The Problem
Two hundred lines waiting to snag something
A former activity centre quarry, once used as a floating aqua park, was being prepared for underwater research and development work. Left behind from the aqua park were an estimated 150 to 200 soft lines, mostly buoyant ropes tied to concrete-filled barrels, spread across the quarry at depths of up to 80 metres. Many were tangled together and stood up through the water column.
For anyone planning to put a diver or an ROV in that water, each line was an entanglement waiting to happen. Silt kept visibility low, the depth made everything slower, and the lines themselves could shift and foul sunken vehicles and structures the site wanted to keep. The quarry needed clearing properly, and the clearance needed proving.
The Operation
Plan, cut, verify
Risk assessment first
A full risk assessment and method statement, built around the site layout, where the debris was and the conditions on the day, so the team knew where every line was likely to be before the ROV went in.
Zone by zone
In Zone 1 the Defender worked to 75 metres, using sonar and the altimeter to find each vertical and horizontal line and cut it clean at the barrel so nothing re-snagged. In Zone 2 the tangled lines were handled and removed with the HD cameras and manipulators, and a freediving platform was moved to a safe spot for later removal.
Grid searches across both zones
Semi-autonomous grid searches at a range of depths, using sonar, HD video and tether monitoring, to confirm no hazards had been missed.
Our Solution
A Defender with sonar, altimeter and manipulators
The team used a VideoRay Mission Specialist Defender ROV. Sonar and the altimeter found the lines in low visibility and kept the vehicle at the right height off the quarry bed, the HD cameras gave the pilot a clear view of each cut, and the manipulators did the handling.
Working from an ROV meant no one was in the water among the lines while they were being cut. The vehicle could also run the pre-programmed grid searches that proved the job was finished.
The Scope
What the operation covered
Lines on sinkers
Buoyant ropes tied to concrete-filled barrels, cut at the barrel and removed.
Tangled midwater lines
Intertwined lines standing up through the water column, handled and cleared with the manipulators.
Freediving platform
Relocated to a safe position so the zone could be cleared and the platform removed later.
Low visibility
Silt throughout the quarry, so lines were found by sonar and altimeter before the cameras confirmed them.
Depth to 75 metres
Lines cleared across the full working depth in a quarry that reaches 80 metres.
Verification
Semi-autonomous grid searches across both zones to confirm the clearance.
From the ROV
The Result
A cleared site, and the proof it was cleared
In total 208 lines were cut and removed, more than the original estimate, and every known entanglement risk in both zones was gone. The grid searches that followed confirmed no visible hazards remained, which is what the site needed before diving, ROV work and research could start.
Nobody entered the water while the lines were being cut, and the freediving platform that had been a hazard in Zone 2 was moved clear for later removal.
Delivered to the client
- 208 soft lines cut and removed across two zones
- Clearance to 75 metres in low visibility
- Freediving platform relocated to a safe position
- Semi-autonomous grid searches confirming the clearance
- Sonar and HD video record of the work
- Risk assessment and method statement for the operation
Clearance work is only finished when it can be shown to be finished. Running the verification grids from the same vehicle that did the cutting is what gave the site that confidence.
Why Atlantas Marine
What made the difference
No one in the water
Every line was cut and handled by the ROV, so the entanglement risk never applied to a person.
Sonar in silt
Sonar and altimeter found the lines when the cameras could not, and kept the vehicle at a safe height off the bed.
Planned, then proven
A method statement up front and grid searches at the end, so the client had evidence the quarry was clear, not an assurance.
Operational experience
Cutting line under tension at depth without re-snagging it takes practice. The team does this kind of work regularly.
208 lines gone. Nobody in the water. Save time. No dive. No risk.
Need an underwater site cleared or surveyed?
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