R. Panneer Selvam

Parameter identification of a compliant nonlinear SDOF system in random ocean waves by reverse MISO method

Abstract The determination of the drag and inertia coefficients, which enter into the wave force model given by Morisons 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.

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

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. @DOI: 10.1115/1.1493201#

System Identification of a Coupled Two DOF Moored Floating Body in 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.

Study of manoeuvrability of container ship by static and dynamic simulations using a RANSE-based solver

The numerical study of manoeuvrability of surface ships necessitates the determination of the hydrodynamic derivatives in the equations of motion. Standard manoeuvring tests are simulated to evaluate the ships manoeuvring qualities. This paper deals with the estimation of linear, nonlinear and roll-coupled hydrodynamic derivatives of a container ship by numerically simulating static and dynamic tests at different roll angles using a RANSE solver. The mathematical model suitable for the nonlinear roll-coupled steering model for high-speed container ships is considered here. In order to include the effect of roll on the ship, the roll-dependent derivatives are estimated by using static and dynamic tests numerically performed at discrete heel angles. Standard definitive manoeuvres such as turning circle and zig-zag tests are numerically simulated by solving the equations of motion and the results are verified with those obtained by using experimental values.

Response analysis of tension-based tension leg platform under irregular waves

Tension Leg Platform (TLP) is a hybrid structure used as oil drilling and production facility within water depths of 1200 m. The extension of this TLP concept to deeper waters is a challenge and warrants for some innovative design concepts. In this paper, a relatively new concept of TLP which is christened as Tension-Based Tension Leg Platform (TBTLP) and patented by Srinivasan (1998) has been chosen for study. Response analysis of TLP with one tension base under irregular waves for three different sea states has been performed using hydrodynamic tool ANSYS® AQWA™. Results are reported in terms of RAOs, response spectrums for surge, heave and pitch degrees of freedom from which spectral statistics have been obtained. The statistics of TBTLP have been compared with TLPs (without tension base) for two different water depths to highlight the features of the new concept. The effect of viscous damping and loading effects on the RAOs are also investigated.

Hydrodynamic Analysis of Tension Based Tension Leg Platform

Tension Leg Platforms (TLPs) are one of the best options for offshore industry in deep waters due to proven motion response characteristics. These are water depth sensitive structures and the motion responses in vertical plane motions (heave, roll and pitch) are critical for a TLP. Tension Based TLP (TBTLP) is a new concept and finds application in much deeper waters. A provision of a tension base at mid-depth results in an economical design of TLP. In fact, the TLP installed at a certain depth without any modifications can be made to be deployed in much deeper water depths by means of a tension base. In this paper, the concept of TBTLP is highlighted and hydrodynamic analysis of the chosen platform has been carried out using ANSYS AQWA package. The motion responses in terms of Response Amplitude Operators (RAOs) of TBTLP with one Tension Base in surge, heave and pitch have been obtained and compared with a TLP without a tension base.© 2012 ASME

Study of Maneuverability of Container Ship With Nonlinear and Roll-Coupled Effects by Numerical Simulations Using RANSE-Based Solver

The examination of maneuvering qualities of a ship is necessary to ensure its navigational safety and prediction of trajectory. The study of maneuverability of a ship is a three-step process, which involves selection of a suitable mathematical model, estimation of the hydrodynamic derivatives occurring in the equation of motion, and simulation of the standard maneuvering tests to determine its maneuvering qualities. This paper reports the maneuvering studies made on a container ship model (S175). The mathematical model proposed by Son and Nomoto (1981, “On Coupled Motion of Steering and Rolling of a High Speed Container Ship,” J. Soc. Nav. Arch. Jpn., 150, pp. 73–83) suitable for the nonlinear roll-coupled steering model for high-speed container ships is considered here. The hydrodynamic derivatives are determined by numerically simulating the planar motion mechanism (PMM) tests in pure yaw and combined sway–yaw mode using an Reynolds-Averaged Navier–Stokes Equations (RANSE)-based computational fluid dynamics (CFD) solver. The tests are repeated with the model inclined at different heel angles to obtain the roll-coupled derivatives. Standard definitive maneuvers like turning tests at rudder angle, 35 deg and 20 deg/20 deg zig-zag maneuvers are simulated using the numerically obtained derivatives and are compared with those obtained using experimental values.

Sensitivity Study of Hydrodynamic Derivative Variations on the Maneuverability Prediction of a Container Ship

The study of maneuverability of a ship involves the determination of the hydrodynamic derivatives in the equations of motion. The standard maneuvers are simulated by integrating the equations of motion and the maneuvering parameters are checked for compliance with appropriate standards set by IMO. The numerically or experimentally predicted hydrodynamic derivatives may differ from actual values of the built and operated ship. Hence, it is worth to understand the sensitivity of these variations on the actual maneuvering performance of the ship. This paper deals with a study on the sensitivity of the hydrodynamic derivatives in the equations of motion of a container ship (S175). The sensitivity analysis of all the hydrodynamic derivatives is performed by deviating each derivative in the range of −50% to +50% from the experimentally derived values, in steps of 10%. The standard maneuvering tests like turning tests at rudder angle, δ = 35° and 20°/20° zig-zag maneuvers are performed for each case and their effects on the standard maneuvering parameters are estimated. The hydrodynamic derivatives that are important and which have to be estimated with high level of accuracy in maneuvering studies for a container ship are identified through this study.Copyright

Hydrodynamic Analysis of an Inverted Catenary Coldwater Pipeline of a LTTD Plant

Drinking water is a precious commodity with growing demand, motivating the researchers to explore innovative and cost effective measures to augment the available resources. Low Temperature Thermal Desalination (LTTD) is one among the ideas that utilizes the ocean thermal gradient for the production of freshwater. The cold water from deep sea is drawn and transported to the coast or to a floating platform to aid the conversion of surface seawater to fresh water. The behavior of this cold water pipeline in open waters is to be studied through analytical, numerical and experimental studies before actual field implementation.This paper presents the preliminary analysis of a flexible cold water pipe made up of High Density Polyethylene (HDPE) catering for an island based desalination plant. The water is drawn at a depth of 400 m and transferred to the shore. The site with coral reef demands an inverted catenary profiled pipeline to minimize damages and is connected to sump at the shallow end and a clump weight at the deep end. Preliminary hydrodynamic analysis of the cold water pipeline is carried out using OrcaFlex software under regular waves. Obtaining the effect of flexible pipeline under regular waves besides carrying out vortex induce vibration analysis and modal analysis forms the scope of this study. Variation of VonMises stresses, and effective tension at salient points (top end, middle and bottom end) are included.Copyright

System identification of coupled heave–pitch motion of ships with forward speed in random ocean waves

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.