Hydrodynamic coupling between the ship and moonpool responses is investigated. Dedicated experime... more Hydrodynamic coupling between the ship and moonpool responses is investigated. Dedicated experiments are carried out in the Ocean Basin at Sintef Ocean. Three different moonpool sizes are studied, where the moonpool length is 1/20, 1/10 and 1/2 of the ship length. The two former are square moonpools, while the latter is a rectangular moonpool with a width 1/2 of the ship's beam. Numerical simulations are carried out using WAMIT. The coupling between the ship and moonpool is seen to be highly dependent on the volume of the moonpool relative to the submerged ship volume. WAMIT greatly over-predicts the moonpool responses in the proximity of resonance, which indicates that wave radiation damping is small, and that damping due to flow separation at sharp edges at the moonpool entrance is dominant. Two main nonlinear effects are observed; Swirling-type sloshing and secondary resonance. These nonlinear effects are excited in a range of wave periods where it is expected that a realistic sea environment will contain significant wave energy. This is most prominent in the largest moonpool. The piston mode shape in freely floating conditions is signficantly different relative to the one in forced heave.
The dynamic stability of a surface-piercing plate advancing with high forward speed in horizontal... more The dynamic stability of a surface-piercing plate advancing with high forward speed in horizontal plane is investigated in the scope of linear theory. The hydrodynamic forces on the plate in sway and yaw are presented in terms of frequency and forward speed dependent added mass and damping coefficients. Flow separation from the trailing edge of the plate is considered. A time domain boundary integral method using linear distribution of Rankine sources and dipoles on the plate, free surface and a vortex sheet is used to calculate these hydrodynamic coefficients numerically. Comparison between the current numerical results and previous numerical and experimental results are presented. Using linear dynamic stability analysis the influence of hydrodynamic coefficients on the plate’s stability is investigated as a simplified alternative to a semi-displacement vessel.
Calculation of wave drift forces in the context of mooring analysis is usually performed by poten... more Calculation of wave drift forces in the context of mooring analysis is usually performed by potential flow boundary element methods. For monohulls with large waterplane area, such as floating, production, storage and offloading vessels (FPSOs), the assumption is that heave and pitch damping is dominated by inviscid radiation of waves. The present study investigates the effects of heave and pitch wave frequency viscous damping on the horizontal wave drift forces of a FPSO. Experimental evidence indicates the presence of non-negligible viscous damping in case of moderate and severe seastates. Such observation is confirmed by computational fluid dynamic (CFD) simulations of forced harmonic motions with different periods and amplitudes. On a second step, a numerical analysis shows that the surge wave drift forces increase significantly at the low frequency range when realistic heave and pitch viscous damping is considered. The frequency range where standard potential flow calculations appear to underestimate wave drift forces coincides with the range where design seastates have most of the energy. Finally, the wave drift coefficients calculated with realistic additional viscous damping are compared with empirical coefficients identified from model test data. The identification procedure follows a second order signal analysis technique known as cross bi spectral analysis.
Modelling the fluid response in a gap using conventional frequency domain potential flow solvers ... more Modelling the fluid response in a gap using conventional frequency domain potential flow solvers is a challenge due to its resonance behavior and the importance of viscous effects. Damping panels, or lids, are usually used to represent the main viscous effects, when more direct method of calculation is not applicable due to limitations on time or computation resources. Although, it is known that the main contributor to the damping is the flow separation at the gap inlet, these methods usually relate the magnitude of the effective damping to the vertical fluid velocity at the surface. In the present study, the application of dipole damping surfaces, as introduced in potential flow code WAMIT, is studied for modelling the flow separation effects more consistently. The relative fluid velocity at the inlet is used to model the damping effect of flow separation. Simplified empirical models are proposed to estimate the magnitude of the damping. The experimental study of flow between two side by side barges by Molin et a. 2009 [1] has been selected to evaluate the present modelling approach. The numerical results show good agreement with the measurements while providing a more consistent basis to apply stochastic linearization of the quadratic damping due to flow separation at the gap inlet.
