160 lb (in water) array to depths of 500 m in the ocean with a 

 maximum vertical excursion of il m in the wave period spectrum 

 (5 to 20 sec) up to Sea State 6, b) all equipment had to be port- 

 able and capable of being launched from the deck of an AGOR 

 class research vessel at sea, and c) the array had to be self- 

 contained with internal recording because, owing to drift, a 

 small spar can not function properly with an umbilical data 

 cable to the ship. 



Literature on spar systems showed that these objectives re- 

 quired a cross between a FLIP style vessel and a wave pole. The 

 FLIP has enough buoyancy to safely support submerged packages and 

 has enough draft to obtain a natural period of oscillation much 

 longer than the expected wave periods. Wave poles are built with 

 portability in mind, but have little excess buoyancy. 



The resulting spar configuration is shown in Fig. 1. This spar 

 is 106 ft long with a main diameter of 5% in. The surface 

 piercing section is reduced to about 3 in. diameter. Two flooded 

 55 gal drums are connected beneath the spar for virtual mass. 

 Weight in air for the total system, including the captured water 

 and the array, is 2327 lb. This ratio of weight-to-surface cross- 

 sectional area tunes the natural period of the system to about 

 31.4 sec. A pressure-releasable ballast is suspended from the 

 bottom of the virtual mass containers. This additional buoyancy 

 will return the spar to the surface in the event of a structural 

 failure or a seal failure in any spar section. Mounted on the 

 top of the spar are navigational aids for station-keeping by the 

 mother ship. 



This spar system has evolved as a small part of a Navy-funded 

 project to measure ocean temperature structure. Prime concern on 

 this project was to obtain a seaworthy, stable support for the 

 array by isolating the system from surface wave energy as much 

 as possible. No attempt has been made to instrument the spar for 

 actual motion analysis. All measurements obtained have been 

 recorded as pressure fluctuations at the thermistor array or by 

 visual observations. Mathematical analyses of spar buoy motions 

 have been developed by several investigators, but are beyond the 

 scope of this paper. 



THE BUOY SYSTEM 



Spar Tuning 



The basic principle of the spar float is to tune the natural 

 period of oscillation outside the spectrum of the expected wave 

 period. This spectrum for wave energy is normally accepted to be 

 from 5 to 20 sec. A reasonable "ballpark" period of 30 sec was 

 used. The system response can be approximated by the equation for 



168 



