Pop goes the weasel

The weasel waits…

Acoustic Release

An accoustic release is a tool used to remotely drop an anchor weight onto the seafloor. This allows the mooring or anchor to be lowered at a controlled rate all the way to the seafloor while remaining attached to the ship’s line.

The release uses soundwaves transmitted through the water to allow two-way communication between the ship and the release. When the proper code is sent by the ship the release will trigger, releasing its payload, and send a response back to the ship confirming release.

During the first leg of Endurance 17 the recovery of an inshore surface mooring did not go according to plan. The early steps of recovering this mooring (which sits in approximately 30 meters of water), connecting to the buoy and cabling that attaches it to the MFN (Multi-Function Node) went smoothly.

With the buoy attached by line to the ship, the acoustic releases on the MFN were triggered to allow the MFN to float to the surface. However, even though both releases reported that they triggered successfully, the MFN did not appear on the surface. This was a problem. The crew decided to attempt to coax the MFN from the bottom by gently pulling on it via the buoy and attached cable. Unfortunately, even with ~1500 pounds of force, it did not budge.

The deck lead and his OOI team ponder their options

Flow directions of sediment from Columbia river. Image courstesy of Washington State Department of Ecology

So, why would an object that is built to be buoyant be so unwilling to come to the surface? For that we need to look at the location of the mooring and environment it is in. This inshore mooring is located within what is known as the Columbia River Littoral Cell, the area of the ocean close to shore that is fed by the sediment load of the Columbia River. This region is the deposition zone for the sand and other sediment that the river brings with it downstream and dumps into the Pacific. The currents along the coast carry a large portion of that sediment north along the coast toward the Olympic Peninsula. In layman’s terms: a ton of sand gets dumped along the coast. The high sedimentation rate resulted in the MFN and its anchor being buried by depositing sediments over their 6-month stint on the seafloor.

It was time to pop the weasel.

The weasel is an emergency line that allows the ship to directly connect to the MFN in just this eventuality. It is simply a small float attached to a line packed onto the MFN. When its acoustic trigger is fired, the float is released pulling the line to the surface.

The weasel successfully fired, and the float shot to the surface, appearing just to the starboard of the buoy. At this point the winch line from the ship, that was attached to the buoy, had to be moved to the weasel line. It was time to launch the small boat!

The weasel float next to the buoy

The ship carries two small craft that can be lowered into the water. One is deployable by the ships crane and the other is mounted to a winch on the side of the ship. The boat was winched down and manned by the crewmembers from the ship and two members of the OOI team.

OOI’s Linus Stoltz uses a scientific reaching wand to detach the winch line from the buoy

They made quick work of moving the winch line to the MFN, and even had some time to have a little fun letting the boat stretch its legs.

The small boat crew practicing for the UNOLS R/V Small Boat Racing Tournament

After a few donuts and with a boat full of broad smiles, the small boat was brought back aboard the Thompson and winched back into its home.

With the line attached the ship was able to apply enough tension to dislodge the MFN from the seafloor and bring it and the buoy on board. There was only one piece left to recover, the mooring anchor.

MFN being hauled aboard with the attached buoy patiently waiting its turn

These 6500-pound steel plates hold the buoyant MFN to the sea floor and are drawn to the surface by the ship via a line carried to the surface by the MFN. This line is rated for a tension load of 34,000 lbs.

As tension was brought onto the line connected to the anchor, it was clear that it was going to be difficult to budge. The tension continued to increase as the heavy lift winch continued to pull. At a little over 16,000 lbs. of tension the line snapped, leaving a slack line dangling limply into the water and a 6500 lb. anchor buried in the sands of the Columbia River.

OOI’s Jeff Wood shows the one that got away…