R Panner Selvam

Tension Leg Platforms (TLPs) are one of the reliable structures in the offshore industry in deep waters because of its motion characteristics in heave, roll and pitch degrees of freedom (dof). Heave motion is very important in offshore facilities and have to be kept as minimum as possible. As the water depth increases TLPs suffers from some limitations and hence has to be modified to cater to deeper waters. One such concept proposed is Tension Based Tension Leg Platform (TBTLP). In this paper, experimental investigations carried out on a 1:150 scaled model of a Tension Based Tension Leg Platform in regular waves in 3 m water depth is reported. These are the first ever experiments which were carried out on a scaled model of the new concept. To investigate the effects of Tension Base, experiments were also conducted on the TLP (without Tension Base). Responses have been compared in terms of Response Amplitude Operators (RAOs) for surge, heave and pitch dof for TBTLP and TLP. Numerical modeling of the TLP and TBTLP responses using ANSYS® AQWA™ software is included as well for comparisons.

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.

This paper examines the results in case of breaking wave groups reported in Sriram et al. (2015) against a wave breaking model called Tian–Barthelemy model (Seiffert and Ducrozet, 2017). In this paper, Tian–Barthelemy model is extended to incorporate (Schäffer, 1996) corrections to linear wave making signals for generating focused wave groups and implemented in an open source computer code for high order spectral (HOS) method based numerical wave tank (NWT) viz., HOS-NWT. The developed model is then studied in the context of wave breaking in dispersively focused intermediate water wave groups with particular focus on multiple wave breaking events. The adapted model performs well in conformity with experimental observations in Sriram et al. (2015). A range of kinematic thresholds are studied with this model to calibrate the right threshold for the model against the experiments. We find strong evidence of a non-breaking wave group in intermediate depths crossing the kinematic threshold of 0.85 (stated in Seiffert and Ducrozet (2017)), implying that this threshold is not universal for intermediate depths. It is also shown that the model does not interfere with the spurious free wave suppression. However, various dynamic and kinematical aspects of this study reveal that suppression of spurious free wave makes the short waves dominant in the lead up to breaking. Hence, the model detects numerous threshold exceedences implying plausibly more wave breaking instances. It is also shown that this model in general violates the spectral signature of breaking in a dispersively focused wave.

Offshore structures such as jacket platforms have anodes fitted on their steel structural members to prevent corrosion. The anodes are projections on the surface of a tubular structural member and they increase the hydrodynamic forces and hydrodynamic coefficients of the tubular member. The effect of anodes on hydrodynamic forces and coefficients depends on several factors. The industrial practice to account for this effect is to multiply the total hydrodynamic force by a global factor, which may be conservative and uneconomical. The objective of this particular study is to determine the effect of location and orientation of anodes on wave loads and hydrodynamic coefficients of a cylinder, which have not been studied extensively to the authors’ best knowledge. The hydrodynamic forces and coefficients on a bare cylinder and a cylinder fitted with anodes were studied for regular waves of Keulegan-Carpenter (KC) number ranging from 0.42 to 1.91 in this experimental investigation.

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.

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.

In this paper, we extend the non-breaking second-order corrected focused wave experimental results of Sriram et al. (2015) in a numerical wave tank based on HOS in the presence of wind. The wind is modelled using the modified Jeffrey's Sheltering Mechanism (Kharif et al., 2008). We take care to preserve the consistency of application of pressure terms in the hybrid Stokesian-HOS formulations when applying wind. We confirm the results of Kharif et al. (2008) i.e. the focusing points in presence of wind show an amplification of the measure wave heights although no discernible shifting of focusing location is observed. However, compared to Kharif et al. (2008), the sheltering mechanism is found to be very weak since it depends on exceedance of a threshold slope based on constant steepness or constant amplitude spectrum. The energy flux to local phase celerity (Bx) both in absence and presence of the aforementioned wind model for the detection of onset of wave breaking (Barthelemy et al., 2015b; Saket et al., 2017b) are reported. Moreover, in the absence of wind, if Bx is used as parameter the onset of wave breaking, then for intermediate water wave groups its threshold should be close to 0.9 (computations based on Kurnia and van Groesen (2014)).

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 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 ship's 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.