Added mass and wave radiation damping for flow-induced rotational vibrations of skinplates of hydraulic gates

2012 ◽  
Vol 35 ◽  
pp. 213-228 ◽  
Author(s):  
K. Anami ◽  
N. Ishii ◽  
C.W. Knisely
Keyword(s):  
Added Mass ◽  
Radiation Damping ◽  
Wave Radiation

On the Radiation and Diffraction of Surface Waves by Submerged Spheroids

1989 ◽  
Vol 33 (02) ◽  
pp. 84-92
Author(s):  
G. X. Wu ◽  
R. Eatock Taylor
Keyword(s):  
Degrees Of Freedom ◽  
Velocity Potential ◽  
Three Dimensional ◽  
Added Mass ◽  
Wave Radiation ◽  
Legendre Functions ◽  
Incoming Wave ◽  
Six Degrees Of Freedom ◽  
Damping Coefficients ◽  
Exciting Forces

The problem of wave radiation and diffraction by submerged spheroids is analyzed using linearized three-dimensional potential-flow theory. The solution is obtained by expanding the velocity potential into a series of Legendre functions in a spheroidal coordinate system. Tabulated and graphical results are provided for added mass and damping coefficients of various spheroids undergoing motions in six degrees of freedom. Graphs are also provided for exciting forces and moments corresponding to a range of incoming wave angles.

Wave Kinematics and Seakeeping Calculation With Varying Bathymetry

2009 ◽  
Author(s):  
Guillaume de Hauteclocque ◽  
Fla´via Rezende ◽  
Yann Giorgiutti ◽  
Xiao-Bo Chen
Keyword(s):  
Boundary Element Method ◽  
Wave Field ◽  
Incident Wave ◽  
Added Mass ◽  
Radiation Damping ◽  
Floating Body ◽  
Diffraction Radiation ◽  
Constant Depth ◽  
Wave Kinematics ◽  
Radiation Theory

Diffraction/Radiation theory is used to calculate the wave kinematics and the motions of a floating body in area of varying bathymetry. The bathymetry is modeled as a second body, which, without special measures, leads to spurious reflection at the edge of the mesh. A modified formulation of the Boundary Element Method is introduced to model partially transparent panels. Those panels, when properly used to smoothly extend the actual (opaque) bathymetry, allow much more accurate computation. The efficiency of the method is tested with regards of several parameters concerning the bathymetry size and the way to smooth the truncation. Numerical results are satisfactorily compared with a 3D shallow water code based on Green-Naghdi theory. The sensitivity to the slope on the ship response is then investigated (motion, added mass, radiation damping and second order loads). The differences with the constant depth calculations are significant, due to the modified incident wave field, but also due to modified added mass and radiation damping terms. The method presented here could be useful in the context of LNG terminals where the depth is quite shallow and the bathymetric variations significant.

Consistent State Space Modelling of Hydrodynamic Memory

2020 ◽  
Author(s):  
Karl E. Kaasen
Keyword(s):  
Differential Equations ◽  
High Frequency ◽  
Radiation Force ◽  
Added Mass ◽  
Equation Model ◽  
Wave Radiation ◽  
Diffraction Radiation ◽  
Coupled Modes ◽  
Linear Differential ◽  
The Given

Abstract The conventional way to model hydrodynamic memory or radiation force is to use retardation functions. These functions are usually derived from frequency-dependent damping functions that are calculated by a diffraction-radiation code using potential theory. Calculating the retardation functions can be challenging due to lack of information at high frequency. In simulation of wave-driven vessel motion the retardation function is convolved with the velocity to give the wave radiation force, which is time-consuming. The paper describes how the memory effects can be modelled consistently by linear differential equations, such that coupled modes of motion share one set of poles. The coefficients of the differential equations are found by least squares fitting of a certain rational function to the numerical damping function. One advantage of this is that no assumption need to be made about the added mass at infinite frequency. Nor is any conditioning of the given data necessary. Using the fitted model in time-domain simulation is much quicker than using retardation functions. The method is applied to data representing the sway, roll and yaw motions of an FPSO of 238 m length. It was found that a sixth-order differential equation model fitted the given numeric radiation function well. It is shown how the high frequency asymptote for added mass can be estimated with high accuracy, which is valuable when it is not known in advance.

