OPTIMAL LINE LENGTHS WHEN SKIDDING WOOD WITH THE SYNCROFALKE CABLE CRANE IN SLOVENIAN CONDITIONS

Boštjan KOŠIR, University of Ljubljana, Biotechnical Faculty
Department of Forestry and Forest Resources
Ljubljana, SLOVENIA

Abstract

In recent years, five Syncrofalke cable cranes have been in operation in the alpine region of Slovenia. On the basis of two independent studies, the optimal line length of Syncrofalke cable crane lines has been calculated. Optimal line length is dependent upon the layout of cable crane corridors (parallel, fan-shaped), skidding direction (uphill, downhill), and average size of the log and wood concentration. The proportion between setting up and taking down and skidding costs also explains the differences in calculated optimal line length under different conditions. The choice of the line length, which is not equal to the calculated optimum, does not significantly affect the difference between optimal and actual costs in quite a wide range of the line lengths.

Key words: cable crane, Syncrofalke, Slovenia

Introduction

In Slovenia, multidrum cable cranes with towers have been in use since the end of the 1960s. The actual share of cable skidding is not very high, but in mountainous parts of Slovenia this is very often the only reasonable possibility for wood extraction. Classical gravity and long-distance cable cranes are still in use, but during recent years some of the most modern mobile tower yarders were put into practice.

For many years we have performed mostly gravity skidding (uphill and downhill) despite the availability of some devices equipped for all-terrain operations, but only recently have all-terrain cable operations become more frequent. Some of the devices, which are still in use, were produced in Slovenian workshops, but Slovenian forestry is also traditionally very well connected with cable crane and equipment production in Austria. We had been importing many of the Urus series cable cranes, Koller carriages, Gantner cable crane and other equipment, and also Wanderfalke and, most recently, Syncrofalke cable cranes with Sherpa U 3 carriages, among others. There are four Syncrofalke cable cranes in operation in Slovenia today.

Many of the cable cranes were studied very closely (Košir, 1985; Košir et al., 1988; Košir, 1990). Some of the most advantageous devices have not yet been studied to a satisfactory extent. In this article the results of two studies are combined (Valjavec, 1998, Rupnik, 2001), and some new applications are drawn.

When skidding with cable cranes the line lengths are normally very different. They differ from the maximum length of the skyline of one cable crane to one very short length, which is not precisely defined. Many times we consider connecting costs as a category which should not exceed a certain amount, rather than as a category which could be greatly influenced with carefully detailed planning of cable crane corridor layouts. Line length as the main influencing factor on the total wood extraction costs is normally under most thorough consideration, but there are also other factors that are important enough when considering optimal line length and, in connection with this, optimal skidding distance.

In this article we have considered — in addition to line length — the influence of skyline patterns (parallel, fan-shaped), bunching distance, average log size in the load and wood concentration per metre of the line length with regards to total costs of wood extraction with Syncrofalke cable cranes.

Methods

Characteristics of the Syncrofalke cable crane

The Syncrofalke cable crane is produced by Forstbetrieb Mayr-Melnhof (Austria). The all-terrain cable crane is mounted on a truck chassis together with the cabin and grapple loader (see Table 1). The skyline is 600– 700 m long; the main line up to 700 m; haulback line 1 400 m; and the auxiliary line 1 400 m. In the choice of lines, several alternatives are possible. Maximum line tension in the skyline is 89kN (dependent upon line type). The tower is 10.5 m high with four guy-lines (4 × 70 m). Carriage is remote controlled Sherpa U 3 for uphill and downhill skidding. Carrying capacity of the cable crane is 3.0 tonnes.

There were two Syncrofalke mobile tower yarders included in this study. The older machine was put into use in 1996 in the northern region of Slovenia, which is abundant with large-sized conifers. The area is mountainous and is a typical alpine region. The second machine is a little younger (1997) and has also some minor changes in comparison with the first one (like grapple loader and truck specifications, the skyline length is also slightly longer). The area of operations is northwestern Slovenia with its typical alpine conditions, but the main tree species in this case is beech. As usual the performance was expressed in tonnes per day, which levelled the difference between the different weights of conifers and broadleaf.

