Some Aspects of Very Large Offshore Structures 



Also the mooring of a tanker to a large circular storage tank was in- 

 vestigated by means of model tests. 



In this paper the following topics will be discussed successively : 



- the calculation of wave loads and wave diffraction, with a compari- 

 son of theoretical and experimental results; 



- anchoring of floating structures; 



- mooring of a ship to an artificial island. 



The object is not to give practical solutions, but to scan the 

 problems and possibilities which occur in the field of hydrodynamics. 



II. WAVE -STRUCTURE INTERACTION 



We shall consider the following aspects of the interaction bet- 

 ween waves and a structure : 



- the pressure distribution on the surface of the body, which has to be 

 known for the structural design; 



- the total wave excited forces and moments, which are important 

 for the design of an anchor system in the case of a floating structure, 

 or, if the body is fixed, for the stability of the structure : the ampli- 

 tude of the vertical force, for instance, must be smaller than the ap- 

 parent weight of the structure in the case of a submerged structure 

 fixed to the bottom; 



- the wave diffraction : if ships are to moor to the structure, it is 

 important to know in which way the incident waves are deformed by the 

 presence of the structure. 



The interaction between waves and a structure is governed by 

 inertial, gravitational and viscous effects. The relative importance 

 of each of these effects depends on the ratios of the wave height and the 

 wave length to the body dimensions. In figure 1 the regions of influen- 

 ce of the different effects are indicated for the case of a vertical cir- 

 cular cylinder (See ref. [l]). From this figure it appears, that gravita- 

 tional effects must be taken into account if ka is larger than 0. 6, or in 

 general, if the wave length .is smaller than approximately five times the 

 body dimensions. This means that, for the structures with which we 

 are dealing here, both the inertial and gravitational effects must be 

 considered. These phenomena can be described adequately by means 

 of the potential theory; this theory, however, presupposes an inviscid 

 fluid. Fortunately, it can be stated that for large structures the poten- 

 tial forces are predominant to such a degree, that the viscous effects 

 can be neglected. 



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