the feasibility of the concept, and the response variables of the OWT tower mating point (i.e., the displacement and velocity) are analyzed under different loading conditions. The lifting wires and selected lifting crane components are also investigated to understand their force situations. Some interesting conclusions are drawn, which can provide references for introducing the control system to the low-height lifting system and further optimizing the concept. • A comprehensive numerical model for pre-assembled OWT installation is proposed. • The low-height lifting crane's flexibility and the details of the lifted OWT are considered. • The lifting crane's buckling stability under dynamic load conditions is validated. • The responses of the installation concept is calculated under combined wind and wave loads. • A deeper understanding of the installation concept is achieved. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical and numerical analysis based on a model of mangrove forests incorporating stems and roots to determine the nearshore wave angle and wave height for one-dimensional (1-D) and two-dimensional (2-D) problems. The shallow water equations are solved using the two-step Lax-Wendroff method. The resulting wave height and wave direction vectors are displayed and analyzed to assess the impact of mangrove effects on various bathymetries, including constant, sloping, and parabolic profiles. This procedure is also applied to actual bathymetric data obtained from the Hatiya channel to analyze the influence of mangrove forests and shoreline geometry on the amplitude and wave directionality of nearshore waves. The obtained numerical results demonstrate that both coastal geometry and vegetation exhibit non-linear characteristics in wave behavior. Furthermore, these factors significantly contribute to the directionality and attenuation of nearshore waves. • Mangrove models studied for nearshore wave angle and height via theoretical and numerical methods. • Wave vectors explore mangrove impact on various bathymetries: constant, sloping, parabolic, real. • Numerical results show nonlinear wave behavior due to coastal geometry and vegetation. [ABSTRACT FROM AUTHOR]

The offshore wind industry is a growing business. New and alternative installation methods which have the potential to reduce the cost of developing offshore wind farms are urgently needed. In this paper, an innovative concept that is tentatively designed for median or large water-depth sites is studied. The conventional jacked-up installation vessel is replaced by the small waterplane area twin-hull vessel (SWATH) equipped with the dynamic position (DP) system, and the heavy lifting is replaced by the low-height lifting crane with lifting wires for lowering the assembled OWT to the floating foundation. The global dynamic responses of the installation system under environmental loads are investigated by the fully coupled nonlinear numerical models, and the corresponding strength capacity for the lifting crane is validated. Several sea states are selected to study the feasibility of the concept, and the response variables of the OWT tower mating point (i.e., the displacement and velocity) are analyzed under different loading conditions. The lifting wires and selected lifting crane components are also investigated to understand their force situations. Some interesting conclusions are drawn, which can provide references for introducing the control system to the low-height lifting system and further optimizing the concept. • A comprehensive numerical model for pre-assembled OWT installation is proposed. • The low-height lifting crane's flexibility and the details of the lifted OWT are considered. • The lifting crane's buckling stability under dynamic load conditions is validated. • The responses of the installation concept is calculated under combined wind and wave loads. • A deeper understanding of the installation concept is achieved. [ABSTRACT FROM AUTHOR]

This paper introduces a novel model for predicting the performance and conducting sensitivity analysis of design parameters in two-body point absorber wave energy converters. The key novelty lies in the examination of design parameter sensitivity and their interaction effects on performance. The model utilizes the response surface method (RSM) in conjunction with central composite design to establish analytical equations for efficiently predicting the resonance frequency and peak generated power of the two-body wave energy converters based on geometric parameters and wave height. This eliminates the need for complex analytical solutions and boundary element simulations. The accuracy and reliability of the RSM prediction models are validated through state-of-the-art numerical simulations and analysis of variance (ANOVA). Using these RSM prediction models, the study proceeds to design and optimize two-body point absorbers for locations along the coastlines of the six Australian states, focusing on two objectives: maximizing peak output power and maximizing the power-to-weight ratio at each location. By incorporating the second optimization target, it is observed that impressive power output with smaller device dimensions or a higher power-to-weight ratio can be achieved across all six Australian states. The developed response surface method prediction model is a versatile mathematical framework applicable to all two-body point absorber wave energy converters and can be adapted for optimizing other types of wave energy converters with expansive search spaces. Australia was selected as a case study due to its abundant wave energy density along its southern shorelines. Notably, no similar studies addressing these specific aspects have been reported in the literature, making this work the first of its kind. • The performance prediction models of two body wave energy converters have been developed from design parameters. • The prediction models are validated against numerical simulations and the analysis of variance (ANOVA). • The sensitivity of design parameters and their interactions has been analyzed and disclosed. • The optimal dimensions for the maximum power and maximum power to weight ratio are derived for all the Australian seas. [ABSTRACT FROM AUTHOR]

