The Experts below are selected from a list of 10662 Experts worldwide ranked by ideXlab platform
Damien Holloway - One of the best experts on this subject based on the ideXlab platform.
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A high Froude number time-domain Strip Theory for ship motion predictions in irregular waves
2012Co-Authors: Benjamin French, Giles Thomas, Mike Thomas Davis, Damien HollowayAbstract:The prediction of ship motion characteristics early in the design stage in realistic sea conditions are of vital importance for the ship designer. Strip theories are commonly used for this purpose as they are fast and inexpensive. In this paper, an existing two-dimensional time-domain Strip Theory optimised for multi-hull vessels travelling at high Froude numbers is extended to predict motions in an irregular seaway. The encountered wave environment is represented by the superposition of regular sinusoidal waves. A method for decomposing idealised sea spectra into component regular waves of varying frequencies and constant amplitudes is presented. Ship motion predictions in irregular waves are then verified by conducting a spectral analysis on the motions and wave environment and comparing with motion predictions in regular waves. A method of ensemble averaging of spectra over a series of runs is adopted to reduce spectral variance. The extended seakeeping method is then validated by comparing predicted motions of a large high-speed catamaran in irregular seas with scale model results from towing tank experiments.
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Motions of Catamarans in Oblique Seas at Speed
International Journal of Maritime Engineering, 2009Co-Authors: Giles Thomas, Damien Holloway, Mike Thomas Davis, Tim Lilienthal, T Magnuson, Lawrence J. Doctors, P CouserAbstract:Motions, particularly roll, may have significant implications for passenger comfort when operating a catamaran in oblique seas. Designers of high-speed catamarans therefore need tools for predicting a vessel's seakeeping performance. This work presents three numerical methods for predicting vessel motions in oblique seas: a boundary-element method combined with a Strip-Theory approach modified for multihulls, a two-dimensional Green function time-domain Strip-Theory and a modified Strip-Theory method. Experiments to measure the motion response of a catamaran in oblique seas were conducted using an NPL 5b model at three speeds (Fn = 0.203, 0.406 and 0.508). The experiments showed that all motions tended to increase with increasing speed. Generally the boundary-element method and modified Strip-Theory method correctly predicted responses in all of the experimental configurations. The two-dimensional Green function time-domain Strip-Theory was used to predict the motions at the two higher speeds only and was also found, in general, to predict the motions satisfactorily.
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Ship Motion Computations Using a High Froude Number Time Domain Strip Theory
Journal of Ship Research, 2006Co-Authors: Damien Holloway, Michael R DavisAbstract:High-speed Strip theories are discussed, and a time domain formulation making use of a fixed reference frame for the two-dimensional fluid motion is described in detail. This, and classical (low-speed) Strip Theory, are compared with the experimental results of Wellicome et al. (1995) up to a Froude number of 0.8, as well as with our own test data for a semi-SWATH, demonstrating the marked improvement of the predictions of the former at high speeds, while the need to account for modest viscous effects at these speeds is also argued. A significant contribution to time domain computations is a method of stabilizing the integration of the ship's equations of motion, which are inherently unstable due to feedback from implicit added mass components of the hydrodynamic force. The time domain high-speed Theory is recommended as a practical alternative to three-dimensional methods. It also facilitates the investigation of large-amplitude motions with stern or bow emergence and forms a simulation base for the investigation of ride control systems and local or global loads.
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Added mass of whipping modes for ships at high Froude number by a free surface boundary element method coupled with Strip Theory
Anziam Journal, 2004Co-Authors: Damien Holloway, Giles Thomas, Michael R DavisAbstract:Accurate prediction of the whipping response of a ship's structure following a wave impact is fundamental to both the prediction of instantaneous local stresses and global fatigue life assessment. In particular the added mass effect of the surrounding water has a profound effect on the modal frequencies. ``Strip Theory'', routinely used for analysis of rigid body motions of ships in waves, is extended in this paper to include ship flexure. Moreover, the theoretical foundation of the method is discussed and it is shown that, although the Theory becomes invalid for rigid body motions of high-speed vessels, the ship flexure problem is an ideal application of the Theory. The associated two-dimensional free surface gravity wave problem is solved using a boundary element method based on wave functions given by Wehausen and Laitone (1960), which is also described. Results are validated against a fully three-dimensional solution, and incorporation of the added mass into a finite element model is shown to give excellent agreement with full scale measurements.
