V. Anantha Subramanian

Parametric studies on seaworthiness of SWATH ships

Abstract This paper reports seakeeping studies performed on a parametrically varied set of SWATH hull forms. The SWATH form, because of its de-linked nature of design affords many variations of the underwater hull geometry without affecting overall deck length and beam. For a given displacement, the hull form can be varied in terms of length, basic section shape, maximum area of cross section of under water hull and strut water plane shape. Using these variants, a parametric family of hull forms has been generated employing Chebychev polynomial scheme for representing sectional area distribution and using a bi-quintic B-spline based surface definition scheme. Not all designs offer optimal performance in a given sea state. A twin-hull motion analysis program SEDOS has been used to study the motions and other dynamic effects. Setting criteria for operability, these dynamic effects have been quantified into a single value namely, operability index. The approach here fulfils an investigation at the design stage in order to tap the full advantage of the SWATH form. The study brings out a methodology for assessment of the SWATH at the design stage highlighting interesting results related to section shapes and sectional area distribution. Thus, combining a newly developed interactive surface generation scheme with an analysis package, a rapid assessment tool is offered for new design.

An efficient algorithm for strapdown accelerometer-based motion measurement

Abstract The paper is concerned with the measurement of motion of free floating bodies using accelerometers strapped to a cross-arm mounted on the body. The measurement system has been studied with the help of a formulation involving a set of coupled nonlinear initial-value equations involving the angular acceleration components. A PC-based software using the Bulirsch-Stoer technique has been developed to solve the initial-value problem so as to deduce the body motions from the measured accelerations. Suitable filtering strategy has been employed at every stage of numerical integration. The reliability of the strapdown accelerometer system together with software developed has been validated using a “dry” test. Typical motion measurements have been done in all the six degrees of freedom of a tug model in a wave flume. The method is deemed to be an efficient and cost-effective technique suitable for free floating bodies and for large motions.

Simulation of Propeller – Hull Interaction Using Ranse Solver

This work simulates propeller-hull interaction effects by a novel method namely, coupling a Vortex Lattice Method (VLM) with a RANSE solver. The VLM generates the propeller forces based on the inputs of blade geometry, wake at propeller inlet, the required thrust and the propeller revolutions. The propeller forces thus obtained are distributed in the fluid domain at the propeller disk region at specific cells. These cells lie close to the blade coordinates and the forces are assigned to the centroids of cells by means of a user-defined function (UDF). The introduction of the body forces into the fluid domain, emulates the propeller action in the field of flow with consequent influences on the flow kinematics. Therefore the interaction effect between propeller and ship hull is simulated successfully by coupling the two methods. Thereby the kinematical aspects such as effective wake conditions are obtained. The results based on the methodology have been verified by comparison with data from the KCS ship available in recent published literature. The method establishes a successful numerical tank approach in understanding the ship hull-propeller interaction problem.

Estimation of hull-propeller interaction of a self-propelling model hull using a RANSE solver

Viscous flow around the self-propelled hull was investigated at model scale with a RANSE solver using k-ϵ turbulence model by coupling the flow around the hull model with the flow generated by the rotating propeller. Hence, the challenges involved in the coupling of flow past ship hull with propeller flow of lower-order scale have been brought out and addressed here. Propeller rotation is accounted for by mesh motion; the solution domain is represented by a rotating cylindrical domain encompassing the propeller and a fixed rectangular domain around the hull for the remaining part of the solution domain; both these domains are interfaced by sliding interfaces. Comparisons are made between the simulated results and corresponding experimental results for hull resistance, propeller thrust and propeller RPM. It is shown that the flow field in the self-propelling condition is well reproduced in the simulation, and the estimated thrust deduction factor and RPM agree well with the measured ones. Among the various turbulence modelling schemes available, the present study uses the Realizable k-ϵ turbulence model.

On the Prediction of Hydrodynamic Forces Acting on a Ship Moving at Constant Drift

The effect of drift angle on a ship is investigated through towing tank tests and using Computational Fluid Dynamics (CFD). Resistance and wave elevations obtained from the computational study are validated with experimental results. Detailed free surface, mean velocity and pressure flow fields on the hull surface are obtained from the computational study for Fn ranging from 0.16 to 0.22 and for drift angle β = 0, 5 and 10°. The lateral force, yaw moment and asymmetric flow characteristics are brought out in the computational study.Copyright

Development of autonomous system for scaled ship model for seakeeping tests

This paper presents a work carried out to develop an onboard integrated system for a controlled free running ship model using data acquired from a Motion Reference Unit(MRU). Using the reference data, the system performs processing and control through wireless communication (Wi-Fi). The application of free running model is to investigate the sea keeping and maneuvering characteristics of a ship. In such a scenario there are disturbing external forces and hence a time - position data based closed loop control algorithm is essential. The development provides the nucleus for such a controlled autonomous system. The ship based integrated system consists of azimuth propulsion unit, MRU, central processing unit for control, data link and a battery pack for autonomous operation of the ship model. A proportional controller is used with the feedback from the MRU to perform heading control using azimuth propulsion system. Quality of free running model test is improved with this system as compared to traditional facilities with captive model and follower carriage based system.

