Thursday, August 30, 2012

Streaming the Wire

An essential part of the work at sea is deploying various pieces of equipment over the side of the ship to sample the ocean water column. On this cruise the gear going over the side of the ship are the Reeve Net for collecting live animals, the CTD Rosette for measuring physical, chemical, and biological parameters (temperature, salinity, oxygen, fluorescence, light transmittance), the HammarHead to measure broad-band acoustical backscattering, and the Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) to capture zooplankton from 1000 m to the sea surface.

The trawl Winch Drum (right) and Traction Module (left) on the 0-1 deck of the New Horizon. This winch is used to tow the MOCNESS.
All over the side deployments with sampling to depth require a winch and wire on which the sampler is attached. The winches are sophisticated devices that hold thousands of meters of cable that can be paid out and hauled in at speeds typically ranging from a meter or two per minute up to 60 meters per minute. On the base of the drum there is usually a Lebus groove designed for the specific cable diameter to provide a perfect wrap of the first layer of wire on the drum.  Paying out the wire is usually not a problem, but hauling in requires a precision levelwind to lay the cable on the drum wrap after wrap and layer after layer with no gaps or cross-overs. A trawl winch with 0.68" conducting cable is being used to tow the MOCNESS off the stern of the New Horizon. In addition to a drum and levelwind, this winch also has a traction module to keep the wire spooling onto the winch at a constant tension.

The 0.68" cable being streamed over the stern of the New Horizon through the sheave on the A-frame.

At the beginning of this cruise, the first two MOCNESS tows to 1000 m encountered difficulties with the wire on the trawl winch not being properly laid down on the drum while hauling in. The ship's engineeers had to make manual adjustments to get the cable to lay properly, which involved intervals of stopping the winch and sometimes paying the wire back out before hauling in again. This resulted in a less than optimal MOCNESS tow where the object is to sample the water column uniformly as the system is hauled from its maximum depth to the surface while opening and closing nets. Making adjustments to fix the problem while the tow is in progress is difficult and it is even more difficult in port where operating the winch is not possible without extra shore side support. So on the way out to station with the ship steaming at 9 kts, we streamed the wire with just a weight on the end of the cable to allow the engineers the time to make adjustments to the winch tension device and levelwind. We paid out more than 1800 meters of wire and then the process of making adjustments to the traction module and the levelwind started as the wire was being hauled in. Time will tell if the problems were fixed and the towing of the MOCNESS will go smoothly for the rest of the cruise.

The Res Tech Meghan Donohue, Chief Engineer Tom Schuller, and oiler William Bouvier making adjustments to the trawl winch.
The Res Tech Meghan Donohue bringing in the cable termination and weight at the end of streaming the cable.
The winch display as seen inside the main lab. The graph at the bottom shows the tension (in lbs) experienced by the winch during the time over which the cable was being streamed.
Peter Wiebe (text and photos)

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