In order to ensure the safety of submerged floating tunnel(SFT) during operation, it is necessary to carry out SFT response analysis under environmental loads. Given that SFTs are often constructed in valley areas situated in seismic zones, the aseismic performance of SFT needs to be investigated. However, there are important factors of non-uniform excitation and flexible-support boundary which have not been explored thoroughly. Non-uniform excitation affects the load input characteristics while flexible-support boundary affects the dynamic characteristics of structure itself. In this paper, we built a multi-point seismic wave synthesis model which fits well with the target power spectrum and a numerical response analysis model which reflects the dynamic characteristics of SFT structure well. The results based on the proposed model show that vertical non-uniform excitation can significantly increase the peak value of SFT tether tension and tube displacement with flexible-support boundary under whether artificial or natural seismic wave series input. While flexible-support boundary can availably produce vibration control effect on peak value of tether tension and tube response of whether displacement or stress indices under non-uniform wave series input. This work can provide reference for dynamic safety assessment and vibration control design of SFT to improve the development of ocean space. • A synthesis model of multi-points seismic wave input sample series was established to consider the influence of non-uniform excitation effect. • A numerical model verified by the theoretical solution was established to consider the influence of flexible-support boundary effect. • The dynamic response of the SFT system were discussed considering non-uniform excitation and flexible-support boundary effects simultaneously. [ABSTRACT FROM AUTHOR]

The propeller fluctuating force is being recognized as the dominant source of underwater radiated noise, its accurate acquisition is thus necessitated for a reliable acoustic-structure analysis and optimization of marine ships. However, such dynamic operational forces are frequently impossible or expensive to directly measure due to various practical challenges. To alleviate this limitation, this paper presents an inverse method for the indirect estimation of propeller forces using measured shafting vibration responses. The force reconstruction procedure is constructed based on the state-space model of the propeller–shafting system, which can be derived by a data-driven subspace identification algorithm in conjunction with either the numerical or experimental models of the system. To ensure the robustness of the inverse analysis in the presence of inevitable measurement noise, a modified regularization strategy based on homotopy continuation is introduced. The effectiveness of the proposed algorithm is demonstrated through numerical simulations and experimental investigations. The obtained results demonstrate the potential feasibility of indirect measurement for accurately identifying propeller fluctuating forces in the time domain. • An inverse procedure is proposed to identify propeller force from shaft responses. • The system is described by a state-space model identified from a subspace algorithm. • A homotopy regularization is implemented to stabilize the inverse process. • The robustness of the scheme is validated through numerical and experimental cases. [ABSTRACT FROM AUTHOR]

During the launch operation of subsea equipment in the offshore industry, several factors are important for a safe and economical installation within an operating window. One of these factors is the hydrodynamic coefficients of the equipment to be installed. The coefficients can be obtained, experimentally or numerically, by applying an oscillatory motion that represents the effects of sea conditions and vessel motion, but these applications simplify conditions during a real operation and disregard important effects such as ocean currents. This paper investigates numerically the effect of current together with forced oscillation on the hydrodynamic coefficients of disks by analyzing two influence parameters, the Keulegan-Carpenter number (K C) and the reduced current velocity (V r). A numerical model validation process was carried out in stages to overcome the lack of references on the subject. The model was compared with Direct Numerical Simulation (DNS) results for low Reynolds number (R e), up to 600, in qualitative and quantitative analysis, and then applied to broader conditions found in real operations, with K C up to 3 , V r up to 0.3 , and for high R e , up to 240,000. On the model scale, higher V r generates an increase in the drag coefficient (C d) and a decrease in the additional mass coefficient (C a). Under real scale conditions, the results are similar, with variations of C d up to 300% and of C a up to 36% for low K C. Analysis of the flow topology indicates that K C and V r influenced vortex shedding and convection. Correlations for the hydrodynamic coefficients as functions of K C and V r are proposed, with a R 2 higher than 93%. • Selected RANS turbulent models presented results similar to DNS simulations for cases of forced oscillation with current for low R e. • The analysis was carried out at a higher R e , similar to a real scale, indicating an increase in drag coefficient and reduction in added mass for high currents. • The flow topology due to oscillatory movement and current shows an influence on vortex shedding. • For the first time, the influence of higher K C and current on the hydrodynamic coefficients was established. • Correlations are proposed for the hydrodynamic coefficients as functions of K C and V r , with a R 2 > 93 %. [ABSTRACT FROM AUTHOR]

The water hammer effect produced when fragments penetrate the liquid tank on warship will cause integral expansion and damage to the structure of the liquid tank. In this paper, a kind of liquid-filled honeycomb structure reinforced by Kevlar fiber tube is designed, and the mechanism of Kevlar fiber tube reinforced honeycomb liquid-filled structure against fragmentation is analyzed. The numerical model of Kevlar reinforced honeycomb liquid-filled structure is established, and the process of high-speed fragments penetrating the defense structure is analyzed by ALE calculation method, and the armor-piercing test of liquid-filled structure is carried out, which verifies the reliability of simulation calculation. The influence of honeycomb core layer and Kevlar fiber tube on the defense of liquid-filled structure against high-speed fragment is further discussed. In addition, an air compartment is arranged in the honeycomb liquid-filled structure to weaken the water hammer effect. It is found that the air compartment has a slight decrease in the ability to weaken the fragment velocity, but it greatly reduces the damage degree of the structure caused by fragment penetration. • The composite protective liquid tank was designed and tested. • The failure process of composite structure is analyzed by armor-piercing test and numerical simulation. • The influencing factor on preventing fragment penetration are comprehensively compared. [ABSTRACT FROM AUTHOR]

