Torstein Lisland1
1 Mechanical Engineer, Nowegian Forest Research Institute, Norway.
Abstract
Logging on soft ground can be done in two principally different ways. One is to use normal mechanized logging, which easily leads to damage to the ground, and repair the damages afterwards. The other is to log in a way that leads to no damage. Environmentally, the last alternative is the one preferred, but it is also the most expensive.
At the Norwegian Forest Research Institute we have been working on the possibilities of using cable cranes on soft ground. We have used small hydrostatic driven interlocked yarders with slack pulling carriage. Maximum reach is 400 m, but on flat ground it is hard to reach more than 200 m. For longer spans we have developed intermediate supports.
During the project we have tried a lot of different patterns as full trees, tree-length and short wood. Both single-tree-choking and bundles have been tried. To get an acceptable production the cycle time must be reduced, and the load size increased. Roughly 50 percent of the cycle time is normally used in connection with hooking and lateral yarding. To shorten this time, one of the possibilities is to pre-choke. And to get bigger average loads, pre-bunching is a good solution.
The best results we obtained were from letting a processor do at least the felling and delimbing in the field. The rest of the conversion can either be done in connection with the delimbing or at roadside. It is cheaper to do the total conversion in the terrain, but it is cheaper to yard tree-lengths than short wood by cable crane. The total costs are, however, of the same size. Thus far it seems that, if it is possible to deliver tree length, this would be the most preferable method.
However, this is a fairly new method in Norway, and there is ample room for improvements. The environmental aspects, combined with the industry's demand for a constant flow of wood, makes it necessary to look for more environmentally friendly logging methods. And the use of cable cranes on soft ground could be one of the solutions.
Introduction
This paper describes some of the development work concerning the use of cable cranes on soft ground in Norway. The demand of more environmentally friendly logging methods has become greater during recent years, and that is one of the reasons for the projects on more gentle methods.
Logging on soft ground can surely be done without any damage if it is possible to carry it out on frozen ground. However, there are many areas with no frozen ground at all, and considering recent winters with very varying conditions, it is a real challenge to base the logging possibilities on frozen ground. The wood-consuming industry demands a constant flow of wood all year round, and there are considerable amounts of wood standing on soft areas which are interesting to harvest.
The areas with low bearing capacity (soft areas) are especially sensitive to damage from normal mechanized logging equipment. One possibility is to log those areas with normal mechanical methods. This leads to damage to the ground, and that has to be repaired. The degree of necessary repair depends on who is looking, that means it depends on how close you are to public areas. The other possibility is to develop logging methods that do not lead to any damage. Principally it is an economic question. Everybody has the same goal - the owners, the loggers, as well as the public, want logging to be done in a way so that it looks as nice as possible when it is finished. Unfortunately it normally costs more to make everybody satisfied, and the big question is who is willing to pay the bill.
At the Norwegian Forest Research Institute a lot of work has been done to develop methods and equipment for using cable crane on soft ground. To achieve acceptable costs it is necessary to investigate time consumption and see where it is possible to reduce costs. We have studied normal cable operations with motormanual felling, choking and transport of full trees to the landing, and conversion there. The ideal place for a landing is in connection with a truck road but, due to a relatively wide road spacing in Norway, we very often have to transport the logs from landing to truck road by forwarder.
When looking upon time distribution on normal cable operations, 50-55 percent of the cycle time is hooking (including lateral pulling and skidding) and unhooking. This means that the carriage is standing still more than half of the cycle time. The time for load transport and haulback depends upon the distance, but very little on the load weight as long as it is kept within the capacity of the equipment. In order to improve the production, it is necessary to reduce hooking/unhooking time, and optimize load size. In addition, it is very important to reduce time consumption for rig up, rig down and changing of roads. This takes an average of 20 percent of the workplace time, is not productive time, and has to be minimized.
In Norway we very seldom use standing skyline, we mostly concentrate on systems with energy recycling, that is the so-called "interlocked winches". Road density has increased, and correspondingly modern Norwegian cable cranes are normally constructed for 350-400 m. The best results are obtained on distances of up to 250-300 m. We have also developed artificial supports for running skyline systems (Figure 4), but it is important to try to avoid the use of supports.
Good planning is the key to success
All the operations mentioned in this paper were run with hydrostatic driven, interlocked cable cranes. The hydrostatic drive with variable pumps leads to versatile equipment, easy to learn to control, and safe working conditions. This makes it easy and quick to spot the carriage exactly in the field in order to give the hooker optimum conditions.