Parameter Identification of a Large Floating Body in Random Ocean Waves by Reverse MISO Method

2003 ◽  
Vol 125 (2) ◽  
pp. 81-86 ◽  
Author(s):  
S. K. Bhattacharyya ◽  
R. Panneer Selvam
Keyword(s):  
Nonlinear System Identification ◽  
Ocean Waves ◽  
Added Mass ◽  
Radiation Damping ◽  
Floating Body ◽  
Mooring Line ◽  
Nonlinear Stiffness ◽  
Numerical Illustration ◽  
Frequency Dependent ◽  
Single Output

Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the Reverse Multiple Inputs-Single Output (R-MISO) method, to a single-degree-of-freedom system with linear and cubic nonlinear stiffnesses. The system mass is split into a frequency independent and a frequency dependent component and its damping is frequency dependent. This can serve as a model of a moored floating system with a dominant motion associated with the nonlinear stiffness. The wave diffraction force, the excitation to the system, is assumed known. This can either be calculated or obtained from experiments. For numerical illustration, the case of floating semi-ellipsoid is adopted with dominant sway motion. The motion as well as the loading are simulated with and without noise assuming PM spectrum and these results have been analyzed by the R-MISO method, yielding the frequency dependent added mass and radiation damping, linear as well as the nonlinear stiffness coefficients quite satisfactorily.

System Identification of a Coupled Two DOF Moored Floating Body in Random Ocean Waves

2005 ◽  
Vol 128 (3) ◽  
pp. 191-202 ◽  
Author(s):  
R. Panneer Selvam ◽  
S. K. Bhattacharyya
Keyword(s):  
System Identification ◽  
Mass Matrix ◽  
Ocean Waves ◽  
Added Mass ◽  
Radiation Damping ◽  
Floating Body ◽  
Mooring Line ◽  
Frequency Dependent ◽  
Linear And Nonlinear ◽  
Random Ocean Waves

Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the reverse multiple input-single output (R-MISO) method, to coupled surge-pitch response (two-degrees-of-freedom) of a large floating system in random ocean waves with linear and cubic nonlinear mooring line stiffnesses. The system mass matrix has both frequency independent and frequency dependent components whereas its damping matrix has only frequency dependent components. The excitation force and moment due to linear monochromatic waves which act on the system are assumed to be known that can either be calculated or obtained from experiments. For numerical illustration, a floating half-spheroid is adopted. The motion as well as the loading are simulated assuming Pierson-Moskowitz (PM) spectrum and these results have been analyzed by the R-MISO method yielding frequency dependent coupled added mass and radiation damping coefficients, as well as linear and nonlinear stiffness coefficients of mooring lines satisfactorily.

Finite Element Analysis of Subsea Pipelines Subjected to Underwater Explosion

2013 ◽  
Author(s):  
F. Van den Abeele ◽  
P. Verleysen
Keyword(s):  
Shock Wave ◽  
Finite Element ◽  
Transient Response ◽  
Acoustic Pressure ◽  
Underwater Explosion ◽  
Structural Response ◽  
Added Mass ◽  
Radiation Damping ◽  
Time Response ◽  
Subsea Pipelines

Underwater mines and explosives, left in ports and harbours after World War II, can still pose a threat to subsea pipelines. In case of an accidental explosion, or even during controlled detonation, such explosives can cause significant damage to subsea pipelines. To assess the safety of pipelines exposed to an underwater explosion, finite element analyses are performed to predict the transient response of the pipeline to an acoustic pressure shock wave. This type of problem is characterized by a strong coupling between the structural response of the pipe and the acoustic pressure on the wetted interface between the pipe surface and the surrounding seawater. The spherical pressure wave induced by an underwater explosion is characterized by a very steep wave front, where the maximum pressure is attained over an extremely short rise time. The pressure then drops off exponentially over a significantly longer period of time. As a result, the structural behaviour is a combination of a long time response, dominated by an added mass effect (low frequency), a short time response, governed by radiation damping (high frequency), and an intermediate time-frequency response, where both added mass and radiation damping effects are present. In this paper, a finite element model is presented to simulate the transient response of a subsea pipeline subjected to an underwater explosion. The close coupling between acoustic pressure and structural response gives rise to numerical challenges like the accurate formulation and representation of the shock wave, the mesh requirements for the acoustic domain, and the position of the surface based absorbing radiation boundaries. An explicit dynamic solver is used to tackle these challenges, and to predict the behaviour of subsea pipelines exposed to an underwater explosion. The numerical results are compared to published experimental data, and can be used to assess the safety of submerged pipelines in the vicinity of explosives.

