There are both opportunities and challenges associated with the start of any very new industry which rise in tandem with the scale of the equipment – which needs safe handling.
Certainly, when it comes to deep sea mining the field is wide open “as no one yet is doing it on a commercial basis” explained Dylan Wells of Canadian custom engineering company Hawboldt Industries. This meant that when a first-mover decided to test the potential by carrying out subsea drilling exploration there was no established pattern to follow, although there were some challenging parameters to meet.
The drill unit itself, said Mr Wells, weighs about 15 tonnes while in air “but at full 3km depth with all the umbilical this mounts up to 27 tonnes,” he explained, so the deployment winch and A frame has to be sized accordingly.
More, the drill has to be landed gently on the seabed even in rough weather, so a very capable AHC (active heave compensation system) was needed to compensate for the vessel motion. While a similar non-AHC unit might have required between 300hp and 400hp, “you’ve got heavy umbilical on the drum and a big weight on the bottom,” he explained: “It means there’s a lot of inertia to overcome and this has more than doubled the power requirement.” Therefore the winch has actually been delivered with a very hefty 800hp, potentially enabling heave compensation in up to sea state 6, depending on the vessel.
The power is supplied by four independent 200hp motors working on a ‘master and slave’ basis, which are finally tied together by a large orbital ring gear. Each has its own variable frequency control, so in case of failure the ‘master’ role will simply pass on to one of the others, giving the system a large measure of redundancy. There’s also a ‘passive-on, active-off’ failsafe mechanical brake mechanism in the form of a spring cylinder operated band brake.
The make-up of the line adds a few more challenges. A 40mm, four-layer armoured umbilical provides the electrical supply for the drill head as well as feedback via fibre-optic cables so it’s both a power-communications link and strength member. All together this means it isn’t particularly flexible when it comes to running onto a winch; in fact the minimum bend radius has resulted in a drum core of 1.8m across with a flange diameter of 2.5m. At this size, “everything gets quite a lot heavier” added Mr Wells.
More, there are other, less obvious issues. He explained: “You have to realise this much armour tends to insulate the cable and a 3.5km length of umbilical can act like a big resistor, so just running the drill can heat it up.” To counter this effect, water sprays have been added to make sure the umbilical temperature is kept within certain limits.
While the winch itself is electric, the sizeable, 7m tall A-frame associated with it has a number of different hydraulic functions: the luffing, boom extension, head control, slew or rotation and opening or closing of the docking mechanism comes via an electro-hydraulic pack. To stop any unwarranted movement, the drill’s docking head is engaged by pulling against a 2m diameter rubber ‘donut’, which provides the necessary pressure against the latch fingers, giving the mechanism a firm grip on the head.
Access has also been given some thought and a second tier platform has been introduced all around the A-frame: a necessity given the height of the drill and the need to be able to get to it for maintenance, even in potentially nasty conditions.
The joystick control and monitoring of line speed, load and payout length has been integrated into the operator’s cabins, and there’s another neat trick too, the over-deck handling of the drill head can be tricky, so the winch has a low tension ‘follow’ mode with just enough pull to keep the line rendering and paying out evenly explained Mr Wells.
Interestingly, the system has been designed to be deployed on a number of vessels: so all of the winch’s electronics are water sealed and integrated into its frame: the whole thing is skid mounted so it can be craned aboard, making the system as close to ‘plug-and-play’ as possible.
By Stevie Knight