When a wind turbine is working in a cold and humid environment, icing may occur which lead to its performance reduction or even blades fracture. In this paper, a CFD-WTIA (Wind Turbine Integrated Analysis) coupled method is established to analyze the blade icing process and its influence on the overall dynamic performance of an integrated jacket-support offshore wind turbine. Firstly, motions of the blades are calculated by the WTIA method and used as input into CFD. Then, dispersed multi-phase model and melting-solidification model are used to simulate the icing growth phenomenon of three-dimensional blades. The k-ε turbulence model is used to calculate the aerodynamic performance before and after icing. Finally, the aerodynamic results after blade icing are returned to WTIA for integrated dynamic response acquisition. At the same time, the dynamic response of the wind turbine under the combined influence of ice and sea ice is analyzed. Results show that the blade ice-accretion increases linearly along the blade span-wise direction and is mainly concentrated on the leading edge of the blade. Lift and drag coefficients are seen deceased and increased respectively after icing. Power production, generator torque, rotor speed, as well as blade vibration are quantitatively studied. The methodology and findings of this paper can provide a good reference for the safety performance evaluation of an icing offshore wind turbine. • A coupling of the CFD-WTIA method is established to investigate icing impact on the integrated offshore wind turbine. • Ice accretion is mainly concentrated on the leading edge. The most serious ice accretion is observed at the blade tip. • Before rated power, icing reduces power generation, rotor speed and torque, and the vibration of structure. • Over rated power, the wind turbine is maintained at rated power, the icing of blades increases structural vibration. • Under the combined load of blade icing and sea ice, the vibration of wind turbine structure is more severe. [ABSTRACT FROM AUTHOR]

This paper presents a numerical modelling methodology for an innovative installation method of floating wind turbines. The study analyses the influence of hydrodynamic and environmental modelling factors on the dynamics of a multibody system consisting of a catamaran with three wind turbines and a spar foundation with a mooring system. The research focuses on features such as viscous drag on the spar, hydrodynamic interaction between floating bodies, long- and short-crested waves, first- and second-order wave loads, wave directions, and wave peak periods. The study found that hydrodynamic modelling factors had a significant effect on the horizontal motion of the installation system but a negligible effect on vertical motion and mooring system tension. Therefore, it is recommended to consider viscous drag forces, hydrodynamic interaction, and first- and second-order wave forces when developing mechanical damping and control systems to reduce horizontal motion. The study also suggests using both long- and short-crested waves for numerical analysis when estimating the weather window for installation operability analysis. Overall, this study contributes to the development of an efficient and practical installation method for floating wind turbines by analysing the impact of hydrodynamic and environmental factors on the dynamics of the multibody system. • Numerical modelling method for an innovative onsite installation concept is presented. • Effects of hydrodynamic and environmental modelling features on complex multibody system are investigated. • Viscous drag, hydrodynamic interaction, and second-order wave forces are important in horizontal response analysis. • Even the simplest numerical model is sufficient for vertical response analysis. • Short-crested wave as well as long-crested wave should be taken into account in weather window estimation. [ABSTRACT FROM AUTHOR]

The launch and recovery of equipment such as remotely operated vehicles (ROVs) is a critical task that defines the operability limits of many marine operations. This paper considers the analysis of control systems that are designed to maximize the operability limits for launch and recovery of a ROV from a small unmanned surface vessel (USV). We use numerical simulation for the analysis, where the method combines recent approaches for wave compensating dynamic positioning, active heave compensation, and positioning control of the ROV with multi-body dynamic simulation of the surface vessel and ROV, including hydrodynamic forces and dynamic interactions from wires that depend on the ROV depth and moonpool. The results show that the choice of control algorithms and their tuning parameters has a significant effect on the system's operability, and should be carefully designed and tuned to optimize the operability limits for any given sea state, weather and operational setup. The results show that numerical analysis with a system's simulation is an effective tool to verify operability and its sensitivity to various parameters for the given ROV recovery application. • The choice of ROV control system, unmanned surface vessel dynamic positioning and active heave compensation control algorithms and their tuning parameters has a significant effect on the system's operability. • Control systems should be carefully designed and tuned to optimize the operability limits for any given sea state, weather and operational setup. • A numerical simulation model for ROV recovery in the moonpool of a small USV is presented, validated and used in analyzing operability. • Numerical system simulation that integrates all physical elements and control systems is an effective method to verify operability from a systems perspective. [ABSTRACT FROM AUTHOR]

