The slow drift oscillations of a moored, large-volume structure are studied in a wave flume. The recordings of the irregular wave input and the resulting mooring forces are analyzed by means of the spectral technique. The experimental results are compared to the existing theory, which is outlined briefly. The results are found to be very sensitive to a coefficient (the reflection coefficient) which therefore should be determined very carefully.
Two major trends in the offshore activity have motivated the present study: The development of large-volume concrete structures and the entrance of the offshore activity into deeper water areas like the northern part of the North Sea. This paper deals with the analysis of the behaviour of moored, large-volume structures in an irregular wave field.
The analysis of this problem has turned out to be complicated. Two different diciplines are involved. Firstly, the properties of the wave field has to be understood. Secondly, the response of the structure to the specified wave field has to be considered. The motion of the structure turns out to take place within two different time scales, the first time scale corresponds to the period of the waves, while the second has periods much longer than the periods associated with the water waves. The oscillations on the latter time scale have been denoted the "slow-drift oscillations". Even though the origin of these oscillations is a second-order effect, it has turned out 1, 2, 3 that these oscillations may be responsible for the major part of the loads on a mooring system. The analysis of these "slow-drift oscillations" will therefore be considered in this paper exclusively.
Analysis of model tests on these oscillations has largely 1, 3 been carried out in the time domain which requires that actual wave records are at hand. Based on a suggestion by Pinkster 2, the analysis of these oscillations will be carried out entirely in the frequency domain throughout this paper. Such an analysis is believe to be advantageous because it would then be sufficient to specify only the statistical properties of the sea surface rather than the whole wave trace.
In order to simplify the analysis of the slow-drift oscillations the surge motion of the structure has been considered only. Theory and results from laboratory tests will be explained, applying regular waves, regular wave groups and irregular waves as well.
An introductory description of this problem has been given by Hsu and Blenkarn 3, while the methods of analysis have been adapted from Remery and Hermans 1 and Pinkster 2. For a more elaborate description of the theory outlined in this paper refer to their papers.