Behaviour of water column inside turret and annular space of FPSO vessel

The water column inside the turret of an FPSO typically undergoes sloshing due to speed as well as wave effects. Sloshing can have detrimental effects and therefore needs to be investigated and minimized. Sloshing occurs at the free surface region. As a result of sloshing, the volume of entrapped water in the turret of an FPSO increases and its acceleration due to the ships motions induces inertia loads, which could affect the overall balance of the loads in the system. This paper analyzes the behavior of the water column inside the turret and annular space of an FPSO vessel subjected to different sea conditions. The results from CFD simulations have been compared with experiments.

Investigation on Harmonic Generation of Submerged Elastic Plate Using Wave Flume Experiments

The focus of this research work is towards the understanding of the hydraulic and hydrodynamic performance of submerged rigid and elastic plates when exposed to regular waves and quantifying the energy loss attributed to elasticity. For this purpose, four different plates with different flexural rigidities have been chosen for the wave flume experiments for studying the hydroelastic problem over a range of rigidity parameters. Furthermore, variation of pressures, reflection and transmission characteristics due to regular waves acting on flexible plate were investigated. Based on the systematic experimental study and investigation of the results it was found that the elasticity in the plate significantly improves the performance of the horizontal plate as a wave barrier in the lower B/L (Relative plate width) region where the rigid plate is not effective. The enhancement in energy loss due to higher harmonic generation is about 20–30%.

Foreword to the Special Issue of the International Journal of Ocean and Climate Systems

This Special Issue of the International Journal of Ocean and Climate Systems (IJOCS) brings a select list of research papers which have been short listed from among the large numbers of papers presented at the Third International Conference in Ocean Engineering (ICOE-2009) held at the Indian Institute of Technology Madras (IITM) India during February 2009. Ten papers have been selected from among more than one hundred papers originally submitted under the eight themes in the Conference. The papers have been selected to conform to the aim and scope of the IJOCS. The IJOCS represents a peer-reviewed Journal whose aim is to provide visibility to research in ocean science and technology with focus on systems approach in general. The Journal encourages presentation of works which involve the use of systems techniques to understand ocean processes, to develop ocean resources and facilities and to describe changes in ocean climate and their impact on human activities. The themes of interest include sustainable energy and resource extraction from the oceans, phenomena related to ocean hazard like cyclones and tsunamis and analysis of ocean data related to waves, wind, tides, currents, tsunami and storm surges. While looking to the aspect of ocean resources, research related to transportation is of related significance and IJOCS encourages visibility to quality works in all these areas. The selected papers presented in this Volume conform to the above general themes of interest supported by the Journal. A well balanced set of papers are included. Polnikov presents a numerical model for wind-waves which has been verified for increased simulation accuracy on the basis of comparison with wave data from buoys. Field measurements and modelling of the morphodynamics of the Wadden sea in the German North sea coast are presented by Albers and Lieberman. The behaviour of offshore compliant structures in terms of their ringing response is presented by Chandraseksharan and Jain. The challenging preparatory test performed by a new class of autonomous underwater vehicle is described by Bose, Lewis and Adams. Under coastal hydrodynamics, the breaking wave impact pressure on a vertical wall is presented by Rajasekharan, Sannasiraj and Sundar. Under research related to transportation, Janardhanan and Krishnankutty present a methodology for application in surface ship manoeuvring using RANS based CFD. Bertram, Kim and co-authors present hydro-elastic simulation of stern slamming and whipping investigated for an LNG carrier. Continuing in the same theme area, Senthil and Subramanian present a numerical method for ship hull-propeller interaction using a RANSE solver. Spectral fatigue analysis of Jacket structures in realistic field environment in the Bombay High fields is presented by Nallayarasu, Goswami and co-authors. A method for accurate computation of run-up height for Tsunami occurrence is presented by Behera, Murali and Sundar with validation based on the real scenario of Tsunami occurrence in the south Indian east coast. The study of bimodal data from laboratory generated abnormal waves is presented by Petrova and Soares. The peer reviewed camera ready prints submitted by the authors are brought out in this volume. It is hoped that readers of IJOCS belonging to the research community as well as the general readers will immensely benefit from this balanced mix of contributions presented.

Experimental Study on Hydrodynamic Behavior of Jacket Spar Platform

This paper presents the experimental study on the hydrodynamic behavior of a jacket spar platform designed for 10 MLD (million liters per day) desalination plant in the water depth of 375 m with the pay load of 50×103 kN. The surge, heave and pitch natural periods from free decay tests are presented and validated with the numerical results obtained using NAOS (Non-linear Analysis of Offshore Structures) developed in IIT Madras. The jacket spar is modeled using sixty beam elements and each mooring lines by ten beam elements with axial prestress to model the pretension. The beam elements with axial prestress do not take any compressive load. The surge, heave and pitch response and mooring line tension of the jacket spar platform for three wave height range of 2.6m to 8.75m in the wave period range of 7.82s to 19.00s are presented. The maximum surge of 0.5% of water depth, the maximum heave RAO of 0.1 and maximum pitch of 1.54° are within the operational limits. The maximum tension in mooring line is only 10% of breaking strength.Copyright