2021 Vol.35(5)
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2021, 35(5): 631-645.
doi: 10.1007/s13344-021-0060-x
Abstract:
The breakwaters have experienced many changes during their construction history. These changes have been considered to improve their performance, depending on their environmental conditions and applications. Numerical modelling was conducted using FLOW-3D software. In this study, the wave overtopping from composite berm breakwater as new conceptual structure and the pressure imposed on the composite berm breakwater are considered and investigated. The results show a decrease of 84.01, 70.88 and 61.42 percent of the wave overtopping in the composite berm breakwater, in comparison to the rubble mound breakwater, horizontally caisson breakwater and caisson breakwater, respectively. Also, the pressure applied to the composite berm breakwater with the pressure applied to the horizontally caisson breakwater was compared and evaluated. Composite berm breakwater compared with horizontally caisson breakwater in P1, the amount of the obtained pressure decreased by 52.09%, in P2 the amount of the obtained pressure decreased by 63.07%, in P3 decreased by 76.09% and in Pu, this pressure reduced by 53.92%. For the composite berm breakwater, the impact of three types of berms, homogenous berm (Type 1), a berm consisting of armor-filter (Type 2) and multi-layer berm (Type 3) with the aim of optimizing the hydraulic responses and wave interaction on the caisson of the breakwater was examined and evaluated. In total, Type 3 will be recommended with a significant reduction in the overtopping values and maximum pressure.
The breakwaters have experienced many changes during their construction history. These changes have been considered to improve their performance, depending on their environmental conditions and applications. Numerical modelling was conducted using FLOW-3D software. In this study, the wave overtopping from composite berm breakwater as new conceptual structure and the pressure imposed on the composite berm breakwater are considered and investigated. The results show a decrease of 84.01, 70.88 and 61.42 percent of the wave overtopping in the composite berm breakwater, in comparison to the rubble mound breakwater, horizontally caisson breakwater and caisson breakwater, respectively. Also, the pressure applied to the composite berm breakwater with the pressure applied to the horizontally caisson breakwater was compared and evaluated. Composite berm breakwater compared with horizontally caisson breakwater in P1, the amount of the obtained pressure decreased by 52.09%, in P2 the amount of the obtained pressure decreased by 63.07%, in P3 decreased by 76.09% and in Pu, this pressure reduced by 53.92%. For the composite berm breakwater, the impact of three types of berms, homogenous berm (Type 1), a berm consisting of armor-filter (Type 2) and multi-layer berm (Type 3) with the aim of optimizing the hydraulic responses and wave interaction on the caisson of the breakwater was examined and evaluated. In total, Type 3 will be recommended with a significant reduction in the overtopping values and maximum pressure.
2021, 35(5): 646-661.
doi: 10.1007/s13344-021-0057-5
Abstract:
Coastal wave energy resources have enormous exploitation potential due to shorter weather window, closer installation distance and lower maintenance cost. However, impact loads generated by depth variation from offshore to nearshore and wave−current interaction, may lead to a catastrophic damage or complete destruction to wave energy converters (WECs). This objective of this paper is to investigate slamming response of a coastal oscillating wave surge converter (OWSC) entering or leaving water freely. Based on fully nonlinear potential flow theory, a time-domain wave−current−structure interaction model combined with higher-order boundary element method (HOBEM), is developed to analyze the coupled hydrodynamic problem. The variable-depth seabed is considered in the model to illustrate the shallow water effect on impact loads and free surface profiles in coastal zone. A domain decomposition approach is utilized to simulate the overlapping phenomenon generated by a jet falling into water under gravity effect. Through a series of Lagrangian interpolation methods, the meshes on boundaries are rearranged to avoid the mismatch between element size on free surface and body surface. The present model is validated against the existing experimental and numerical results. Simulations are also provided for the effects of wave−current interaction and uneven local seabed on the slamming responses. It is found that the length of the splash jet increases for a following current and decreases for an opposing current, and that the slamming response of the OWSC device is sensitive to the geometric features of the uneven seabed.
