PART II. THE FEASIBILITY OF HYDRODYNAMIC FORMS 

 FOR STATION KEEPING OF A LARGE FLOATING PLATFORM 



by 



R. H. Fashbaugh, Ph.D. 



INTRODUCTION 



This section is a preliminary study to determine whether a hydro- 

 dynamic form for extracting energy from ocean waves is economical in 

 providing partial station keeping forces for the buoyant column configura- 

 tion of the Mobile Ocean Basing System (MOBS). 



A brief explanation of the method in which the hydrodynamic forms 

 extract energy from ocean waves is appropriate. A sketch of the subject 

 hydrodynamic form is shown in Figure 1. The form cross section is 

 crescent in shape which provides the characteristic that water flowing 

 around the form in a direction that is into the concave side causes a 

 drag force on the form that is higher than the drag force caused by 

 water flowing at the same flowrate in the opposite direction. Associated 

 with an ideal gravity wave on the surface of a deep body of water is a 

 fluid motion in which the water particles move in circular closed paths. 

 The diameter of these paths decays exponentially with increase distant 

 from the water surface. The force on a circular cylinder, extending 

 vertically into a body of water, that is caused by surface waves is an 

 oscillating force. If the variation in water surface height is neglected 

 the wave force on the cylinder will have peak values in the wave direction 

 and opposite to the wave direction that are equal. When the circular 

 cylinder is replaced by a hydrodynamic form which is oriented so that 

 the concave side faces in the direction of wave travel the oscillating 

 force due to the waves will have a larger peak value in the direction 

 opposite to the direction of wave travel. The oscillating force on 

 the form will therefore have an impulse over a wave period that is 

 opposite to the direction of wave travel which will cause the form and 

 any associated structure to move in that direction. The intent of this 

 study is to estimate the magnitude of this force impulse and to assess 

 its utilization for MOBS station keeping. 



The problem of determining the magnitude of oscillating wave forces 



on the hydrodynamic form is by nature empirical. Testing is therefore 



required to determine empirical coefficients. Adequate coefficients 



were obtained from the NCEL tests described in Part I. In addition to 



the NCEL test results, the results of tests conducted at the University 



12 

 of California-^' on circular and flat piles were utilized. An analytical 



solution for the wave force in which these data are used is presented. 



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