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Application of Two Dimensional Boundary Element Methods to Ship Motion Predictions
Computational Fluid Dynamics 2002, 2003Co-Authors: Damien Holloway, Michael R DavisAbstract:This paper outlines and compares two boundary element solutions for the two dimensional hydrodynamic problem of floating ship-like sections, one for periodic motion, and one for arbitrary motion. These solutions are suitable for use in ‘Strip Theory’ ship motion predictions. For traditional Strip Theory, a periodic 2D solutions is required, while for a high speed Strip Theory developed by the authors a boundary element solution capable of describing arbitrary 2D motion beneath a free surface is required. The boundary element solutions make use of source functions that automatically satisfy the appropriate free surface boundary condition.
Owen Hughes - One of the best experts on this subject based on the ideXlab platform.
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a practical method to apply hull girder sectional loads to full ship 3d finite element models using quadratic programming
Ships and Offshore Structures, 2014Co-Authors: Chengbi Zhao, Owen HughesAbstract:Interest in the seakeeping loads of vessels has increased dramatically in recent years. While many studies focused on predicting seakeeping loads, little attention was given on how loads are transferred to 3D finite-element models. In current design practice, methods for predicting seakeeping motions and loads are mainly based on the potential flow Theory, either Strip Theory methods or 3D-panel methods. Methods based on Strip Theory provide reasonable motion prediction for ships and are computationally efficient. However, the load outputs of Strip theories are only hull girder sectional forces and moments, such as vertical bending moment and vertical shear force, which cannot be directly applied to a 3D finite-element structural model. Methods-based 3D panel methods can be applied to a wide range of structures, but are computationally expensive. The seakeeping load outputs of panel methods include not only the global hull girder loads, but also panel pressures, which are well suited for 3D finite-element...
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applying Strip Theory based linear seakeeping loads to 3d full ship finite element models
ASME 2013 32nd International Conference on Ocean Offshore and Arctic Engineering, 2013Co-Authors: Chengbi Zhao, Owen HughesAbstract:Panel based hydrodynamic analyses are well suited for transferring seakeeping loads to 3D FEM structural models. However, 3D panel based hydrodynamic analyses are computationally expensive. For monohull ships, methods based on Strip Theory have been successfully used in industry for many years. They are computationally efficient, and they provide good prediction for motions and hull girder loads. However, many Strip Theory methods provide only hull girder sectional forces and moments, such as vertical bending moment and vertical shear force, which are difficult to apply to 3D finite element structural models. For the few codes which do output panel pressure, transferring the pressure map from a hydrodynamic model to the corresponding 3D finite element model often results in an unbalanced structural model because of the pressure interpolation discrepancy. To obtain equilibrium of an imbalanced structural model, a common practice is to use the “inertia relief” approach to rebalance the model. However, this type of balancing causes a change in the hull girder load distribution, which in turn could cause inaccuracies in an extreme load analysis (ELA) and a spectral fatigue analysis (SFA). This paper presents a method of applying Strip Theory based linear seakeeping pressure loads to balance 3D finite element models without using inertia relief. The velocity potential of Strip sections is first calculated based on hydrodynamic Strip theories. The velocity potential of a finite element panel is obtained from the interpolation of the velocity potential of the Strip sections. The potential derivative along x-direction is computed using the approach proposed by Salvesen, Tuck and Faltinsen (1972). The hydrodynamic forces and moments are computed using direct panel pressure integration from the finite element structural panel. For forces and moments which cannot be directly converted from pressure, such as hydrostatic restoring force and diffraction force, element nodal forces are generated using Quadratic Programing. The equations of motions are then formulated based on the finite element wetted panels. The method results in a perfectly balanced structural model. An example is given to compare the “ordinary Strip Theory” to the proposed direct pressure integration method. The accuracy proves the validity of this new method.