Coastal wave energy resources have enormous exploitation potential due to shorter weather window, closer installation distance and lower maintenance cost. However, impact loads generated by depth variation from offshore to nearshore and wave−current interaction, may lead to a catastrophic damage or complete destruction to wave energy converters (WECs). This objective of this paper is to investigate slamming response of a coastal oscillating wave surge converter (OWSC) entering or leaving water freely. Based on fully nonlinear potential flow theory, a time-domain wave−current−structure interaction model combined with higher-order boundary element method (HOBEM), is developed to analyze the coupled hydrodynamic problem. The variable-depth seabed is considered in the model to illustrate the shallow water effect on impact loads and free surface profiles in coastal zone. A domain decomposition approach is utilized to simulate the overlapping phenomenon generated by a jet falling into water under gravity effect. Through a series of Lagrangian interpolation methods, the meshes on boundaries are rearranged to avoid the mismatch between element size on free surface and body surface. The present model is validated against the existing experimental and numerical results. Simulations are also provided for the effects of wave−current interaction and uneven local seabed on the slamming responses. It is found that the length of the splash jet increases for a following current and decreases for an opposing current, and that the slamming response of the OWSC device is sensitive to the geometric features of the uneven seabed.
2021, 35(5): 662-675.
doi: 10.1007/s13344-021-0058-4
Abstract:
A mathematical model has been developed to analyze the influence of extreme water waves over multiconnected regions in Visakhapatnam Port, India by considering an average water depth in each multiconnected regions. In addition, partial reflection of incident waves on coastal boundary is also considered. The domain of interest is divided mainly into two regions, i.e., open sea region and harbor region namely as Region-I and Region-II, respectively. Further, Region-II is divided into multiple connected regions. The 2-D boundary element method (BEM) including the Chebyshev point discretization is utilized to solve the Helmholtz equation in each region separately to determine the wave amplification. The numerical convergence is performed to obtain the optimum numerical accuracy and the validation of the current numerical approach is also conducted by comparing the simulation results with existing studies. The four key spots based on the moored ship locations in Visakhapatnam Port are identified to perform the numerical simulation. The wave amplification at these locations is estimated for monochromatic incident waves, considering approximate water depth and different reflection coefficients on the wall of port under the resonance conditions. In addition, wave field analysis inside the Visakhapatnam Port is also conducted to understand resonance conditions. The current numerical model provides an efficient tool to analyze the amplification on any realistic ports or harbors.
A mathematical model has been developed to analyze the influence of extreme water waves over multiconnected regions in Visakhapatnam Port, India by considering an average water depth in each multiconnected regions. In addition, partial reflection of incident waves on coastal boundary is also considered. The domain of interest is divided mainly into two regions, i.e., open sea region and harbor region namely as Region-I and Region-II, respectively. Further, Region-II is divided into multiple connected regions. The 2-D boundary element method (BEM) including the Chebyshev point discretization is utilized to solve the Helmholtz equation in each region separately to determine the wave amplification. The numerical convergence is performed to obtain the optimum numerical accuracy and the validation of the current numerical approach is also conducted by comparing the simulation results with existing studies. The four key spots based on the moored ship locations in Visakhapatnam Port are identified to perform the numerical simulation. The wave amplification at these locations is estimated for monochromatic incident waves, considering approximate water depth and different reflection coefficients on the wall of port under the resonance conditions. In addition, wave field analysis inside the Visakhapatnam Port is also conducted to understand resonance conditions. The current numerical model provides an efficient tool to analyze the amplification on any realistic ports or harbors.
