S. Nallayarasu

Bio: S. Nallayarasu is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Spar & Buoy. The author has an hindex of 9, co-authored 30 publications receiving 241 citations. Previous affiliations of S. Nallayarasu include Indian Institutes of Technology & National University of Singapore.

CFD approach to roll damping of ship with bilge keel with experimental validation

Abstract: The most common method of reducing roll motion of ship-shaped floating systems is the use of bilge keel which act as damping elements. The estimation of the damping introduced by bilge keel is still largely based on empirical methods. The present work adopts the CFD approach to the estimation of roll damping, both without and with bilge keel and validates the results with experiments conducted in a wave flume. Specifically, free oscillation tests are conducted at model scale to obtain roll damping, both by experiments and CFD simulation and reasonably good comparisons are obtained. The experiments also include PIV study of the flow field and attempt has been made to correlate the measured flow field with that obtained by CFD. The CFD methodology has the potential to determine rationally the size and orientation of bilge keels in design with reasonably accurate estimate of the additional roll damping that it provides to ship's roll motion.

Numerical prediction of roll damping of ships with and without bilge keel

Abstract: Estimation of roll damping of ships from free roll decay experiments or using appropriate semi-empirical formulae is a common practice. The total roll damping of a ship or a barge can be strongly nonlinear consisting of linear and nonlinear components; especially when bilge keel is fitted to the hull as a damping device and also when angles of roll are large. CFD simulations of nonlinear roll motion with experimental validation is gaining acceptance in the recent past. The present work reports three dimensional simulations of roll decay of a ship model, both with and without bilge keel, with experimental validation. A good comparisons of roll damping, natural period and added mass with experiments are obtained. The use of large eddy simulation on this class of problems is explored and commented upon. Ikeda's semi-empirical method over-predicts roll damping at larger roll angles. The pressure aft of bilge keel is found to be deviating whereas the pressure in front of bilge keel showed a close resemblance with semi-empirical calculations. Effect of bilge keel thickness, degrees of freedom and scaling on roll motion and damping is found to be insignificant for a barge-like ship model chosen in this study.

Estimation of incident and reflected waves in regular wave experiments

Abstract: This paper presents a laboratory technique for measurement of reflected waves by a submerged horizontal plate. A simple method is proposed to decompose the composite wave record obtained from the wave reflection experiments in the wave flume. The data collected were used to compute the reflected wave height by means of two and three probe methods proposed by other investigators and compared with the directly computed reflected wave using the measured incident wave. The experiment was carried out for a horizontal plate of 1 m length spanning the full width of the flume with wave periods ranging from 0.8 to 1.8 sec with 0.1 sec increments and wave heights of 5, 10 and 15 cm. The methods using two and three probes with phase measurement yield better results than the three probe method without phase measurement, and in general they have a wider range of application. The method using three probes without the phase measurement generally fails due to numerical instability of the scheme. The results obtained by the proposed method are compared with the two and three probe methods and a comparative analysis of the four methods is presented and discussed.

Influence of Heave Plate on Hydrodynamic Response of Spar

Abstract: Spar technology has been used in offshore oil and gas exploration successfully for number of years for drilling, production and storage in deepwater. The motion response of floating structures especially the heave response in particular, is very important to the selection of suitable drilling and production equipments. Reduction of heave response can be achieved by attaching a heave damping plate to the keel of a Spar. This has been used in the past. Experimental and numerical studies on such devices of various diameters under regular waves has been carried out and presented. The experiments were conducted on a 1: 100 scale model of Spar designed for a water depth of 245m with a payload of 10000 tonnes and the numerical analysis was carried out using ANSYS AQWA software. Numerical and experimental values of RAOs for surge, pitch and heave compares reasonably well. Measured and numerical results of RAOs for surge, pitch and heave and the effect of diameter ratio, wave steepness, and mooring line pretension were presented and discussed. An optimum heave response is achieved when the heave plate diameter is 20% to 30% larger than the diameter of the spar.Copyright © 2011 by ASME

Water-wave scattering by submerged elastic plates

Abstract: We present a solution to the water-wave interaction with a submerged elastic plate of negligible thickness by the eigenfunction-matching method. The eigenfunction expansion depends on the solution of a special dispersion equation for a submerged elastic plate and this is discussed in detail. We show how the solution can be calculated for the case of normal incidence on a semi-infinite plate in two spatial dimensions and then extend this solution to obliquely incident waves, to a plate of finite length and to a circular finite plate in three dimensions. Numerical calculations showing various properties of the solutions are presented and a near-orthogonality relation for the eigenfunctions is used to derive an energy-balance relation.