Table 1. Some basic technical characteristics of the studied cable crane

Setting up and taking down

Assembling and dismantling the first Syncrofalke cable crane was studied in detail by Valjavec (1998) according to the methodology that had been used before (Košir, 1985). There were ten corridors included in the study. In all cases they worked with a three-man crew. The installations were regardless of the skidding direction with the haulback line. In some cases the cable cranes were operating in thinnings of different stages, but there were also cases of regeneration felling and even cutting in stands damaged by the windthrow. Productive line lengths varied between 90 and 276 m when skidding uphill and between 82 and 370 m when skidding downhill. Actual line lengths were in most cases at least 50 m longer (to reach the end anchor). Intermediate supports were rare and altogether we measured times of setting up and taking down of three supports. Wood volume also varied significantly and was between 0.54 and 2.38 m³ per metre of the line (approx. 0.50–2.0 tonnes per metre). The weather was mostly favourable and that was true also for average terrain conditions (see Table 2). In all cases (up- and downhill) the crane operated with three lines: skyline, mainline and haulback (auxiliary) line.

Time studies were done with a continuous method for every one of the three man crews. The times were later added together to get total time duration of the work. Setting up and taking down times were also added together to get a unique time for fixing one line. On average the share of the setting up times in total assembling and dismantling times was 73 percent (uphill), 75 percent (downhill).

We supposed that total setting up and taking down time is dependent upon the influence of the line length (setting up and down the lines), constant times for setting up and taking down the machine with a tower, type of layout and the time for setting up and taking down intermediate supports. The number of supports is on average dependent on the line length, as we may suppose that on longer lines we can expect more terrain difficulties and, therefore, more supports. For the purpose of this study we derived an equation that was based on the following consideration:

Time (h) that is dependent upon the line length for parallel lines:

T1 = 0.0000106*LL²+0.000934*LL

N = 10
R² = 0.51

Constant times dependent upon the layout system are: 3.79 h for uphill and 4.10 h for downhill lines. These times should be added to the previous equation to get times for setting up/taking down parallel lines for uphill and downhill skidding directions. For fan-shaped lines the above times should be reduced with the coefficient (Košir, 1990):

k = 0.0004*LL + 0.72

Time (h) for setting up/taking down the supports is also (indirectly) dependent upon the line length and is (average of the measurement):

T2 = 0.00991*LL

Skidding the wood

The basic methodology of field studies was taken from previous studies (Košir, 1990) and is therefore comparable with older time studies. There were 349 loads included for uphill skidding and 362 loads for downhill skidding on several different lines. For the purpose of this study we derived the following statistical equations:

Uphill skidding (minutes per load):

T = (3.671 + 0.00603*VLA + 0.1440*ZBI - 1.994*MAS)*1.38
N = 349         R² = 0.710

Downhill skidding (minutes per load):

T = (3.584 + 0.007844*VLA + 0.134*ZBI - 1.347*MAS)*1.38
N = 362         R²=0.762

Where:

Table 2. Basic characteristics of the studied cable corridors

min = time in minutes of the whole crew (3 workers)

The above times were divided by the load size to get the time per load time. The load size is dependent upon the average size of the piece and was calculated as:

Uphill skidding (tonne per load):

Tonne = 1.604-0.0943/MAS
N = 349         R²=0.615

Downhill skidding (tonne per load):

Tonne = 1.753-0.139/MAS
N = 362         R²=0.660

Optimal line length computation

Optimal line length computation is based on a calculation of total costs that are the sum of setting up and taking down and skidding costs. We can see from the example in Figure 1 that optimal line length is defined by both components as minimal costs in specific configurations. It is obvious from the previous section that the total costs are dependent upon the following parameters: line length, layout pattern, wood concentration, skidding direction, skidding and bunching distances and the size of the timber. There are also other factors whose impact we have not included in the equations because of the lack of measurements or for other reasons. In any case, there are many combinations among the listed factors, and we were forced to make some restrictions. We decided to compute the costs for a moderate step for the line length up to 580 m and wood concentration from 0.2 to 1.1 tonne per metre of line for different settings and skidding directions. Basic cost calculations were received from the Tolmin Forestry Enterprise. All costs were calculated in Euro.