2021, 35(5): 676-686.
doi: 10.1007/s13344-021-0059-3
Abstract:
In the current study, the treatment of air/water interface has been made on dam-break induced tsunami-like wave by the Coupled Level Set and Volume of Fluid (CLSVOF) three-dimensional modelling. The overall CLSVOF method adopts a Tangent of Hy-perbola for INterface Capturing (THINC) scheme with the Weighted Linear Interface Calculation (WLIC) and Level Set (LS) function for capturing interface and calculating normal vector, respectively. As far as THINC/WLIC scheme is concerned, since the convection problem of the VOF function can be solved well, the numerical diffusion can be avoided. The spatial terms in the LS equation were discretized by the Optimized Compact Reconstruction Weighted Essentially Non-Oscillatory (OCRWENO) scheme with fourth-order accuracy, which can avoid false oscillation of LS solution. By combining CLSVOF method with Immersed Boundary (IB) method, the simulation of dam-break induced tsunami-like wave impacting on a stationary breakwater can be carried out. Grid sensitivity, mass error and free-surface profile are first calculated for the tsunami-like wave problem to validate the proposed numerical algorithm, which shows excellent agreement between the numerical results and experimental data. Tsunami-like waves with varied tailgater levels are then investigated. Calculations of velocity magnitude, free-surface profile and wave elevation of the tsunami-like wave are conducted to investigate its dynamics and kinematics.
In the current study, the treatment of air/water interface has been made on dam-break induced tsunami-like wave by the Coupled Level Set and Volume of Fluid (CLSVOF) three-dimensional modelling. The overall CLSVOF method adopts a Tangent of Hy-perbola for INterface Capturing (THINC) scheme with the Weighted Linear Interface Calculation (WLIC) and Level Set (LS) function for capturing interface and calculating normal vector, respectively. As far as THINC/WLIC scheme is concerned, since the convection problem of the VOF function can be solved well, the numerical diffusion can be avoided. The spatial terms in the LS equation were discretized by the Optimized Compact Reconstruction Weighted Essentially Non-Oscillatory (OCRWENO) scheme with fourth-order accuracy, which can avoid false oscillation of LS solution. By combining CLSVOF method with Immersed Boundary (IB) method, the simulation of dam-break induced tsunami-like wave impacting on a stationary breakwater can be carried out. Grid sensitivity, mass error and free-surface profile are first calculated for the tsunami-like wave problem to validate the proposed numerical algorithm, which shows excellent agreement between the numerical results and experimental data. Tsunami-like waves with varied tailgater levels are then investigated. Calculations of velocity magnitude, free-surface profile and wave elevation of the tsunami-like wave are conducted to investigate its dynamics and kinematics.
2021, 35(5): 687-699.
doi: 10.1007/s13344-021-0061-9
Abstract:
Based on the elastic foundation beam theory and the multi-floating-module hydrodynamic theory, a novel method is proposed to estimate the dynamic responses of VLFS (Very Large Floating Structure). In still water, a VLFS can be simplified as an elastic foundation beam model or a multi-floating-module model connected by elastic hinges. According to equivalent displacement of the two models in static analysis, the problem of rotation stiffness of elastic hinges can be solved. Then, based on the potential flow theory, the dynamic responding analysis of multi-floating-module model under wave loads can be computed in ANSYS-AQWA software. By assembling the time domain analysis results of each module, the dynamic responses of the VLFS can be obtained. Validation of the method is conducted through a series of comparison calculations, which mainly includes a continuous structure and a three-part structure connected by hinges in regular waves. The results of this paper method show a satisfactory agreement with the experiment and calculation data given in relative references.
Based on the elastic foundation beam theory and the multi-floating-module hydrodynamic theory, a novel method is proposed to estimate the dynamic responses of VLFS (Very Large Floating Structure). In still water, a VLFS can be simplified as an elastic foundation beam model or a multi-floating-module model connected by elastic hinges. According to equivalent displacement of the two models in static analysis, the problem of rotation stiffness of elastic hinges can be solved. Then, based on the potential flow theory, the dynamic responding analysis of multi-floating-module model under wave loads can be computed in ANSYS-AQWA software. By assembling the time domain analysis results of each module, the dynamic responses of the VLFS can be obtained. Validation of the method is conducted through a series of comparison calculations, which mainly includes a continuous structure and a three-part structure connected by hinges in regular waves. The results of this paper method show a satisfactory agreement with the experiment and calculation data given in relative references.