This paper presents a detailed investigation on the influences of different water depths and mooring parameters on the nonlinear responses of mooring systems and motion behaviors of a 10 MW semi-submersible wind turbine. To reduce computational effort, a developed mooring design and analysis program MOOR_SAD is used for quasi-static analysis, which is validated through comparing with the simulation results in OrcaFlex. The fully coupled dynamic numerical analysis is performed using OrcaFlex-FAST and validated through the comparison of numerical and experimental results of the NREL OC5 DeepCwind. Then, thirty mooring cases with different water depths and mooring parameters are studied for analyzing the nonlinear responses of chain mooring lines and motion performances of the 10 MW FOWT in shallow water. The results show that when the chain properties and pretension of the mooring lines are kept the same, the shorter mooring line leads to a stronger nonlinearity in mooring restoring force and stiffness. Besides, the weight of the mooring line has little effect on the mooring shape. This study could be expected to provide an engineering approach for mooring analysis and design of FOWTs in shallow water. • The effects of water depths on the nonlinear responses of mooring systems and motion behaviors of a FOWT are investigated. • Sensitivity analysis for the nonlinear effects of mooring parameters is conducted. • The design recommendations for the catenary mooring system of a semi-submersible FOWT in shallow water are proposed. [ABSTRACT FROM AUTHOR]

The low efficiency of the floating breakwater (FB) in attenuating middle and long-period waves is a problem which is urgent to be addressed. To improve the wave attenuation performance, various new configurations have been proposed by scholars. We proposed a new configuration which was designed with wing structure and openings, and it was experimentally proved that the novel design having a significant improvement in attenuating incoming waves. Unfortunately, the flow characteristics around and inside the FB as well as the wave attenuation mechanisms did not be intensively studied. In this paper, CFD analysis is conducted to investigate the hydrodynamic performance of FBs with complex configuration. The accuracy of the numerical method is verified by comparing with the experiment results. Through the numerical studies, the novel wave attenuation mechanisms of the FB with wing structure and openings are revealed. The wave slamming induced by wing structure and the water spraying induced by opening are two main factors which improve wave attenuation efficiency. After that, the optimization design which aims to improve wave attenuation performance is conducted. • A numerical analysis method is applied to investigate performances of floating breakwaters with complex configuration. • The water spraying, wave slamming, reversed flow and vortex phenomena are observed in the simulation. • Novel wave attenuation mechanisms of the floating breakwater are found and analyzed. • Improved design and analysis of the floating breakwater is conducted and the wave attenuation performance is improved. [ABSTRACT FROM AUTHOR]

This article investigates an improved adaptive dynamic event-triggered control scheme for autonomous underwater vehicles (AUVs) under the quaternion-based attitude representation. The main innovation in this paper is to improve the dynamic event-triggered (DET) algorithm, which makes the trade-off between the control performance of the AUV system and the communication frequency in the controller-to-actuator (CA) channel easier to be achieved. Primarily, to describe the attitude representation of the AUV in a unique form, a unified auxiliary variable confining the scalar quaternion's initial error value is developed to avoid the potential unwinding phenomenon. Subsequently, based on the unified auxiliary variable, an event-based adaptive control law is tactfully designed to mitigate the adverse effects of lumped external perturbations and unknown model dynamics. Moreover, by introducing the modified trigger adjustment algorithm, the trigger frequency can be adapted to the performance requirements of the AUV system at different evolutionary stages. Finally, theoretical analysis and numerical simulations are presented to demonstrate the stability and effectiveness of the provided method. • An initiative resetting dynamic variable algorithm is provided to govern the trigger sequence. • The proposed DET method provides a more flexible and general way to balance communication frequency and system control performance. • A global and unique dynamics description of AUVs can be achieved with guaranteed anti-unwinding characteristics. [ABSTRACT FROM AUTHOR]

Research on the engine room's natural gas leakage and diffusion has great significance for the LNG Powered ships' design, the fire risk assessment and explosion. In this paper, a model experiment system with a scale ratio of 1:10 was established. Helium gas was used instead of natural gas to study the evolution law of light gas concentration distribution in the model engine room over time. The CFD model of light gas diffusion in the indoor thermal environment is verified by experimental data and used to simulate the natural gas diffusion behavior in a full-size ship's engine room. The influence of leakage amount, cabin temperature, ventilation conditions, leakage location and other factors on the distribution of natural gas concentration was analyzed. The results show that the established CFD model has good predictive power for the diffusion behavior of light gas in the ship's engine room, and the numerical simulation results of the trend of leakage gas concentration with time are consistent with the experiment, and the maximum error is not more than 20%. The amount of leakage has a great influence on the concentration near the leakage location. The larger the initial volume flow and the higher the temperature of the leakage, the larger the dangerous area in the ship's engine room. Increasing ventilation flow is an effective way to reduce the diffusion area and control the concentration of natural gas in the engine room. When the ventilation flow is doubled, the diffusion area of natural gas reduces to 1/3. The leaked natural gas will gather at the location nearing the obstacles which block the jet flow and the upper layer of the engine room. The research results provide a basis for accurately predicting the spatial and temporal distribution model and method of gas diffusion concentration in the engine room environment. • The experimental study on the leakage and diffusion characteristics of light gas in the ship engine room model is carried out. • The CFD engine room model is used to realize the mutual verification between numerical simulation and experiment. • Based on the full-size ship engine room, the effects on the diffusion characteristics of natural gas leakage is studied. [ABSTRACT FROM AUTHOR]

