Assessment of Winch-Assist Skidder in Gisborne, New Zealand
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Executive Summary: Winch-assist systems are commonly used to expand ground-based operations onto steeper terrain. Skidders tethered to winch-assist machines are becoming popular in New Zealand as a method of extraction, however little is known about the productivity and soil disturbance effects from this system. The aim of this research was to improve the industry’s knowledge of winch-assisted skidder systems. An investigation was carried out in Emerald Forest, about 30km south-west of Gisborne, of an operation using a Falcon Winch Assist machine and a six-wheeled Tigercat 635G grapple skidder over a period of three days. Objectives were to determine the productivity of the winch-assisted skidder operation, to carry out a soil disturbance assessment on the slopes, and to evaluate the benefits of utilising this system.
Productivity was assessed by measuring 121 skidder cycles over two different skidder paths using standard time study methods. The skidder paths followed the same main trail from the landing for 150m before diverging down different sides of a ridge. Bunches were extracted from different distances along the paths. Skidder Path 1 had a maximum slope of 33 degrees (65%) and was 300m long. Skidder Path 2 had a maximum slope of 30 degrees (58%) and was 315m long. As expected, as the extraction distance increased, the cycle time of the skidder increased and productivity decreased. The average skidder productivity over the study period was 65.0 cubic metres per productive machine hour (m3 /PMH). The average productivity per scheduled machine hour (SMH) was 48.7 m3 /SMH. Machine utilisation was 75%. This was largely attributed to a significant mechanical delay which stopped the skidder from working for half of the second day of the study. The soil disturbance assessment was carried out post-harvest using a line transect method collecting 902 data points over three different sites. The assessment combined the effect on site of the winchassist skidder, the tracked felling machine and the shovel logger. The three sites resulted in deep disturbance across 17%, 11% and 10% of the sites measured, where deep disturbance was defined as exposure of subsoil. Shallow disturbance, defined as mixing of litter and topsoil, occurred in 27%, 20% and 21% of the three sites respectively. In this particular setting, the use of the winch-assisted skidder contributed to saving the construction of 720m of road, a large culvert crossing and two skid sites that were originally planned. A forest area of 19.3 hectares which was planned to be harvested by cable yarder was scheduled for this winch-assisted skidder system, with significant savings in logging cost. Other benefits of using this system included increasing the utilisation of the winch machine by tethering both the felling machine and skidder. By enabling more days logging due to the ability to work in poor weather conditions, overall system productivity was improved. The ability to extract stems away from waterways resulted in the riparian strips being left intact, reducing the impact of harvesting on the waterways. It was evident that maintaining a mix of extraction distances throughout the day was necessary to ensure that the processor on the landing had adequate buffer stock and was not waiting for wood throughout the day.