The Twenty-fifth International Ocean and Polar Engineering Conference, Jun 21, 2015
The roll damping of a conventional ship cross-section, equipped with bilge-keels, is studied usin... more The roll damping of a conventional ship cross-section, equipped with bilge-keels, is studied using an efficient, hybrid numerical code called PVC3D, based on Finite Volume Method, for different oscillation periods and amplitudes. It is shown that the linear free-surface boundary condition can be adopted. However, the exact body boundary condition is necessary in order to capture the correct flow separation behavior, which requires mesh deformation techniques. The influence of the geometrical parameters such as bilge-keel height, center of rotation, and section's draft on roll damping is investigated. The normal forces and bending moments acting on the bilge-keels are presented. Using the developed efficient tool, parameter studies are made possible which help to form a better understanding of geometrical characteristics influence on roll damping. With more validation studies, PVC3D can be developed into a reliable and efficient design tool.
Estimating moonpool piston mode response at resonance is important for operation safety. This is ... more Estimating moonpool piston mode response at resonance is important for operation safety. This is a difficult task, in particular, due to nonlinear nature of the moonpool response connected to the damping imposed by the flow separation at the moonpool's inlet. In the present work, the applicability of a simplified model, based on decomposing the problem into potential and viscous components, is investigated. The moonpool piston mode response is modeled as an additional degree of freedom. The coupling terms between this new degree of freedom and other vessel's modes of motions are calculated based on potential flow calculations. Radiation and diffraction problems are considered separately. A finite volume solver with linearized boundary conditions is used to obtain the moonpool response under forced vertical motions. A quadratic damping model is fitted to the obtained responses and added to the free-surface condition of the potential flow formulation. The problem is solved both in frequency and time-domain. The validity of the obtained model is investigated by model test comparison for a dummy vessel with moonpool undergoing regular and irregular forced oscillations, as well as an offshore operation vessel with moonpool exposed to irregular waves. The benefits and shortcomings of the model are discussed. It is suggested that this method can be used as a practical tool to address moonpool piston mode response in irregular waves.
The piston and first sloshing modes of two-dimensional moonpools with recess are investigated. De... more The piston and first sloshing modes of two-dimensional moonpools with recess are investigated. Dedicated forced heave experiments are carried out. Di↵erent recess lengths are tested from 1/4 to 1/2 of the length of the moonpool at the mean waterline. A theoretical model to calculate the natural frequencies is developed based on linearized potential flow theory and eigen-function expansion. Two numerical methods are implemented; a Boundary Element Method (BEM) and a Navier-Stokes solver (CFD). Both the BEM and CFD have linearized free-surface and body-boundary conditions. As expected, the BEM over-predicts the moonpool response significantly, in particular at the first sloshing mode. The CFD is in general able to predict the maximum moonpool response adequately, both at the piston and first sloshing modes. Both numerical methods fail to predict the Du ng-type behaviour at the first sloshing mode, due to the linearized free-surface conditions. The Du ng behaviour is more pronounced for the largest recess. The main source of damping in the proximity of the first sloshing mode is discussed.
In linear Rankine panel method, the discrete linear dispersion relation is solved on a discrete f... more In linear Rankine panel method, the discrete linear dispersion relation is solved on a discrete free-surface to capture the free-surface waves generated due to wave-body interactions. Discretization introduces numerical damping and dispersion, which depend on the discretization order and the chosen methods for differentiation in time and space. The numerical properties of a linear Rankine panel method, based on a direct boundary integral formulation, for capturing two and three dimensional freesurface waves were studied. Different discretization orders and differentiation methods were considered, focusing on the linear distribution and finite difference schemes. The possible sources for numerical instabilities were addressed. A series of cases with and without forward speed was selected, and numerical investigations are presented. For the waves in three dimensions, the influence of the panels' aspect ratio and the waves' angle were considered. It has been shown that using the cancellation effects of different differentiation schemes the accuracy of the numerical method could be improved.
