Some Aspects of Very Large Offshore Structures 



From the measurements of the total horizontal wave force 

 on the cylinders, the mean value which represents the constant resis- 

 tance or drift force, was also determined. In figure 4 the results are 

 given for a particular case, together with the calculated values. 



In order to check a more extreme case, calculations and 

 measurements were performed for a pyramid-like structure, of 

 which the shape is given in figure 5. Due to the sharp edges, it is 

 difficult to represent this object by means of a source distribution. 

 The number of sources, applied in the computer calculations, amounts 

 to 92. 



The results of the calculations and the measurements of the 

 horizontal wave force on the structure are given in figure 6. Even in 

 this case the agreement is reasonable. 



Some aspects of the interaction between structure and waves 

 were studied in greater detail for a circular model, which - at a sca- 

 le ratio of 1 : 100 - can be regarded as the representation of a cylin- 

 drical island, for instance a storage tank, 96 m in diameter, fixed 

 to the bottom in 50 m deep water and extending to above the water 

 surface. The pressure distribution on this model was determined in 

 regular waves with varying periods. To this end the model was pro- 

 vided with four very sensitive pressure gauges. These gauges were 

 placed on a vertical line at regular distances, to obtain the distribu- 

 tion of the pressure over the water depth. The measurement of the 

 variation of the pressure along the circumference of the cylinder was 

 established by rotating the model. In figures 7 and 8 the results are 

 given for ka = 2 and ka = 3, which for a scale ratio of 1 : 100, corres- 

 pond to wave periods of 8 and 10 seconds. In general, the measured 

 pressures closely approximate the calculated values. The diffraction 

 of the waves by the cylinder was calculated with the potential theory 

 and also measured in the basin in a large number of points around 

 the model. Figure 9 shows the calculated wave pattern for ka = 1. 4. 

 The lines in this figure connect the points with equal values of the ra- 

 tio of resulting wave height to incident wave height. In figures 10 and 

 11 the results are given of the calculated and measured wave height 

 behind and in front of the cylinder for ka = 4. Again, the experiments 

 confirm the theoretical calculations. 



II. 6 Wave loads in high, irregular and breaking waves. 



Up till now only sinusoidal waves of low amplitude were 

 taken into consideration. However, for the design of offshore struc- 



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