The wave glider powered by waves has a unique dual-body architecture and can perform long-duration operational tasks in the ocean. In this paper, to accurately predict its maneuverability, the horizontal plane 3-degree of freedom (DOF) dynamic models of the surface float and the submerged glider are established separately. The influence of ocean currents is considered, and the influence of the torsional moment of the umbilical cable is considered for the first time. The hydrodynamic coefficients of the submerged glider will vary with different rudder angles and angles of attack of incoming flow. The computational fluid dynamics (CFD) method is used to analyze and calculate added mass coefficients and damping coefficients corresponding to different rudder angles based on planar motion mechanism (PMM) test simulation and oblique navigation dynamic test simulation. The flow fields are analyzed under different working conditions. The structural parameters of the wave glider and the hydrodynamic coefficients obtained from the simulation are brought into the dynamic model. The rotary motion and the Z-shaped motion under different sea conditions and currents are numerically analyzed. The difference of the hysteresis of the surface float relative to the submerged glider in yaw motion are analyzed under different ocean currents and rudder angles. Sea trial and simulation results show that the established dynamic model and obtained hydrodynamic parameters can accurately basically predict the maneuvering performance of the wave glider. • Considering tension and torsion of umbilical and currents, the dynamic model of WG in the horizontal plane is established. • The computational fluid dynamics method is used to analyze and calculate added mass coefficients and damping coefficients. • Sea trial and simulation results show that the established dynamic model can accurately predict the maneuvering of the WG. [ABSTRACT FROM AUTHOR]

A slender body's impact and water entry generate a violent flow with the free surface, time-dependent hydrodynamic loads, and motion or structural responses. The strong three-dimensional effects of the two-phase flow amplify the computational cost of predicting the hydrodynamic flows accurately with the free surface. This paper proposed a high-efficiency Smoothed Particle Hydrodynamics (SPH) model to simulate the oblique water entry of a cylinder. The number of particles is almost 50 million. The numerical model is validated by comparison with the experimental data and the convergence study of particle resolution. The results show that the cavity may shift transversely due to the unbalanced pressure gradients. The transverse shift is stronger with a smaller entry angle. The results are compared with Logvinovich's model, which shows that Logvinovich's model can accurately predict the shape of the cavity away from the splash, whereas the SPH model can accurately predict violent deformation, sealing, and breaking of the free surface in all stages of cavity evolution. The splash dynamics are quantified, benefiting from the sharp interface computed by the SPH method of high resolution. The splash can obtain more kinetic energy when the entry angle is small. Besides, The pinch-off pattern is more complicated than the vertical water entry due to the transverse shift of the cavity. Two columnar bubbles are separated from the cavity due to the transverse shift of the cavity when the entry angle is small. [Display omitted] • The oblique water entry is computed by multi-resolution two-phase SPH method. • The pressure gradient generated transveral shift of the cavity is found. • The changes of the potential and kinetic energy of splashes are presented. [ABSTRACT FROM AUTHOR]

This paper aims to extend the implementation of DualSPHysics-Project Chrono coupling to hydroelasticity problems involving free-surface flows and deformable elastic boundaries. In the present model, the fluid sub-domain of the phenomenon is based on the WCSPH model of DualSPHysics. The solid sub-domain, simulated by the Project Chrono engine, is modeled using the set of rigid bodies attached by hinges with torsional and damping stiffness. An analytical solution of Euler-Bernoulli beam theory is employed to adapt the rigid-multibody system to the mechanical characteristics of the elastic sub-domain via the torsional stiffness values of hinges. The stability and accuracy of the present model are tested by three cases with different flow characteristics and mechanical properties for solid sub-domain, involving a newly-designed experimental setup and two benchmark cases in the literature. Obtained results show reasonable accuracy with experimental measurements and other numerical model computations. • This paper presents the application of DualSPHysics-Project Chrono coupling to hydroelasticity problems. • The solid sub-domain of the hydroelasticity phenomenon is numerically modeled using a set of rigid bodies attached by hinges with torsional stiffness. • An analytical solution of Euler-Bernoulli beam theory is employed to adapt the rigid-multibody system to mechanical characteristics of solid sub-domain through torsional stiffness values. • The present model is validated by a newly-designed experimental setup and two benchmark cases in the literature. [ABSTRACT FROM AUTHOR]

