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CHAPTER 2. LITERATURE REVIEW

Existing Forest Harvesting Models

Computer simulation of logging systems began in the late 1960s. The goal of most logging system simulations has been to determine productivity, costs and the effect of changes to the system on productivity and costs (Goulet, Iff and Sirois, 1979). The following is a synopsis of many previous and existing forest harvesting models in chronological order.

Georgia Tech Model (1968)

This model is a General Purpose Simulation Software (GPSS) simulation of the production of 5’3” pulpwood. The model requires 19 variables and 15 distribution parameters to describe the system. The output is a GPSS summary with average performance values (Goulet et al. 1979).

Auburn Pulpwood Harvesting System Simulator (1969)

This model is based in FORTRAN and is a time-oriented simulation of southeastern pulpwood operations. It consists of two phases – production and transportation. The production phase includes: felling, delimbing and topping, skidding, bucking, bunching and loading. The transportation phase is simply the movement of trucks between the landing and mill (Goulet et al. 1979)
This model is composed of components – groups of like operations. Each component is assigned to a particular harvesting function and interactions among components are modeled using buffer inventories. A uniform time increment is used, and component cycle times must be evenly divisible by the uniform time increment for the model to run (Goulet et al. 1979).
The systems modeled are eight shortwood and six tree-length harvesting configurations. The output of the model is in the form of three reports: end of hour, end of day and end of week. End of day and end of week reports include a cost per log and production rates (Hool et al. 1972, Goulet et al. 1979).
All times used as inputs to the model are average cycle times that reflect the occurrence of variation due to delays. The model is deterministic in nature but can be modified to incorporate non-deterministic features using the Monte Carlo or other simulation techniques (Hool et al. 1972, Goulet et al. 1979).

Timber harvesting model for Appalachia (1973)

This model was one of the early attempts to provide a flexible productivity and cost estimator that could simulate a range of harvesting systems from cable skidders to cable yarders. All felling was assumed to be with a chainsaw (Biller and Johnson 1973).
Production study data was used to develop production functions for the model and generic cost information was used in cost calculation per unit of production and time. The output included information about distribution of time in elemental activities for general system components. The model included the capability to change the system to gauge the effect of possible changes on productivity and cost (Biller and Johnson 1973).

Simulation Applied to Logging Systems – SAPLOS (1973)

SAPLOS is a general logging simulation model, which is based in FORTRAN/GASP IV (Goulet et al. 1979; Goulet et al. 1980). It identifies five typical points where bottlenecks occur in a logging operation: stump, skid road, landing, prehaul deck and processing point. The activities that deliver logs to each point are modeled by identifying equipment and end-of-service events at each point. The model can simulate a very wide range of logging systems from small pulpwood crews to cable yarding crews (Goulet et al. 1979; Goulet et al. 1980).
Inputs to this model consist of cost and system configuration data and two subroutines. The first subroutine represents tree characteristics such as volume and height. The second subroutine represents stand conditions such as slope and stocking. The output gives estimates of average production and cost, with specific production and cost statements for each activity and the system as a whole (Goulet et al. 1979; Goulet et al. 1980).

Appalachian Logging Simulator (1973)

This model was developed with the goal of improving the logging system. It models different system configurations and reports productivity averages. Average productivities are used to determine where imbalance may lay and the magnitude of the productivity imbalance. The user has the capacity to change the number of each type of machine in the system. Costs are reported using the machine rate methodology (Martin 1973).

Forest Harvesting Simulation Model – FHSM (1975)

FHSM is a FORTRAN/GASP II timber and pulpwood-harvesting simulator. It consists of modules for: felling, limbing, bucking and limbing, bucking at the stump, skidding, bucking at the landing, loading, hauling and unloading. Modules can be selected and combined to tailor the model to a specific system. Interactions among modules are modeled with wood inventories and equipment operating capacities. The operational times of modules are created using elemental operating times for the operations. There is no inclusion of stochastic delays in the model (Goulet et al. 1979; Goulet et al. 1980).
Inputs to the model are standard cruise data, and frequency distributions from time and motion studies. This makes the model stochastic; the user controls the level of detail. The output from the model is in the form of a single report containing production and processing time by machine and total wood volume delivered to mill. There is no economic analysis included (Goulet et al. 1979; Goulet et al. 1980).

