Leg 5a: A Blur of Activity at Axial Base
Tuesday, September 16, 2014
Installation of a 2-legged mooring at Axial Base
The Axial Base vertical mooring EOM cable terminates approximately 10m above the anchor (left) in a "strongback" (right)-- a cage designed to protect the EOM cable termination and provide a strong mechanical link to the mooring. The strongback is connected to the anchor via a pair of acoustic releases, a 31" syntatic foam float and short lengths of chain. The anchor slid into place approximately 100m from the secondary node, causing some nervous moments as that distance was at the limit of the cable reach.
Leg 5a of Visions'14, the OOI cabled observatory installation cruise, was a short, but continuous effort to install a unique mooring at the base of Axial Seamount. The activity onboard the R/V Thompson in the past two days is the culmination and realization of a multi-year effort to rethink and re-engineer the way a mooring works and the types of data that it can provide. The mooring we just deployed is unique in that it is a deep-water, offshore mooringpowered and controlled from shore by a fiber optic cable. The centerpiece of this mooring is a large (12'x12'x10') bright orange platform installed at ~ 200 water depth that allows for two instrument packages to be attached. One package hosts an underwater winch that raises and lowers an instrument "pod" to create chemical and biological profiles of the water column, and the second package has a suite of instruments that take biolgogical, physical and chemical oceanographic measurements at the platform, including a digital still camera and upward-looking acoustics measurements.
To accommodate such a large platform, which tis connected to the seafloor by a tension-bearing fiber optic cable (EOM--electrical-optical-mechanical), the OOI-RSN team at the UW-APL and UW-College of the Environment-Oceanography designed a unique two-legged mooring, one leg is "dumb" with a synthetic line attaching the float to an anchor, the other leg "smart" with the platform-anchor connection made by the EOM. This two-legged design creates a very stable position for the platform, one that is not subject to rotations that would compromise the fiber optic connection.
Conceptually this design is elegant, however the physical implementation of a new kind of mooring took a monumental effort by the UW-APL vertical mooring team to get the components ready and a careful deployment plan developed. The past 24+ hours were when all of these preparatory activities came together and the team assembled on the Thompson, led by Eric Boget, were given the opportunity to deploy this complex mooring in the NE Pacific.
Mooring operations began Sunday, September 14th with the pre-tensioning of the winches (HLW--Heavy Lift Winch with the mechanical synthetic leg, the MLW--Medium Lift Winch with the EOM cable, and the ship's trawl wire winch) so that carrying the load of the huge 12,000 lb anchors would not cause fouling of the drum. After an early wake up, on-site mooring operations began at 0600 hrs on Monday September 15th with the deployment of the "dumb" anchor using the HLW and the 2800m synthetic line. This was the first time that this newly delivered Sound Ocean Systems winch was used, and the heavy load was a good test of its capabilities.
After several deck-edge transfers to attach gear (links of chain, syntactic foam floats, acoustic releases), the platform was moved into place and attached to both the mechanical leg and the EOM leg (via the "strongback"--a titanium cage that protects the cable termination). The platform went into the water at around 1530 hrs, 9 hours after the deployment began. Once the platform was in the ship slowly started moving south, and the EOM cable was spooled off the MLW into the surface ocean. This was the "fun" part of the deployment, as teams of five rotated in and out to bolt on syntactic foam football floats to the EOM cable. This mooring had 85 floats, and by the end the teams developed NASCAR level pit-crew chemistry and were attaching floats in less than 30 seconds. By 2200 the EOM cable was stoppered off to the deck and the termination to the anchor was prepared.
At 0130 Sep 16, the "smart" anchor was lowered over the side with the ship's trawl wire, and navigated into place, landing on the seafloor just meters away from the desired location at 0430. At 0630 ROPOS was in the water to deliver and connect the 120 m cable between the secondary node and the EOM anchor. There were several nervous moments while making the laying out the cable because the anchor had slid slightly beyond the comfortable reach of the cable, and there was the possibility that the cable would not reach. The "Axial Base Vertical Mooring and Shallow Profiler" were connected at 1130, about 30 hours after the deployment began, marking a successful end to an extremely complex and efficient operation to get this one-of-a-kind system in place.
Many congratulations and acknowledgments are in order for this massive and successful deployment -- Eric Boget led the deployment team and directed the whole operation; Avery Snyder, Keith Magness, and Paul Aguilar worked tirelessly on deck for 20 hours; James Tilley did his thing filling in wherever needed; Skip Denny provided the good weather; ROPOS provided the navigation and deck support; and none of this could have happened without the professionalism of the Captain and Crew of the R/V Thompson.