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Mooring Magic

NEPTUNE Canada, its Highland Technology contractors, the IOS mooring team, and the fabulous R/V Thompson crew co-operated Tuesday in a complex deployment of the Northwest Regional Circulation Mooring (RCM-NW) in the axial valley of the Endeavour segment of the Juan de Fuca Ridge. The RCM-NW mooring is part of a planned 4-mooring array at Endeavour designed to establish a big picture view of currents and water column characteristics in the axial valley of this hydrothermally active spreading centre. Each mooring consists of a large anchor and a suite of tethered instruments suspended above the anchor on cables connected to floats.

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NEPTUNE Canada research theme integrator Steve Mihaly with a mooring anchor, 13 September 2011.

2010 Deployment: 2-ship operation

The North-East mooring was deployed on our Fall 2010 NEPTUNE Canada installation/maintenance cruise thanks to a combined effort from our team on the R/V Thompson and a DFO team on the CCGS Tully: the Tully crew deployed the mooring itself while ROPOS (deployed from the R/V Thompson nearby) waited for the mooring at depth. This setup proved very efficient as the mooring could be guided within 1m of its planned landing site and connected as soon as it landed.

2011 Deployment: 1 ship only

This year, the deployment was performed entirely from the R/V Thompson. The plan involved carefully lowering floats and instruments one by one off the aft deck, using the ship's crane and A-Frame. Once strung out behind the ship, the 750kg anchor was lowered into the water on the A-Frame, pulling the instruments into a vertical configuration. Then, the entire assembly was lowered to the seafloor where it was detached using an acoustic release. The process was very delicate – working on the aft deck with incredibly heavy objects, ropes and cables strung everywhere, two cranes overhead, and sensitive scientific instruments in between was like performing a ballet in hardhats and lifejackets! Deck operations alone took over six hours, not including the days of planning and preparation beforehand, and the ROPOS dive to connect the instrument afterwards. The pictures below show the step-by-step operations as they took place.

Step-by-Step: 1-Ship Mooring Deployment

Deployment Phases

The deployment plan had three main phases:

  1. First, there was a lot of preparation.
  2. Second, the mooring was lowered into the water. At the end of that phase, the mooring would be standing freely in the water column supported at the surface by two large deployment floats.
  3. Third, these two floats would be removed and the mooring transferred to the deep-sea winch cable so it could be slowly lowered to the seafloor at a depth of 2300m.

Step 1: Preparation

As a first step, floats, instruments and cables for the 270m tall mooring had to be arranged and secured on the deck so that they would be accessible in order of deployment without risk of tangling. The engineering team did this work the previous day, then the night shift reviewed the set-up and added some safety artefacts.

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Top float of the mooring (left) and two floats used in the deployment.

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Orange floats on the cable, the acoustic release used in the deployment (silver cylinder in the centre), the L-Box (orange canister at right) and the concrete anchor.

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Overhead view of the entire setup.

Step 2: Dunking the Floats

The first step in the deployment was to lower the three large spherical floats to the water using the ship's crane. The top float for the mooring, which is made of material that will not compress under immense pressure at depth, weighs over 350kg in air!

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The first float is lowered off the aft deck.

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Three floats bobbing with a cormorant observing.

Step 3: Dunking the Instruments

Next, the instruments on the long mooring cable were deployed off the deck one-by-one. An upward-looking Acoustic Doppler Current Profiler (ADCP) was slowly lowered into the water via crane. Then, 3 tandem instrument packages (Acoustic Current Meter (ACM) + conductivity-temperature-depth (CTD) gauge) with interspersed floats were lowered into the water.

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The upward-looking ADCP goes over the edge.

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A float at the surface with an instrument package below.

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The final ACM/CTD package goes out.

Step 4: Dunking the Anchor

Next, the anchor and L-box (electronics canister) were carefully raised up and over the edge using the A-Frame on the R/V Thompson.

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The mooring anchor and L-box are lifted over the water.

The anchor was then lowered and released to assume its natural position at the bottom of the mooring line. Doing this also pulled the entire assembly into vertical configuration.

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The anchor is lowered on the winch cable.

Step 5: Removing Two Floats

At this point, the second phase of the deployment began. The crew used a pole to hook the floats and pull them around to the aft deck. The top two floats were removed and lifted back on deck using the A-Frame. This left only the main mooring float atop the array in the water.

Step 6: Beacon Attachment

A beacon was attached to the anchor before it was deployed, and a second beacon was secured to the mooring float. The first beacon reports the position of the mooring during the descent and helps position it on the seafloor. The second beacon is a precautionary measure: should the cable snap, or some other mishap occur (e.g. the anchor falling off and instruments floating away), the mooring can be tracked and retrieved.

An acoustic release was used to connect the float to the winch cable. This release is triggered from the ship by sending a "ping" through the water, telling the release to unlatch when the mooring reaches its target depth.

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The top float of the mooring with beacon and acoustic release.

Watching & Waiting

The deck operations were a successful co-operative effort on this bright sunny day at sea. Our mooring was deployed and sent off down through the water.

As the float disappeared from view, we all waited anxiously for the next ROPOS dive to see how it would look on the seafloor, and more importantly, whether it would start sending data on connection.

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The top float of the mooring disappears below the surface.

All's Well that Lands Well!

As it turned out, the mooring was successfully deployed. When ROPOS went down to inspect, the mooring was found intact, and upon connection, we discovered that all 9 connected instruments activated and began sending data!

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Mooring anchor and L-box with cable connection at the seafloor (depth: 2145m).

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Plot of data from four temperature sensors on the mooring.

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