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Journal of Fluids and Structures

Journal of Fluids and Structures 27 (2011) 1134–1135

doi:10.1

journal homepage: www.elsevier.com/locate/jfs

Book review

Wave Forces on Offshore Structures, Turgut Sarpkaya, Cambridge University Press, Cambridge (2010). (£60.00,hardcover), ISBN: 9780521896252

This new book by Sarpkaya on time-dependent flows is a long-awaited follow-up to his first book ‘‘Mechanics of Wave

Forces on Offshore Structures’’ by Sarpkaya and Isaacson, 1981. This new book is much more than an up-dated version of theSarpkaya and Isaacson book and this new version will be very difficult to supplant. There is a historical aspect to the newbook, which offers additional physical insight. The Sarpkaya and Isaacson book became out of-print some years ago. Itsunavailability as of late had driven up its purchase price to $850 on Amazon.com, which is an amazing price compared toits original purchase price and its longevity. It also had become the most stolen book from libraries in the USA, which is asignificant measure of its success as an engineering reference book.

Sarpkaya begins this new tome with a routine introduction to the subject of offshore structures and the equationswhich govern the hydrodynamics on these structures. It is his discussion of added mass in Chapter 2 that makes thissection distinctive. He starts with a review of the history of added mass, dating back to DuBuat and then to Bessel’sexperiments with pendulums, in both air and water, at very low amplitude and Reynolds number and discusses the addedmass, or added inertia, concept in great detail. The reader is referred to the discussion in the book for the specific details.What is important to mention here is that he debunks the concept of the vortex force, which was suggested by Lighthill(1986) and further developed by Leonard and Roshko (2001). The Morison equation expression for a time-dependent forceof an oscillating flow on a circular cylinder is similar in concept to the Lighthill description. In the Morison equation, bothdrag and inertia coefficients are determined based on cyclic averaging which does not yield either a constant inertia (oradded mass) coefficient or constant drag coefficient. This Lighthill approach considers the time-dependent force to becomposed of an ideal inertia force and a velocity-square-dependent drag force. In this description, the term ideal inertiaforce is based on the inertia coefficient having the mathematically determined values of 2.0 for a circular cylinder and 1.5for a sphere for the case of the body at rest and the fluid accelerating. As Sarpkaya so eloquently states, this is a conceptthat was shown to be invalid by Stokes (1851). The viscosity and the changing state of the wake do affect the added masswhich means that the added mass is not a constant value, but is clearly time-dependent. The actual added mass dependsfully on the time-dependent nature of the flow in a time-dependent flow. Practically, what this means is that, ‘‘once theflow is set in motion, disturbed by separation and vortex-shedding, the instantaneous forces acting on the cylinder cannotbe predicted, except by 2-D (sic 3-D) numerical simulations.’’ This reviewer suggests that anyone wanting a clearunderstanding of added mass should read this section of Sarpkaya’s book with care.

Chapter 3 considers the general idea of separation and time-dependent flows. Of particular interest to the offshoreindustry is oscillating flows. Sarpkaya discusses in detail the early confusion and lack of organization in analyzing data todetermine the drag and inertia coefficients in the Morison equation. It was not until he used the ratio of Reynolds numberto Keulegan–Carpenter number (Sarpkaya, 1976) to define a frequency parameter (which I will call the Sarpkaya number)was any coherence found in the data. He discusses in detail the concept of unsteady separation and the complexity ofpredicting separation. Different cases are discussed and the time dependence of a separation point in an unsteady flow isemphasized. Anyone with an interest in boundary layer separation must read Chapter 3. The major topic addressed in thischapter is unsteady flow past a circular cylinder, the Morison equation and the drag and inertia coefficients from theMorison equation. A main feature of this chapter is the introduction of the Sarpkaya number, which allowed the data to becorrelated in a manner which lead to a greatly improved physical understanding of the force coefficients.

In Chapter 4, the combination of waves and currents is introduced. It begins with a discussion of water wave theoriesand then delves into wave–structure interaction. There is a strong focus on the Cd and Cm values from the Morisonequation. A discussion of surface roughness and its effect on Cd and Cm is a feature of this chapter. Special effects such ascurrents, pile orientation, interference, and wave impact are also addressed.

Wave forces on large bodies represent the focus of Chapter 5. This is a study of wave diffraction and its influence onwave forces. Linear and higher order effects are discussed in this brief chapter.

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Book review / Journal of Fluids and Structures 27 (2011) 1134–1135 1135

Vortex-induced vibrations (VIV), Chapter 6, is the longest chapter in the book. Sarpkaya discusses the phenomenon ofVIV, what its cause is, and what parameters govern this very important aspect of offshore design. A feature of the chapter isself-excited vibrations, which in the opinion this reviewer, is the main focus of VIV on an offshore platform. Suppressiondevices are briefly discussed. The future of VIV analysis, which is the application of the methods of Computational FluidDynamics (CFD) to these complex flow problems, is discussed. CFD offers a capability of understanding VIV, which greatlyexceeds that of experimental studies. However, the CFD approach to describe VIV is still not a mature method of analysisand improved computational methods are still being developed to provide better understanding.

The last chapter is on hydrodynamic damping which is a subject which deals primarily with small amplitudeoscillations of a body. This is a situation in which the body oscillation is caused by ‘‘excitation that comes from outside, i.e.,not caused by the flow itself’’. An example of this is an oscillation at one location, where the current might be zero, causedby VIV at another location. There is no fluid forcing at this one location; it is simply a motion caused by VIV from anadjacent location. This typically occurs at very large Sarpkaya numbers. Hydrodynamic damping is affected by a largenumber of physical parameters. The Honji instability and symmetry breaking are two interesting phenomena that arisefrom damping.

I consider this book to represent a major contribution to the understanding of forces on offshore structures.It represents a summary of the major factors that influence the design of offshore structures. Any self-respecting offshoredesigner needs to have this book in his/her book shelf as a reference. I know that I will use mine.

References

Sarpkaya, T., 1976. Inline and transverse forces on smooth and sand-rough cylinders in oscillatory flow at high Reynolds numbers. NPS Technical Report,NPS-69SL76062, Monterey, CA.

Leonard, A., Roshko, A., 2001. Aspects of fluid-induced vibration. Journal of Fluids and Structures 15, 415–425.Stokes, G.G., 1851. On the effect of internal friction on the motion of pendulums. Mathematical and Physical Papers, Cambridge University Press,

Cambridge, UK.Lighthill, M.J., 1986. Fundamentals concerning wave loading on offshore structures. Journal of Fluid Mechanics 173, 667–681.

Charles DaltonDepartment of Mechanical Engineering,

University of Houston,

N207, Engineering Building 1,

Houston, TX 77204-4792, USA

E-mail address: Dalton@uh.edu