An engineering model to estimate and incorporate quadratic damping of the piston-mode moonpool re... more An engineering model to estimate and incorporate quadratic damping of the piston-mode moonpool responses in the proximity of the piston mode period is proposed. The model provides a physical-based equivalent linearized damping coefficient. The method is not limited to forced motion, but applicable to freely floating moonpool vessels. Further, it is not limited to moonopools, but can be generalized to gap resonance problems, such as side-by-side operations. The soundness of the proposed physical-based method is demonstrated using the panel code WAMIT with a linear damping term in the free-surface boundary condition inside the moonpool using two existing moonpool experiments as case studies; (1) a two-dimensional rectangular box with a moonpool subject to forced heave, and (2) a freely floating offshore vessel in incident waves. The WAMIT computations using the proposed method reconstructs the experimentally obtained piston-mode and vessel responses well. We suggest that the proposed method can be used with fair degree of confidence in an early design or operational analysis phase, in the (often) case that the quadratic damping is not known from either experiments or CFD. To our knowledge, this is the first general, physical-based piston-mode damping model that does not require any tuning from experiments. NOMENCLATURE B Beam of vessel mid-ships. ε Fluid damping coefficient. ε m Fluid damping coefficient for moonpools. η ia Motion amplitude in i-direction. D Draft. FDS Fluid damping surface. ω Circular frequency. g Gravitational constant. H Incident wave height. K Pressure drop coefficient. κ Scale factor. L Length of vessel.
This report presents results from hydrodynamic analysis of interaction between glacial ice and se... more This report presents results from hydrodynamic analysis of interaction between glacial ice and semisubmersible drilling units, as part of the Petroleum Safety Authority's project "NORD ST20 Loads, design and operation of floaters in the northern area". The overall objective of the study has been to study the hydrodynamic behaviour of smaller masses of glacial ice (growlers or bergy bits) exposed to environmental loads, and to assess the hydrodynamic interaction and possibility of collision with a semisubmersible drilling unit. Comprehensive numerical analysis of ice mass response in waves have been performed, with focus on nonlinear Froude-Krylov forces and hydrodynamic interactions due to the near presence of a larger semisubmersible. Position of impact, impact velocity and impact energy have been estimated in cases when collision between the ice mass and semisubmersible occurred.OCE2018 A-116 - Loads, design and operation of floaters in the Arctic - Ptil – NORD ST20publishedVersio
Journal of offshore mechanics and Arctic engineering, Oct 5, 2015
The dynamic stability of a surface-piercing plate, advancing with high forward speed in the horiz... more The dynamic stability of a surface-piercing plate, advancing with high forward speed in the horizontal plane, is investigated in the scope of linear theory. The hydrodynamic forces on the plate in sway and yaw are presented in terms of frequency and forward speed-dependent added mass and damping coefficients. Flow separation from the trailing edge of the plate is considered. A time-domain boundary integral method using linear distribution of Rankine sources and dipoles on the plate, free surface, and vortex sheet is used to calculate these hydrodynamic coefficients numerically. Comparison between the current numerical results and previous numerical and experimental results is presented. Using linear dynamic stability analysis, the influence of hydrodynamic coefficients on the plate's stability is investigated as a simplified alternative to a semidisplacement vessel.