Hydrodynamic coefficients and pressure loads on heave plates for semi-submersible floating offshore wind turbines: A comparative analysis using large scale models

Abstract: Hydrodynamic forces on heave plates for a semi-submersible floating offshore wind turbine are discussed herein. A model of one of the platform columns has been built. This allows for the fitting of either a plain solid plate or the real heave plate prototype design. The latter is equipped with a vertical flap at its edge. The influence of the flap on the hydrodynamic coefficients is investigated through a results comparison with the plain solid one. The model plate diameter is 1 m, thus becoming, to the authors' knowledge, the largest for which results have been published. Results from experiments, in which added mass and damping coefficients have been measured, are presented. This experimental campaign also comprised the direct measurement of dynamic pressures on both heave plates, a fundamental magnitude for the structural design, which, until now, had not been experimentally explored for this type of system. For comparative reasons, numerical simulations were also conducted following common industry standards, both with a wide-spread frequency domain panel method (WADAM) and a RANS CFD commercial code (ANSYS CFX). Finally, results are compared with literature and consistent non-dimensionalizations are sought, with the aim of making these results useful for preliminary design purposes. The authors believe this research could benefit the offshore wind industry by improving the hydrodynamic design of the concept.

Ship roll damping control

Abstract: The technical feasibility of roll motion control devices has been amply demonstrated for over 100 years. Performance, however, can still fall short of expectations because of difficulties associated with control system designs, which have proven to be far from trivial due to fundamental performance limitations and large variations of the spectral characteristics of wave-induced roll motion. This tutorial paper presents an account of the development of various ship roll motion control systems together with the challenges associated with their design. It discusses the assessment of performance and the applicability of different mathematical models, and it surveys the control methods that have been implemented and validated with full scale experiments. The paper also presents an outlook on what are believed to be potential areas of research within this topic.

Numerical study of fixed Oscillating Water Column with RANS-type two-phase CFD model

Abstract: Various studies investigated the behaviour and the performance of Oscillating Water Columns (OWCs) suggesting many alternative design concepts to improve the efficiency of the device. The OWCs examined here are fixed on the seabed and have a slit opening at the seaward side. The present study investigates the applicability of a multiphase Reynolds Averaged Navier-Stokes (RANS) numerical model for simulating the interaction between an OWC and regular and irregular waves. An initial validation of the open-source computational fluid dynamics (CFD) software package OpenFOAM with the wave generation and absorption toolbox waves2Foam is performed against experimental results obtained at the COAST laboratory of the University of Plymouth. The main aim of the study is to complement to the validation of RANS CFD models and later employ the broadly used numerical tool for further studies for better understanding the behaviour of the OWCs. A method based on mechanical damped oscillations for calculating the eigen frequency of the device from a decay test is presented and compared with the performance curve. The strength of CFD modelling for obtaining better insight to the hydrodynamics of OWCs is also demonstrated.

Design of Welded Tubular Connections : Basis and Use of AWS Code Provisions(構造)(学位論文要旨)

Abstract: 1. Introduction to Tubular Structures. Attributes of Tubes. Architectural and Structural Forms. The Need for an Integrated Approach. Author's Viewpoint for this Monograph. Tubulars as Structural Members. Simple Welded Joints. 2. Conceptual Basis for the Design Rules. Definitions. Failure modes of tubular connections. General procedures for analysis and design. Design simplifications. Stresses in welds. 3. Static Strength of Circular Section Joints. Early work on T-, Y-, and K-connections. Lessons from field failures. Overcoming the beta paradox. Empirical punching shear design equation. Understanding the sources of reserve strength. Further evolution of the A.W.S. code. Final form. Design charts. 4. Fatigue Design. Levels of analysis. Hot spot stress. Stress concentration factors (SCF). S-N curves. Comparisons with international rules and data. 5. Tubular Connections Involving Non-Circular Sections. Box connections - existing A.W.S. rules. Box connections - proposed new rules. Hybrid connections. Special topics. Fatigue box connections. 6. Special Topics for Circular Section Joints. Overlapping connections. Multi-planar connections. Grouted connections. Internally stiffened tubular connections. 7. Fatigue Size and Profile Effects. Fracture toughness. Lamellar tearing. Role of redundancy. Non-destructive inspection criteria. 8. Construction System. Material selection. Tube manufacture. Connection layout. Welded joint design. Welder and procedure qualification. Prefabrication. Assembly. Inspection. Appendices. 1. Symbols and notation. 2. Design revisions, tubular structures. 3. Notch toughness task group, proposed revisions for AWS D1.1. Index.