2021, 35(5): 700-711.
doi: 10.1007/s13344-021-0062-8
Abstract:
The maximum predicting error of the commonly used passive truncated mooring system method may reach 30% due to the difference of dynamic characteristics between the truncated and full-depth mooring line. In this paper, the experimental strategy called three-parameter (displacement, velocity and acceleration) active control method at the truncated point of mooring line is established to implement the synchronous equivalent of motion and force, and the realization of active truncated mooring system for model test is studied theoretically. The influences of three-parameter and one-parameter (displacement) active control strategies on the compensation effects are compared by numerical study. The results show that the established three-parameter active control method can feasibly realize the static and dynamic equivalent of truncated and full-depth mooring system, laying a good foundation for the following physical model test of active truncated mooring system.
The maximum predicting error of the commonly used passive truncated mooring system method may reach 30% due to the difference of dynamic characteristics between the truncated and full-depth mooring line. In this paper, the experimental strategy called three-parameter (displacement, velocity and acceleration) active control method at the truncated point of mooring line is established to implement the synchronous equivalent of motion and force, and the realization of active truncated mooring system for model test is studied theoretically. The influences of three-parameter and one-parameter (displacement) active control strategies on the compensation effects are compared by numerical study. The results show that the established three-parameter active control method can feasibly realize the static and dynamic equivalent of truncated and full-depth mooring system, laying a good foundation for the following physical model test of active truncated mooring system.
2021, 35(5): 712-723.
doi: 10.1007/s13344-021-0063-7
Abstract:
Two marine structures arranged side by side with a narrow gap may suffer from violent free-surface resonance, which would cause green water on deck, dramatically raise hydrodynamic loads on structures and seriously threaten the operation safety. The CFD-based open-sourced software, OpenFOAM®, is employed to simulate the two-dimensional fluid resonance inside a narrow gap between a fixed box and a vertical wall induced by regular waves with different wave heights. The topographies with various plane slopes are placed in front of the wall. The focus of this article is on the influences of the incident wave height and the topographic slope on the nonlinear characteristics of various hydrodynamic parameters (including the wave height in the gap, the vertical wave force, and the horizontal wave force on the box) during gap resonance. The ratios of their high-order to the corresponding 1st-order components under different sets of the incident wave height and the topographic slope are analyzed. It is found that the relative importance of all the high-order components increases gradually with the incident wave height for all the three parameters. The topographic influence on them closely depends on the type of the parameters and the incident wave height. In addition, the occurrence of the 2nd-order gap resonance phenomenon can cause the 2nd-order wave height and horizontal force to be significantly larger than the corresponding 1st-order components.
Two marine structures arranged side by side with a narrow gap may suffer from violent free-surface resonance, which would cause green water on deck, dramatically raise hydrodynamic loads on structures and seriously threaten the operation safety. The CFD-based open-sourced software, OpenFOAM®, is employed to simulate the two-dimensional fluid resonance inside a narrow gap between a fixed box and a vertical wall induced by regular waves with different wave heights. The topographies with various plane slopes are placed in front of the wall. The focus of this article is on the influences of the incident wave height and the topographic slope on the nonlinear characteristics of various hydrodynamic parameters (including the wave height in the gap, the vertical wave force, and the horizontal wave force on the box) during gap resonance. The ratios of their high-order to the corresponding 1st-order components under different sets of the incident wave height and the topographic slope are analyzed. It is found that the relative importance of all the high-order components increases gradually with the incident wave height for all the three parameters. The topographic influence on them closely depends on the type of the parameters and the incident wave height. In addition, the occurrence of the 2nd-order gap resonance phenomenon can cause the 2nd-order wave height and horizontal force to be significantly larger than the corresponding 1st-order components.
2021, 35(5): 724-735.
doi: 10.1007/s13344-021-0064-6
Abstract:
As a type of autonomous underwater vehicle (AUV), underwater gliders (UG) are getting increasing attention in ocean exploration. To save energy and satisfy the mission requirements of a longer voyage, shape optimization for UGs has become a key technique and research focus. In this paper, a conventional UG, including its fuselage and hydrofoil, is optimized, which aims to decrease the average resistance in one motion cycle. To operate the optimization progress for the complex object, multiple free form deformation (FFD) volumes are established for geometric parameterization. High-fidelity simulation models are employed for objective function evaluation and gradients calculation. And sequential quadratic programming (SQP) method is adopted as an optimization algorithm. The optimization results show that there exists a UG with symmetrical and non-horizontal hydrofoils that has lower resistance.