Optimal line length is equal to the minimum costs, which can be obtained by computation from the above-listed equations or by deriving the proper equation by the line length. The last method would be more accurate, but also impractical. We chose the simpler way and computed the optimal line length with the help of a simple programme, which exceeded all expectations.

Results

Influence of the cable corridor layout on total costs

Setting up and taking down costs are dependent upon the line characteristics. This means that these costs are independent of the bunching distance, which could be connected with the space between the corridors, and other variables such as average timber size (see Table 3).

Skidding costs are on the other hand influenced almost entirely by variables such as bunching distance, log size and others.

Table 3. Average relative costs in dependence on the cable crane layout, bunching distance and average size of the log

We can see from Table 3 that bunching distance and log size do not affect the setting times. Differences between the setting costs of different skidding directions are up to 4 percent where downhill skidding is more expensive. The difference between the setting costs of parallel and fan-shaped lines within the same skidding direction is 16–17 percent; fan-shaped lines are cheaper.

The influence of the bunching distance on skidding costs is on average 23–24 percent. The influence of the log size is much greater and lies between 69 and 86 percent. Altogether the differences in total costs are between 53 and 67 percent regarding the log size. Regarding the skidding direction there is almost no differences in total costs.

Influence of the line length and wood concentration on total costs

We have already seen that the line length affects the setting up/taking down and skidding costs. In the model, the greater line length also means greater timber volume per setting, as the wood concentration per metre of line is constant. This also means that the costs per unit are smaller on longer lines. The costs of setting are composed of fixed (i.e. tower anchoring) and variable parts (i.e. setting up and down the lines). In Figure 1 the costs per lines up to 580 m are shown for wood concentration 1.0 tonne per metre of line. The influence of the fixed part of setting costs is very strong at shorter lines, while on longer lines, i.e. above 300 m, the influence is no longer very strong.

Skidding costs are influenced by previously mentioned variables among which line length is the most important. In parallel settings of lines we can calculate that average skidding distance is half of the line length, and two-thirds of the line length in fan-shaped layouts. Longer lines mean also higher skidding costs (see Figure 1).

As a result we can compute the total costs, which show in this case the minimum costs at the line length of 260 m. Up to 260 m the influence of setting costs is greater than skidding costs, while at longer lines the influence of skidding costs prevail. The biggest difference between the costs at optimum line length and the most expensive case is 26 percent, but the optimal range is quite flat; within 5 percent difference in costs from optimum lay lines from 160 to 420 m.

Figure 1. Influence of the line length to the setting and skidding costs

Timber concentration affects only setting up and taking down costs. Skidding costs are constant, as we suppose that the bunching distance remains the same, if we have a small or great concentration of felled trees per metre of the line. In any case, wood concentration is among the most influential variables despite the fact its relative influence slowly decreases with increasing wood concentration.

Optimal line length

Line lengths where minimal costs of one combination of the variables were calculated are shown in Table 4. The range of optimal line lengths is quite wide. The proportion between maximal and minimal optimal line length varies according to the change of wood concentration in a range between 1:1.70 and 1:2.17 (+ -0.04). The maximal difference is at an uphill direction and fan-shaped lines with small wood, and the minimal difference is at downhill skidding, fan-shaped lines with large wood. Smaller wood concentration causes the optimal line length to be longer, while on the other hand small wood skidding means a shorter optimal line length at the same layout. Comparison of the skidding directions shows that at downhill skidding we can expect up to 8 percent longer optimal distances.