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applying sectional seakeeping loads to full ship structural models using quadratic programming
2012Co-Authors: Owen Hughes, Chengbi ZhaoAbstract:Interest in the seakeeping loads of vessels has increased dramatically in recent years. In current design practice, methods for predicting seakeeping motions and loads are mainly in two categories, Strip Theory methods and 3D- panel methods. Methods based on Strip Theory provide reasonable motion prediction and are computationally efficient. However, many Strip Theory methods provide only hull girder sectional forces and moments, such as vertical bending moment and vertical shear force, which cannot be directly applied to a 3D finite element structural model. For panel based methods, the outputs include not only the global hull girder loads, but also panel pressures, which are well suited for 3D finite element analysis. However, because the panel based methods are computationally expensive, meshes used for hydrodynamic analyses are usually coarser than the mesh used for structural finite element analyses. Consequently, the panel pressure calculated from a hydrodynamic model mesh has to be transferred to the structural model mesh. The resulting discrepancy of the pressure map, regardless of what interpolation method is used, causes an imbalanced structural model. To obtain equilibrium of an imbalanced structural model, a common practice is to use the "inertia relief" approach (1) . However, this type of balancing causes a change in the hull girder load distribution, which in turn could cause inaccuracies in an extreme load analysis (ELA) and a spectral fatigue analysis (SFA). This paper presents a method to balance the structural model without using inertia relief. The method uses quadratic programming to calculate corrective nodal forces such that the resulting hull girder sectional loads match those calculated by seakeeping analyses, either by Strip Theory methods or 3D- panel methods. To validate the method, a 3D panel linear code, MAESTRO-Wave (2) , was used to generate both panel pressures and sectional loads. A model is first loaded by a 3D-panel pressure distribution with a perfect equilibrium. The model is then loaded with only the accelerations and sectional forces and moments. The sectional forces and moments are converted to finite element nodal forces using the proposed quadratic programming method. For these two load cases, the paper compares the hull girder loads, the hull deflection and the stresses, and the accuracy proves the validity of this new method.
Michael R Davis - One of the best experts on this subject based on the ideXlab platform.
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Ship Motion Computations Using a High Froude Number Time Domain Strip Theory
Journal of Ship Research, 2006Co-Authors: Damien Holloway, Michael R DavisAbstract:High-speed Strip theories are discussed, and a time domain formulation making use of a fixed reference frame for the two-dimensional fluid motion is described in detail. This, and classical (low-speed) Strip Theory, are compared with the experimental results of Wellicome et al. (1995) up to a Froude number of 0.8, as well as with our own test data for a semi-SWATH, demonstrating the marked improvement of the predictions of the former at high speeds, while the need to account for modest viscous effects at these speeds is also argued. A significant contribution to time domain computations is a method of stabilizing the integration of the ship's equations of motion, which are inherently unstable due to feedback from implicit added mass components of the hydrodynamic force. The time domain high-speed Theory is recommended as a practical alternative to three-dimensional methods. It also facilitates the investigation of large-amplitude motions with stern or bow emergence and forms a simulation base for the investigation of ride control systems and local or global loads.
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Added mass of whipping modes for ships at high Froude number by a free surface boundary element method coupled with Strip Theory
Anziam Journal, 2004Co-Authors: Damien Holloway, Giles Thomas, Michael R DavisAbstract:Accurate prediction of the whipping response of a ship's structure following a wave impact is fundamental to both the prediction of instantaneous local stresses and global fatigue life assessment. In particular the added mass effect of the surrounding water has a profound effect on the modal frequencies. ``Strip Theory'', routinely used for analysis of rigid body motions of ships in waves, is extended in this paper to include ship flexure. Moreover, the theoretical foundation of the method is discussed and it is shown that, although the Theory becomes invalid for rigid body motions of high-speed vessels, the ship flexure problem is an ideal application of the Theory. The associated two-dimensional free surface gravity wave problem is solved using a boundary element method based on wave functions given by Wehausen and Laitone (1960), which is also described. Results are validated against a fully three-dimensional solution, and incorporation of the added mass into a finite element model is shown to give excellent agreement with full scale measurements.
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Application of Two Dimensional Boundary Element Methods to Ship Motion Predictions
Computational Fluid Dynamics 2002, 2003Co-Authors: Damien Holloway, Michael R DavisAbstract:This paper outlines and compares two boundary element solutions for the two dimensional hydrodynamic problem of floating ship-like sections, one for periodic motion, and one for arbitrary motion. These solutions are suitable for use in ‘Strip Theory’ ship motion predictions. For traditional Strip Theory, a periodic 2D solutions is required, while for a high speed Strip Theory developed by the authors a boundary element solution capable of describing arbitrary 2D motion beneath a free surface is required. The boundary element solutions make use of source functions that automatically satisfy the appropriate free surface boundary condition.
