Sea Technology

MAY 2013

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The series of deck units used to transmit, receive and process test data. is 4,400 meters, where the higher output of the oil-flled transducer can be fully exercised. A mooring comprised of three Teledyne Benthos Model 865-A transponding acoustic releases, and equipment to record sound velocity and raw audio signal were deployed to the full 4,400-meter depth. Each release was confgured to operate in key frequencies of interest. These frequencies were selected to highlight the best and worst operational cases for legacy acoustic equipment. One of the release units was confgured to operate with 7.0 kilohertz as the receive frequency, which is one of the two most diffcult operational frequencies for the 865-A release. This diffculty is due to the capabilities of the piezoelectric ceramic element: because of its size, more power is required to achieve the same output levels across the various frequencies, and since the output power of the transmitter remains at 20 watts, the lower frequencies will be at lower output levels. Another release was confgured to operate at 12.0 kilohertz, which is considered one of the most successful operational frequencies in the release unit's feld history. The last was confgured to operate at 14.0 kilohertz, which is considered the second-most diffcult operational frequency for the 865-A release. The diffculty is again due to the capabilities of the piezoelectric ceramic element: the size of the element is larger and thus is not as effcient at generating the higher frequency 14.0-kilohertz signals. The sea trials were conducted with a pair of Teledyne Benthos Universal Deck Box UDB-9000 surface command units confgured with identical software—one confgured with an oil-flled transducer and the other confgured with a potted transducer. Various slant ranges were tested to achieve the maximum operational range using each transducer type. The maximum attempted slant range was 14,570 meters while the minimum slant range was approximately 5,000 meters. The 865-A acoustic release uses a constant waveform tone to transpond and a frequency shift keying (FSK) signal scheme to command the unit to perform tasks such as enable and disable transponding, release and indicate tilt. Each release was designated to receive three transpond requests from each of the surface UDB-9000 units. The response from the 865-A units was used as an indicator of real-world performance for each system. Each unit performed well at the various ranges at the 12.0-kilohertz frequency due to the higher output of both transducer types. However, when operating at the outerlimit frequencies of 7.0 and 14.0 kilohertz, there was a 34 st / May 2013 noticeable improvement when using the oil-flled transducer. Of the three transpond commands transmitted to each of the three release units at the 7.0and 14.0-kilohertz frequencies, the oil-flled transducer had two failures out of the total 30 transpond attempts, while the potted transducer had 17 failures out of the total 30 transpond attempts. The oil-flled transducer saw a 93 percent success rate and the potted transducer a 43 percent success rate with transpond commands. During the command transmission portion of the testing, again at the 7.0- and 14.0-kilohertz frequencies, the oil-flled transducer was able to command the acoustic releases after one or two retransmissions of the FSK commands, whereas the potted transducer had a near 100 percent failure rate. These results were expected because of the difference in the output of the oil-flled transducer and the potted transducer types. At the extreme ranges of 14,570 and 11,951 meters, the potted transducer performed poorly. At these ranges, the potted transducer was only able to transpond from the releases one time and completely unable to command the releases. At the 12.0-kilohertz frequency, the potted transducer was able to achieve reasonable reliability, but multiple retries were required, and multiple command transmissions were required at the various tested ranges. The oil-flled transducer was very reliable at all ranges, and no complete failures to transpond or command the release were observed. This is again due to the increased output and relatively low occurrence of multipath noise in this environment. While these results may have differed greatly in environments with shallow-water depths or shorter operational ranges, in deepwater applications, the oil-flled transducer demonstrated an ability to exceed its specifed maximum range of 10,000 meters. Conclusion Given the results seen with this testing, it is clear that in deepwater environments the higher-output transducer is preferred. However, in high-multipath environments, such as shallow water, the potted transducer may be preferred, though this test program did not provide for that further analysis. While these trials demonstrated the clear advantage of one transducer type, that does not imply it is ideal for all applications. Manufacturers should continue to offer advice on the selection of the encapsulation method that is best suited for user applications. In this case, tank and subsequent sea trials provide significant validation of engineering analyses and allow users to have more certainty in their selection for deepwater acoustic communications. n Adam Lipper is an application engineer at Teledyne Benthos with almost nine years of experience. He has deployed moorings with customers, partners and third-party users, which gives him broad knowledge of the application of acoustic equipment in real-world scenarios. His primary technology focus is with acoustic telemetry modems and acoustic release products. Joe Borden is the product line manager for acoustics and positioning at Teledyne Benthos. He leads the development and application of products for acoustic data telemetry and subsea positioning in commercial, scientifc and government markets. www.sea-technology.com

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