WAVE ENERGY EXTRACTION BY CRESCENT SHAPED COLUMNS FOR STATION KEEPING 

 OF FLOATING OCEAN PLATFORMS - HYDRAULIC MODEL AND FEASIBILITY STUDY 



Technical Note N-1156 



ZF 38,512.001.017 

 03-001 



by 



C. L. Liu, Ph.D. and R. H. Fashbaugh, Ph.D. 



ABSTRACT 



Fixed or floating column-supported platforms in the ocean experience 

 direct and reverse wave drag caused by the oscillating water particle 

 velocity. The drag coefficient for the direct flow can be different 

 from that for the reverse flow if the cross-sectional area of the sup- 

 porting column is not symmetric about the column axis. Therefore, a net 

 wave drag force theoretically can be produced. The purpose of this 

 study was to determine whether this concept can be utilized to reduce 

 the power requirements for positioning large floating platforms such as 

 the proposed Mobile Ocean Basing System (MOBS). 



Two support column cross sections were chosen for evaluation; a 

 semi-circle and a crescent shape which is formed by two intersecting 

 circles (the larger circle having a radius 1-1/2 times that of the smal- 

 ler circle) . A circular cross section was included to provide a refer- 

 ence. Free drift velocity tests of three small scale tri-column plat- 

 forms with the three different cross sections were made in a small wave 

 tank as a preliminary experiment. Based on the results of this drift 

 experiment wave drag measurements were made with larger single column 

 models of 4 inch diameter and 8 foot length in a large wave tank. Free 

 drift velocity tests were also conducted with a 14 inch by 17 inch rec- 

 tangular model floating platform with 36 supporting columns to aid in 

 evaluating the concept for MOBS use. 



The results of the tests show that there is a net wave force oppo- 

 site to the direction of wave travel on the column with the crescent 

 cross section. However, an analytical study based on the test data 

 shows the rate of energy extraction from waves by crescent shaped columns 

 for sea state 5 to be about 67„ of the energy required for station keeping 

 of a large floating platform. Hence, the concept does not seem applica- 

 ble to large floating ocean platforms. 



Approved for public release; distribution unlimited. 



