NOAA Teacher at Sea
Aboard NOAA Ship Oscar Dyson
July 23 – August 11, 2012
- Mission: Alaskan Pollock Midwater Acoustic Trawl Survey
Geographical Area: Bering Sea
Date: August 7, 2012
Latitude: 60°25’90” N
Longitude: 177°28’76” W
Ship speed: 3 knots (3.45 mph)
Weather Data from the Bridge
Wind Speed: 5 knots (5.75 mph)
Wind Direction: 45°
Wave Height: 2-4 ft with a 2 ft swell
Surface Water Temperature: 8.6°C (47.5 °F)
Air Temperature: 8°C (46.4 °F)
Barometric Pressure: 1019 millibars (1 atm)
Science and Technology Log:
In my last blog, we learned about how the scientists onboard the Oscar Dyson use some very sophisticated echo-location SONAR equipment to survey the Walleye pollock population.
Can the Walleye pollock hear the “pings” from the SONAR?
No. Unlike in the movies like “The Hunt for Red October” where submarines are using sound within the human audible range to “ping” their targets, the SONAR onboard the Oscar Dyson operates at frequencies higher than both the human and fish range of hearing. The frequency used for most data collection is 38 kHz. Human hearing ranges from 20 Hz to 20 kHz. Walleye pollock can hear up to 900 Hz. So, the pollock cannot hear the SONAR used to locate them…
Can the Walleye pollock hear the ship coming?
Normally, YES! Fish easily hear the low frequency noises emitted from ships.
If you are operating a research vessel trying to get an accurate estimate on how many fish are in a population, and those fish are avoiding you because they hear you coming, you will end up with artificially low populations estimates! The International Council for the Exploration of the Seas (ICES) established noise limits for research vessels that must be met in order to monitor fish populations without affecting their behavior. Fish normally react to a threat by diving, and that reduces their reflectivity or target strength, which reduces the total amount of backscatter and results in lower population estimates (see my last blog).
That is why NOAA has invested in noise-reducing technology for their fish survey fleet. The Oscar Dyson was the first of five ships build with noise-reducing technology. These high-tech ships have numerous strategies for reducing noise in the range that fish might hear.
There are two main sources of engine noise onboard a ship: machinery noise and propeller noise.
The best acoustic ship designs are going to address the following:
1) Address hydrodynamics with unique hull and propeller design.
2) Use inherently quiet equipment and choose rotating rather than reciprocating equipment.
3) Use dynamically stiff foundations for all equipment (vibration isolation).
4) Place noisier equipment toward the centerline of the ship.
5) Use double-hulls or place tanks (ballast and fuel tanks) outboard of the engine room to help isolate engine noise.
6) Use diesel electric motors (diesel motors operate as generators while electric motors run the driveshaft.
The U.S. Navy designed the Oscar Dyson’s hull and propeller for noise quieting. This propeller is designed to eliminate cavitation at or above the 11 knot survey speed. Not only does cavitation create noise, it can damage the propeller blades.
The Oscar Dyson’s hull has three distinguishing characteristics which increase its hydrodynamics and reduce noise by eliminating bubble sweep-down along the hull. The Oscar Dyson has no bulbous bow, has a raked keel line that descends bow to stern, and has streamlined hydrodynamic flow to the propeller.
To reduce a ship’s noise in the water, it is absolutely crucial to control vibration. The Oscar Dyson has four Caterpillar diesel gensets installed on double-stage vibration isolation systems. In fact, any reciprocating equipment onboard the Oscar Dyson is installed on a double-stage vibration isolation system using elastomeric marine-grade mounts.
Since the diesel engines are mounted on vibration isolation stages, it is necessary to also incorporate flexible couplings for all pipes and hoses connecting to these engines.
Any equipment with rotating parts is isolated with a single-stage vibration system. This includes equipment like the HVAC, the electric generators for the hydraulic pumps, and the fuel centrifuges that remove any water and/or particles from the fuel before the fuel is pumped to the diesel generators.
Low Noise Equipment:
The only equipment that does not use vibration isolation stages are the two Italian-made ASIRobicon electric motors that are mounted in line with the prop shaft. Both are hard-mounted directly to the ship because they are inherently low-noise motors. This is one of the benefits of using a diesel-electric hybrid system. The diesel motors can be isolated in the center of the ship, near the centerline and away from the stern. The electric motors can be located wherever they are needed since they are low noise.
Even the propeller shaft bearings are special water-lubricated bearings chosen because they have a low coefficient of friction and superior hydrodynamic performance at lower shaft speeds resulting in very quiet operation. They use water as a lubricant instead of oil so there is a zero risk of any oil pollution from the stern tube.
Acoustic Insulation and Damping Tiles:
The Oscar Dyson uses an acoustic insulation on the perimeter of the engine room and other noisy spaces. This insulation has a base material of either fiberglass or mineral wool. The middle layer is made of a high transmission loss material of limp mass such as leaded vinyl.
The Oscar Dyson also has 16 tons of damping tiles applied to the hull and bulkheads to reduce noise.
All of these noise-reducing efforts results in a fully ICES compliant research vessel able to survey fish and marine mammal populations with minimal disturbance. This will help set new baselines for population estimates nationally and internationally.
As you can see from the graph above, The Oscar Dyson is much quieter than the Miller Freeman, the ship that it is replacing. You can see the differences in the hull design from the picture below.
Next blog, I will write about new, cutting edge technology that might reduce the need for biological trawling to verify species.
Special thanks to Chief Marine Engineer Brent Jones for the tour of the engineering deck and engine room, and for the conversations explaining some of the technology that keeps the Oscar Dyson going.
I found out drills aboard ships are serious business! Unlike a fire drill at school where students meander across the street and wait for an “all clear” bell to send them meandering back to class, fire drills on a ship are carefully executed scenarios where all crew members perform very specific tasks. When out at sea, you cannot call the fire department to rescue you and put out a fire. The crew must be self-reliant and trained to address any emergency that arises. When we had a fire drill, I received permission from Commanding Officer Boland to leave my post (after I checked in) and watch as the crew moved through the ship to locate and isolate the fire. They even used a canister of simulated smoke to reduce visibility in the halls similar to what would be experienced in a real fire!
Late last night, we finished running our transects! Our last trawl on transect was a bottom trawl which brought up some crazy creatures! Here are a couple of photos of some of the critters we found.
Next blog will probably be my last from Alaska. T-T