R Panner Selvam

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.

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, speciakally 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. Copyright © 2006 by ASME.

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 (usually cubic nonlinearity characterised by a constant) 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 assuming PM spectrum and these results have been analysed by the R-MISO method, yielding the frequency dependent added mass and radiation damping, linear as well as the nonlinear stiffness coefficients quite satisfactorily.

The determination of the drag and inertia coefficients, which enter into the wave force model given by Morison's equation, is particularly uncertain and difficult when a linear spectral model is used for ocean waves, and the structure is compliant and has nonlinear dynamic response. In this paper, a nonlinear System Identification method, called Reverse Multiple Inputs-Single Output (R-MISO) is applied to identify the hydrodynamic coefficients as well as the nonlinear stiffness parameter for a compliant single-degree-of-freedom system. Four different types of problems have been identified for use in various situations and the R-MISO has been applied to all of them. One of the problems requires iterative solution strategy to identify the parameters. The method has been found to be efficient in predicting the parameters with reasonable accuracy and has the potential for use in the laboratory experiments on compliant nonlinear offshore systems. © 2001 Elsevier Science Ltd. All rights reserved.

A new frequency domain system identification method for estimation of hydrodynamic derivatives embedded in linear steering equations for ship maneuvering in calm seas is presented. The frequency domain multiple input-single output models developed for identification involves determination of constant, 'zero-frequency' hydrodynamic derivatives. The method is robust, non-iterative and computationally light and it does not require any starting estimates. In this method, the time domain operations are converted to linear operations in the frequency domain. The responses of the ship in a few standard maneuvers have been simulated in the numerical examples and the proposed method is applied to this data in order to estimate the hydrodynamic derivatives for all possible 'identifiable' combinations.

The estimation of the motion response of a floating semisubmersible type offshore platform for a desalination plant, of capacity 10 million litres per day (MLD) of fresh water, is the focus of the study. The platform needs station keeping by a mooring system using a spar moored in deep water. To cater to these requirements, several design configurations of the semisubmersible were tried out for their hydrodynamic performance in order to choose the best among them, in all cases keeping the cost of components at reasonable levels. The hydrodynamic analysis of the platforms was carried out using the software WAMIT. The natural heave period was the main criterion in finalizing the configuration of the semi-submersible because it has greatest impact on keeping the downtime of platform operation to a minimum. WAMIT yields unrealistically high RAO that is attributed to the effect of viscous damping not being incorporated in the analysis. Experimental investigation was carried out to find the viscous damping coefficient of the configuration of the semi-submersible that was finally chosen among a few alternatives. A 1:50 scale model was tested in the 4m flume at the Department of Ocean Engineering, I.I.T Madras. Free oscillation tests were carried out to find the damping coefficient and natural heave period. Use of experimentally obtained damping values in WAMIT yielded excellent comparison with experimentally obtained response values (RAOs). On the basis of the present work, configuration of a semi-submersible having six rectangular columns and two rectangular pontoons has been finalized for the desalination plant. Copyright © 2011 by ASME.

Ship dynamics in ocean waves involve frequency-dependent added mass and radiation damping which can be estimated either by potential theory-based calculations or by experiments. With uniform forward speed, the added mass and damping matrices become asymmetric. In this paper, we attempt to use a system identification approach, specifically the reverse multiple input-single output (R-MISO) method, for coupled heave-pitch response (two degrees of freedom) of a ship moving with uniform forward speed in random ocean waves. The system mass matrix has both frequency-independent and frequency-dependent components, whereas its damping matrix has only frequency-dependent components. The frequency-dependent components of the mass and damping matrices are asymmetric. The excitation force and moment due to linear monochromatic waves which act on the system are assumed to be known and can be either calculated or obtained from experiments. For numerical illustration, a ship whose hydrodynamic behaviour has been computed by strip theory has been considered. The motion, as well as the loading, is simulated assuming Pierson-Moskowitz spectrum in conjunction with response amplitude operators from the strip theory code, and these results are analysed by the R-MISO method yielding frequency-dependent asymmetric coupled added mass and radiation damping coefficients satisfactorily. © 2010 Taylor & Francis.

High density polyethylene pipe has been used to draw coldwater for low temperature thermal desalination plants since the year 2006 at Lakshadweep Islands, India. The pipeline endured in-line oscillations due to shear current in one of the desalination plants, and this gave an opportunity to observe the response parameters of the prototype pipeline. The recorded oscillation parameters are utilized as a benchmark to verify the results of the vortex induced vibration (VIV) analysis in frequency domain and time domain, and the results show good concurrence. The in-line fatigue damage assessment is carried out for a range of current profiles showing the need for mitigation of pipeline VIV. The helical strakes on a partial length of the pipeline is modeled numerically showing significant suppression of VIV response as well as substantial increase in fatigue life of the pipeline. The benchmarking of the VIV response analysis with prototype measurements enhances the reliability of using numerical VIV fatigue analysis and mitigation measures.