9780521896252_frontmatter

WAVE FORCES ON OFFSHORE STRUCTURES

A thorough understanding of the interaction of waves and currents withoffshore structures has now become a vital factor in the safe and eco-nomical design of various offshore technologies. There has been a sig-nificant increase in the research efforts to meet this need. Althoughconsiderable progress has been made in the offshore industry and inthe understanding of the interaction of waves, currents, and wind withocean structures, most of the available books concentrate only on prac-tical applications without a grounding in the physics. This text strivesto integrate an understanding of the physics of ocean–structure inter-actions with numerous applications. This more complete understandingwill allow the engineer and designer to solve problems heretofore notencountered and to design new and innovative structures. The intent ofthis book is to serve the needs of future generations of engineers design-ing more sophisticated structures at ever-increasing depths.

Dr. Turgut “Sarp” Sarpkaya is an internationally recognized authority influid mechanics research and was named by Cambridge University as oneof the world’s one thousand greatest scientists. “Sarp,” as he is known tofriends and colleagues, is the recipient of the Turning Goals into RealityAward by NASA, and he was selected Freeman Scholar by the Amer-ican Society of Mechanical Engineers (ASME). Sarpkaya received hisPh.D. from The University of Iowa, followed by postdoctoral work at theMassachusetts Institute of Technology. He was the Thomas L. FawickDistinguished Professor at the University of Nebraska and taught at theUniversity of Manchester. He was named Professor and Chairman ofMechanical Engineering at the Naval Postgraduate School in 1967 andDistinguished Professor in 1975. His research over the past 50 years hascovered the spectrum of hydrodynamics. His oscillating flow tunnel andthe vortex-breakdown apparatus are two among several unique researchfacilities he has designed. Sarpkaya has published more than 200 papersand has explored for the Defense Advanced Research Projects Agency(DARPA) numerous classified projects dealing with the hydrodynamicsand hydroacoustics of submarines.

He served as chairman of the Executive Committee of the FluidsEngineering Division of ASME and the Heat Transfer and FluidMechanics Institute. He is a Fellow of the Royal Society of Naval Archi-tects and Marine Engineers, Fellow of the ASME, and Associate Fellowof the American Institute of Aeronautics and Astronautics.

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WAVE FORCES ON OFFSHORESTRUCTURES

Turgut “Sarp” Sarpkaya






cambridge university pressCambridge, New York, Melbourne, Madrid, Cape Town, Singapore,Sao Paulo, Delhi, Dubai, Tokyo

Cambridge University Press32 Avenue of the Americas, New York, NY 10013-2473, USA

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c© Turgut Sarpkaya 2010

This publication is in copyright. Subject to statutory exceptionand to the provisions of relevant collective licensing agreements,no reproduction of any part may take place without the writtenpermission of Cambridge University Press.

First published 2010

Printed in the United States of America

A catalog record for this publication is available from the British Library.

Library of Congress Cataloging in Publication data

Sarpkaya, Turgut.Wave forces on offshore structures / Turgut Sarpkaya.

p. cm.Includes bibliographical references and indexes.ISBN 978-0-521-89625-2 (hardback)1. Offshore structures – Hydrodynamics. 2. Wave resistance(Hydrodynamics) 3. Ocean waves – Mathematical models. I. Title.TC1665.S27 2010627′.98–dc22 2009034862

ISBN 978-0-521-89625-2 Hardback

Cambridge University Press has no responsibility for the persistence oraccuracy of URLs for external or third-party Internet Web sites referredto in this publication and does not guarantee that any content on suchWeb sites is, or will remain, accurate or appropriate.






The book is dedicated to the memory of Herman Schlichting,

who shared with me his time and inspiration for an unforgettable year at

AVA/Gottingen in 1972.






Contents

Preface page xi

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Classes of offshore structures 3

2 Review of the fundamental equations and concepts . . . . . . . . . . . . . . 7

2.1 Equations of motion 72.2 Rotational and irrotational flows 92.3 Velocity potential 112.4 Euler’s equations and their integration 132.5 Stream function 152.6 Basic inviscid flows 162.7 Force on a circular cylinder in unseparated inviscid flow 172.8 Slow motion of a spherical pendulum in viscous flow 202.9 Added mass or added inertia 222.10 An example of the role of the added inertia 302.11 Forces on bodies in separated unsteady flow 312.12 Kinetic energy and its relation to added mass 33

3 Separation and time-dependent flows . . . . . . . . . . . . . . . . . . . . . . . 39

3.1 Introduction and key concepts 393.2 Consequences of separation 423.3 Body and separation 473.4 Strouhal number 523.5 Near wake and principal difficulties of analysis 563.6 Lift or transverse force 583.7 Free-stream turbulence and roughness effects 583.8 Impulsively started flows 64

