Sea Technology

NOV 2018

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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30 ST | November 2018 R ecent hydrographic observations have revealed warming of bottom water around the world ocean, caused by warming of the Earth's climate system. These observations involve thousands of conductivity, tempera- ture and depth (CTD) casts, in which research vessels lower a CTD instrument package from the sea surface to the ocean floor on a cable. Researchers must conduct as many CTD casts as possible within their limited ship- board time while ensuring safety and produc- tivity. Measurements for climate studies also must be of the highest possible quality in the deep sea, where the signal-to-noise ratio is generally low. A persistent problem affecting the quality of CTD data is rotation and related motions of the CTD package. In this article, we de- scribe the use of a compact underwater slip ring swivel to ensure high efficiency, safety and quality for CTD data in the deep ocean. The Problem of Rotation Most cables are prone to some degree of rotation due to imperfectly balanced torque, thus a CTD package lowered on a cable will also be forced to rotate. But even a perfect ca- ble risks being deformed when the rotation of an imperfectly balanced CTD package forces it to twist. CTD packages rotate as a result of several factors: the location of the center of buoyancy and center of gravi- ty, the righting moment, the package's dynamic stability during free fall, and hydrodynamic resistance. Rotation and related motions (pitch and roll) of the CTD package are common in practice. Attaching a stabilizing fin to the package helps reduce rotation, but the risk remains of the cable developing hockles and other damage. CTD observations can be compromised by kinking, bird-caging or crushing of cables, which may in turn cause insulation ruptures, broken conductors, poor winding and corrosion. During cruise 73 of RV Knorr, for example, several instances of severe kinking, defor- mation and loss of signal in the armored cable occurred, even though the damage was limited to the 10 m at the end of the cable. These mishaps are commonly treated during cruises by detaching the cable from the CTD package and un- twisting it on the ship's deck to remove hockles and loops, or forestalled by attaching a weight to the cable and em- ploying a swivel above the weight, which allows the ca- ble to rotate freely and avoid hockles or kinks. However, for CTD measurements, the need to supply power to and transmit data from the CTD package through the cable rules out the use of a simple swivel. Underwater Slip Ring Swivel A combination slip ring and swivel enables pow- er supply and signal transmission for underwater CTD sensors while preventing problems arising from cable torque. One such apparatus, by Hanayuu Co. Ltd., has been in use since 2017 to support CTD observations during cruises of RV Mirai. Its compact size (21 cm long Compact Underwater Slip Ring Swivel Minimizing Effect of CTD Package Rotation on Data Quality By Dr. Hiroshi Uchida • Yosaku Maeda • Soichi Kawamata A rectangular fin is attached to the CTD frame to resist its rotation. The slip ring swivel is between the cable termination and the CTD.

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