Using CFD to Assess Low Frequency Damping

2012 ◽  
Author(s):  
Alessio Pistidda ◽  
Harald Ottens ◽  
Richard Zoontjes
Keyword(s):  
Standard Procedure ◽  
Low Frequency ◽  
Added Mass ◽  
Model Tests ◽  
Viscous Damping ◽  
Viscous Effects ◽  
Floating Bodies ◽  
Time Step ◽  
Damping Coefficients ◽  
Quadratic Component

During offshore installation operations, floating bodies are often moored using soft mooring which are designed to withstand the environmental forces. Large amplitude motions often occur due to excitation by slowly varying wind and wave drift forces. To analyze these motions the dynamic system has to be accurately described, which includes an estimation of the added mass and damping coefficients. In general, the added mass can be accurately calculated with traditional potential theory. However for the damping this method is not adequate because viscous effects play an important role. Generally these data are obtained using model tests. This paper validates the CFD methodology as an alternative to model tests to evaluate the viscous damping. The aim is to define a standard procedure to derive viscous damping coefficients for surge, sway and yaw motion of floating bodies. To estimate viscous damping in CFD, a 3D model of the launch and float-over barge H-851 was used. For this barge, model test data is available which could be compared with the results of the CFD analysis. For the simulations, the commercial package STAR-CCM+ with the implicit unsteady solver for Reynolds-Averaged Navier-Stokes (RANS) equations was used. The turbulence model implemented was the k-Omega-SST. Numerical errors have been assessed performing sensitivity analysis on time step and grid size. Damping has been investigated by performing decay simulations as in the model tests, taking the effect of coupling among all motions into account. The P-Q fitting method has been used to determine the linear and quadratic component of the damping. Numerical results are validated with those obtained from the towing tank. Results show that CFD is an adequate tool to estimate the low frequency damping in terms of equivalent damping. More investigations are required to determine the linear and quadratic component.

URANS Predictions of Low-Frequency Damping of a LNGC

2019 ◽  
Author(s):  
Frédérick Jaouën ◽  
Arjen Koop ◽  
Lucas Vatinel
Keyword(s):  
Low Frequency ◽  
Offshore Structures ◽  
Viscous Damping ◽  
Wave Loads ◽  
Viscous Effects ◽  
Offshore Structure ◽  
Time Step ◽  
Cfd Simulations ◽  
Damping Coefficients ◽  
Hydrodynamic Damping

Abstract The horizontal motions of a moored offshore structure in waves are dominated by the resonance phenomena that occur at the natural frequencies of the system. Therefore, the maximum excursions of the structure depend on both the wave loads and the damping in the system. At present, potential flow calculations are employed for predicting the wave loads on offshore structures. However, such methods cannot predict hydrodynamic damping which is dominated by viscous effects. Therefore, the current practice in the industry is to obtain the low-frequency damping based on model testing. Nowadays, CFD simulations also have the potential to predict the low-frequency viscous damping of offshore structures in calm water. To obtain confidence in the accuracy of CFD simulations, a proper validation of the results of such CFD calculations is essential. In this paper, the flow around a forced surging or swaying LNGC is calculated using the CFD code ReFRESCO. The objective is to assess the accuracy and applicability of CFD for predicting the low-frequency viscous damping. After a description of the code and the used numerical methods, the results are presented and compared with results from model tests. Both inertia and damping coefficients are analyzed from the calculated hydrodynamics loads. Extensive numerical studies have been carried out to determine the influence of grid resolution, time step and iterative convergence on the flow solution and on the calculated damping. The numerical uncertainty of the results are assessed for one combination of amplitude and period for the surge motion. The CFD results are compared to experimental results indicating that the calculated damping coefficients agree within 5% for both surge and sway motion.

