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18/06/2008
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The solution to last month’s challenge, on digging holes at depth, comes from Aberdeen, in the form of Rotech Subsea’s ‘Mass Flow Excavators’
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These are essentially large T-shaped assemblages of pipes, with two contra-rotating impellers – one at the end of each arm of the T, and a nozzle at the base of the upright. Hydraulic motors, through a flow divider, drive them, so the two impellers go round at the same speed, using biocompatible hydraulic oil.
According to sales executive Jamie Ross, they deliver sea water at only about 0.5 bar. However, because of the inertia associated with up to 8,000 litres of sea water – and its more than eight tonnes weight impinging on the sea bottom every second – they can shift sand, gravel, silt, rock dump, drill cuttings or even firm clay – although the latter does require a special nozzle.
The impellers impart gyroscopic horizontal stability, while the weight of the unit imparts vertical stability. As the machines are non-contact and produce no reaction force, they impart no stress on supports, and cause no damage to pipelines, cables or structures on the sea bed. The 8,000 litres per second T8000 unit is the largest, but the company also makes T4000 and R2000 units. Mass flow excavators have been used for pipeline trenching for repair work down to 230m in the Gulf of Mexico. Monitoring is by multi-beam sonar.
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Author
Tom Shelley
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