A mini ROV-based method for recovering marine instruments at depth

Abstract

Instruments are often deployed at depth for weeks to years for a variety of marine applications. In many cases, divers can be deployed to retrieve instruments, but divers are constrained by depth limitations and safety concerns. Acoustic release technology can also be employed but can add considerable expense and acoustic releases will at times fail. Here, we report a simple method that utilizes a commercially available mooring hook integrated with a mini remotely operated vehicle to attach lines to instruments deployed on the sea floor, which can then be winched to the surface. The mooring hook apparatus was tested in a pool setting and then used to retrieve acoustic telemetry receiver bases (50 kg) or fish traps (30-50 kg) from the northern Gulf of Mexico continental shelf at depths between 28 and 80 m. During 2013-2019, 539 retrievals (100% success rate) were made of receiver bases (n = 239) and traps (n = 300) on 30 sea days using this approach. This method could easily be applied to other types of instruments, or recovery and salvage of objects that are too deep for standard diving operations.

Fig 1. Remotely Operated Vehicle (ROV) ballast system.

A) VideoRay Pro4 remotely operated vehicle (ROV) with skids labeled. B) Underside of the ROV with skids hinged open to reveal ballast weights. Also visible is the skid plate between the skids with tapped holes utilized for attaching ROV accessories.

Fig 2. Mooring hook apparatus.

A) Top view of the disassembled mooring hook apparatus that consists of a 120-mm stainless steel mooring hook and cradle (Suncor, Inc., patent US00D391474S), a 0.3-cm thick aluminum mounting plate, and syntactic foam block for buoyancy. B) Top view of assembled mooring hook apparatus. C) Side view of assembled mooring hook apparatus. D) Assembled mooring hook apparatus with aluminum mounting plate attached to the skid plate of a VideoRay Pro4 remotely operated vehicle (ROV) with the mooring hook and cradle extending in front of the ROV.

Fig 3. Retrieval technique.

A-C) Digital images of a VideoRay Pro4 remotely operated vehicle (ROV) approaching a trap submerged in a University of Florida pool and then snatching one of the trap’s buoy lines with a mooring hook, which then released from its cradle as the ROV was flown in reverse away from the trap. Images D-F) display the same process from the perspective of the ROV’s forward camera as the mooring hook was attached to a buoy line during the process of trap retrieval from the northern Gulf of Mexico continental shelf.

Fig 4. Acoustic telemetry bases and fish traps.

Digital images of A&B) an acoustic telemetry base with receiver and C-H) experimental lionfish traps observed in a University of Florida pool or on the northern Gulf of Mexico continental shelf. Nylon lines (450-kg lifting strength) attached to bases or between traps and floats were snatched with the mooring hook retrieval apparatus described in the text and winched to the surface via the retrieval line attached to the mooring hook.

Fig 5. Summary of telemetry base and fish trap deployments.

Box plots of depth distributions for acoustic receiver array (n = 3) deployments (n = 25, 120, and 112 individual receiver base deployments, respectively) and experimental lionfish trapping (n = 300 trap sets) in the northern Gulf of Mexico. The horizontal line inside each box indicates the median, while lower and upper sides indicate the 25^th and 75^th percentiles and extended bars indicate 5^th and 95^th percentiles. Symbols indicate acoustic receiver base deployments or traps sets that were beyond the 5^th or 95^th percentiles of the respective depth distributions.