J J Jensen - One of the best experts on this subject based on the ideXlab platform.
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Slamming Simulations in a Conditional Wave
Volume 4: Offshore Geotechnics; Ronald W. Yeung Honoring Symposium on Offshore and Ship Hydrodynamics, 2012Co-Authors: Sopheak Seng, J J JensenAbstract:A study of slamming events in conditional waves is presented in this paper. The ship is sailing in head sea and the motion is solved for under the assumption of rigid body motion constrained to two degree-of-freedom i.e. heave and pitch. Based on a time domain non-linear Strip Theory most probable conditional waves are generated to induce short term extreme responses of 4500 MNm sagging and hogging vertical bending moment (VBM) amidships on a modern 9,400-TEU post-Panamax container ship and 3000 MNm (sag) on a Panamax container ship. The results of the Strip Theory are compared to the results of free surface NS/VOF CFD simulations under the same wave conditions. In moderate seas and no occurrence of slamming the structural responses predicted by the methods agree well. When slamming occurs the Strip Theory overpredicts VBM but the peak values of VBM occurs at approximately the same time as predicted by the CFD method implying the possibility to use the more accurate CFD results to improve the estimation of slamming loads in the Strip Theory through a rational correction coefficient.Copyright © 2012 by ASME
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PREDICTION OF WAVE-INDUCED ROLLING RESPONSES BY A TIME-DOMAIN Strip Theory
2001Co-Authors: Z-h Wang, J J Jensen, J-z XiaAbstract:This paper presents a time-domain Strip Theory formulation for predicting wave loads and 5-DOF motions of a ship at constant forward speed and arbitrary wave heading. The hydrodynamic memory effect due to the free surface is approximated by a set of higher-order ordinary differential equations, which can automatically include the non-linear memory effect and the momentum-slamming force. The formulation is generalized from the time-domain Strip Theory for vertical wave loads and ship responses. Based on time-domain Strip Theory formulation, a linear method for predicting wave-induced anti-symmetric motions and loads of rigid-body ships is developed. Numerical calculations are presented for a panamax container ship for the roll motion and torsional moment and are compared with experimental results.
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Non-linear wave loads and ship responses by a time-domain Strip Theory
Marine Structures, 1998Co-Authors: Jinzhu Xia, Zhaohui Wang, J J JensenAbstract:Abstract A non-linear time-domain Strip Theory for vertical wave loads and ship responses is presented. The Theory is generalized from a rigorous linear time-domain Strip Theory representation. The hydrodynamic memory effect due to the free surface is approximated by a higher order differential equation. Based on this time-domain Strip Theory, an efficient non-linear hydroelastic method of wave- and slamming-induced vertical motions and structural responses of ships is developed, where the structure is represented as a Timoshenko beam. Numerical calculations are presented for the S175 Containership and are systematically compared with the experimental results given by Watanabe et al. (1989, J. Soc. Naval Architects Japan, 166) and O’Dea et al. (1992, Proc. 19th Symp. on Naval Hydrodynamics). The agreement between the present predictions and the experiments is very encouraging.
Chengbi Zhao - One of the best experts on this subject based on the ideXlab platform.
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a practical method to apply hull girder sectional loads to full ship 3d finite element models using quadratic programming
Ships and Offshore Structures, 2014Co-Authors: Chengbi Zhao, Owen HughesAbstract:Interest in the seakeeping loads of vessels has increased dramatically in recent years. While many studies focused on predicting seakeeping loads, little attention was given on how loads are transferred to 3D finite-element models. In current design practice, methods for predicting seakeeping motions and loads are mainly based on the potential flow Theory, either Strip Theory methods or 3D-panel methods. Methods based on Strip Theory provide reasonable motion prediction for ships and are computationally efficient. However, the load outputs of Strip theories are only hull girder sectional forces and moments, such as vertical bending moment and vertical shear force, which cannot be directly applied to a 3D finite-element structural model. Methods-based 3D panel methods can be applied to a wide range of structures, but are computationally expensive. The seakeeping load outputs of panel methods include not only the global hull girder loads, but also panel pressures, which are well suited for 3D finite-element...
