out-of-roundness and the structure needs to be checked for conformance 

 with specifications. Local deviations are not too difficult to determine 

 but overall deviations from cross-sectional geometry are quite difficult 

 to determine. For concrete structures, the failure or instability shape 

 is dependent more on overall out-of-roundness deviations than local devi- 

 ations. (For thin-walled metallic structures the reverse is true where 

 the instability shape is controlled by local deviations.) 



Operation 



Inspection . Instrumentation and nondestructive test methods need 

 to be developed to aid in inspecting and monitoring the long-term integrity 

 of the structure and quality of the materials. Long-term strain measure- 

 ments can be obtained with vibrating strain gages and Carlson stress 

 gages; however, these measurements monitor the behavior at discrete loca- 

 tions. Advanced systems that monitor the integrity of structural compo- 

 nents are desirable. Acoustic emission technology is an approach that 

 has potential for monitoring large segments of the OTEC structure (whether 

 fabricated of concrete or steel). When concrete is overloaded or fatigued, 

 material breakdown progresses from microcracking to macrocracking . Every 

 propagating crack releases strain energy and results in an emission of 

 acoustic energy. By monitoring the sound amplitude and number of occur- 

 rences of cracks an indication is obtained as to the severity of material 

 distress. Sensors can be placed at various locations on the structure and 

 by use of triangulation, the location of distress can be identified for 

 more detailed investigation. 



39 



