Reducing tension headaches

It is quite simple says Ross Cowie, project engineer of ACE Winches’ design team: “Electrically powered lifting gear, with electronic interfaces, offer better control.” And this is exactly what some projects demand.

The company has recently provided several winches that, amongst other things, will be used to help spool umbilical products onto a large carousel sitting in the hold of a new lay ship.

Mr Cowie explained: “Clients wanted a system which could be completely controlled by a single operator so if at any point there was an issue, it would be possible to hit the emergency stop and safely bring the loading process, and the interlinked equipment, to a standstill with no tension building up that could strain the product.”

Its sophisticated technology: the seven winches (ranging from 10 tonnes up to 40 tonnes working load) are driven by variable frequency converters to give a smooth power uptake: further a number have a high-speed function which gives them the ability to increase their line speed from the normal 15m/m all the way to a very fast 60m/m. Moreover, as the equipment interfaces with the vessel’s global control system, ACE Winches had to meet an exacting set of standards in order to allow the entire umbilical laying set to ‘clip together’ on installation.

Most of these units will be used for more than helping spool product into the carousel. Mr Cowie explained that while using the main pedestal crane for heavy lift operations it’s normal to attach a pair of winch tagging lines at corners of the load – this prevents the payload turning in mid-air. Unfortunately, a vessel’s rolling or pitching motion has been known to cause the wire to snap.

These winches avoid the issue by including an automated control function: the operator enters a set point tension; a load cell then checks the actual tension in the wire against this set point and pays out or brings in the lines to match, sharing the load between them.

However, fast response times are key to maintaining uniform tension in the wire rope. “The winch can accelerate from zero to full speed - which with a four pole motor is 1,800 rpm - in less than a second. And a lesser variation will give you an almost instantaneous reaction time,” said Mr Cowie. Interestingly he added: “In fact you can get an electric motor to accelerate faster than this, but in our experience if you speed up the reaction much beyond this it results in the winch being put in constant overdrive. If you drop it down a little it gives you a better overall damping effect.”

Less sophisticated but very practical was the ACE Winches-supplied A frame; this has been positioned to allow a line from the 10 tonne winch to be diverted down through a moonpool, to deploy or retract smaller tools or messenger wires without tying up the large scale handling equipment.

One last point: the vessel will potentially be deployed in Arctic waters. “When you design winches for -40°C it becomes a lot more difficult because you have to specify components that will survive in these kinds of temperatures. As you can imagine most are not as readily available as those that are capable of working in North Sea conditions down to -20°C. Even the steel work requires additional testing and certification to prove it will be resistant to brittle fractures at such temperatures.”

Mr Cowie concluded that despite these issues, worldwide deployment introduces an additional consideration for hydraulically driven kit: “Hydraulic oil must be changed out for a grade suitable to the climate. I’d say global operations present a strong argument for having all electric equipment.”