ABSTRACT

Irregular water wave kinematics have recently been measured in the wave flume at Norwegian Hydrodynamics Laboratones (NHL) in Trondheim, using Laser Doppler Velocimetry (LDV) equipment (Skjelbreia et al, 1989 and 1991) The use of Wheeler stretching theory to estimate water wave kinematics in Irregular waves is recommended to represent the best fit to the measurements Reviewing available literature on water wave kinematics, Gudmestad (1992) waves at the same conclusion

Uncertainties in the estimate of water wave kinematics is, however, shown to represent one of the man sources of uncertainties for the calculations of forces on slender structures like jackups m relatively deep water The need to further determine the uncertainty in the water wave kinematics prediction is pointed out (Haver and Gudmestad, 1992)

The data from the NHL tank tests have recently been reassessed and revised estimates of the uncertainties in wave kinematics are presented Both the deviation from Wheeler theory and the spreading of the results have been estimated as function of depth below the surface The influence of the revised estimates of the uncertainty in water wave kinematics on the overturning moment of a drag dominated structure is reviewed

INTRODUCTION

In determining the response of offshore structures, it is of utmost importance to determine, in the most correct manner, all factors which contribute to the total force acting on these structures Applying the Monson formula (Monson et al, 1950) to calculate forces on offshore slender structures, uncertainties related to the understanding of the wave climate, the hydrodynamic force coefficients and the kinematics of ocean waves represent the most important contributions to the uncertainties in the prediction of the total forces on these structures (Haver and Gudmestad, 1992)

Traditional calculation of forces on offshore structures involves the use of regular waves with the following non-linearity incorporated

  • use of regular wave theories incorporating higher order terms

  • use of Monson equation having a nonlinear drag term

  • inclusion of the effect of the free surface by integrating all contributions to total forces and moments from the sea floor to the free surface of the waves

In order to describe the sea more realistically, the ocean surface is to be described as an menular sea surface represented by its energy spectrum The associated decomposition of the sea surface is given as a linear sum of linear waves The total force is found by integrating the contribution from all components in the wave spectrum to the free surface The kinematics of each component must therefore be determined It can easily be shown, however, that the higher frequency wave components obtain unrealistic high velocity values above the free surface when a linear Airy velocity profile is assumed (Gudmestad, 1990) This problem has been given particular attention by the oil industry which is constantly improving its models for calculation of forces on offshore structures

A real sea surface may deviate from the Gaussian surface obtained from the superposition of Airy waves The effect of a possible deviation from the Gaussian assumption is herein assessed by showing results for various values of the coefficient of skewness

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