The hardest work by cable logging is the hooking, and it is of greatest importance to give the hooker the best working conditions
The normal way of cable crane operations in Norway is to transport whole trees or tree lengths to the landing, and to do the conversion there. This means it is impossible to do any kind of bunching on the terrain. Experience from skidding short wood with skidders tells us that logs are easily lost during transport. On the other hand, it would obviously lead to bigger loads when pre-bunching. A pilot operation in our research forest was encouraging. We put together loads of short wood, used a 3.5 m choker around, and attached it to the hoist line by means of a snatch link that could pass through to the hoist line drum on the carriage. This reduced the hooking time essentially, and the production increased. After that we used a two-grip processor to fell, convert and bunch on the terrain. We tried different ways to locate the loads, and we experimented with different snap links. When comparing this to normal full tree logging we got out more loads per hour but, due to small loads, there was no rise in production.
On soft ground it is often possible to drive once, at least if it is possible to drive on a bed of branches. That means it is possible to let a processor fell and process the trees, and make bundles. To avoid damage from transport, this can be done by cable crane. On a relatively flat and soft area we tried two different systems, both however were based on bunching in order to get bigger loads. One system was based on processing short wood on the terrain, the other one on tree length. From experience and available equipment we decided to use a single-grip harvester for the purpose.
The area was planned beforehand, and corridors, tailtrees, and anchorings were marked. It was planned to log short wood and tree-length every second row, and it was yarded to the truck road (Figure 1). In order to make the hooking as easy as possible, it is important to mark the corridors beforehand. The processor operator can then spot the loads better and thus help the hooker. The transport starts with the processing, and it is very important that the operator knows what is possible to do with the cable crane, and what the optimum conditions are. The goal is to put the loads in a herringbone pattern to the corridor, but it depends on the terrain. The operator must always have in mind that a choker shall be put under the load. Using the short wood system, an attempt was made to keep sawlogs and pulp in different piles. Each pile could consist of more loads. On difficult conditions one log was first put on the ground, and then the remainder placed upon this at a certain angle. This was done to get the choker easier under the loads. It can be desirable to place the sawlogs closest to the corridor, but it is not a must. By tree length this is simpler. The only aim here is to spot the loads so that hooking can be done in the quickest and safest way.
For choking, 3.5-4 m chokers were used, connected to the hoist line by means of snatch links. While the carriage transported one load to the landing, another load was choked; so when the carriage came back, it was just necessary to disconnect the empty chocker and connect the pre-choked load. The pattern of using only one chocker per load, and pre-choke, reduces the hooking time dramatically.
The cable crane was placed on the truck road. With this type of yarding it is important to locate the guylines so that the crane can be moved sideways without lowering the tower, and without changing the guylines. When yarding short wood, the crane first was placed in the centre of the skyline corridor. In this position the sawlogs were yarded. After that the crane was moved sideways, and the pulp was yarded (Figure 2). This means it is necessary to work along the corridor twice, but it makes the further transport easier and cheaper. Another possibility is that the processor puts sawlogs and pulp in the same piles. That makes the cutting easier, it is easier to get suitable size of loads, but then it has to be sorted at the landing. What is better depends on the equipment available. Yarding short wood rapidly leads to big piles at the landing, which again leads to more time-consuming unhooking. It is important that the crane can be easily moved sideways to spread the loads, as well as to have good contact with the truck drivers to move the wood away as soon as possible.
By yarding tree length, the loads were yarded with the butt end to the roadside (Figure 3). With this method assortments are no problem, and it is easier to get suitable loads. Also by this method it is important to move the yarder sideways to avoid too high piles of trees, and problems with unhooking. Processing was done with the same processor used on the terrain. It worked on the road, and the logs were stacked on both sides.
Figure 1. Bundles of tree length and shortwood, ready transport
Figure 3. Tree-length yarding to roadside
The utilization of automatic releasing chokers has always been a dream, and a lot of development work has been done on it. One often works well, sometimes two, but three chokers or more under bad conditions are so far still a dream. But with pre-bunching and tight lines it should be possible. In addition to normal chokers we tried one automatic releasing choker made by FERIC in Canada (Figure 5). Principally it looks like a normal Barden Hook, but on one side it has a hinged "wing". In normal position this locks the opening so that the choker cannot be released. When the load is lowered to the ground and the carriage is pulled backwards, the "wing" is pulled onto a log. Then it opens the locked part of the hook, and it is released. When transporting single trees this works perfectly, but on bundles the hook might hit between logs, and then it fails.