Coupling between moonpool resonance and vessel motion is investigated in two-dimensional and quas... more Coupling between moonpool resonance and vessel motion is investigated in two-dimensional and quasi three-dimensional settings, where the models are studied in forced heave and in freely floating conditions. The two-dimensional setups are with a recess, while the quasi three-dimensional setups are without recess. One configuration with recess is presented for the two-dimensional case, while three different moonpool sizes (without recess) are tested for the quasi three-dimensional setup. A large number of forcing periods, and three wave steepnesses are tested. Boundary Element Method (BEM) and Viscous BEM (VBEM) time-domain codes based on linear potential flow theory, and a Navier–Stokes solver with linear free-surface and body-boundary conditions, are implemented to investigate resonant motion of the free-surface and the model. Damping due to flow separation from the sharp corners of the moonpool inlets is shown to matter for both vessel motions and moonpool response around the piston mode. In general, the CFD simulations compare well with the experimental results. BEM over-predicts the response significantly at resonance. VBEM provides improved results compared to the BEM, but still over-predicts the response. In the two-dimensional study there are significant coupling effects between heave, pitch and moonpool responses. In the quasi three-dimensional tests, the coupling effect is reduced significantly as the moonpool dimensions relative to the displaced volume of the ship is reduced. The first sloshing mode is investigated in the two-dimensional case. The studies show that damping due to flow separation is dominant. The vessel motions are unaffected by the moonpool response around the first sloshing mode.
Journal of offshore mechanics and Arctic engineering, Oct 9, 2019
Moonpool resonance is investigated in a two-dimensional setting in terms of regular, forced heave... more Moonpool resonance is investigated in a two-dimensional setting in terms of regular, forced heave motions of a model with moonpool with different rectangular-shaped recess configurations. A recess is a reduced draft zone in the moonpool. Dedicated experiments were carried out. The model consisted of two boxes of 40 cm width each, with a distance of 20 cm between them. Recess configurations varying between 5 cm to 10 cm in length and 5 cm in height were tested. Different drafts were also tested. The free-surface elevation inside the moonpool was measured at eight locations. A large number of forcing periods, and five forcing amplitudes were tested. A time-domain Boundary Element Method (BEM) code based on linear potential flow theory was implemented to investigate the resonance periods, mode shapes as well as the moonpool response as predicted by (linear) potential flow theory. Dominant physical effects were discussed, in particular damping due to flow separation from the sharp corners of the moonpool inlet and recess. The effect of the recess on the pistonmode behavior is discussed. BEM simulations where the effect of flow separation is empirically modelled were also conducted. The non-dimensional moonpool response suggests strong viscous damping at piston-mode resonance. The viscous BEM simulations demonstrate improvement over inviscid BEM, although further improvement of the method is needed. The piston mode shapes are clearly different from the near flat free-surface elevation for a moonpool without recess, consistent with recently published theory.
Journal of marine science and technology, Sep 16, 2014
ABSTRACT Transverse force and yaw moment acting on a surface-piercing flat plate with yaw angle a... more ABSTRACT Transverse force and yaw moment acting on a surface-piercing flat plate with yaw angle and forward speed are studied. Following the Froude hypothesis, the problem is decomposed into a tail-separated forward flow and a bottom-tip-separated cross-flow parts. The objective is to quantify the role of different components in the resulted force and moment acting on the plate. A 3D potential flow code using distribution of Rankine sources and dipoles is used to solve the linear potential flow problem. The free-surface boundary condition is linearized based on the undisturbed flow velocity (Neumann–Kelvin linearization). The plate’s trailing-edge flow separation is modeled using a dipole distribution behind the plate on a linearized vortex sheet. The force and moment due to the cross-flow separation from the plate’s bottom tip are calculated by means of a slender body cross-flow method with a rigid free-surface boundary condition. Hence the free-surface effects are confined in the forward flow problem and neglected in the cross-flow problem. Simulations are carried out for different aspect ratios and drift angles. The plate’s thickness is taken into account. Convergence and sensitivity studies are performed carefully. Results are compared with previous experimental and numerical studies. The importance of the second-order forces and validity of the linear assumptions are touched upon. The overall agreement of the results is acceptable. The relative importance of the two viscous and potential components are studied. It is shown that the decomposed problem could follow the experimental results up to a relatively high Froude number.
