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. © Springer India 2015.

Srinivasan Chandrasekaran

Department of Ocean Engineering

Design

DRILLING PLATFORMS

Dynamics

Education

Stochastic systems

Structural dynamics

ENVIRONMENTAL LOADS

EXPERIMENTAL AND NUMERICAL STUDIES

Hydrodynamic response

OFF SHORE PLATFORMS

PERFORATED MEMBERS

Preliminary analysis

STOCHASTIC DYNAMICS

Structural engineer

OFFSHORE STRUCTURES

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Dynamic Analysis and Design of Offshore Structures

Process of deploying wireless sensor network in offshore structures for structural health monitoring is presented in this paper. Offshore structures are subjected to environmental loads with high uncertainties that are capable of causing damage during their service life. Preventive measures to safeguard the offshore platforms from such damages are not only innovative but also inevitable. Structural damages can be readily identified by monitoring their response behavior deploying dense array of sensors. Deploying wireless structural health monitoring (SHM) system ensures reduced installation time, compact design and cost reduction in comparison to that of the traditional wired systems. Wireless sensor network architecture for monitoring offshore platforms, proposed in the present study will acquire data using network of sensor nodes and transmits to the base station for post-processing. Present study compares the structural responses of offshore tension leg platform under different postulated failure cases through experimental investigations on scaled model. Acquired data are also hosted in the webpage after analyzing and post-processing. The alert monitoring system, which is an integral part of SHM triggers alert messages is an indigenous design. © 2018, JASNAOE.

Journal of Marine Science and Technology (Japan)

Health monitoring of tension leg platform using wireless sensor networking: experimental investigations

Increase in drilling operations in the Arctic region intuits to examine the suitability of platforms under ice loads. This paper examines the dynamic response of triceratops under continuous ice crushing. Effect of ice loads on one and adjacent buoyant legs are examined under different ice velocity, crushing strength and ice thickness. Based on the numerical investigations carried out, it is seen that the response of deck in active degrees-of-freedom is nonlinear, showing higher response with the increase in ice crushing strength. Ice loads cause a shift in the mean response. Limit ice load acting on the buoyant legs increases with the increase in the ice thickness. With the increase in ice crushing strength, set-down increases along with the increase in the mean value of surge; mean and total yaw response also increases with the increase in the ice crushing strength. Increased deck response under ice loads is due to transfer of translational response from buoyant legs to deck by ball joints. However, pitch response of the deck is lesser than that of buoyant legs, verifying the restraint offered by ball joints to transfer rotation. It may be an added advantage of triceratops under ice-infested sea loads. Though this may not be the only sufficient requirement, certainly no transfer of rotational response from buoyant leg to the deck enables a comfortable working environment. From the analysis of triceratops under lock-in ice condition, it is found that the response of the deck and buoyant legs is bounded and the resonance build up is not seen in any degrees-of-freedom due to damping present in the structure, which is an additional advantage of a triceratops. © 2019 Elsevier Ltd

Ocean Engineering

ICE-INDUCED response of offshore triceratops

Offshore platforms are of high strategic importance, whose preventive maintenance is on top priority. Buoyant Leg Storage and Regasification Platforms (BLSRP) are special of its kind as they handle LNG storage and processing, which are highly hazardous. Implementation of structural health monitoring (SHM) to offshore platforms ensures safe operability and structural integrity. Prospective damages on the offshore platforms under rare events can be readily identified by deploying dense array of sensors. A novel scheme of deploying wireless sensor network is experimentally investigated on an offshore BLSRP, including postulated failure modes that arise from tether failure. Response of the scaled model under wave loads is acquired by both wired and wireless sensors to validate the proposed scheme. Proposed wireless sensor network is used to trigger alert monitoring to communicate the unwarranted response of the deck and buoyant legs under the postulated failure modes. SHM triggers the alert mechanisms on exceedance of the measured data with that of the preset threshold values; alert mechanisms used in the present study include email alert and message pop-up to the validated user accounts. Presented study is a prima facie of SHM application to offshore platforms, successfully demonstrated in lab scale. © 2018, Harbin Engineering University and Springer-Verlag GmbH Germany, part of Springer Nature.

Journal of Marine Science and Application

Structural Health Monitoring of Offshore Buoyant Leg Storage and Regasification Platform: Experimental Investigations

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. © 2016 Informa UK Limited, trading as Taylor &amp; Francis Group.

