Image courtesy Mark W. Atherton
SONAR IN THE 70S The marriage between sonar and ROVs started in the mid1970s, when Ametek Straza kick started a technological leap in the fledgling ROV business. They launched the first commercial Continuous Tone, Frequency Modulated (CTFM) sonar for ROVs. Although this sonar provided a very low-resolution image, it had a unique audio output that allowed a pilot to acoustically guide an ROV to a target. With CTFM sonar technology, the operator could hear the echoed return from a target. The frequency component of the audio provided a reasonable idea as to the target’s range. A distant target had a high-frequency response that became progressively lower as the ROV closed the gap to the target. The introduction of the 971-series scanning sonar by Mesotech Systems Ltd. (now Kongsberg Mesotech Ltd.) in 1983 was a game changer for the ROV world. With scanning sonar, the transducer turns (using a stepper motor), so that from a fixed position it provides up to 360˚ of acoustic view. The display resolution of the 971 was far better than its CTFM counterpart and the sonar head was small enough to fit on most observation-class and larger vehicles. The 971 was not only an obstacle-avoidance sonar. With a 1.8˚ horizontal beam width, it also had the resolution to define shape and locate small targets. The 971 was soon being used on ROVs in the oil patch of the Gulf of Mexico
for pipe-lay projects, rig support, underwater construction, and search and salvage operations. But scanning sonar technology came with a price – early systems were in the fifty-thousand-dollar range. There was another problem with the early scanning sonar: it used an open-faced transducer attached to a shaft with o-ring seals to keep water out of the electronics housing and motor end. In depths greater than 1000 meters, and sometimes less, even a slight bump to the transducer often resulted in deflection of the shaft/oring and water leaking into the sonar head electronics. Effectively, a minor accident turned the sonar head into a very expensive aquarium!
DILEMMA OF THE DEEP By the late 1980s, oil companies were moving to drill in deeper water, and the operational depth of both ROVs and onboard sensors subsequently increased. The push was on to solve the water ingress problem. The answer turned out to be covering the transducer with an oil-filled dome and using a bulkhead penetrator to isolate the transducer/motor from the electronics. This domed-head design was adopted by scanning sonar manufacturers and significantly increased the depth rating of different scanning head models. Problem solved – sort of!
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