Analysis of Semi-Submersible Under Combined High Waves and Current Conditions Compared With Model Tests

2018 ◽  
Author(s):  
Limin Yang ◽  
Arne Nestegård ◽  
Erik Falkenberg
Keyword(s):  
Simulation Model ◽  
Low Frequency ◽  
Model Tests ◽  
Wave Forces ◽  
Viscous Effects ◽  
Numerical Predictions ◽  
Wave Drift ◽  
Zero Current ◽  
Frequency Excitation ◽  
Wave Drift Forces

Viscous effects on the low-frequency excitation force on column based platforms are significant in extreme waves. The wave drift force as calculated by a zero-current potential flow radiation/diffraction code becomes negligible for such waves. In the present study, the effect of current and viscous contributions on the slowly varying wave forces are adjusted by a formula developed in the Exwave JIP, see e.g. [1], which is validated against model test results. This paper presents numerical predictions of low frequency horizontal motions of a semi-submersible in combined high waves and current condition. In the simulation model, frequency dependent wave drift forces from radiation/diffraction code are modified by the formula. Static current forces and viscous damping are modelled by the drag term in Morison load formula using relative velocity between current and floater and with force coefficients as recommended by DNVGL-RP-C205 [2]. Low frequency surge responses calculated by the simulation model are compared with model tests for waves only and for combined collinear and noncollinear wave and current conditions.

Predicting Roll Added Mass and Damping of a Ship Hull Section Using CFD

2011 ◽  
Author(s):  
Frederick Jaouen ◽  
Arjen Koop ◽  
Guilherme Vaz
Keyword(s):  
Numerical Methods ◽  
Added Mass ◽  
Ship Hull ◽  
Time Step ◽  
Damping Coefficients ◽  
Time Discretisation ◽  
Rolling Body

In this paper, the flow around a forced rolling body is analyzed with MARIN in-house CFD code ReFRESCO. The objective is to assess if the code can correctly predict the added mass and damping coefficients of a rolling vessel. After a description of code and numerical methods, the results for the flow computed around a 2D rolling hull section are presented. Sharp and rounded bilges are investigated for three roll amplitudes and three roll periods. The influence of grid and time discretisation and iterative errors are analyzed. The CFD results with Re-FRESCO are compared to experiments and to results obtained with the commercial CFD package CFX. The results shown here indicate that ReFRESCO is capable of accurately predicting the added mass and damping coefficients. However, it is also shown that fine grids and time-steps are required to obtain a grid and time-step converged solution.

Nonlinear Hydrodynamic Forces on an Accelerated Body in Waves

2005 ◽  
Vol 127 (1) ◽  
pp. 17-30 ◽  
Author(s):  
Motoki Yoshida ◽  
Takeshi Kinoshita ◽  
Weiguang Bao
Keyword(s):  
Perturbation Analysis ◽  
Low Frequency ◽  
Multiple Scale ◽  
Added Mass ◽  
Floating Body ◽  
Floating Bodies ◽  
Low Frequency Oscillations ◽  
Wave Drift ◽  
Decay Test ◽  
Field Radiation

Wave-drift added mass results from nonlinear interactions between waves and low-frequency oscillatory motions of a floating body, in the presence of incident waves. In previous works, wave-drift damping which is the component of wave-drift force in phase with the velocity of low-frequency oscillations was investigated mainly based on a quasi-steady analysis. However, investigations related to wave-drift added mass, the component in phase with acceleration, were very few. In this paper, wave-drift added mass is derived directly from a perturbation analysis with two small parameters and two time scales, using a Cartesian coordinate system that follows the low-frequency oscillations, dynamic oscillation model has been used. Especially, the method to solve higher-order potentials, which are necessary for evaluation of wave-drift added mass, is presented. Analytical solutions and calculated results of wave-drift added mass, and far field radiation conditions for each order of potentials are obtained. Also, wave-drift added mass of floating bodies has been systematically measured from a slowly forced oscillation test or a free decay test in waves. Experimental results are compared with calculated results. Then, for a supplement, the secular behavior that some velocity potentials show is discussed. Applying a multiple scale perturbation analysis to one of these problems, a nonsecular solution is obtained.