Hydrodynamic coupling between the ship and moonpool responses is investigated. Dedicated experime... more Hydrodynamic coupling between the ship and moonpool responses is investigated. Dedicated experiments are carried out in the Ocean Basin at Sintef Ocean. Three different moonpool sizes are studied, where the moonpool length is 1/20, 1/10 and 1/2 of the ship length. The two former are square moonpools, while the latter is a rectangular moonpool with a width 1/2 of the ship's beam. Numerical simulations are carried out using WAMIT. The coupling between the ship and moonpool is seen to be highly dependent on the volume of the moonpool relative to the submerged ship volume. WAMIT greatly over-predicts the moonpool responses in the proximity of resonance, which indicates that wave radiation damping is small, and that damping due to flow separation at sharp edges at the moonpool entrance is dominant. Two main nonlinear effects are observed; Swirling-type sloshing and secondary resonance. These nonlinear effects are excited in a range of wave periods where it is expected that a realistic sea environment will contain significant wave energy. This is most prominent in the largest moonpool. The piston mode shape in freely floating conditions is signficantly different relative to the one in forced heave.
The dynamic stability of a surface-piercing plate advancing with high forward speed in horizontal... more The dynamic stability of a surface-piercing plate advancing with high forward speed in horizontal plane is investigated in the scope of linear theory. The hydrodynamic forces on the plate in sway and yaw are presented in terms of frequency and forward speed dependent added mass and damping coefficients. Flow separation from the trailing edge of the plate is considered. A time domain boundary integral method using linear distribution of Rankine sources and dipoles on the plate, free surface and a vortex sheet is used to calculate these hydrodynamic coefficients numerically. Comparison between the current numerical results and previous numerical and experimental results are presented. Using linear dynamic stability analysis the influence of hydrodynamic coefficients on the plate’s stability is investigated as a simplified alternative to a semi-displacement vessel.
Calculation of wave drift forces in the context of mooring analysis is usually performed by poten... more Calculation of wave drift forces in the context of mooring analysis is usually performed by potential flow boundary element methods. For monohulls with large waterplane area, such as floating, production, storage and offloading vessels (FPSOs), the assumption is that heave and pitch damping is dominated by inviscid radiation of waves. The present study investigates the effects of heave and pitch wave frequency viscous damping on the horizontal wave drift forces of a FPSO. Experimental evidence indicates the presence of non-negligible viscous damping in case of moderate and severe seastates. Such observation is confirmed by computational fluid dynamic (CFD) simulations of forced harmonic motions with different periods and amplitudes. On a second step, a numerical analysis shows that the surge wave drift forces increase significantly at the low frequency range when realistic heave and pitch viscous damping is considered. The frequency range where standard potential flow calculations appear to underestimate wave drift forces coincides with the range where design seastates have most of the energy. Finally, the wave drift coefficients calculated with realistic additional viscous damping are compared with empirical coefficients identified from model test data. The identification procedure follows a second order signal analysis technique known as cross bi spectral analysis.
Modelling the fluid response in a gap using conventional frequency domain potential flow solvers ... more Modelling the fluid response in a gap using conventional frequency domain potential flow solvers is a challenge due to its resonance behavior and the importance of viscous effects. Damping panels, or lids, are usually used to represent the main viscous effects, when more direct method of calculation is not applicable due to limitations on time or computation resources. Although, it is known that the main contributor to the damping is the flow separation at the gap inlet, these methods usually relate the magnitude of the effective damping to the vertical fluid velocity at the surface. In the present study, the application of dipole damping surfaces, as introduced in potential flow code WAMIT, is studied for modelling the flow separation effects more consistently. The relative fluid velocity at the inlet is used to model the damping effect of flow separation. Simplified empirical models are proposed to estimate the magnitude of the damping. The experimental study of flow between two side by side barges by Molin et a. 2009 [1] has been selected to evaluate the present modelling approach. The numerical results show good agreement with the measurements while providing a more consistent basis to apply stochastic linearization of the quadratic damping due to flow separation at the gap inlet.