As a type of autonomous underwater vehicle (AUV), underwater gliders (UG) are getting increasing attention in ocean exploration. To save energy and satisfy the mission requirements of a longer voyage, shape optimization for UGs has become a key technique and research focus. In this paper, a conventional UG, including its fuselage and hydrofoil, is optimized, which aims to decrease the average resistance in one motion cycle. To operate the optimization progress for the complex object, multiple free form deformation (FFD) volumes are established for geometric parameterization. High-fidelity simulation models are employed for objective function evaluation and gradients calculation. And sequential quadratic programming (SQP) method is adopted as an optimization algorithm. The optimization results show that there exists a UG with symmetrical and non-horizontal hydrofoils that has lower resistance.
2021, 35(5): 736-749.
doi: 10.1007/s13344-021-0065-5
Abstract:
Layered soil profiles create challenges for foundation installation and detrimentally affect the foundation performance. This research explored the free-fall penetration behavior of a new dynamically installed plate anchor, the Flying Wing Anchor®, in layered soil profiles. This new concept anchor combines the advantages of low-cost installation of torpedo piles and high efficiency of plate anchors. Anchor is initially installed through free-fall like a torpedo pile, and followed by drag embedment like a plate anchor. The methodology is to perform free-fall penetration tests with a model anchor in a variety of test beds containing marine clays with different profiles of undrained shear strength versus depth. A calibrated prediction model accounting for the effects of strain-rate and stiff layer produces results similar to those from the model test. The design curves were developed based on the calibrated analytical model, and are valuable to estimate the impact velocity thresholds of prototype anchor to penetrate through stiff layers. The free-fall penetration tests indicated that the penetration ability of FWA® increases with the increased impact velocity. This new dynamically embedded plate anchor can penetrate through the stiff layers that would cause problems for the conventional plate anchor, such as the drag embedded anchor, plowing on the top of stiff layer instead of breaking into it. Therefore, the new dynamically embedded plate anchor can provide a possible solution for layered soil profiles in deep water.
Layered soil profiles create challenges for foundation installation and detrimentally affect the foundation performance. This research explored the free-fall penetration behavior of a new dynamically installed plate anchor, the Flying Wing Anchor®, in layered soil profiles. This new concept anchor combines the advantages of low-cost installation of torpedo piles and high efficiency of plate anchors. Anchor is initially installed through free-fall like a torpedo pile, and followed by drag embedment like a plate anchor. The methodology is to perform free-fall penetration tests with a model anchor in a variety of test beds containing marine clays with different profiles of undrained shear strength versus depth. A calibrated prediction model accounting for the effects of strain-rate and stiff layer produces results similar to those from the model test. The design curves were developed based on the calibrated analytical model, and are valuable to estimate the impact velocity thresholds of prototype anchor to penetrate through stiff layers. The free-fall penetration tests indicated that the penetration ability of FWA® increases with the increased impact velocity. This new dynamically embedded plate anchor can penetrate through the stiff layers that would cause problems for the conventional plate anchor, such as the drag embedded anchor, plowing on the top of stiff layer instead of breaking into it. Therefore, the new dynamically embedded plate anchor can provide a possible solution for layered soil profiles in deep water.
2021, 35(5): 750-758.
doi: 10.1007/s13344-021-0066-4
Abstract:
Marine current energy has been increasingly used because of its predictable higher power potential. Owing to the external disturbances of various flow velocity and the high nonlinear effects on the marine current turbine (MCT) system, the nonlinear controllers which rely on precise mathematical models show poor performance under a high level of parameters’ uncertainties. This paper proposes an adaptive single neural control (ASNC) strategy for variable step-size perturb and observe (P&O) maximum power point tracking (MPPT) control. Firstly, to automatically update the neuron weights of SNC for the nonlinear systems, an adaptive mechanism is proposed to adaptively adjust the weighting and learning coefficients. Secondly, aiming to generate the exact reference speed for ASNC to extract the maximum power, a variable step-size law based on speed increment is designed to strike a balance between tracking speed and accuracy of P&O MPPT. The robust stability of the MCT control system is guaranteed by the Lyapunov theorem. Comparative simulation results show that this strategy has favorable adaptive performance under variable velocity conditions, and the MCT system operates at maximum power point steadily.
