To supplement the analytical study a test was conducted in the NCEL 

 wave tank in which the effect of the hydrodynamic forms on the drift 

 velocity of a model floating platform, Figure 2, was determined. The 

 results of this test are presented below. 



ESTIMATION OF HYDRODYNAMIC FORM EFFECTIVENESS 



Force Exerted by Surface Waves 



The force exerted by surface waves on a cylindrical object which 

 extends vertically into a body of water is made up of two components, 

 a viscous drag force and a virtual mass force. The drag force component 

 is proportional to the square of the horizontal component of the water 

 velocity relative to the object and the virtual mass force component is 

 proportional to the relative horizontal acceleration. From Morison-'- the 

 force exerted on a differential section, dz , can be written as: 



dF = 



Cn if ]u 



u + C. 



pffD^ 



du 

 dt 



dz, 



(1) 



where u is the horizontal component of velocity, D the cylinder diameter. 

 p the water mass density, C-q the coefficient of drag, and C^ the 

 coefficient of virtual mass. A theoretical expression is used for the 

 velocity u in relation (1). Since there are several wave theories the 

 choice of which one to use depends upon the degree to which it is 

 desired to approximate non- linear ocean waves. For the purposes of 

 this preliminary study the linear Airy wave theory for deep water will 

 be adequate. Values for the empirical coefficients Cq and Cm are taken 

 from References 1 and 2 to be consistent with the linear wave theory 

 assumption. 



The interest here is obtaining the wave force on a full scale 

 hydrodynamic form by utilizing Equation (1). Because of the unusual 

 shape of the cross section of this form values for the coefficient Cj) 

 and Cm are not available. There is available however from the NCEL 

 model test data which can be used to compare the peak wave force for 

 the crescent shape to the peak wave force for a circular section of 

 equivalent diameter. Utilizing this, the wave force on a hydrodynamic 

 form is considered related to the wave force on a circular cylinder by 

 the relation: 



AF = K F 



cyl 



(2) 



where AF is the difference between positive and negative peak values of 

 the oscillating force on the hydrodynamic form and F^yl is the peak 

 value of force on a circular cylinder subjected to the same wave train. 



44 