The Twenty-fifth International Ocean and Polar Engineering Conference, Jun 21, 2015
The roll damping of a conventional ship cross-section, equipped with bilge-keels, is studied usin... more The roll damping of a conventional ship cross-section, equipped with bilge-keels, is studied using an efficient, hybrid numerical code called PVC3D, based on Finite Volume Method, for different oscillation periods and amplitudes. It is shown that the linear free-surface boundary condition can be adopted. However, the exact body boundary condition is necessary in order to capture the correct flow separation behavior, which requires mesh deformation techniques. The influence of the geometrical parameters such as bilge-keel height, center of rotation, and section's draft on roll damping is investigated. The normal forces and bending moments acting on the bilge-keels are presented. Using the developed efficient tool, parameter studies are made possible which help to form a better understanding of geometrical characteristics influence on roll damping. With more validation studies, PVC3D can be developed into a reliable and efficient design tool.
Estimating moonpool piston mode response at resonance is important for operation safety. This is ... more Estimating moonpool piston mode response at resonance is important for operation safety. This is a difficult task, in particular, due to nonlinear nature of the moonpool response connected to the damping imposed by the flow separation at the moonpool's inlet. In the present work, the applicability of a simplified model, based on decomposing the problem into potential and viscous components, is investigated. The moonpool piston mode response is modeled as an additional degree of freedom. The coupling terms between this new degree of freedom and other vessel's modes of motions are calculated based on potential flow calculations. Radiation and diffraction problems are considered separately. A finite volume solver with linearized boundary conditions is used to obtain the moonpool response under forced vertical motions. A quadratic damping model is fitted to the obtained responses and added to the free-surface condition of the potential flow formulation. The problem is solved both in frequency and time-domain. The validity of the obtained model is investigated by model test comparison for a dummy vessel with moonpool undergoing regular and irregular forced oscillations, as well as an offshore operation vessel with moonpool exposed to irregular waves. The benefits and shortcomings of the model are discussed. It is suggested that this method can be used as a practical tool to address moonpool piston mode response in irregular waves.
The piston and first sloshing modes of two-dimensional moonpools with recess are investigated. De... more The piston and first sloshing modes of two-dimensional moonpools with recess are investigated. Dedicated forced heave experiments are carried out. Di↵erent recess lengths are tested from 1/4 to 1/2 of the length of the moonpool at the mean waterline. A theoretical model to calculate the natural frequencies is developed based on linearized potential flow theory and eigen-function expansion. Two numerical methods are implemented; a Boundary Element Method (BEM) and a Navier-Stokes solver (CFD). Both the BEM and CFD have linearized free-surface and body-boundary conditions. As expected, the BEM over-predicts the moonpool response significantly, in particular at the first sloshing mode. The CFD is in general able to predict the maximum moonpool response adequately, both at the piston and first sloshing modes. Both numerical methods fail to predict the Du ng-type behaviour at the first sloshing mode, due to the linearized free-surface conditions. The Du ng behaviour is more pronounced for the largest recess. The main source of damping in the proximity of the first sloshing mode is discussed.
In linear Rankine panel method, the discrete linear dispersion relation is solved on a discrete f... more In linear Rankine panel method, the discrete linear dispersion relation is solved on a discrete free-surface to capture the free-surface waves generated due to wave-body interactions. Discretization introduces numerical damping and dispersion, which depend on the discretization order and the chosen methods for differentiation in time and space. The numerical properties of a linear Rankine panel method, based on a direct boundary integral formulation, for capturing two and three dimensional freesurface waves were studied. Different discretization orders and differentiation methods were considered, focusing on the linear distribution and finite difference schemes. The possible sources for numerical instabilities were addressed. A series of cases with and without forward speed was selected, and numerical investigations are presented. For the waves in three dimensions, the influence of the panels' aspect ratio and the waves' angle were considered. It has been shown that using the cancellation effects of different differentiation schemes the accuracy of the numerical method could be improved.
