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Engineering Controls Database

Engineering Solutions: Prototype Slack Tank Monitor

In 2008, over 8 billion pounds of seafood was harvested in the United States earning over $4.4 billion. There are approximately 115,000 harvesters in the United States using a variety of different fishing gear and vessels [NOAA 2010]. Species that contributed the most to this revenue include shrimp, Pacific salmon, pollock and lobster.

Commercial fishing is one of the most dangerous occupations in the United States. Many commercial fishing operations are characterized by hazardous working conditions, strenuous labor, long work hours and harsh weather. During 1992-2008, an annual average of 58 deaths occurred (128 deaths per 100,000 workers), compared with an average of 5,894 deaths (4 per 100,000 workers) among all U.S. workers [DOL 2010].
The Nati0nal Institute for Occupational Safety and Health (NIOSH) maintains the Commercial Fishing Incident Database (CFID), a surveillance system for workplace fatalities in the commercial fishing industry in the United States. A review of the data from 2000-2009 found that 504 commercial fishermen died while fishing in the United States and 24 (18%) were initiated by vessel instability.

Having partially full, or “slack” tanks, may contribute to a vessel rolling-over or capsizing. Vessel stability can be adversely affected by shifting liquids or sloshing fish in tanks, holds, or on decks. In these cases the instability is due to the free surface effect where the center of mass is moving as the ship heels. It is good practice to use checkers to keep loose fish on the deck from sliding while still allowing excess water to drain out the scuppers. It is also good practice to use strong bin-boards running the length of the fish holds to keep fish from shifting with each roll.

The key to maintaining ship stability is keeping holds or tanks either completely full or completely empty. A partially full tank allows the contents to shift; changing the ships’ center of gravity reducing the metacentric height, and that in turn indicates reduced stability. Consistently monitoring the level status of the tanks may provide a warning that the check-valve in the drain line is leaking causing slack tanks. The traditional method of monitoring for slack levels has been the use of an array of float-switches stacked at various heights within the tank. As the tank fills or drains, the corresponding mechanical float-switch opens or closes an electrical connection indicating the content level. Mechanical float switches can become easily fouled and stick in either a closed or opened position. This fouling of moving parts becomes a safety issue when erroneous “full” or “empty” tank levels are reported when there is actually a slack-tank condition.
The Alaska Pacific Regional Office (APRO) of NIOSH focuses on safety issues within the commercial fishing industry. In addition to the statistics collected by APRO on fatalities in the fishing industry, NIOSH partners with companies in the industry to test engineering solutions for hazards found in the fleet. The newest partnership, with B&N Fisheries Company, involves sea-testing the reliability of a prototype slack tank monitor.

NIOSH is currently sea-testing prototype slack tank monitors installed on the B&N Fisheries Company’s trawler F/V Epic Explorer. The F/V Epic Explorer has three fish holds being monitored; forward, mid-ship, and aft. Instead of putting an array of float-switches in each hold, three separate pressure transducers were mounted in the tank fill-lines in the engine room. The sensors measure the head-pressure in the tanks and activate the slack-tank alarm at multiple field-adjustable levels. The only exposed moving part on these sensors is a small, sealed, stainless-steel diaphragm. Inspection access to the pressure-transducers is relatively easy even when the tanks are not empty. Wiring is simplified by keeping the controls and sensors in the relatively dry engine room and near an electrical power source. The installation of the prototype system was finished in early January 2011. Testing and refinements of the monitor will continue to be made throughout the fishing season. As it becomes available, additional information on this control as well as other NIOSH research on fishing safety can be found on twitter, @NIOSH Fishing, or on the NIOSH website
One pressure transducer was installed on each of the F/V Epic Explorer

One pressure transducer was installed on each of the F/V Epic Explorer's three fish holds. A valve isolates each sensor for easy inspection or service without emptying the tanks.

NOTE: The information in this Data Base summary report is taken primarily from Woodward [2011].
DOL [2010]. Injuries, illnesses, and fatalities: Census of Fatal Occupational Injuries (CFOI)--current and revised data. Washington, DC: US Department of Labor, Bureau of Labor Statistics. Available at Accessed September 29, 2011.

NOAA [2010]. Fisheries Economics of the United States, 2008. US Department of Commerce. National Oceanographic and Atmospheric Administration. National Marine Fisheries Service. Available at Accessed September 29, 2011.

Woodward CW [2011]. NIOSH Engineering Solutions: Prototype slack tank monitor. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Alaska Pacific Regional Office.
commercial fishing