Marine current energy has been increasingly used because of its predictable higher power potential. Owing to the external disturbances of various flow velocity and the high nonlinear effects on the marine current turbine (MCT) system, the nonlinear controllers which rely on precise mathematical models show poor performance under a high level of parameters’ uncertainties. This paper proposes an adaptive single neural control (ASNC) strategy for variable step-size perturb and observe (P&O) maximum power point tracking (MPPT) control. Firstly, to automatically update the neuron weights of SNC for the nonlinear systems, an adaptive mechanism is proposed to adaptively adjust the weighting and learning coefficients. Secondly, aiming to generate the exact reference speed for ASNC to extract the maximum power, a variable step-size law based on speed increment is designed to strike a balance between tracking speed and accuracy of P&O MPPT. The robust stability of the MCT control system is guaranteed by the Lyapunov theorem. Comparative simulation results show that this strategy has favorable adaptive performance under variable velocity conditions, and the MCT system operates at maximum power point steadily.
2021, 35(5): 759-766.
doi: 10.1007/s13344-021-0067-3
Abstract:
Experimental studies were conducted in a super-large wave flume, aiming at uncovering the hydrodynamic characteristics involved in the turbulent wave boundary layer of full scale environment. An explicit formula of boundary layer thickness on rough turbulent flow was presented based on the measured velocity data of the present study and collected experimental data on wave boundary layer. It was found that the bottom wave-associated nominal stresses under the conditions of prototype scale tests suppress the vertical turbulence scattering upward over the boundary layer, which accounts for thickening of the boundary layer under wave condition. Such effect has yet not been reported in the literatures using oscillatory U-tube or small-sized wave flume. The phase inconsistency in the turbulent boundary layer to the free stream velocity (velocity immediately outside the boundary layer) is within 15°, which is remarkably smaller than the results from oscillatory U-tubes, as well as the larger wave flume experiment presented byXie et al. (2021) , showing that the coarser bed would further reduce the phase lead. The intensity of the vertical turbulent component is approximately 1/2 of the horizontal component, which has larger ratio compared with the value of 1/5 reported by previous studies. Especially, it was also found that the vertical turbulent energy was approximately 3/4 of the turbulent energy in spanwise directions (y-direction). This means that the turbulent fluctuation has similar order in all three-directions (x, y, z) in a full scale environment and highlights that the turbulent components in all the three directions should not be neglected when calculating the total turbulent energy.
Experimental studies were conducted in a super-large wave flume, aiming at uncovering the hydrodynamic characteristics involved in the turbulent wave boundary layer of full scale environment. An explicit formula of boundary layer thickness on rough turbulent flow was presented based on the measured velocity data of the present study and collected experimental data on wave boundary layer. It was found that the bottom wave-associated nominal stresses under the conditions of prototype scale tests suppress the vertical turbulence scattering upward over the boundary layer, which accounts for thickening of the boundary layer under wave condition. Such effect has yet not been reported in the literatures using oscillatory U-tube or small-sized wave flume. The phase inconsistency in the turbulent boundary layer to the free stream velocity (velocity immediately outside the boundary layer) is within 15°, which is remarkably smaller than the results from oscillatory U-tubes, as well as the larger wave flume experiment presented by
Numerical Study on Scale Effect of Form Factor for DTMB5415, KCS, KVLCC2, and 4000TEU Container Ship
2021, 35(5): 767-778.