An engineering model to estimate and incorporate quadratic damping of the piston-mode moonpool re... more An engineering model to estimate and incorporate quadratic damping of the piston-mode moonpool responses in the proximity of the piston mode period is proposed. The model provides a physical-based equivalent linearized damping coefficient. The method is not limited to forced motion, but applicable to freely floating moonpool vessels. Further, it is not limited to moonopools, but can be generalized to gap resonance problems, such as side-by-side operations. The soundness of the proposed physical-based method is demonstrated using the panel code WAMIT with a linear damping term in the free-surface boundary condition inside the moonpool using two existing moonpool experiments as case studies; (1) a two-dimensional rectangular box with a moonpool subject to forced heave, and (2) a freely floating offshore vessel in incident waves. The WAMIT computations using the proposed method reconstructs the experimentally obtained piston-mode and vessel responses well. We suggest that the proposed method can be used with fair degree of confidence in an early design or operational analysis phase, in the (often) case that the quadratic damping is not known from either experiments or CFD. To our knowledge, this is the first general, physical-based piston-mode damping model that does not require any tuning from experiments. NOMENCLATURE B Beam of vessel mid-ships. ε Fluid damping coefficient. ε m Fluid damping coefficient for moonpools. η ia Motion amplitude in i-direction. D Draft. FDS Fluid damping surface. ω Circular frequency. g Gravitational constant. H Incident wave height. K Pressure drop coefficient. κ Scale factor. L Length of vessel.
This report presents results from hydrodynamic analysis of interaction between glacial ice and se... more This report presents results from hydrodynamic analysis of interaction between glacial ice and semisubmersible drilling units, as part of the Petroleum Safety Authority's project "NORD ST20 Loads, design and operation of floaters in the northern area". The overall objective of the study has been to study the hydrodynamic behaviour of smaller masses of glacial ice (growlers or bergy bits) exposed to environmental loads, and to assess the hydrodynamic interaction and possibility of collision with a semisubmersible drilling unit. Comprehensive numerical analysis of ice mass response in waves have been performed, with focus on nonlinear Froude-Krylov forces and hydrodynamic interactions due to the near presence of a larger semisubmersible. Position of impact, impact velocity and impact energy have been estimated in cases when collision between the ice mass and semisubmersible occurred.OCE2018 A-116 - Loads, design and operation of floaters in the Arctic - Ptil – NORD ST20publishedVersio
Journal of offshore mechanics and Arctic engineering, Oct 5, 2015
The dynamic stability of a surface-piercing plate, advancing with high forward speed in the horiz... more The dynamic stability of a surface-piercing plate, advancing with high forward speed in the horizontal plane, is investigated in the scope of linear theory. The hydrodynamic forces on the plate in sway and yaw are presented in terms of frequency and forward speed-dependent added mass and damping coefficients. Flow separation from the trailing edge of the plate is considered. A time-domain boundary integral method using linear distribution of Rankine sources and dipoles on the plate, free surface, and vortex sheet is used to calculate these hydrodynamic coefficients numerically. Comparison between the current numerical results and previous numerical and experimental results is presented. Using linear dynamic stability analysis, the influence of hydrodynamic coefficients on the plate's stability is investigated as a simplified alternative to a semidisplacement vessel.