Streamwise Gate Vibration

2017 ◽  
pp. 204-229
Keyword(s):  
Wave Theory ◽  
Mass Effect ◽  
Added Mass ◽  
Wave Radiation ◽  
Theory Analysis ◽  
Single Degree Of Freedom ◽  
Single Degree ◽  
Long Span ◽  
Mass Spring

Streamwise vibrations of gates due to the bending flexibility of the skinplate of Tainter gates or the weir plate of long-span gates result in pushing-and-drawing of the water in the reservoir. During each cycle of vibration, the gate's motion must accelerate and then decelerate the water mass in contact with the vibrating gate surface, resulting in a substantial added mass effect. From simple single degree-of-freedom mass-spring-damper vibration theory, one understands that the effect of added mass is to lower the frequency of gate vibration. In addition to the push-and-draw effect, streamwise motion can also result in discharge fluctuation for inclined gates, providing a source of gate excitation. Rayleigh's wave theory analysis from the previous chapter is applied to provide an analysis framework for determining the magnitude of wave radiation damping and to calculate the added mass.

scholarly journals Does More Tension Reduce VIV?

2017 ◽  
Author(s):  
J. Kim Vandiver ◽  
Leixin Ma
Keyword(s):  
Damping Ratio ◽  
Response Prediction ◽  
Element Model ◽  
Radiation Damping ◽  
Wave Radiation ◽  
Vibration Energy ◽  
Deepwater Drilling ◽  
Prediction Program ◽  
Sheared Flow ◽  
Damage Rate

In this paper it is shown that for very long risers in sheared flow there is a surprising outcome — the VIV response amplitude in the power-in region does not depend at all on the amount of damping in the power out region, as long as it is sufficient to prevent waves from reflecting at the boundary and returning to the power-in region. In these cases, the response in the power-in region depends on the wave radiation damping and not on the damping in the power-out regions. Tension plays a major role in the determining the radiation damping and in some cases, but not all, pulling harder will indeed reduce response in the power-in region. Numerical simulations are presented in which a finite element model of a long riser is used to compute the VIV response in a sheared flow for which the power-in region is at one end of the riser. The radiated waves are shown to diminish with distance traveled as expected. When ζoutnout, the product of the number of wavelengths to reach the far termination and the damping ratio in the power-out region is greater than 0.18, it is shown that no significant vibration energy returns to the power-in region and the response in the power-in region is independent of the damping in the power-out region. The numerical simulation is used to illustrate the effect of changing tension on the radiation damping and therefore on the VIV response. The VIV response prediction program SHEAR7 is used to evaluate the effect of increasing tension on a realistic deepwater drilling riser in 3000 m water depth. A 20% increase in tension leads to a 12% reduction in fatigue damage rate.

An Investigation Into the Limitations of the Panel Method and the Gap Effect for a Fixed and a Floating Structure Subject to Waves

2016 ◽  
Author(s):  
Blanca Peña ◽  
Aaron McDougall
Keyword(s):  
Time Domain ◽  
Real Life ◽  
Added Mass ◽  
Panel Method ◽  
Gap Effect ◽  
Wave Radiation ◽  
Literature Study ◽  
Floating Structure ◽  
Cfd Analyses ◽  
Wave Induced

The wave-induced motions of vessels moored next to a fixed object and open to the sea impact the operability of many offshore operations, and should be assessed in order to avoid accidents and catastrophes. When analysing vessels moored by a fixed object (e.g. quay-side or platform), time domain simulations have shown numeric instabilities resulting in unreliable outcomes. The origin of the numerical instability might lie in the hydrodynamic added mass and wave radiation damping. This is typically calculated using potential flow methods and influenced by the existence of standing waves in the gap between the two bodies. For certain frequencies, these give negative values, potentially causing instabilities in non-linear (coupled) time domain simulations. In these cases, the vessel can behave unexpectedly, generating energy rather than dissipating it. As such, certain simulations have been disregarded as they are unlikely to accurately represent real-life scenarios. This paper investigates and compares added mass and damping using two different tools and studies the gap effect when conducting diffraction analysis using 3D panel methods. The work covers a literature study into potential theory, multibody analysis, Computational Fluid Dynamics (CFD) and lid techniques. This is followed by a study conducted using both panel method and CFD analyses. The results from both approaches have been compared, showing interesting information and the necessity of researching more into the problem addressed in this paper.

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