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Offshore Support Journal

Offshore Support Journal

Fibre rope crane can lower full load to maximum depth

Mon 12 Oct 2015

Fibre rope crane can lower full load to maximum depth
With the Trident crane, the problem of the weight of the wire eating into hook capacity is eliminated

As reported in OSJ on a number of occasions, using fibre rope rather than conventional steel wire can help cranes lift heavier loads and deploy them to greater depths. This is because, unlike conventional steel wire, fibre rope is neutrally buoyant in water. Because it weighs so much, steel wire significantly reduces hook capacity as more and more wire is paid out to reach greater depths. 

A number of companies have developed hybrid concepts that make use of steel wire and fibre rope, including NOV, which unveiled a fibre rope extension system (FRES) back in 2009. The FRES was a portable, self-contained containerised fibre rope storage system with specially designed hang-off sheaves with locks attached to the end of the crane jib. The system inserted fibre rope in sections of about 1,000m into the wire carrying a load. It was an elegant solution, but NOV has taken the concept an important step further and developed a completely new type of crane that is designed to use fibre rope alone (although the new crane can also use steel wire if required).

This means that, with NOV’s new knuckleboom crane Trident, depth is no longer an issue. Hook capacity remains the same, no matter the water depth. A 150-tonne capacity Trident crane can thus raise and lower a 150-tonne module to a depth of 3,000m, whereas a similar crane with steel wire will only manage about 60–70 tonnes to the same depth. This is because the weight of the steel wire consumes about 50 per cent of the crane’s capacity. At greater depths, the problem becomes even more acute as the weight of the steel wire gets greater and greater. In contrast, a 400-tonne capacity Trident crane can lower a 400-tonne load to whatever depth is required. Equally, it means that a 200-tonne fibre rope crane can do the job that a 400-tonne crane would normally do. Moreover, a smaller crane is less expensive and can be installed on a smaller vessel, reducing costs considerably.

NOV designed the hoisting system for the crane with fibre rope in mind. It includes a new winch and spooling system and a fibre rope protection system that prevents the rope from heating up during use. Bending fibre rope over the sheaves on a crane causes it to heat up, which can affect the fatigue lift of the rope.

“We started developing what became the Trident three to four years ago,” Yngvar Boro, senior manager, winch and crane at NOV, told OSJ. “We looked at the problem areas with existing equipment that used fibre rope – such as spooling problems, bending fatigue and the heating up of the fibre rope – and came up with a new concept.”

One problem with fibre rope is that it can heat up when it is bent on to a spool. The more bending around the spool is involved, the greater the problem. “We knew that two layers was the maximum for the spool – the problem was how to accommodate 4,000m of fibre rope in two layers. The solution was a vertical drum and to wind the rope vertically, rather than horizontally, inside the crane in the pedestal. The pedestal provides ample space to spool the fibre rope, and the need for large drums for wire below deck or a ‘backpack’ on the crane that eats in to deck space when revolving the crane is eliminated. The crane is electrically powered using permanent magnet motors with the ability to regenerate power during braking, lowering and in active heave-compensated mode. Another obvious advantage is that an electrically powered crane uses 30 per cent less energy than a hydraulic crane.

The other key challenge NOV needed to tackle was cooling the fibre rope. Cooling fibre rope is not so much of an issue in regions such as the North Sea, but where temperatures are higher, above 70°C, Mr Boro explained, changes occurs in the structure of the rope because of a phenomenon known as crystallisation. This weakens the rope. The problem is, of course, that many of the most important deepwater developments are in deep water in parts of the world where ambient temperatures are much higher – such as in West Africa, Brazil and Australasia.

NOV needed to find a way to ensure that fibre rope wasn’t affected in this way. Its solution is, first, to cover the rope and the winch and then to provide it with active cooling in the form of air conditioning. In an environment such as West Africa, this air conditioning would be ‘on’ permanently, whether the crane is in use or not. Because the air conditioning system is a closed-cycle system, power consumption is not a big issue. In the new design, all of the key components are mounted inside the crane, eliminating the risk of corrosion.

“We set out to be able to reduce the temperature of the fibre rope to an acceptable level at all times,” Mr Boro explained. “This means that, when you start operating the crane, you have cold rope. When the crane and the heave compensation system are brought into operation, we use water cooling to the sheaves in the active heave compensation system in order to prevent the rope from heating up above acceptable limits. You have active cooling of the drum and jibs via the air conditioning so that the temperature is maintained at 20°C all of the time, including when the crane is parked, and you have water cooling of the sheaves in continuous and active heave compensated mode.”

NOV has tested the concepts used in the Trident over a three-year period using fibre rope from a number of manufacturers. It has found that at least two of them are suitable for use with the new crane, although it would not divulge which ones.

The new concept seems to have few drawbacks. The overall footprint of the crane is not dissimilar from a conventional crane. The drum may be a little larger, but not significantly so, and the slewing ring is more or less the same as a conventional machine. As highlighted above, the new crane is also prepared for a kinetic energy regenerating system, and its novel design means that its centre of gravity is lower than a conventional crane with the winch above deck. The NOV Trident has an optional integrated boom rest and would be delivered fully tested from the workshop, with internal access for maintenance through the pedestal.

“We have already had a number of enquiries and the crane has been specified for at least five subsea vessels and a drillship. As off today we have a contract for delivery of the first Trident crane in the first quarter of 2017.  We have also had enquiries from related industries, such as seabed mining,” said Mr Boro, noting that there is no reason why vessels should not also be refitted with NOV Trident in place of conventional units.

“This is the beauty of the system,” Mr Boro concluded. “If you have to lower a 100-tonne load to 3,000m, you can use a 100-tonne crane rather than a larger, more expensive 200-tonne crane. You can do the same job with a smaller vessel. We are offering NOV Trident cranes in the 150–400-tonne range and can go larger if need be,” he concluded.

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