Time for some deep thinking

Pipelines and cables can be laid on the bottom, of course, but are safest when buried in trenches. Anything just left lying about is liable to get caught up by something else. And then there are 'things' that have been buried, either deliberately or inadvertently, which have to be dug up for maintenance, just as they do on land – and that's not forgetting well heads or jacket legs that need to be dug round. Manual digging in a diver's suit it not on. And while dredgers are the traditional way of digging up material from the sea or riverbed, they cannot reach the bottom in deep water. Yes, there are ploughs that can be pulled through the seabed material. However, while this is fine when burying submarine cables, they cannot excavate enough material for anything big. And the kinds of machines that dig trenches and big holes on land –whether they are excavators with single buckets or machines with a series of buckets on chains – sadly do not work under water. The Challenge Our challenge this month is for you to come up with a viable means to dig those underwater holes! Not perhaps in the great ocean depths, but deeper than a dredger might accomplish – and without having to spend too much. The main requirements are simplicity, ruggedness, ease of use and environmental friendliness. And the machine should not cause damage to anything else of importance already down there, unlike a land-based excavator or mechanical dredger. Our offshore engineering readers may know all about our preferred solution. If they don't, they should. It is a good solid piece of British engineering and one of its most ingenious features is that it is self-stabilising. While not suitable for the true ocean deeps, it is regularly used down to a few hundred metres. No electronics are involved, apart from the monitoring systems. Chances are, it will seem all too obvious when you read about it below. For those without access to the web, the solution will be described fully in our June edition. But you may have other ideas, thanks to some 'deep' thinking. If so, let us know. Solution 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'. These are essentially large 'T' shaped assemblages of pipes, with two contra-rotating impellors – 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 impellors 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 impellors 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.