R. Eatock Taylor

SECOND ORDER DIFFRACTION FORCES ON A VERTICAL CYLINDER IN REGULAR WAVES

Reexamen de la theorie de Lighthill et Molin et derivee des resultats definitifs. Expressions analytiques pour obtenir la force au second ordre sur un cylindre vertical perce en surface

Variability of hydrodynamic load predictions for a tension leg platform

Abstract A survey of diffraction/radiation analyses has been performed on behalf of the International Ship and Offshore Structures Congress, Committee I.2. This paper presents the results from the survey, including data from 17 organisations plotted in a common format. The variability of the computed hydrodynamic loads and responses is discussed, and it is concluded that there is a degree of uncertainty in results predicted on the basis of commonly used idealisations. Even in long period waves the variability between the computed results is considerable. To obtain reliable results at the shorter periods associated with heave, pitch and roll resonances of a deep water TLP, much finer meshes than those used by most of the participating organisations would be required.

Radiation and diffraction of water waves by a submerged sphere at forward speed

A submerged sphere advancing in regular deep-water waves at constant forward speed is analysed by linearized potential theory. A distribution of sources over the surface of the sphere is expanded into a series of Legendre functions, by extension of the method used by Farell (J. Ship Res. 17, 1 (1973)) in analysing the wave resistance on a submerged spheroid. The equations governing the velocity potential are satisfied by use of the appropriate Green function and by choosing the coefficients in the series of Legendre functions such that the body surface condition is satisfied. Numerical results are obtained for the wave resistance, hydrodynamic coefficients and exciting forces on the sphere. Some theoretical aspects of a body advancing in waves are also discussed. The far-field equation of Newman (J. Ship Res. 5, 44 (1961)) for calculation of the damping coefficients is extended, and a similar equation for the exciting forces is derived.

The exciting force on a submerged spheroid in regular waves

The hydrodynamic problem of a submerged spheroid in waves is analysed based on linearized potential theory. An analytic formulation is derived and demonstrated by considering the problem of a stationary spheroid in head or following seas. Tabulated numerical results are obtained for a spheroid whose major axis is six times the minor axis, submerged at a depth twice the minor axis. Figures for many other cases are also provided. It is suggested that the present method can be extended to the problem of oscillating bodies at forward speed.

MEAN DRIFT FORCES ON AN ARTICULATED COLUMN OSCILLATING IN A WAVE TANK

Abstract An analytical solution is presented for calculating the regular wave induced respones of an articulated column in a wave tank. Extension of the procedure leads to calculation of second order mean drift forces and moments. Hydrodynamic interaction between a cylindrical column and the parallel walls of the tank is shown theoretically to be highly significant and experimental data are presented to corroborate this finding.

DYNAMICS OF OFFSHORE STRUCTURES, PART I: PERTURBATION ANALYSIS

This is the first of two companion papers concerned with an assessment of the non-linear effects implied by the modified Morison equation. In this paper a spectral approach is adopted. The wave excitation is taken as a non-linear function of the Gaussian wave kinematics, while the implied hydrodynamic damping is taken to be linear but time varying. The solution is obtained by a perturbation technique. Results are compared with those of linearization and time domain simulation. The appearance of higher order convolutions of the wave spectra in the present approach leads to increased excitation near the fundamental natural frequency of typical space frame structures, in comparison with linear solutions. The implied hydrodynamic damping, however, plays a major part in limiting resonant responses.

HYDRODYNAMIC FORCES ON SUBMERGED OSCILLATING CYLINDERS AT FORWARD SPEED.

The hydrodynamic problem of submerged oscillating cylinders at forward speed is analysed by linearized potential theory. The numerical method used combines a finite-element approximation of the velocity potential in a region surrounding the cylinder with a boundary integral equation representation of the outer region. This method avoids the calculation of the second-order derivatives of the steady potential due to forward speed, which appear in the body surface condition for the unsteady potential due to the oscillation of the cylinder. Numerical results from the present method for test cases of a circular cylinder in an unbounded fluid and below a free surface are in excellent agreement with the analytical solutions. Further results for elliptic cylinders are provided and the influence of forward speed on the hydrodynamic force on a submerged cylinder is investigated.

BARGE MOTIONS IN RANDOM SEAS - A COMPARISON OF THEORY AND EXPERIMENT

This paper describes a comparison of experimental data & theoretical results for the motions in waves of large flat-bottomed barges having zero forward speed. It was shown that potential-flow theory is in reasonable agreement with experimental data for all motions except roll near resonance. These roll-motion discrepancies due to vortex shedding from bilge-keel edges, are discussed.

DYNAMICS OF OFFSHORE STRUCTURES, PART II: STOCHASTIC AVERAGING ANALYSIS

This is the second of two companion papers concerned with an investigation into the non-linear dynamics of drag dominated offshore structures excited by waves. The equations of motion are approximated by a linear system with a time varying damping term; the excitation however is a non-linear function of the wave particle kinematics. To study the effect of the time varying damping, the resonant response is here examined by means of a narrow band idealization. A stochastic averaging of the governing equations is applied, and the Fokker-Planck equation for the response amplitude probability densities is thereby obtained. From the stationary solution of this equation, second order statistics are derived, together with an expression for the hydrodynamic damping which is equivalent. to the time varying term. Numerical results are given to illustrate the magnitude of this term for a typical framed structure, and to provide comparisons with results obtained by the conventional technique of linearization.

WAVE UPWELLING EFFECTS IN TLP AND SEMISUBMERSIBLE STRUCTURES

Mutual hydrodynamic interaction effects between closely spaced large columns can cause substantial increases in local wave height and in other kinematic quantities. This phenomenon of upwelling is here analysed theoretically, by using a high order hybrid element technique. Results are presented as contour plots and isometrics of local free surface amplitudes in regular waves; linear transfer functions of wave amplitude at a point; mean square wave amplitudes in long crested random seas; and local components of horizontal velocity in regular waves. It is suggested that wave upwelling is an important phenomenon which should be considered in design.