Influence of Draft Variation on Heave Added-Mass and Hydrodynamic Damping Coefficients of Floating Bodies

1990 ◽  
Vol 34 (03) ◽  
pp. 172-178
Author(s):  
Marc Vantorre
Keyword(s):  
Numerical Data ◽  
Integral Equation Method ◽  
Added Mass ◽  
Boundary Integral ◽  
Hydrodynamic Coefficients ◽  
Floating Bodies ◽  
Damping Coefficients ◽  
Hydrodynamic Damping ◽  
Harmonic Components ◽  
Heave Motion

A general nonlinear theory for solving the radiation problem for floating or immersed bodies in a periodic heave motion, composed of a number of harmonic components, is applied for calculating the influence of draft variations on the linear hydrodynamic coefficients for heave. It is shown that a calculation method for added-mass and damping coefficients of axisymmetric bodies based on a boundary integral equation method can easily be modified to obtain numerical values of the first and second derivatives with respect to draft of the hydrodynamic coefficients as well. The method is illustrated by experimental and numerical data for a floating cone.

Prediction of Roll Damping Using Viscous Flow Solvers

2012 ◽  
Author(s):  
Manuel Manzke ◽  
Thomas Rung
Keyword(s):  
Domain Size ◽  
Building Blocks ◽  
Sensitivity Study ◽  
Convergence Criterion ◽  
Parameter Study ◽  
Floating Bodies ◽  
Time Step ◽  
Roll Damping ◽  
Damping Coefficients ◽  
Ranse Solver

This article illustrates the use of a RANSE solver coupled to a motion solver to predict the free roll decay and the associated damping coefficients of floating bodies. The necessary building blocks to perform such a prediction are described briefly. A sensitivity study for the convergence criterion, the time step, the domain size and the grid resolution is performed for a simple 2-dimensional barge. The results are compared to results from experiments. Furthermore a simulation for a free roll decay of a Navy Combatant is performed, considering the results of the parameter study for the Barge. Overall results indicate that the natural roll frequency can be well predicted, while the prediction of the roll damping coefficients is afflicted with some uncertainties.

Reliable Estimation of Low-Frequency Viscous Damping for a Turret-Moored FLNG in Current

2018 ◽  
Author(s):  
Z. Huang ◽  
S. Ryu ◽  
D. Lee ◽  
C. S. Hughes
Keyword(s):  
Low Frequency ◽  
Test Method ◽  
Inertia Force ◽  
Total Force ◽  
Viscous Damping ◽  
Damping Force ◽  
Damping Coefficients ◽  
Calm Water ◽  
Oscillation Test ◽  
Measured Force

For a turret-moored Floating Liquefied Natural Gas Plant (FLNG), it is important to use confidently derived low frequency viscous damping coefficients in the prediction of its motions and mooring loads in wind, wave and current conditions. In this paper we present our recent experimental work on the low frequency sway and yaw viscous damping in calm water and in current. In general, damping force is a relatively small portion of the total hydrodynamic force on an oscillatory model. In a previous ExxonMobil damping test in calm water (Huang et al., 2010), i.e. without current and wave, a deeply submerged double-body model was forced to oscillate to avoid surface wave contamination. An inertia compensation system was also designed to cancel the inertia force and the restoring force during oscillations, then the measured force was mainly damping force. Because of the schedule constraints of the present study, it was not possible to perform the submerged oscillation test. Instead, a forced oscillation test in water surface was performed based on KC-number and β-number. In order to obtain reliable damping coefficients, we had to carefully design the test conditions, i.e. current speeds, oscillation amplitudes and frequencies so that an adequate portion of damping force within the total force could be achieved with no significant surface waves that could contaminate the damping results being generated by the oscillating model. Good damping results were obtained. To check the acceptance of the test method based on Froude scaling, a limited number of tests were performed in which the oscillation amplitudes and frequencies were scaled down based on the Froude scaling. Magnitudes of the measured force and moment are significantly low. The time series of the measurements have drifting and significant noise. We could not confidently determine viscous damping results from the measurements.