3.8.1 Introductory comments 643.8.2 Representative impulsively started flows 65

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viii Contents

3.9 Sinusoidally oscillating flow 693.9.1 Introduction 693.9.2 Fourier-averaged drag and inertia coefficients 753.9.3 Experimental studies on Cd and Cm 763.9.4 Transverse force and the Strouhal number 853.9.5 Roughness effects on Cd, Cm, CL, and St in SOF 903.9.6 A critical assessment of the Morison equation 953.9.7 Oscillatory plus mean flow or the in-line oscillations of a

cylinder in steady flow 983.9.8 Forced oscillations of a cylinder in a trough 1043.9.9 Oscillatory flow in a smaller U-shaped water tunnel 107

4 Waves and wave–structure interactions . . . . . . . . . . . . . . . . . . . . . 109

4.1 Surface gravity waves 1094.1.1 Linear wave theory 1104.1.2 Higher-order wave theories 1154.1.3 Character of the forces predicted 1174.1.4 Random waves 1194.1.5 Representative frequency spectra 121

4.2 Wave–structure interaction 1224.2.1 Principal factors of analysis and design 1234.2.2 Design wave and force characterization 1254.2.3 Force-transfer coefficients 1274.2.4 A brief summary of the literature giving explicit Cd and

Cm values 1354.2.5 Suggested values for force-transfer coefficients 1384.2.6 Effects of orbital motion, coexisting current, pile

orientation, interference, and wall proximity 1394.2.7 Pipe lines and wall-proximity effects 1554.2.8 Wave impact loads 166

5 Wave forces on large bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

5.1 Introduction 1725.2 The case of linear diffraction 1755.3 Froude–Krylov force 1765.4 The case of a circular cylinder 1775.5 Higher-order wave diffraction and the force acting on a vertical

cylinder 1815.6 Closing remarks 184

6 Vortex-induced vibrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

6.1 Key concepts 1866.1.1 Nomenclature 190






Contents ix

6.2 Introduction 1926.3 Added mass, numerical simulations, and VIV 2036.4 Governing and influencing parameters 204

6.4.1 Parameter space 2046.4.2 Uncertainties 2076.4.3 Mass and structural damping 2086.4.4 fvac, fcom, and added mass 209

6.5 Linearized equations of self-excited motion 2116.6 Unsteady force decomposition 213

6.6.1 Lighthill’s force decomposition 2156.7 Limitations of forced and free vibrations 217

6.7.1 General discussion 2176.7.2 Amplitude and phase modulations 219

6.8 Experiments with forced oscillations 2236.8.1 A brief summary of the existing contributions 2236.8.2 Detailed discussion of more recent experiments 227

6.9 The wake and the VIV 2326.10 Self-excited vibrations 2406.11 Discussion of facts and numerical models 2506.12 Suppression devices 2526.13 Evolution of numerical models 2556.14 Experiments with advanced models 262

7 Hydrodynamic damping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

7.1 Key concepts 2657.2 Introduction 2657.3 Elements of damping 267

7.3.1 Stokes canonical solutions 2687.4 Previous investigations 2707.5 Representative data 272

7.5.1 Solid cylinders 2727.5.2 Perforated cylinders 2747.5.3 Three-dimensional instabilities 2757.5.4 Closing remarks 280

References 285

Index 321






Preface

In 1857, William Thomson (Lord Kelvin) wrote to Sir George GabrielStokes, “Now I think hydrodynamics is to be the root of all physical science,and is at present second to none in the beauty of its mathematics.” How-ever, 117 years later, Sir Geoffrey Ingram Taylor had a thoughtful reminder(1974): “Though the fundamental laws of the mechanics of the simplest flu-ids, which possess Newtonian viscosity, are known and understood, to applythem to give a complete description of any industrially significant process isoften far beyond our power.”

The ultimate requirement for the understanding and quantification of theocean–structure interaction is the understanding of the physics of turbu-lence, separation, and computational fluid dynamics. This is particularly truefor the extraction of oil and gas. Computers (virtual simulations) and experi-ments (i.e., what can be calculated and what can be measured) have and willcontinue to have unsurpassable limitations. Even if one were able to calcu-late a given fluid–structure interaction, with sufficient time and resources,one will not be able to cover the entire parameter space. Thus, approx-imations, experiments, experience, successes and failures, empirical equa-tions, and virtual modeling will continue to define the limits of our powerto design structures that serve the intended purposes. Experience, physicalinsights, computers, past regrets, and future failures will continue to definethe form and the substance of ocean–structure interactions. Developing thetechnology and carrying out the physical processes to discover and extractoil and gas, particularly from greater depths at increased risks and cost,have become one of the major challenges facing the industrialized countries,notwithstanding the efforts to develop alternate sources of energy.

