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Patra P.,Indian Register of Shipping | Bhattacharya B.,Indian Institute of Technology Kharagpur
Bulletin of Earthquake Engineering | Year: 2011

The current Indian Standard (IS) code for seismic design of structures (IS 1893:2002) specifies the use of time history analysis for structures with height greater than 40m. However, for structures less than 40m it recommends the concept of equivalent static analysis. This study attempts to investigate the adequacy of the current design code when it comes to the actual evaluation of structures shorter than 40 m subjected to seismic loading using dynamic analysis as opposed to the code specified static analysis. Incremental dynamic analysis, which subjects a structure to a progressively increasing series of intensity measures, has been adopted here for the purpose. Three 2D moment resisting steel structures under the 1991 Uttarkashi and the 2001 Bhuj earthquakes (both of which predate the current IS1893) have been studied-a single storeyed portal frame, a 2 storey 3 bay frame and a 3 storey 2 bay frame. While it can be argued that two records are never enough for any generalization, and that only a full probabilistic analysis can determine if the limiting collapse prevention probability has been exceeded for these structures, the IS code in both cases does significantly under predict the seismic demands on the structures. At the same time, and perhaps why the codal provisions usually work, the structural capacities are in most cases underestimated as well. These suggest that a thorough study is in order and that there is scope for rationalization in the IS codal provisions. © 2010 Springer Science+Business Media B.V. Source

Rajesh G.,Indian Institute of Technology Madras | Giri Rajasekhar G.,Indian Register of Shipping | Bhattacharyya S.K.,Indian Institute of Technology Madras
Journal of Ship Research | Year: 2010

This paper deals with the application of nonparametric system identification to the nonlinear maneuvering of ships using neural network method. The maneuvering equations contain linear as well as nonlinear terms, and one does not attempt to determine the parameters (or hydrodynamic derivatives) associated with nonlinear terms, rather all nonlinear terms are clubbed together to form one unknown time function per equation, which are sought to be represented by neural network coefficients. The time series used in training the network are obtained from simulated data of zigzag and spiral maneuvers. The neural network has one middle or hidden layer of neurons and the Levenberg-Marquardt algorithm is used to obtain the network coefficients. Using the best choices for number of hidden layer neurons, length of training data, convergence tolerance, and so forth, the performances of the proposed neural network models have been investigated and conclusions drawn. Source

Ganesan T S.,Indian Register of Shipping | Sen D.,Indian Institute of Technology Kharagpur
Applied Ocean Research | Year: 2015

In this paper, motion response of a moored floating structure interacting with a large amplitude and steep incident wave field is studied using a coupled time domain solution scheme. Solution of the hydrodynamic boundary value problem is achieved using a three-dimensional numerical wave tank (3D NWT) approach based upon a form of Mixed-Eulerian-Lagrangian (MEL) scheme. In the developed method, nonlinearity arising due to incident wave as well as nonlinear hydrostatics is completely captured while the hydrodynamic interactions of radiation and diffraction are determined at every time step based on certain simplifying approximations. Mooring lines are modelled as linear as well as nonlinear springs. The horizontal tension for each individual mooring line is obtained from the nonlinear load-excursion plot of the lines computed using catenary theory, from which the linear and nonlinear line stiffness are determined. Motions of three realistic floating structures with different mooring systems are analyzed considering various combinations of linear and approximate nonlinear hydrodynamic load computations and linear/nonlinear mooring line stiffness. Results are discussed to bring out the influence and need for consideration of nonlinearities in the hydrodynamics and hydrostatics as well as the nonlinear modelling of the line stiffness. © 2015 Elsevier Ltd. Source

Ganesan T. S.,Indian Register of Shipping | Sen D.,Indian Institute of Technology Kharagpur
Applied Ocean Research | Year: 2016

The coupled system of two side-by-side fixed and/or floating bodies interacting with a large amplitude nonlinear wave is studied using a direct time domain solution method. The numerical model is based on a three-dimensional mixed Eulerian-Lagrangian (MEL) method under certain simplifying approximations permitting Rankine panel scheme to be implemented over a time-invariant boundary surface to solve the boundary value problem for the unknown velocity potentials. A 4th order Adams-Bashforth-Moulton scheme is used for time marching of rigid-body motion histories of the individual bodies and evolution of the free-surface including the gap region in which large resonant fluid motions occur. A systematic study has been carried out to evaluate the performance of the developed time domain method in simulating the forces and motions as well as the fluid motion in the gap region for the two body system under various arrangements and in different wave-headings. At first, the computed numerical results have been validated and verified with computational and experimental results available in literature for standard geometries such as vertical truncated cylinders and rectangular boxes. Secondly, effectiveness of the damping lid model which is introduced to suppress wave resonance in the gap region is investigated including its influence on maximum sway forces on fixed and floating rectangular barges in side-by-side configurations. Thirdly, comparative studies on absolute and relative motion response for two cases (two rectangular barges, and a FLNG-FPSO + shuttle tanker) in side-by-side arrangement are detailed to bring out the importance of nonlinearities arising due to steep nonlinear incident waves. Finally, coupled motions of the two-body system of an FPSO and a shuttle tanker floating in side-by-side configuration in a steep nonlinear wave field are studied in which the two bodies are connected through hawsers, and also the FPSO is moored to the ground. Additionally there is a fender between the two bodies. © 2016 Elsevier Ltd. Source

Dhavalikar S.S.,Indian Register of Shipping
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2011

Over the years many methods are evolved for computing ship motions and loads, i.e. seakeeping analysis. All of these methods are known as potential flow methods where fluid is assumed to be irrotational. In these methods velocity vector is represented as a gradient of scalar potential phi (f) known as velocity potential. One of the oldest methods is strip theory method where ship is descretized into number of 2D strips. Other methods are panel methods where entire hull is descretized into number of panels. These are also known as Green's function methods. Various formulations of Green's function do exist. In a recent development Green's function methods are extended to Rankine Panel method (RPM) where free surface in the vicinity of vessel is panelized for computations. In RPM Green's function computations are made easy. Again these methods are divided into frequency domain and time domain. Time domain methods take into account various nonlinearities which generally cannot be handled by frequency domain methods. For zero and non-zero forward speed of the vessel different formulations exist. All these methods have their own advantages and disadvantages. Hence, it is very important in the initial design stage to decide on method of seakeeping analysis. Here an attempt is made to compare the results of seakeeping analysis using various tools based on various seakeeping methods. Copyright © 2011 by ASME. Source

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