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applying Strip Theory based linear seakeeping loads to 3d full ship finite element models
ASME 2013 32nd International Conference on Ocean Offshore and Arctic Engineering, 2013Co-Authors: Chengbi Zhao, Owen HughesAbstract:Panel based hydrodynamic analyses are well suited for transferring seakeeping loads to 3D FEM structural models. However, 3D panel based hydrodynamic analyses are computationally expensive. For monohull ships, methods based on Strip Theory have been successfully used in industry for many years. They are computationally efficient, and they provide good prediction for motions and hull girder loads. However, many Strip Theory methods provide only hull girder sectional forces and moments, such as vertical bending moment and vertical shear force, which are difficult to apply to 3D finite element structural models. For the few codes which do output panel pressure, transferring the pressure map from a hydrodynamic model to the corresponding 3D finite element model often results in an unbalanced structural model because of the pressure interpolation discrepancy. To obtain equilibrium of an imbalanced structural model, a common practice is to use the “inertia relief” approach to rebalance the model. However, this type of balancing causes a change in the hull girder load distribution, which in turn could cause inaccuracies in an extreme load analysis (ELA) and a spectral fatigue analysis (SFA). This paper presents a method of applying Strip Theory based linear seakeeping pressure loads to balance 3D finite element models without using inertia relief. The velocity potential of Strip sections is first calculated based on hydrodynamic Strip theories. The velocity potential of a finite element panel is obtained from the interpolation of the velocity potential of the Strip sections. The potential derivative along x-direction is computed using the approach proposed by Salvesen, Tuck and Faltinsen (1972). The hydrodynamic forces and moments are computed using direct panel pressure integration from the finite element structural panel. For forces and moments which cannot be directly converted from pressure, such as hydrostatic restoring force and diffraction force, element nodal forces are generated using Quadratic Programing. The equations of motions are then formulated based on the finite element wetted panels. The method results in a perfectly balanced structural model. An example is given to compare the “ordinary Strip Theory” to the proposed direct pressure integration method. The accuracy proves the validity of this new method.
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applying sectional seakeeping loads to full ship structural models using quadratic programming
2012Co-Authors: Owen Hughes, Chengbi ZhaoAbstract:Interest in the seakeeping loads of vessels has increased dramatically in recent years. In current design practice, methods for predicting seakeeping motions and loads are mainly in two categories, Strip Theory methods and 3D- panel methods. Methods based on Strip Theory provide reasonable motion prediction and are computationally efficient. However, many Strip Theory methods provide only hull girder sectional forces and moments, such as vertical bending moment and vertical shear force, which cannot be directly applied to a 3D finite element structural model. For panel based methods, the outputs include not only the global hull girder loads, but also panel pressures, which are well suited for 3D finite element analysis. However, because the panel based methods are computationally expensive, meshes used for hydrodynamic analyses are usually coarser than the mesh used for structural finite element analyses. Consequently, the panel pressure calculated from a hydrodynamic model mesh has to be transferred to the structural model mesh. The resulting discrepancy of the pressure map, regardless of what interpolation method is used, causes an imbalanced structural model. To obtain equilibrium of an imbalanced structural model, a common practice is to use the "inertia relief" approach (1) . However, this type of balancing causes a change in the hull girder load distribution, which in turn could cause inaccuracies in an extreme load analysis (ELA) and a spectral fatigue analysis (SFA). This paper presents a method to balance the structural model without using inertia relief. The method uses quadratic programming to calculate corrective nodal forces such that the resulting hull girder sectional loads match those calculated by seakeeping analyses, either by Strip Theory methods or 3D- panel methods. To validate the method, a 3D panel linear code, MAESTRO-Wave (2) , was used to generate both panel pressures and sectional loads. A model is first loaded by a 3D-panel pressure distribution with a perfect equilibrium. The model is then loaded with only the accelerations and sectional forces and moments. The sectional forces and moments are converted to finite element nodal forces using the proposed quadratic programming method. For these two load cases, the paper compares the hull girder loads, the hull deflection and the stresses, and the accuracy proves the validity of this new method.
