Srinivasan Chandrasekaran

This paper addresses the parametric studies on the impact response of offshore triceratops subjected to low-velocity impact. Triceratops is the object of the present impact study which is one of the new generation offshore complaint platforms with three buoyant legs connected to deck by ball joints. The innovative geometric form and compliant characteristics distinguish triceratops from conventional offshore platforms. The buoyant legs in triceratops are usually designed as orthogonally stiffened cylindrical shell structures. Numerical studies are carried out using Ansys Explicit analysis solver to predict the impact response and local damage of buoyant leg of triceratops. From the impact force time history obtained through explicit analysis, the hydrodynamic time response analysis is carried out in Ansys Aqwa to predict the response of deck and buoyant legs. Parametric studies are mainly focussed on the effect on the location of the indenter, size of the indenter, type of indenter, number of stiffeners and strain-rate definition. Force displacement curves are reported along with detailed numerical observations.

One of the effective ways to reduce the forces on the damaged member is to encompass it with a perforated cover. A scaled model of double-walled perforated outer cylinder, encompassing the inner cylinder is numerically investigated using computational fluid dynamics (CFD) package STAR CCM+ version 7.06.009. Viscous and the turbulence effects due to perforation are focused. With the aid of non-dimensional plots, viscous effects on the scaled model are highlighted. For the structural members encompassed by perforated cover, the developed design charts will be useful in the preliminary design to quantify the velocity (and the forces) for different sea states. Copyright © 2014 by the International Society of Offshore and Polar Engineers (ISOPE).

Effect of free convection on the visco-elastic fluid (Walter - B' type) flow past an infinite vertical plate accelerating in its own plane with constant heat flux is examined analytically. It is found that for given values of Grashof number, Prandtl number and Newtonian parameter; flow velocity at any point increases with the increase in time and non-Newtonian parameter, however, it decreases with both, the heating and cooling of the plate. © 2010 Elsevier Inc. All rights reserved.

This paper describes an innovative method of using a non-buoyant body to harness ocean waves. All the point absorbers are buoyant in nature and move up due to buoyancy and come down because of gravity. The point absorbers are designed to move along the waves to make the device efficient. These devices face excessive stress during the rough weather on account of the extreme motion of waves and cause the total device failure. The present study shows that using a non-buoyant body for conventional point absorber principle is much efficient and safer than any other device proposed till today. A small scale wave energy converter with non-buoyant body was designed, fabricated and tested in small scale wave maker. An electrical generator was coupled with the device to generate electrical energy from harnessed waves. The generator was electrically loaded and the generated power was measured. It was found from the experiments that the proposed device showed a significant improvement in electricity generation and safety during extreme conditions. In addition to the electricity generation, the characteristics of the device were also studied by using various wave and device parameters. © 2012 Higher Education Press and Springer-Verlag Berlin Heidelberg.

Hydrocarbon fire accidents in offshore deck structures are of serious concern due to the confined environment of offshore oil drilling and production platforms, which limits the escape routes. One of the major structural components that are highly affected during fire accidents is the deck plates. The deck plates are usually designed as stiffened plates with high strength steel. The increase in temperature during fire changes the mechanical properties of steel significantly, which may result in strength and geometric instability of the structure. The present study aims to investigate the behavior of a stiffened steel deck plate under different hydrocarbon fire scenarios using sequential thermal and structural analyses using Ansys. The stress distribution and deformation of the plate in time scale are presented, discussing the highly affected region concerning the source of the fire. The fire rating for the steel deck under hydrocarbon fire is also suggested for different fire scenarios considered in the study. It was observed that the deformation of the deck plate varies to the area of deck plate in direct contact with fire, boundary conditions of the plate, and the area of steel plate with elevated temperature beyond which the mechanical properties of steel are highly affected. Also, the numerical investigations of the deck plate under the combined action of mechanical load and hydrocarbon fire had shown an increased deformation in the deck plates, which in turn reduces the fire rating of the plate under all fire scenarios. The effect of creep strain at high temperature on the response of the steel deck plate is also studied for the different fire loading cases.