Timber Harvesting and Transportation Simulator – THATS (1975)

THATS is a FORTRAN based model that simulates standard harvesting configurations. It consists of felling-limbing-topping, bunching, skidding, bucking, loading, and hauling. There is also a road construction component (Goulet et al. 1979; Goulet et al. 1980).
Inputs to this simulation include: cost, stand data, average and standard deviations for the operation being modeled. The averages and standard deviations are used to generate events within the model. Random variable event times are assumed to be normally distributed; a logarithmic function is applied to the distribution when the time-study data shows a skewing (Goulet et al. 1979; Goulet et al. 1980).
Wood flow through the system is modeled by volume. The input to the operation is a tree and the output is a volume. An in-process inventory exists between operations. The output of the model is a basic table from which the user constructs the performance variables of interest. There is also a cost accounting component (Goulet et al. 1979; Goulet et al. 1980).

Full-Tree Chipping and Transport Simulator – FCTS (1976)

FCTS consists of a chipping model and a transport model both are built in GPSS. The model simulates feller-bunchers, skidders, a chipper and trucks in the field, and the trucks at the mill (Goulet et al. 1979; Goulet et al. 1980).
This is one of the earlier graphical simulation models where the Cartesian coordinates and volume of every tree in the stand must be specified. Tree identity is maintained until it has been chipped. The skidder movements are modeled such that spatial location of each skidder is known at all times and they can move around one another (Goulet et al. 1979; Goulet et al. 1980).
The feller-buncher and skidder types can also be specified to describe machine characteristics, so that a specific machine can be simulated in the system. The output of the model is a production and cost summary for each machine and an energy consumption estimate (Goulet et al. 1979; Goulet et al. 1980).

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Harvesting System Simulator – HSS (1976)

HSS is a FORTRAN based simulation of a harvesting system. The user specified system configuration contains a maximum of 14 machines divided into 6 aggregations of similar machines (Goulet et al. 1979; Goulet et al. 1980; Reisinger et al. 1986).
Differences in stand types, volume per acre, species composition, skidding distance and terrain can be modeled by dividing the tract into a maximum of 14 blocks. There is no constraint on the acreage or volume that each block can cover. A production rate is specified for each harvesting block and machine combination (Goulet et al. 1979; Goulet et al. 1980; Reisinger et al. 1986).
Non-productive periods can be modeled at specified times or applied stochastically. Repairs to machines that suffer from a breakdown can be made at the site of breakdown, by bringing the machine to the landing or hauling the machine to the shop. There are two types of delay; a minor delay leaves the machine in place while a major delay brings the machine back to the landing (Goulet et al. 1979; Goulet et al. 1980; Reisinger et al. 1986).
There are up to 27 variable/parameter inputs to the model, all or some of which can be used for a given run. The data used for inputs can be empirical, averages or theoretical distributions (Goulet et al. 1979; Goulet et al. 1980; Reisinger et al. 1986).
The output of the model is very detailed and includes time, production, cost and revenue. The reports tabulate this information by machine, phase and system on a weekly basis (Reisinger et al. 1986). Discounted cashflow and return on investment analyses can be made (Goulet et al. 1979; Goulet et al. 1980).

Residues for Power – REPO (1976)

This model was generated to simulate the specific process of transferring chipped harvesting residues to a power plant. Though it was developed for another purpose it can be applied more generally to materials handling to simulate a timber harvesting and transportation system (Goulet et al. 1979).
It is based in SIMCOMP and uses a fixed-time increment. Combinations of skidding, loading, transporting, unloading, sorting and shipping machines can be modeled. Flow of materials between operations is controlled by rate functions and materials-in-process inventories are used to show influence of one process on another (Goulet et al. 1979).

