NOAA Teacher at Sea
Onboard NOAA Ship Rainier
June 1-12, 2009
Mission: Hydrographic survey
Geographical area of cruise: Trocadero Bay, Alaska; 55°20.990’ N, 33°00.677’ W
Date: June 8, 2009
Weather Data from the Bridge
Temperature: Dry Bulb: 13.9°C (57°F); Wet Bulb: 12.2C (54°F)
Cloudcover: Overcast 8/8
Wave Height: 0-1
Visibility: 10 nautical miles
Wind: 325, 4 kts.
Sea Wave Height: 0-1
Sea water temperature: 12.2°C (54°F)
Science and Technology Log
Before I explain the science we did today, I will answer the question posed on log number 3: What is a patch test and why is it run? A patch test is used to find offsets in sonar setup (continuous errors) and timing errors. It is done whenever physical alterations occur with the sonar. Such problems can occur when it is mounted skewed, or not in perfect line with the ship.
The Patch Test checks pitch, roll, and yaw. The ship is run out from the shore and back into shore, along the same line. The computer has all offsets set to zero. A swath is sliced at the edge so the computer is looking at the outer beams from the side. With a roll offset it must be changed from an X pattern to a single, flat line. With a pitch offset, a nadir view is taken, the angle is adjusted until the two lines (one out from shore, one into shore) form a single straight line. Yaw also has two lines offset so they must be combined to one line configured identical to the two lines. The Patch Test takes approximately one to two hours to complete.
Today I was involved in obtaining data relevant to tides. We used a tide gauge, levels, GPS, and a staff placed in the water, with a nitrogen being pumped under it. The tide gauge measures the unit of pressure it takes for a nitrogen bubble to be squeezed out. The greater the amount of water covering it the greater pressure is required to release a bubble. To get water depth, someone reads the staff water level and continually records this information for three hours. This will enable us to know the difference in the pressure gauge readings and the staff water level. A Global Positioning System (GPS) is used to determine where the staff is by getting a good constellation (satellites in orbit) reading.
It was my job to hold a level rod on the primary benchmark location while someone else recorded measurements using the surveyor’s level (The level rod is measured in centimeters). The surveyor’s level has three lines, or stadia, inside the level. These lines are read as upper, middle, and lower. I placed a smaller bubble level against the rod to make sure it was straight up and down. Once my location was recorded a second person, also holding a level rod, placed hers on a staff (a triangular wood structure) set in the water. The surveyor’s level was disturbed, then a second reading was taken at the staff, and a second reading was taken at my benchmark. If the numbers did not match within a certain range, or historical data, the measurements had to be repeated.
We went through this process for five benchmarks. These benchmarks were placed in specific locations based on elevation and stability of the ground above high tide level. This procedure is completed to ensure the benchmarks and the staff have not been moved, by either human disturbance or a natural occurrence, such as an earthquake. As a side note, in some locations these benchmark rods have had to be drilled down 125 feet. In the Arctic they may be drilled 25 feet into the permafrost. When drilling the holes for the benchmarks care must be given to ensure the surface is smooth, not skewed, or at an unusual angle.
Sometimes a temporary benchmark, called a turtle, must be used. This is a small, heavy, circular piece of equipment, which can be placed anywhere solid. The person holding the staff can turn all the way around it to allow for different measurements. All data collected in this activity are sent to NOAA’s tides office.
After viewing the ship’s photography server I have become interested in what actually goes on in the dry dock. The dry dock is in Seattle where the ship goes for repairs, restoration, and refitting. After the launches and other things are removed from the ship it goes to the dry dock. Gates close off the ends. On the floor of the dock are blocks for the ships hull to sit on, placed exactly in line for this particular ship. Divers go down to check for precise placement before the water is drained. The ship is tied to the dock to stabilize it (prevent it from tilting or falling off the blocks) and if you are not off the ship before it goes into dry dock you are not getting off anytime soon! On the hull are numbers used to identify the ship’s sections. When it is being retrofitted the workers must know where each section came from so the ship can be put back together correctly.
Dry dock operations can take from two months to over a year, depending on the work needing to be done. Crew stays on board as long as possible. When berths are being refurbished they stay in local hotels. Other personnel either work on other NOAA ships or go to other project sites. (All dry dock photographs courtesy of Rainier picture server)
For more information visit these sites:
http://www.osha.gov/SLTC/shipbuildingrepair/drydocking.html http://en.wikipedia.org/wiki/Dry_dock (has history of ancient dry docks)