Analysis of a mechanised cut-to-lenth harvesting operation through use of discrete-event simulation

Thesis (MScFor)--Stellenbosch University, 2015.

ENGLISH ABSTRACT: Abstract Mechanised timber harvesting operations are rapidly increasing in South Africa, particularly in Eucalyptus pulpwood production. There are however still considerable inefficiencies in implementation and evidence of unnecessary operational variability in current mechanised systems. A typical South African cut-to-length operation for harvesting Eucalyptus pulp logs utilising two excavator-based harvesters and one purpose-built forwarder was studied. The ability of performing discrete-event simulation using R was tested. One of the harvesters and the forwarder were simulated individually and alternative work methods were modelled and compared against the original work method. The changes in productivity based on productive machine hours and cost were recorded. The input data was negatively affected by the large variation in stand and individual tree characteristics. This led to a decrease in model validity. Machine simulation models where however able to capture trends found by other authors. The current method of felling a combination of the four and five tree wide swaths yielded the highest productivity of 11.43 m3 hr -1. Tree size had a notable effect on both the harvester and forwarder productivity. Increasing the tree size from 0.14 m3 to 0.20 m3 and 0.3 m3 led to productivity increases of 5.26 m3 hr -1 and 13.14 m3 hr -1, respectively. When comparing the original mean stack sizes of 2.5 m3, stacks with a mean volume of 5.4 m3 yielded an increase in productivity of 5.91 m3 hr -1. Fitting a larger grapple (1 m2 vs. original 0.8 m2 opening) to the forwarder showed increased productivities across all stack sizes. Forwarder productivity decreased by up to 17.8% with an increase in extraction distance. The largest decrease in forwarder productivity was noted when increasing the on-road travel (both loaded and unloaded) distance from 0 m to 100 m (single road scenario); decreasing by 6.1% when using the standard grapple and 7.6% when using the larger grapple. When using both roads the largest productivity decreases were found when increasing the on-road extraction distance from 0 m to 200 m; decreasing by 15.3% when using the standard grapple and 17.8% when using the larger grapple. Costing of each individual machine was carried out per alternative scenario. Decreases in harvester cost were noted between increasing tree sizes, and forwarder cost increased with extraction distance. Harvester cost decreased by as much as R35.24 m-3 when increasing mean tree volume from 0.14 m3 to 0.3 m3, whereas forwarding cost decreased by a maximum of R1.13 m-3 when extracting larger stacks (5.4 m3), when compared to the observed system (2.5 m3 stacks). Removal of all road travel elements and piling directly at roadside, showed savings of up to R10.21 m-3 when compared to the observed system. R proved to be useful for carrying out discrete-event simulations, however, dedicated simulation probability distributions need to be developed before it can be said that R is highly suitable for discrete-event simulation.

AFRIKAANSE OPSOMMING: Aangesien die gebruik van meganiseede hout ontginning operasies in Suid-Afrika vinnig toeneem, veral in die produksie van Eucalyptus pulp, is die implementering van produktiewe sisteme dringend nodig. Die implementering van hierdie sisteme is nog baie oneffektief en daar is tans baie variasie in die toepassing. ’n Tipiese Suid-Afrikaanse sny-na-lengte ontginnings operasie is ondersoek, dit het twee laaigraaf-baseerde enkelgreepontginners en een ekstraksie voertuig (“forwarder”) wat spesiaal gebou is, ingesluit. Die vermoë om afsonderlike gebeurtenis simulasie met gebruik van R uit te voer, is getoets. Een van die enkelgreepontginners en die ekstraksie voertuig is individueel nageboots. Behalwe vir die huidige sisteem is alternatiewe metodes gemodelleer en met die oorspronklike werksmetode vergelyk. Die veranderinge in produktiwiteit en koste is aangeteken. Die insetgegewe is weens die groot variasie in kenmerke van groepe bome negatief beïnvloed. Dit het gelei tot ’n afname in die geldigheid van die model. Die masjien modelle het egter dieselfde neigings getoon as wat die ander outears beskryf. Die huidige metodes om vier tot vyf tree wye stroke af te kap, het gelei tot die hoogste produktiwiteit van 11. 43 m3 per uur -1. Die grootte van die boom het ’n merkwaardige effek gehad op die produktiwiteit van die enkelgreepontginner asook die produktiwiteit van die ekstraksie voertuig. Die verhoging van die boomgrootte vanaf 0.14 m3 tot 0.20 m3 en 0.3 m3 het gelei tot ’n toename in die produktiwiteit van 5.26 m3 per uur -1 en 13.14 m3 per uur -1 onderskeidelik. Intussen het stapels met ’n volume van 5.4 m3 gelei tot ’n middelterm van 5.4 m3 en ’n toename in produktiwiteit van 5.91 m3 per uur. Die gebruik van ’n groter gryphaak (1 m2) het met betrekking tot alle stapelgroottes gelei tot hoër produktiwiteit. Die produktiwiteit van die ekstraksie voertuig het as gevolg van die toename in vervoer afstand met tot 17.8% afgeneem. Die grootste afnames is tussen afstandtussenposes 0 m en 100 m (enkelpad scenario’s) asook 0 m en 200 m (dubbelpad scenario’s) opgemerk. Die kosteberekening van elke individuele masjien is per scenario gedoen. Afnames in die koste van die enkelgreepontginner is opgemerk by toenames in boomgrootte, en die koste van die enkelgreepontginner het toegeneem met die vervoer afstand. Ontginnings kostes het met ’n maksimum van R35.24 afgeneem met ’n toename in boomvolume, terwyl die ekstraksie voertuig koste tot ’n maksimum van R1.13 per siklus afgeneem het wanneer groter stapels vervoer is. ’n Afname in die vervoer tot by die kant van die pad lei tot ’n besparing van tot R10.21 per siklus. Daar is bewys dat R tot ’n mate geskik is vir die simulasie van afsonderlike gebeurtenisse.