WHOI Mooring Recovery

Guest Blogger: Jeff Lord (WHOI)

The WHOI mooring was deployed from the RV Knorr on September 14, 2012. It was recovered in two phases on September 30 and October 1. Prior to the recovery, the workboat was deployed with technicians to remove some of the delicate meteorological instruments that could be damaged during the recovery. At that time a pickup sling and floating line were shackled to the buoy’s lifting bale to make recovery easier.

At 0840 UTC on September 30, the command was sent to the acoustic release to decouple the mooring from the anchor that has held it in place for the past year. Fifty minutes later, the large cluster of glass balls that serve as backup recovery were spotted on the surface. The ship approached the glass ball cluster to begin the recovery process.

If a mooring has backup recovery installed, the recovery us done from the bottom to the top, and the buoy is recovered near the end of the processes. Recovering the majority of the mooring and instruments before the buoy keeps working tensions at a minimum through the recovery. This is the safest way to work.

A grapnel was thrown into the cluster of glass balls, and they were brought through the a-frame using the ship’s mooring winch.

Once the glass balls were safely on deck, and the mooring line leading back to the buoy was secured, all hands assisted with breaking down the twisted cluster of balls and chain and getting them stowed.

As soon as the deck was clear, the recovery continued by hauling up the 4700 meters of synthetic line (rope) using an electric capstan. The line was stowed in wire baskets.

 

 Once most of the line was recovered, the mooring line/load was transferred to the mooring winch for the remainder of the recovery. Recovery of wire rope and instruments is more controlled, and therefore safer, using the winch.

As the wire rope was pulled in, instruments clamped to the wire were removed. Instruments in cages or on titanium load bars were removed by transferring the tension of the mooring to “stopper lines” just below the instrument to be removed. Once the instrument was removed, the wires were shackled back together and the winch continued hauling in. This procedure was repeated until all the instruments, wire, and chain up to 40 meters below the buoy were removed.

 

At this point, it was late in the day and the weather forecast for the following day was for more good weather. We decided to stick with our original plan to set the buoy and remaining mooring line adrift and finish the recovery the next morning.

The ship stood by the mooring overnight, as it drifted freely.

 

On the morning of October 1, the ship approached the buoy from the port side. A grapnel was tossed over the line that had been attached to the buoy the previous day. This line was hauled in slowly, and when the buoy was close enough, the pickup sling was transferred to the hook on the ship’s crane.

 The crane lifted the buoy out of the water and brought it to rest against the side of the ship. Lines from three air tuggers were attached to point on the buoy to control the swinging of the 5,000-pound buoy as it was lifted on board.

Once the buoy was on deck and secured, the remaining mooring was stopped off and detached from the buoy. The crane was used to pull 8-meter segments of the mooring with instruments straight out of the water for recovery. Each segment was stopped off at the deck, and as the crane lowered the segment down, it was disconnected and moved away from the buoy. The last part of the mooring to be recovered was an 18.5-meter segment of wire rope that was pulled up by hand.

As soon as the buoy and remaining instruments were on deck and secured the cleaning commenced. All instruments that had biofouling needed to be cleaned before they were brought into the lab for data recovery.