Underwater Inspection Of Naval Structures

Nondestructive testing (NDT) performed underwater is one of the major maintenance methods to assure the efficient operation of naval structures such as piers, navigational aids, etc. Methods used for NDT of naval structures include general methods such as visual testing, ultrasonics and radiography. Visual testing, supplemented by instruments such as TV cameras and stereo-photography, offers good capability for surface inspection to detect anomalies such as corrosion, impact damage and surface-breaking defects. Ultrasonic and radiographic methods can be applied to a wide range of materials and offer capability to detect both external and internal defects. Recent advances include focused ultrasonic systems and tomography; both hold promise to further improve underwater NDT capability. Vibration analysis and acoustic emission are attractive for maintenance of large structures such as offshore towers. Methods for NDT of metals include magnetic particle, eddy currents and electrical potential. These methods provide information about surface and near-surface defects and about the effectiveness of cathodic protection systems. Coring methods are used for both concrete and wood structures underwater. In addition, mechanical methods for surface hardness and strength and magnetic methods to locate metal rebar are used for underwater concrete inspection. Presently visual inspection is necessary in any underwater maintenance program. Pulse-echo ultrasonics has been used successfully to determine wall thickness and to detect and characterize defects. Techniques such as low frequency ultrasound look attractive for testing of concrete and wood structures. Magnetic particle methods are used to locate surface and near-surface defects on underwater steel structures. Gamma ray tomography can characterize structural members of all materials. Improved signal processing and data handling for all methods will be necessary to obtain the most from underwater NDT. Techniques such as adaptive learning and digital signal processing are expected to play strong roles in future testing systems.