The expansion of the offshore wind industry in areas with high seismicity has led to engineering challenges related to the design of the offshore wind turbines (OWTs). Monopiles, i.e., tubular steel piles of large outer diameter, low aspect ratio (penetration depth over outer diameter), and relatively thin pile wall, are traditionally the preferred foundation type for OWT due to fabrication, transportation, and installation standardization. For all bottom-founded systems, soil–structure interaction (SSI) plays a crucial role in the system's response. Additional challenges arise in the case of seismic SSI as, not only the system's response, but also the seismic ground motion itself are affected by the soil characteristics. Furthermore, uncertainties related to soil properties, as derived from the soil testing campaign and interpretation, need to be thoroughly considered for OWT load calculations and the design of the support structure. The uncertainty in soil interpretation may have a large impact on the characteristics of the input seismic motion. Subsequently, SSI will affect the seismic loads acting on the support structure and the OWT. This knock-on effect of the interpretation of the soil parameters is unknown, but may be significant to account for. In fact, when a "best estimate" soil parameter set is used, the resulting seismic load may not necessarily correspond to the most probable load for the assumed seismic event. This paper investigates the influence of the uncertainty in soil parameters, as they may result from the soil interpretation, on the seismic loads. It demonstrates the skewed distribution of OWT seismic loads using a realistic design case study on a commercial OWT. Results are presented in the form of transfer functions, response spectra at mudline and normalized bending moments along the support structure. Three distinct structural components of interest are selected to evaluate the results. It is concluded that, for the analysis of OWT under seismic loading conditions in particular, it cannot be decided a priori which soil properties would result in conservative or progressive design. Based on the obtained results, recommendations are given which aim to de-risk and enhance the current design practice. • Consideration of uncertainty in soil properties is important for the seismic design. • At least two sets of soil properties should be accounted for the seismic design. • Transfer functions (bedrock to mudline) are used to identify the critical eigenmodes. • Response spectra (at mudline) are used to detect the critical set of soil properties. • A sensitivity study is advised if the seismic load is close to the ULS design load. [ABSTRACT FROM AUTHOR]

Riser systems usually encounter large offset and huge environmental loads when emergency disconnection. However, the two-dimensional recoil theory and research still lack. This paper consequently establishes the two-dimensional riser recoil dynamics theory and two-dimensional mud discharge theory. The interaction mechanism between the riser and discharged mud is discussed, and the types of discharged load and the contribution of internal fluid (FI) to the riser finite element matrix are clarified. Combining the method of integration and coordinates reconstruction, the actual ordinate of the riser and elongation of the tensioner are retrieved. Numerical analysis results show the FI mass and gravity should not be included in the axial part of the riser element matrix during recoiling. The additional inertial load of FI due to the movement of the riser element influences the transverse displacement. The extremums of VM and bending moment appear at two ends of the riser system during early recoil stage. Transverse deformation of the riser system significantly affects the longitudinal displacement, which cannot be ignored. The dynamic responses of recoil in different two-dimensional scenarios are explored, including in different wave and current environments, at different vessel offset points, and with different vessel transverse velocities. • The two-dimensional (2D) riser recoil dynamics theory and the 2D mud discharge theory are established. • Interaction mechanism between riser and discharged mud is discussed, and the contribution of internal fluid to the finite element matrix of the riser element are clarified. • Mass and gravity of internal fluid should not be included in the axial part of the riser element matrix during recoiling. • Transverse deformation of the riser system significantly affects the longitudinal displacement. [ABSTRACT FROM AUTHOR]

This paper analyzes the propulsion performance and the correlating flow details of a waterjet–propelled vehicle in straight ahead and steady turning conditions. The viscous flow field of the fully discretized waterjet – propelled vehicle is numerically solved utilizing RANS method. Drift angles ranging from 0° to 30° with an interval of 10° are considered. The theoretical control volume of waterjet system under oblique flow and the corrected thrust equation are proposed. The results show that the enhanced bilge vortex in drift alters the ingested flow pattern of the waterjet-propelled vehicle. A steady hydraulic performance of the pump is observed under non-cavitating conditions, and the inlet is one of the major source of the energy loss of the internal flow. It is problematic to evaluate the thrust and the inlet efficiency due to the irregular shape of capture area. The tangential velocity component is in proportion to the drift angle at pump inflow plane, and leads to the port/starboard asymmetry of the axial velocity and vorticity. The swirling flow rolls up near the windward inlet duct and local vortices align to the spiral axial flow. The streamwise vortex occurs and distributes near the lower-windward side of the inlet in steady turning conditions. • The theoretical control volume of waterjet system under oblique flow and the correction equation are proposed. • A stable hydraulic performance of the pump is observed under steady turning maneuvering. • Streamwise vortex occurs and distributes near the lower-windward side of the inlet in steady drift. [ABSTRACT FROM AUTHOR]