Parallel and fan-shaped lines also differ in optimal line length. In parallel settings optimal lines are up to 40 percent longer. In this case the setting up and taking down costs are relatively higher than in fan-shaped settings and this causes the need for longer lines. The average total costs are also higher in this case.

The case with the log size influence is similar: large timber also causes up to 40 percent longer optimal distances, as the skidding costs are lower than with the small wood with the same setting up and taking down costs. If bunching distance enters the regression equation in a linear combination, it does not affect the proportion between skidding and setting up and taking down costs of different line lengths. The influence on the optimal line length therefore does not exist (see Table 3).

In Table 5 the indexes of total costs are computed if the top left cell is set to 100 percent. We can observe quite significant differences between combinations. Total costs are almost the same in both skidding directions, which is surprising according to some previous reports (Valjavec, 1998). Total costs in parallel setting are up to 4 percent higher because the setting up and taking down costs are higher in parallel, but on the other hand average skidding costs are higher because of longer skidding distances in fan-shaped lines. The share of the setting up and taking down costs in total costs explains differences in optimal line length under constant wood concentration and different settings very well. At longer optimal distances the share of the setting up and taking down is also greater.

There is still the question of the sensitivity of the optimal line length calculation. In all cases it is relatively low, but it should be carefully examined, as we can in practice make a miscalculation in two directions: i) to plan a shorter; or ii) too long line than optimal. In the first case we can make a much greater mistake than in the second case. The illustration is shown in Figure 2.

Figure 2. Optimal line length, upper and lower limits of 5 percent higher costs for parallel lines, uphill skidding and average log size of 0.1 tonne.

Table 4. Optimal line lengths according to described variables

Table 5. Indexes of total costs according to described variables

Figure 3. Choosing line length for parallel lines, uphill, ZBI = 10 m. MAS = 1.0 tonne

Discussion

The problem of optimal opening of the forest area is reflected in several interconnected questions. Line length is in low correlation with the forest road network (Košir et al., 1988), but the question of optimal line length also demands the knowledge of cable crane set-up and take down times, as well as performance when skidding wood. In the paper these relations are observed from a merely theoretical point of view. Bunching distance is, in our case, an insignificant variable for calculating the optimal line length, and we could assume that optimal density of the cable crane corridors and optimal line length are not directly related. The condition for this to be true is that wood concentration would remain equal in sparse and dense line layouts, which in practice could not be confirmed. An integrated model should include optimization of the line length and distance between the lines. This demand also implies the necessity of studying the influence of the bunching distance on skidding costs more thoroughly.

References

Kosir, B. 1985. Influence of setting up and down on the total skidding costs with cable cranes on the example from Trnovo Forest - Strokovna in znanstvena dela 78, Ljubljana. p. 117.

Kosir, B., Gorican, E. & Koren, I. 1988. Analysis of some aspects of cable crane skidding on the basis of operational plans. Gozdarski vestnik 4, p.174–179.

Kosir, B. 1990 Economic organizational aspects of working area delimitation of cable cranes and tractors in wood skidding. Univ. of Ljubljana, Biotechnical Fac. Dep. of Forestry. Doctor thesis, Ljubljana, 350 pp.

Rupnik, A. 2001. The effects of wood extraction by Syncrofalke Cable Crane in Tolmin area. University of Ljubljana, Biotechnical Faculty, Dep. of Forestry and Forest Resources, Graduation thesis, Ljubljana, 56 pp.

Valjavec, B. 1998. The analysis of wood extraction in alpine conditions with mobile Syncrofalke Tower Yarder equipped with Sherpa - U3 Carriage. University of Ljubljana, Biotechnical Faculty, Dep. of Forestry and Forest Resources, Graduation thesis, Ljubljana, 33 pp.