Low Frequency Second Order Wave Exciting Forces on Floating Structures

AuthorsJ.A. Pinkster
Conference/JournalPhD-thesis, Delft University of Technology
DateDec 31, 1979

In this thesis the mean and low frequency second order wave drift forces on bodies moored or stationed in waves are analyzed. Expressions are derived for the second order forces based on direct integration of pressure acting on the wetted part of the body. It is shown that the second order forces in irregular waves may be determined from knowledge of the mean forces in regular waves and the low frequency forces in regular wave groups. In order to calculate the mean and low frequency forces on bodies of arbitrary shape use is made of a three-dimensional linear potential theory computer program. The form of the body is approximated by a distribution of plane facet elements representing a source distribution. For a hemisphere the results of computations of the mean second order horizontal forces in regular waves are compared with analytical results. This comparison demonstrates the accuracy of the computations and the equivalence of the expressions for the second order forces developed in this thesis with respect to an already existing expression based on momentum and energy considerations. In order to demonstrate the validity of the present theory with respect to realistic hull forms, results of computations of the mean second order forces in regular waves are compared with results of experiments on a tanker, a semi-submersible, a rectangular barge arid a submerged horizontal cylinder. For the first three hull forms the mean horizontal forces are compared. For the submerged cylinder the mean vertical forces are compared. The correlation found between results of computations and experiments confirms the general applicability of the theory for predicting the second order forces on a wide range of hull forms. A detailed analysis of components of the mean second order forces shows that for floating vessels the horizontal forces are dominated by a contribution dependent on the relative wave elevation around the waterline of these vessels. For the tanker and the semi-submersible results of computations of the low frequency horizontal force in regular wave groups are compared with experimental results obtained from model tests in regular wave groups and irregular waves. The experimental results from tests in irregular waves are analyzed by means of crossbi-spectral methods. Results of this comparison indicate that, provided certain conditions are fulfilled, the mean second order force in regular waves may be used to approximate the low frequency force in irregular waves. Finally, for a dynamically positioned vessel the results of computations are used to demonstrate the effectiveness of a wavefeed-forward control signal based on relative wave elevation measurements for reducing low frequency horizontal motions induced by drift forces in irregular waves. The results show that model tests confirm the theoretical predictions.

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