Gas freeing is one of the most hazardous and critical operations on oil/chemical tankers. The process aims to remove toxic gases from tanks and bring oxygen to a healthy level (21%) in the tank atmosphere. Significant shipboard operations such as hot work or entry into the tank after tank cleaning require a successful gas-freeing process; otherwise, the consequences may be devastating for human life, cargo, ship, and the marine environment. Therefore, proactive planning and comprehensive risk assessment are essential to enhance operational safety onboard tanker ships. In this study, the potential hazards of the gas-freeing process are evaluated under the Fine–Kinney based intuitionistic fuzzy TODIM (Tomada de Decisão Interativa e Multicritério) approach. While the Fine Kinney and TODIM methods are used for calculating the risk score with the probability, exposure, and consequence parameters and determining the risk priority, intuitionistic fuzzy sets (IF) help to overcome uncertainty in human decision-making. The research findings are anticipated to guide maritime professionals, safety inspectors, and shipboard personnel in enhancing the gas-freeing process's safety and potential mitigating risks on oil/chemical tanker ships. • Numerical risk assessment under Fine-Kinney and TODIM for gas freeing process is performed. • Intuitionistic fuzzy sets are modified to overcome uncertainty. • The paper recommends preventive measures according to highest risk factors. [ABSTRACT FROM AUTHOR]

Leader–follower formation tracking is surveyed for underactuated unmanned surface vehicles (USVs) with velocity constraints in this paper. The formation is expected to track a planned trajectory, and the planned trajectory is only accessible to the root leader. Each follower carries sensors for obtaining reference information. To tackle collision and connectivity constraints, error transformation is carried out on the dynamic surface with prescribed performance functions, and a concise dynamic surface that satisfies finite-time stability is created with K-class functions and barrier Lyapunov functions. Then, neural networks are utilized to approximate unknown disturbances, and a bounded dynamic controller is proposed based on the velocity estimation. Given that the planned trajectory might cause velocity saturation due to external disturbances and maneuvers for formation acquisition, a virtual leader is introduced to conduct trajectory improvement for anti-saturation control. Based on the saturated information, a feedback mechanism is proposed to adjust the velocity of the virtual leader and velocities of USVs simultaneously. Mathematical and numerical analyses are conducted to prove the feasibility of the controller and the mechanism. • Formation tracking with velocity and error constraints is surveyed for USVs. • A concise finite-time controller is proposed based on dynamic surfaces and Lyapunov functions. • A feedback mechanism is established for velocity constraints. • The velocity of the trajectory is revised dynamically for anti-saturation control with the feedback mechanism. • Stability of the control law with the feedback mechanism is given through comparison and mathematical analysis. [ABSTRACT FROM AUTHOR]

The aim of the paper is to study the influence on the structural strength assessment, in terms of fore end weight, in the conversion process from a conventional to an inverted bow hull of a platform supply vessel under bow flare impact loads. Linear and elastic buckling analyses of the corner bracket connection (deck beam with web frame) located in the fore body are obtained from modifications to its geometry and the inclusion of different stiffening methods. A series of numerical and experimental analyses are undertaken with the design and construction of a new auxiliary structure. The most effective combination (bracket geometry and stiffening method) is established and alternative geometries that optimize the local weight of the corner bracket are proposed by means of topology optimization, obtaining an innovative bracket geometry (omega bracket) that minimises weight while maintaining constant buckling strength. The results obtained establish useful and valuable design criteria that save material and weight to the ship's structure. [Display omitted] • Comparison between conventional bow and inverted bow were established, in terms of bow flare pressure distribution. • The buckling strength assessment of the corner bracket connection in the fore end structure was determined. • A local model of each geometry was carried out to validate the numerical results. • A topology optimization procedure was applied to obtain a new corner bracket geometry (omega bracket). [ABSTRACT FROM AUTHOR]

Characterization of structural behaviors on a submerged floating tunnel (SFT) under various wave conditions is an important task for the realization of an emerging structure. An SFT can be designed as a single or dual section type by considering its structural, cost-effective, and constructional aspects. However, few experiments have been performed under irregular waves conditions and research on dual section SFTs is hard to find, even though this section type is considered in many preliminary designs. In this paper, single and dual type SFTs were experimentally investigated by using a 2-D wave flume. For measuring the behavior under various wave conditions, 16 regular waves and 5 irregular waves with different combinations of wave heights and periods were determined. Generated waves were verified, and tensile forces on mooring lines were primarily analyzed. After that, tunnel displacements and vibration characteristics were investigated to support additional reliability of the study, followed by a comparison of the structural responses with numerical analyses. It was found that the behavior of SFTs is affected by wave height, period, characteristic dimension, and that the decreased motion of dual SFTs does not mean a better result. Through the experimental study, characteristics and possibilities for realization of the SFT are examined. • Characterization of single and dual SFT through a hydraulic experiment. • Comparison of structural responses between section types and load parameters. • Investigation of structural safety according to regular and irregular wave loads. [ABSTRACT FROM AUTHOR]