Coupled CFD Simulation of the Response of a Calm Buoy in Waves

2005 ◽  
Author(s):  
Peter Woodburn ◽  
Paul Gallagher ◽  
Mamoun Naciri ◽  
Jean-Paul Borleteau
Keyword(s):  
Cfd Simulation ◽  
Viscous Damping ◽  
Mooring System ◽  
Viscous Effects ◽  
Floating Structure ◽  
Time Step ◽  
Natural Period ◽  
Model Scale ◽  
Waves And Currents ◽  
Work Done

This paper describes work done within the EU FP5 Project EXPRO-CFD to develop a system to couple commercial CFD software to existing hydromechanics tools to allow prediction of the response of floating structures in waves and currents, including viscous effects. Its focus is the use of this system to improve the prediction of CALM buoy response in waves. The Atkins EXPRO-CFD system is made up from the CFD code CFX, coupled to the AQWA-LINE and AQWA-NAUT hydromechanics codes. In this system, CFD provides the complete set of hydrodynamic forces and moments at each time step in the motions simulation, with the dynamics of the floating structure, its moorings and riser/export lines modeled in the AQWA-NAUT software. AQWA-NAUT returns the structure’s displacements and velocities to the CFD model and a moving grid algorithm uses these to couple motions and fluid flow in an accurate and stable manner. The motions of a CALM buoy were studied to test the capabilities of the system. The CALM buoy geometry (based on current designs by SBM) is 23m in diameter with a 2m wide skirt attached 1m above the keel; the effects of flow separation off this skirt and the associated viscous damping on the motions of the buoy were expected to be significant, especially around its natural period. A series of 1:40 model scale tests were carried out by Sirenha using a simplified mooring system with no risers. The results from the model tests, from AQWA-NAUT alone (carried out by SBM), and from the coupled EXPRO model were compared directly at model scale. The same AQWA-NAUT model was used in both the AQWA-NAUT-only simulations and the coupled simulations, allowing direct comparison between the results. The EXPRO-system simulations were carried out ‘blind’, i.e. without access to the experimental data. The three sets of RAOs showed reasonable agreement in long or short waves (within the limits of the specification of the mooring system). However, around the natural period, the AQWA-NAUT-only model significantly over-predicted the response in heave, and in particular in pitch. Although the EXPRO-CFD system slightly over predicted the heave and pitch responses, the results were close to the experimental measurements throughout. Further tests indicate that the weakness in the potential flow approach appears to be in the formulation of added viscous damping rather than the choice of model values for drag coefficients.

Experimental Identification of an Advanced Spar’s Low Frequency Drag Damping in Waves

2020 ◽  
Author(s):  
Christopher Wright ◽  
Haruki Yoshimoto ◽  
Ryota Wada ◽  
Ken Takagi
Keyword(s):  
Renewable Energy ◽  
Low Frequency ◽  
Domain Model ◽  
Viscous Drag ◽  
Viscous Effects ◽  
Floating Platform ◽  
Large Area ◽  
Floating Bodies ◽  
Weakly Nonlinear ◽  
Decay Tests

Abstract Climate change and increasing population growth are accelerating the need for new clean renewable energy generation. One such proposed type of renewable energy is offshore floating wind, which has yet to reach a convergence as to the optimum type of floating platform to support a wind turbine. To date there has been numerous proposals of novel floating platforms with unique hull characteristics. One such type is the advanced spar, which has a large area of sharp edges and therefore considerably different hydrodynamic viscous effects than typical cylindrical platforms. Prediction of a floating body’s low frequency drag damping is crucial to successfully predicting the horizontal motions. Calculation of the viscous effects have been seen to have the most uncertainty. Published literature shows that the viscous drag effects of floating bodies either increase or decrease with increasing wave severity as compared to still water decay tests. In this paper a combined experimental and numerical method for identifying low frequency viscous damping effects on the hull of a moored platform is introduced. An initial offset is applied to the platform, which is then released in still water and regular sinusoidal waves. The experimental results will then be compared to a weakly nonlinear time domain model in order to identify how the drag coefficients vary with wave conditions. Discussions on Keulegan–Carpenter (KC) number dependent damping are given. Finally simulations using results from these experiments are compared against a full scale deployed floating platform in multi-directional waves, current and wind.

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