Coupling between moonpool resonance and vessel motion is investigated in two-dimensional and quas... more Coupling between moonpool resonance and vessel motion is investigated in two-dimensional and quasi three-dimensional settings, where the models are studied in forced heave and in freely floating conditions. The two-dimensional setups are with a recess, while the quasi three-dimensional setups are without recess. One configuration with recess is presented for the two-dimensional case, while three different moonpool sizes (without recess) are tested for the quasi three-dimensional setup. A large number of forcing periods, and three wave steepnesses are tested. Boundary Element Method (BEM) and Viscous BEM (VBEM) time-domain codes based on linear potential flow theory, and a Navier–Stokes solver with linear free-surface and body-boundary conditions, are implemented to investigate resonant motion of the free-surface and the model. Damping due to flow separation from the sharp corners of the moonpool inlets is shown to matter for both vessel motions and moonpool response around the piston mode. In general, the CFD simulations compare well with the experimental results. BEM over-predicts the response significantly at resonance. VBEM provides improved results compared to the BEM, but still over-predicts the response. In the two-dimensional study there are significant coupling effects between heave, pitch and moonpool responses. In the quasi three-dimensional tests, the coupling effect is reduced significantly as the moonpool dimensions relative to the displaced volume of the ship is reduced. The first sloshing mode is investigated in the two-dimensional case. The studies show that damping due to flow separation is dominant. The vessel motions are unaffected by the moonpool response around the first sloshing mode.
Journal of offshore mechanics and Arctic engineering, Oct 9, 2019
Moonpool resonance is investigated in a two-dimensional setting in terms of regular, forced heave... more Moonpool resonance is investigated in a two-dimensional setting in terms of regular, forced heave motions of a model with moonpool with different rectangular-shaped recess configurations. A recess is a reduced draft zone in the moonpool. Dedicated experiments were carried out. The model consisted of two boxes of 40 cm width each, with a distance of 20 cm between them. Recess configurations varying between 5 cm to 10 cm in length and 5 cm in height were tested. Different drafts were also tested. The free-surface elevation inside the moonpool was measured at eight locations. A large number of forcing periods, and five forcing amplitudes were tested. A time-domain Boundary Element Method (BEM) code based on linear potential flow theory was implemented to investigate the resonance periods, mode shapes as well as the moonpool response as predicted by (linear) potential flow theory. Dominant physical effects were discussed, in particular damping due to flow separation from the sharp corners of the moonpool inlet and recess. The effect of the recess on the pistonmode behavior is discussed. BEM simulations where the effect of flow separation is empirically modelled were also conducted. The non-dimensional moonpool response suggests strong viscous damping at piston-mode resonance. The viscous BEM simulations demonstrate improvement over inviscid BEM, although further improvement of the method is needed. The piston mode shapes are clearly different from the near flat free-surface elevation for a moonpool without recess, consistent with recently published theory.
Journal of marine science and technology, Sep 16, 2014
ABSTRACT Transverse force and yaw moment acting on a surface-piercing flat plate with yaw angle a... more ABSTRACT Transverse force and yaw moment acting on a surface-piercing flat plate with yaw angle and forward speed are studied. Following the Froude hypothesis, the problem is decomposed into a tail-separated forward flow and a bottom-tip-separated cross-flow parts. The objective is to quantify the role of different components in the resulted force and moment acting on the plate. A 3D potential flow code using distribution of Rankine sources and dipoles is used to solve the linear potential flow problem. The free-surface boundary condition is linearized based on the undisturbed flow velocity (Neumann–Kelvin linearization). The plate’s trailing-edge flow separation is modeled using a dipole distribution behind the plate on a linearized vortex sheet. The force and moment due to the cross-flow separation from the plate’s bottom tip are calculated by means of a slender body cross-flow method with a rigid free-surface boundary condition. Hence the free-surface effects are confined in the forward flow problem and neglected in the cross-flow problem. Simulations are carried out for different aspect ratios and drift angles. The plate’s thickness is taken into account. Convergence and sensitivity studies are performed carefully. Results are compared with previous experimental and numerical studies. The importance of the second-order forces and validity of the linear assumptions are touched upon. The overall agreement of the results is acceptable. The relative importance of the two viscous and potential components are studied. It is shown that the decomposed problem could follow the experimental results up to a relatively high Froude number.
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Papers by Babak Ommani