This paper presents the experimental results for a 1:50 scaled model of an immersed tunnel element suspended from a twin-barge in tank tests. A compliant four leg catenary mooring system using steel link chains in the physical tests is investigated. The corresponding numerical model is developed to simulate the mooring characteristics and the fully coupled dynamic responses of the tunnel-barge system, using the hydrodynamic properties from the diffraction/radiation potential code. To validate the numerical model, preliminary static, quasi static, decaying test and regular wave test were carried out for the comparison. The main findings demonstrate that the mooring tensions of the tunnel-barge system are comparatively lower than that of the tunnel suspended from a fixed platform, particularly when subjected to long period irregular waves. The maximum mooring load of the tunnel element is significantly affected by the natural frequencies of both the tunnel and twin-barge. For the method of the tunnel element immersion presented in this paper, large local peaks in mooring tensions occur at excitation periods of waves close to the tunnel-barge resonance of roll, which should be avoided during design for an appropriate mooring system. • A moored tunnel element suspended from a twin-barge model is numerically investigated to study the effect of different wave conditions and immersion depths on the mooring characteristics of the tunnel element. To validate the numerical model, preliminary static, quasi static, decaying test and regular wave test were carried out for the comparison. • The mooring tensions of the tunnel-barge system are comparatively lower than that of the tunnel suspended from a fixed platform, particularly when subjected to long period irregular waves. • The influence of the dynamic response of the floating twin-barge on the tunnel mooring loads introduces an important aspect in the coupled behavior that needs to be considered in the early design phase. • The mooring tensions of the tunnel element in the coupled system increased with the wave period until reached a local peak and then occurs a decreasing trend. [ABSTRACT FROM AUTHOR]

This paper addresses the method for estimating maneuvering mathematical models in waves, and uses captive model tests to verify it. To obtain the wave force and moment, two kinds of captive model tests are performed. One is straight towing tests with different towing directions to consider various incident wave angles, while the other is circular motion tests at different regular wave frequency. These captive model tests provide added resistance and wave-induced steady force considering both different wave direction and ship speed, and different wave drift angles. The wave drift forces and moment obtained are validated by performing the maneuvering simulation in waves, and comparison with the results of free running model tests. This paper introduces the application of the Estimation-Before-Modeling (EBM) approach, one of the System Identification methods, to estimate mathematical models for ship maneuverability targeting KRISO Very Large Crude oil Carrier (KVLCC2). The most important force component for the prediction of the maneuverability of a ship is the accuracy of the wave-induced steady force acting on the ship. Here, the EBM approach is divided into two steps: First, the motion variables and forces are estimated by Extended Kalman Filter and Modified Bryson-Frazier smoother. Second, the wave-induced steady forces are determined by removing the hydrodynamic force acting on the ship during motion. Also, during the second step of the EBM approach, the database consisting of wave length and incident wave angles is constructed. All estimated variables are based on data obtained by free running model tests in regular waves. Finally, the estimated wave-induced steady force is interpolated, added to the hydrodynamic force component of the 3-DoF equations of motion, and the turning simulation of the ship in waves is then performed. Comparison of the results with the results of free running model tests confirms the validity of the established model. • It is revealed that some of the research contents described in this paper were presented by the author at MASHCON 2019. • We received the results of the 1st round review from reviewers on February 7, 2021. • The revised manuscript results for the first review will be submitted on April 1, 2021. [ABSTRACT FROM AUTHOR]

This numerical study aims to analyze the hydrodynamic characteristics of in-series structures with two cross-sections, as well as their power characteristics under different spacing (α) and radius (φ) ratios respectively. The results show that as the spacing ratio increases, the in-series circular/square-cylinder system displays VIV, whereas the in-series square-cylinder system can effectively suppress vibration within the interval of α = 5.3 – 5.7, 6 ≤ U r ≤ 8 when φ = 1. It is worth noting that as the radius ratio increases, the vibration of the downstream square cylinder transits from VIV to galloping. Subsequently, the short-time Fourier transform and wavelet transform are employed to perform time-frequency analysis of different cases. In terms of energy harvesting, the in-series circular-cylinder system shows its maximum power of 48.83 mW at U r = 9 when α = 6, φ = 1. Additionally, the power of the in-series square-cylinder system at φ = 2 is higher than that in the other two cases (φ = 1 and 3), and in these three cases, the power at α = 6 is much greater than that of the case α = 8 and 4 regardless of the radius ratio. As to conversion efficiency, the case α = 6 and φ = 2 demonstrates the optimal energy harvesting performance. Thus, flexible control of the radius ratio and spacing ratio in a special range can not only suppress vibration but also enhance energy harvesting efficiency. • This paper studies the WIV of series square cylinders under different spacing ratios α and radius ratios φ. • For the series square-cylinder system, the vibration of the downstream cylinder can be effectively suppressed when α = 5.3–5.7, 5 ≤ U r ≤ 8. • As the radius ratio increases, the vibration of the downstream vibrating square-cylinder transitions from VIV to galloping. • Short-time Fourier transform and Morlet wavelet transform are performed to analyze the characteristics of time-frequency. [ABSTRACT FROM AUTHOR]