Table 1. Stand and load figures for the operations
|
Operation No. |
m3/day |
m3/tree |
m3/load |
tree/load |
|
1 |
20 |
0.34 |
0,85 |
2.5 |
|
2 |
10.2 |
0.22 |
0.63 |
2.8 |
|
3 |
19 |
0.37 |
0.58 |
|
|
4 |
26.2 |
|
0.89 |
6 logs |
|
5 |
26.2 |
0.36 |
1.35 |
3.6 |
The first operations were more or less to develop auxiliary equipment, these are now working well. The production figures are from the last five operations. To avoid the influence time lost due to different conditions, the production figures are related to MAIN TIME. Operations No. 1, 3, 4 and 5 were carried out with the same type of crane. Operation No. 2 had an older type and, since the standing volume also was lower, the results cannot be compared directly. The production figures are related to 100 m transport distance. The time for hooking and unhooking is independent from transport distance, and is calculated as average figures for the different operations at average speed for each operation. The time for transport and haulback varies little in the different operations, except time for haulback in operation No. 3. The reason for low speed there is that the distance was short, and the driver used only a low gear both in haulback and in transport. The reason for the long unhooking time in operation No. 2 is that the driver was situated in the cabin, and had to go out for every unhooking. In operations No. 1 and No. 3 the processors took away the trees continuously, and thus made more efficient unhooking. In operations No. 4 and No. 5 the wood was yarded to piles, and that meant more time-consuming unhooking.
Table 2. Time distribution per load by 100 m transport distance. Main time
|
Operation No. |
Haulback |
Hooking |
Transport |
Unhooking |
Extraction |
|||||
|
min |
% |
min |
% |
min |
% |
min |
% |
min |
% |
|
|
1 |
31 |
10 |
141 |
61 |
58 |
19 |
31 |
10 |
|
|
|
2 |
34 |
9 |
151 |
42 |
57 |
16 |
109 |
30 |
12 |
3 |
|
3 |
54 |
23 |
82 |
36 |
63 |
27 |
32 |
14 |
1 |
0 |
|
4 |
29 |
12 |
80 |
34 |
60 |
26 |
48 |
21 |
16 |
1 |
|
5 |
32 |
15 |
83 |
38 |
54 |
25 |
39 |
18 |
10 |
4 |
The most interesting time is the hooking time. In operations No. 1 and No. 2 whole trees were yarded, while in operations No. 3, No. 4 and No. 5 pre-choked bundles were yarded. With the last system there was a 65 min reduction in hooking time. With a cycle time of 225 min this means seven loads more per hour (main time).
The use of automatic releasing chokers will reduce the unhooking time to a certain degree, but there is not too much to gain. However, when building high log piles, unhooking is time-consuming and also dangerous, and for this purpose automatic release is good. The reason for the low unhooking time is that the operator has a movable control unit which he places as close to the unhooking spot as possible, taking into consideration safety as well as oversight of the operation.
Conclusions
During the project period we have developed equipment and methods which make it possible to use cable cranes on soft ground. Running skyline systems with relatively small forces makes it possible to use light anchoring equipment, and thus make the set-ups and row changing quicker and simpler.
Table 3. Production on different operations, 100 m transport distance. Main time
|
Operation No. |
Assortment |
min/load |
Load hour |
m3/hour |
|
1 |
Tree length |
309 |
19.8 |
16.8 |
|
2 |
Tree length |
363 |
16.5 |
10.4 |
|
3 |
Logs, bunched |
232 |
25.8 |
15.0 |
|
4 |
Logs, bunched |
233 |
25.8 |
23.0 |
|
5 |
Tree length, bunched |
218 |
27.5 |
37.1 |
Figure 4. Artificial support for running skyline
Figure 5. Automatic releasing choker
Figure 6. Production per hour main time, transport distance 100 m
Looking at the results (Figure 6), it is clear that there has been an increase in production during the project. To get the cycle time down and production up, it is necessary to bundle and preferably pre-choke. Whether it is preferable to work with tree length or short wood depends on different conditions. The tree-length method depends first of all on mechanical felling and bunching. The short wood system should also be preferably combined with mechanical felling and conversion, but can also be carried out with motormanual cutting. Earlier motormanual cutting and hand skidding was done to increase the output of forwarders, and it is of course possible also for cable cranes.
Which method is the best one depends on different factors. According to our studies it takes 25 percent more time to process short wood than tree length on the terrain. But when adding the time for converting the tree length at roadside, the total time is 29 percent higher for the tree length method. On the other hand, the output for yarding tree length was 60 percent higher than for short wood. This is surely related to too small loads for the short wood system. If it is possible to deliver tree length, this seems to be the best solution.
The logging costs vary also with time consumption, but since the costs are different for the different equipment this has to be calculated separately. With a transport distance of 100 m it is slightly cheaper to work with short wood, while the opposite is true when the distance is 300 m. The difference is however within US$ 1.5. Compared to normal mechanized logging it is 30-50 percent more expensive, that means US$ 5 per m3 on average.
It is important to be aware that using cable crane on soft ground is just in the beginning. It is 25 years since we started with skidders. At that time an average yearly production was 2500 m3. Today those figures are totally different. We have the same tendency for the forwarders. Looking upon the increase during a short project period, this seems promising for the future. But there are still many unanswered questions, and more practical experience is needed.