Geometric forms of offshore complaint structures deployed for ultra-deep waters are modified significantly to conform to higher degree of compliancy. One such recent innovation is the stiffened triceratops, which is discussed in this study. The platform comprises of three buoyant leg structures (BLS), supporting the deck. Each single buoyant leg unit is interconnected with a finite number of stiffeners to make it monolithic and thereby reducing the effect of encountered wave loads. However, BLS units are not interconnected so that their compliancy by design is not compromised. Ball joints are introduced between the buoyant legs and the deck to restrain transfer of rotation from the BLS units to the deck and vice versa. In this study, experimental investigations carried out on the scaled mode of stiffened triceratops to estimate its dynamic response characteristics under regular waves are discussed in detail. Results show that the translational and rotational motion of the deck in all active degrees-of-freedom decreased significantly due to the presence of ball joints while stiffening of BLS units increased the tether tension. It is also seen that the pitch response decreases significantly with the increase in wave period, while this effect is marginal on surge and heave responses.

Presence of the outer perforated cylinder reduces the direct wave impact on the inner cylinder, which has been testified by many researchers. However, the force reduction mechanism, which is complicated due to the wave-porous structure interaction, needs to be addressed in detail. The present study explains the mechanism with the aid of the computational fluid dynamics (CFD) tool STAR CCM+. This package is chosen for its capabilities to simulate viscous and turbulence effects caused by passage of waves. For the present study, flow fields around the twin cylinders with different orientations are examined with and without the outer perforated cover. Mechanism contributing to the reduction of force on the existing structure is explained in physical terms, and force reduction is quantified. The present study has direct application in the retrofitting application of offshore members.

This book attempts to provide readers with an overall idea of various types of offshore platform geometries. It covers the various environmental loads encountered by these structures, a detailed description of the fundamentals of structural dynamics in a class-room style, estimate of damping in offshore structures and their applications in the preliminary analysis and design. Basic concepts of structural dynamics are emphasized through simple illustrative examples and exercises. Design methodologies and guidelines, which are FORM based concepts are explained through a few applied example structures. Each chapter also has tutorials and exercises for self-learning. A dedicated chapter on stochastic dynamics will help the students to extend the basic concepts of structural dynamics to this advanced domain of research. Hydrodynamic response of offshore structures with perforated members is one of the recent research applications, which is found to be one of the effective manner of retrofitting offshore structures. Results of recent research, validated by the experimental and numerical studies are presented to update of the readers. Integration of the concepts of structural dynamics with the FORM-evolved design of offshore structures is a unique approach used in this book. The book will prove useful to the practicing and consulting offshore structural engineers, as also to students and researchers working in the field.

In order to reduce the direct wave impact, coastal and offshore structures are often constructed with one or more perforated layers. Several permeable breakwaters and docks have been deployed in coastal protection measures to reduce direct impact due to encountered wave loads and to reduce wave reflection in front of these structures as well; compared with the traditional ones, such structural forms are found to be more economical. Such structures result in lesser surface fluctuation in harbours due to the low reflection, which is vital for loading and unloading of ships. Deploying permeable breakwaters posses other advantages namely: i) increasing water circulation; ii) retaining water quality and iii) enhances coastal protection. Emerged and submerged perforated cylindrical structures reduce wave-structure interactions and scouring problems considerably, but their use on floating structures is scarce in the literature. This study is focused on detailed experimental investigations carried out on impermeable inner cylinder encompassed by a larger outer cylinder with perforatios along its length. By varying the porosity and perforatio diameter, their influence on the hydrodynamic response of the cylindrical member is highlighted through the current study. The conclusions on the comparison of forces in the cylinder with and without perforated cylinder coverage are presented. Copyright © 2011 by ASME.