doi: 10.1007/s13344-021-0068-2
Abstract:
Four ships, a twin-propeller naval ship, two single-propeller container ships, and a single-propeller very large crude carrier (VLCC), were studied to investigate the scale effect of the form factor. The viscous flow fields of the ships at different scales were solved numerically via the Reynolds-averaged Navier–Stokes method combined with the shear stress transport k–ω turbulence model. The numerical method was validated through comparisons with experimental data, and numerical uncertainty analysis was carried out based on the ITTC recommended procedure. On this basis, scale effects of the form factor were analyzed using different friction lines, and scale effects of flow fields and the mean axial wake fractions were further analyzed in details. The results showed that the form factor exhibited scale effects when adopting the ITTC-1957 line, and it increased with the increase in the Reynolds number. The scale effect of the form factor reduces the prediction precision of the full-scale ship resistance. The friction line has a significant effect on the form factor. The form factor exhibits little dependence on the Reynolds number when using the numerical friction line or the Katsui line, which is useful for full-scale ship resistance predictions. With the increasing Reynolds number, the boundary layer thickness becomes thinner and the axial velocity contour contracts toward the center plane, and there is nearly a linear relationship between the reciprocal of mean axial wake fraction on propeller disc and Reynolds number in logarithmic scale for the three types of ship forms.
Four ships, a twin-propeller naval ship, two single-propeller container ships, and a single-propeller very large crude carrier (VLCC), were studied to investigate the scale effect of the form factor. The viscous flow fields of the ships at different scales were solved numerically via the Reynolds-averaged Navier–Stokes method combined with the shear stress transport k–ω turbulence model. The numerical method was validated through comparisons with experimental data, and numerical uncertainty analysis was carried out based on the ITTC recommended procedure. On this basis, scale effects of the form factor were analyzed using different friction lines, and scale effects of flow fields and the mean axial wake fractions were further analyzed in details. The results showed that the form factor exhibited scale effects when adopting the ITTC-1957 line, and it increased with the increase in the Reynolds number. The scale effect of the form factor reduces the prediction precision of the full-scale ship resistance. The friction line has a significant effect on the form factor. The form factor exhibits little dependence on the Reynolds number when using the numerical friction line or the Katsui line, which is useful for full-scale ship resistance predictions. With the increasing Reynolds number, the boundary layer thickness becomes thinner and the axial velocity contour contracts toward the center plane, and there is nearly a linear relationship between the reciprocal of mean axial wake fraction on propeller disc and Reynolds number in logarithmic scale for the three types of ship forms.
2021, 35(5): 779-788.
doi: 10.1007/s13344-021-0069-1
Abstract:
In the leg-lowering process, the offshore jack-up platform is in a floating state, and the spudcan may collide with the seabed due to the platform motion in waves, thereby threatening the safety and stability of the platform. This paper first analyzed the hydrodynamic response of a jack-up platform under different sea states. Then a finite-element model was established which considered the material and geometrical nonlinearity in the structure and the nonlinear interaction between soil and structure to investigate the collision response between the spudcan and seabed during lowering jack-up legs. The results show that the dynamic response of the platform decreases and gradually tends to be stable as the wave period increases. The motion response of the platform reach the largest value when the wave period is 8 s and the wave direction is 0°. The collision angle between the spudcan and seabed has a large influence on the collision response, while the velocity of leg-lowering has little influence on the collision response. The collision response is also significantly affected by soil conditions. For clay, the increase in undrained shear strength and Young’s modulus will increase the impact force.
In the leg-lowering process, the offshore jack-up platform is in a floating state, and the spudcan may collide with the seabed due to the platform motion in waves, thereby threatening the safety and stability of the platform. This paper first analyzed the hydrodynamic response of a jack-up platform under different sea states. Then a finite-element model was established which considered the material and geometrical nonlinearity in the structure and the nonlinear interaction between soil and structure to investigate the collision response between the spudcan and seabed during lowering jack-up legs. The results show that the dynamic response of the platform decreases and gradually tends to be stable as the wave period increases. The motion response of the platform reach the largest value when the wave period is 8 s and the wave direction is 0°. The collision angle between the spudcan and seabed has a large influence on the collision response, while the velocity of leg-lowering has little influence on the collision response. The collision response is also significantly affected by soil conditions. For clay, the increase in undrained shear strength and Young’s modulus will increase the impact force.
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