The impetus for the offshore industry came from the tragic failure of theTexas Tower No. 4 (a U.S. Air Force radar platform) in January 1961, whichwas quite recent, considering the fact that humans have occupied this planetfor millions of years. Such tragedies have precipitated the need to under-stand wave–structure interaction, tower dynamics in random seas, and gusts

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xii Preface

at ever-increasing depths in friendly as well as hostile environments. Thepre-computer era and unsophisticated electronic age had to go through theirown evolutions to alleviate the frustrations of the offshore engineer. Nowthe attention of the offshore engineer is engaged with the problems of howto study, measure, and observe the ocean environment and how to exploitits riches. These tasks require not only a periodic assessment of the state-of-the-art and definition of research goals for the foreseeable future, butalso the understanding and use of virtual modeling, because the fundamen-tal and physical research and numerical simulations are cheaper than regretand unknown conservatism.

The principal difficulties stem from the problems associated with the de-termination of the state of the sea, the three-dimensional and random natureof the ocean–structure interactions, quantification of the forces exerted onstructures, and the lack of fluid-mechanically satisfying closure model(s) forturbulence. The collective creative imagination of many paved the way tophysics-based laws, approximate models, and enlightening explanations ofinfinitely complex (but beautiful) phenomena.

Considerable progress has been made during the past few decades. Theseminal experimental studies of Keulegan and Carpenter in 1956 inspiredmuch experimental work and led, in the mid-1970s, to a better understand-ing of the vortex trajectories about bluff bodies, evolution of the transversevortex street, three-dimensional instabilities and damping at very low K val-ues, effects of the aspect ratio and three-dimensionality of flow along a cylin-der, flow about tube bundles, vortex-induced oscillations, dynamic response,separation-point excursions, and to the demonstration (by this writer) of thedependence of the force coefficients not only on K (the Keulegan–Carpenternumber) but also on β = Re/K = D 2/νT (first conceived and used by Sarp-kaya in 1976). This brought a new age of experiments in the laboratoriesand in the field, reliable data analysis, model tests, new journals, and ever-increasing practical and engineering research with the help of computers(starting with memories as large as 264 bytes in 1956 with an IBM 650). Thiswriter is fortunate to have been an active part of this evolution.

Since the 1970s, the dependence of the force coefficients on K, k/D,and β has led to a better understanding of the vortex trajectories aboutbluff bodies, evolution of the transverse vortex street, separation–pointexcursion, three-dimensional instabilities, flow separation and damping atvery low K values, effects of aspect ratio and three-dimensionality of flowalong a cylinder, coherent and quasi-coherent structures, flow about multi-tube bodies, vortex-induced vibrations, numerical simulations, large-eddysimulation (LES) (an ingenious compromise based on “multilayer” filter-ing), Reynolds-averaged Navier–Stokes (RANS) equations, and detachededdy simulation (DES), which emerged to alleviate the difficulties thatare inherent in both LES and RANS solutions; see, e.g., Mittal and Moin






Preface xiii

(1997) and Breuer (1998). Other models, such as DESIDER (detachededdy simulation for industrial aerodynamics), UFAST (unsteady effects inshock-wave-induced separation), POD (proper orthogonal decomposition),unsteady-RANS, and others are relatively more recent entries into the art ofmodeling.

The friendly and enlightening input of many of my colleagues (Drs. C.Dalton, G. E. Karniadakis, C. C. Mei, D. J. Newman, O. H. Oakley, B. M.Sumer, J. K. Vandiver, M. M. Zdravkovich, and many others) is sincerelyappreciated. I have had the privilege to be assisted by several colleagueswith specific talents and information. Professor Charles Dalton (a longtimefriend) read each chapter and provided valuable input and encouragement.Many graduate students have worked with me: Dr. Gerardo Hiriart Le Bert,Dr. Farhad Rajabi, Dr. Ray L. Shoaff, Dr. Frank Novak, and others workingtoward their M.S. or Ph.D. degrees. Mr. Jack McKay’s ingenuity permeatedthe design and construction of numerous research facilities. I express my sin-cere appreciation for the extensive research support and willing cooperationextended to me through the years by the Office of Naval Research and theNational Science Foundation. Finally, I am truly grateful to Mrs. Irma Fink,Ms. Greta Marlatt, and Mr. Jeff Rothal, who have found and delivered withincredible speed hundreds of references. For all of these I express to themand the Naval Postgraduate School my special gratitude. I owe my specialthanks to Peter C. Gordon, Senior Editor of Cambridge University Press,for his friendly advice and encouragement during the past two years.

I am deeply indebted to Professor Gunther F. Clauss for his graciouspermission to use the drawings and photographs of his fixed platforms andguyed and compliant towers.

This book is dedicated to the designers and builders of offshore structuresand to the researchers in this field. Their concern for the advancement of thestate of the art motivated my work. I sincerely hope that my efforts, modestrelative to their monumental achievements, will meet their approval.

T. “Sarp” Sarpkaya, California

www.omae2006.com

A video is presented by Sarpkaya in the above Web site in black and white.It depicts the motion of vortices for about two minutes and may be playedover and over again.

In addition, there is a PowerPoint presentation by Sarpkaya on fluiddamping (Chapter 7). It is provided as a courtesy of Dr.-Ing. Walter L.Kuehnlein ([email protected]), and [email protected] [email protected]. To them, I express my gratitude.






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9780521896252_frontmatter