BREAKING OUT

Fibre rope in running skyline logging:

A NORWEGIAN CASE STUDY

THE STRENGTH OF ROPE REQUIRED

to successfully extract trees from the forest over a long distance results in larger diameter ropes being used. Wire rope has traditionally been used for cable logging because of strength and durability characteristics, but it is very heavy for forest workers to handle, making it timeconsuming and physically demanding to work with. Rigging accessories required are also large and robust, and the weight of the rope and carriage over a longer reach is detrimental to payload capability.

Modern cable logging operations are fast and productive, but this movement of the rope and the higher loading means they are susceptible to fatigue. Improvements in fibre rope provide opportunities for a significantly lighter rope to be used. In New Zealand synthetic rope is already common as strawline on yarders, in central Europe as guylines and in Eastern Europe for cable skidders.

Synthetic rope has several benefits compared to traditional steel rope; a very high strength to weight ratio, reduced manual workload and easier to handle and repair. However, synthetic rope has low abrasion resistance and costs more.

Put to the test

Professor Rien Visser from the University of Canterbury was able to support his Norwegian colleague, Dr Halvor Torgesen, in evaluating an Ultra-High-MolecularWeight Polyethylene (UHMWPE) fibre rope in a downhill, running skyline, cable logging operation. The equipment used was a forwarder mounted Owren T3 cable crane with a running skyline, owned and operated by Rørvik Taubanedrift AS from Norway. The yarder has a pulling force of 5 tonnes and a drum diameter of 400mm.

The fibre rope tested was 12mm diameter, 0.11kgm and with a minimum breaking load of 17.7 tonnes. This was used for both the main and haulback lines. The cost of the fibre rope used in this study was just over twice that of the equivalent steel rope.

By using synthetic rope, the running skyline configuration could use a customised slack-pulling carriage weighing only 35kg, with a 50m line for lateral hauling. This was a significant weight saving as the carriage required for a wire rope was a 360kg Owren drum carriage.

The extraction was whole trees; run by a team consisting of a winch operator, a harvest machine operator, one chainsaw worker and one choker-setter. The yarding study took place in two different forests (Frøyset and Herje, west coast Norway) harvesting Norway spruce. At one site logging was more challenging as it was close to live 22kV power lines, but the use of synthetic rope reduces the risk of arcing. The average tree sizes were 0.41 m3/tree and 0.51 m3/tree on; small trees by New Zealand

The forwarder mounted Owren T3 cable crane with a running skyline. The yarder has a pulling force of 5 tonnes and a drum diameter of 400mm.

Fibre rope in running skyline logging: Story: Prof Rien Visser and Dr Halvor Torgesen

12mm HMPE rope on the mainline drum. The amount of rope in use can be adjusted by using the storage compartment (on left) so that the drum can generate the largest force through to the rope by keeping the effective diameter low.

standards. As they were building loads using typically 3 chokers, loads were 1.7-2.6m3 which is similar to many larger scale grapple operations here. The extraction distances were between 240 and 440m and lateral hauling was up to 50m.

Synthetic rope chokers were also used. During operations, there were a few breaks at the end of the dropline where the sliding sling lines (chokers) were being attached. To solve this, the rope was spliced to double thickness at its end and this proved very effective. Splicing the synthetic rope proved very easy, especially in comparison to wire rope.

Promising results

The harvest system operated with the new synthetic ropes for a period of five months and was monitored over that time for wear. The first rope failure occurred after 6,680 m3 was harvested, or after 1,280 winch hours. At that stage the rope was clearly worn as the rope needed to be spliced a further three times over two days. The rope was replaced with a wire rope and the trail completed.

During operations, it was noted that there were some problems with spooling onto the drum, as well as some risk of abrasion as the rope spooled onto and off the drum under tension and at speed. To overcome this, the rope was lubricated with oil in an attempt to improve spooling and reduce abrasion during spooling. However, the lubrication caused problems with the rope jamming on the winch drum (i.e. pinching into the spool). Therefore, the rope was rinsed to get the original surface friction back.

In comparison, previously used wire rope in the same configuration lasted for approximately 10 to 20,000m3, or one year depending on the setup. As such we can conclude the useful life of a synthetic rope as a running skyline under these test conditions is approximately half to a third of a wire rope.

Using a rope that costs at least twice as much and only lasts half as long might not seem like a good deal. However, rope is only 3-5% of the total hourly operating cost for a cable yarder, so a payload increase of only 5% will offset this – easily obtained with a weight saving in rigging of 1200kg. There is also the ergonomic benefit for the chokersetter with the light and flexible rope. Benefits when shifting location, changing rope and splicing were also appreciated by the crew, as was a reduced fuel use. This does need to be balanced with breakage risk during operation.

This study confirmed that fibre rope has a place in cable logging. While it is very well suited when the rope is less exposed to friction, it is expensive and can have a short lifespan. In a bigger forestry perspective; cable logging needs technical progress to cope with the limitations of today’s technology. Wheel-based machines are becoming more steep- terrain capable, so cable logging needs to keep innovating to stay competitive and remain the preferred logging method with lowest ground impact.

About the authors: Rien Visser is a Professor at the School of Forestry University of Canterbury, Christchurch, and Dr Halvor Torgesen is an engineer at the Norwegian Skogbrukets verdiskapningsfond that also funded the study. NZL The chokers are also synthetic rope and are simply clipped on to the dropline. Rope breaks are readily repaired with a pocket knife and a knot.