Harvesting Analysis Technique – HAT (1981)

This model began development in 1967. The simulator has three components: stand generator, machine simulator and harvesting system simulator (Stuart 1981).
This simulation is geared towards loblolly pine plantations and the actual HAT is a combined form of three models. The stand is defined using the Pinus taeda (PTAEDA) model. Individual trees are identified by their Cartesian location and have an associated physical description of size in the model. The generalized machine simulator (GENMAC) is included in the program set to model harvesting machine activities. The Harvesting System Simulator (HSS) is used to simulate harvesting systems which can be composed of up to 14 machines (Stuart 1981).
The HAT is used by first generating a stand using the PTAEDA model. The GENMAC model is run to generate a simulation summary report for each machine, a BUNDLE file describing the form and location of felled trees, PRODUCTION AND OPERATIONS file containing production distributions and a RESIDUAL file describing the remaining stand, if any. The PRODUCTION AND OPERATIONS file from GENMAC is used as input to the HSS model (Stuart 1981).

Program For Logging Cost Estimation – PROLOG (1982)

PROLOG is a menu driven cost analysis program, similar to LCAP (section 2.2.15), but based in FORTRAN. It was initially developed for students and consists of five sections to describe the system: general assumptions, equipment descriptions, labor wage information, reporting options and file management (Reisinger et al. 1986).

Geometric Modeling Of Thinning System Performance(1983)

This model was developed to model mechanized thinning to characterize the productivity and costs of thinning. It is applied in evaluating existing, proposed and conceptual aspects of feller-buncher design (Fridley and Jorgensen 1983).
Elemental equations are used to generate cycle times and are functional upon the characteristics of the feller-buncher and key site and stand parameters (Fridley and Jorgensen 1983).

Model for predicting logging machine productivity (1983)

This is a numerical model designed to provide estimates of productivity, and variation in productivity for existing and developing machine concepts. The model calculates volume produced and elemental production times. It returns the average and variance of the production and time (Meng 1983).

Logging Cost Analysis Package – LCAP (1984)

This is a Beginners All-purpose Symbolic Instruction Code (BASIC) menu driven program for calculating logging costs. It consists of four sections: machinery costs, labor costs, miscellaneous costs and summary table. Cost calculations are based on machine rate (Reisinger et al. 1986).
Inputs are costs and rates entered by the user for each of the required data. Output is given by hour, day, month and year, for each of the three cost sections and summarized in the summary table section (Reisinger et al. 1986).

CHAPTER 1. INTRODUCTION.
1.1 Objectives
CHAPTER 2. LITERATURE REVIEW
2.1. Existing Forest Harvesting Models
2.2. Types of Forest Harvesting Models
CHAPTER 3. STELLA SOFTWARE
CHAPTER 4. HARVEST SYSTEM ASSIGNMENT (HSA) MODEL
4.1. Objective
4.2. Scope
4.3. Structure
4.4. Inputs
4.5. Productivity calculation
4.6. Delays
4.7. Simulation length
4.8. Outputs
CHAPTER 5. MACHINE ALLOCATION (MA) MODEL
5.1. Objective
5.2. Scope
5.3. Structure
5.4. Inputs
5.5. Simulation calculations
5.6. Simulation Length
5.7. Outputs
CHAPTER 6. MODEL VALIDATION.
6.1. HSA Model
6.2. MA model.
CHAPTER 7. EXAMPLE SIMULATIONS.
7.1. HSA Model
7.2. MA Model
CHAPTER 8. SUMMARY AND CONCLUSIONS
8.1. Harvest System Assignment Model
8.2. Machine Allocation Model
REFERENCES