SOME ASPECTS 

 OF VERY LARGE OFFSHORE STRUCTURES 



G. van Oortmerssen 



Netherlands Ship Model Basin 

 Wageningenj Netherlands 



ABSTRACT 



Due to the fast development of the offshore industry, 

 there is a rapidly increasing demand for very large 

 unconventional offshore structures, both floating and 

 fixed to the bottom, to be applied for storage and pro- 

 duction purposes. The general hydrodynamic aspects 

 of these big objects will be summarizedin this paper. 

 In the case of floating structures, the drift force is 

 relatively important and consequently resonance phe- 

 nomena can occur in the anchor lines. Therefore, in 

 rather shallow water a structure fixed to the bottom 

 will be preferred in many cases. 



From calculations and model experiments it appeared 

 that the wave loading on a large object and the wave 

 pattern around it can be calculated with great accura- 

 cy with a diffraction theory. 



As an example a cylindrical storage tank - 96 m in 

 diameter, fixed to the bottom in 50 m deep water and 

 extending above the water surface - will be discussed. 



This example is hardly hypothetical, since structures 

 with comparable dimensions are in the design stage 

 or under construction at present. The wave pressure 

 on the tank and the wave diffraction as calculated with 

 the potential theory are compared with measurements. 

 The agreement is very good. 



From the wave pattern around the tank it was 

 found, that it can be advantageous to moor a tan- . 

 ker immediately to the tank. Model tests were con- 



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