Ships and Offshore Structures

Dynamic analyses of stiffened triceratops under regular waves: experimental investigations

In offshore structural engineering, buoyant leg storage and regasification platform (BLSRP) is one of the recent innovative structural forms that is archived to suit industrial requirements. Proposed platform consists of a deck, which is connected to six buoyant leg structures (BLSs) through the hinged joints while BLS units are connected to the seabed using taut-mooring tethers. The conceived structural form is a hybrid concept, which restrains transfer of both rotational and translational responses from the BLS units to the deck and vice versa. The main advantage is the improved functionality in terms of increase in the storage and regasification capacity of liquefied natural gas. Experimental investigations are carried out on a scaled model of BLSRP (1:150) under regular waves for two different wave approach angles. Free vibration tests are carried out on the scaled model to estimate the natural periods; results show that BLSRP resembles a tethered compliant structure like tension leg platform (TLP) except showing relatively higher stiffness in the yaw degree-of-freedom. Geometric design of BLS units ensures a good recentring capability of the deck in all the translational degrees-of-freedom. Lesser heave response of deck, in comparison to that of the BLS units ensures comfortable and safe operability. Attempted study is a prime-facie towards design and development of offshore production and process platforms that can reduce the cost of oil and gas exploration. © 2016 Informa UK Limited, trading as Taylor &amp; Francis Group.

Dynamic analyses of buoyant leg storage regasification platform (BLSRP) under regular waves: experimental investigations

Dynamic analysis and design of offshore structures

Diffraction of linear waves around a group of dual porous cylinders consisting of a thin and porous outer cylinder with an impermeable inner cylinder is investigated analytically based on the eigenfunction expansion method proposed by Spring and Monkmeyer [Spring BH, Monkmeyer PL. Interaction of plane waves with vertical cylinders. In: Proceedings 14th international coastal engineering conference. 1974. p. 1828-47] and further modified by Linton and Evans [Linton CM, Evans DV. The interaction of waves with arrays of vertical circular cylinders. Journal of Fluid Mechanics 1990;215:549-69]. The present formulation is an extension of the work of Wang and Ren [Wang KH, Ren X. Wave interaction with a concentric porous cylinder system. Ocean Engineering 1994;21(4):343-60], wherein; the interaction of linear waves with a single concentric porous cylinder system was studied. This paper aims at investigating the influence of multiple interactions between the cylinders in the group on the hydrodynamic wave forces, wave run-up and free-surface elevation in their vicinity. Further, the study focuses on the variation of the forces and run-up on the individual cylinders within the group compared to that on isolated cylinders. © 2008 Elsevier Ltd. All rights reserved.

Applied Ocean Research

Interaction of regular waves with a group of dual porous circular cylinders

The wave force on a seawater intake structure consisting of a perforated square caisson of 400 mmx400 mm size encircling a vertical suction pipe of 160-mm diameter is investigated using physical model studies. The porosity of caisson was varied from 1.6 to 16.9%. Regular and random waves of wide range of heights and periods were used. It is found that the force ratio (ratio of the force on perforated caisson to the force on caisson with zero percent porosity) reduces to an extent of up to 60% with increase in porosity of the caisson from 1.6 to 16.9%. The force ratio was found to increase with increase in relative wave height and reduces with increase in relative width. Multiple regression analysis of the measured data points was carried out and predictive equations for wave force ratios are obtained both for regular and random waves. The results of this investigation can be used in the hydrodynamic design of perforated caissons, which are widely used as seawater intake structures. © 2002 Elsevier Science Ltd. All rights reserved.

Wave forces on a seawater intake caisson

The tension-leg platform (TLP) is a hybrid type of offshore platform, fixed for the vertical degrees of freedom and compliant for the horizontal degrees of freedom. The platform structure is connected to the sea-bed foundation by vertical moorings (tension legs) which are kept in tension under all conditions by the excess buoyancy over weight of the platform. Hydrodynamic analysis is an important stage of the design of these structures; model tests are usually done for this purpose. This paper gives the details of an experimental study to analyze the behavior of a 1:50 TLP model subjected to the action of regular and irregular waves. Platform motions in surge, sway and heave directions were measured using accelerometers. Tether tensions were measured using specially developed load cells. Details of instrumentation, testing and analysis to obtain response amplitude operators (RAOs) are explained. The connection of tether to the column, arrangement for adjusting pretension from top, connection of load cell to the tether and provision for changing the angle of incidence of waves are some special features of this experimental study. The work reported will serve as a useful practical guide for similar types of experimental study. In addition, the conclusions on the comparison of RAOs of the model for regular and irregular waves will be useful in planning hydrodynamic model tests. © 1993 Society for Experimental Mechanics, Inc.

Journal	Data powered by TypesetDynamic Analysis and Design of Offshore Structures
Publisher	Data powered by TypesetSpringer India
Open Access	No