management

Open-pit project

management Christos P. Roumpos, Public Power Corporation S.A., Greece, and Filippos-Markos P. Spanidis, ASPROFOS Engineering S.A., Greece, present a project management approach to open-pit lignite mine planning and exploitation.

L

Lignite is used as a raw material in power plant operation. In Greece, electricity production by the Greek Public Power Corporation S.A. (PPC) is mainly based on lignite exploitation in open-pit mines. The country’s lignite reserves amount to 6.7 billion t, of which 4.2 billion t (59%) is economically recoverable.1 Exploitation projects usually take more than two decades. Establishing a project management plan to achieve production targets is essential, as lignite exploitation projects are complex and encompass scope, time, cost and quality.2 The key elements of an exploitation project management system are availability of resources, investment cost and organisation of resource allocation.

Belt conveyor transporting lignite from a mine to a power station bunker.

found. Mining specialists plan excavation so that the time and costs involved in the removal of overburden can be controlled. Following removal of the overburden, excavation of the lignite is carried out. The need for a high production rate and selective mining (corresponding to the deposit characteristics) resulted in the adoption of the terrace mining method (TMM). The exploitation system is divided into three main stages: excavation (E), transportation (T) and dumping (D); the EDT model.

The lignite mining fields are subdivided into segments known as ‘benches’, the thickness of which varies between 10 - 30 m, depending on the type of equipment available. The overburden/interburden waste materials are transferred to pre-selected disposal areas, while the lignite is transported to lignite yards situated close to the power plants.

Mining equipment The TMM combines the use of bucket

Open-pit lignite mine exploitation systems The mining method Greek lignite deposits develop on numerous horizontal seams of varying thickness, located between ‘interburden waste beds’ of varying lithological and petrographical content. On top of these geological formations, other waste material (overburden) is

Figure 1. Typical exploitation model (longitudinal section, not to scale). 55

World Coal • April 2003

Table 1. Technical characteristics of the lignite mine Lignite reserves

630 million t

Excavation area

34 km2

Average exploitation ratio

4:1 m3/t

[waste/lignite] 3040 million m3

Total excavations

Figure 2. AOA network for lignite exploitation project.

Mine organisation and planning Let us assume that a detailed mining study for the whole lignite field has been issued, outlining the following data:3 - 7 Optimised final pit limits. Strategic plan of mine development, comparing alternatives based on decision-making criteria.

Multi-seam lignite layers (overburden, interburden and lignite layers) in a pit slope after the cut of a bucket wheel excavator.

wheel excavators for excavation, belt conveyors for transportation, and spreaders for dumping. Bucket wheel excavators are the most suitable excavation machines for the selective mining of lignite layers with a thickness greater than 0.5 m. Preparatory site works are supported by a variety of conventional equipment, such as graders, rippers, elevating graders, trucks, mobile crushers, drilling machines and bulldozers. In addition, mining shovels are used when excavation of hard formations is required.

Geometrical data of excavation and dumping benches, according to the equipment (bucket wheel elevators, belt conveyors and spreaders). Bench and sector qualitative/quantitative data.

mass

Mechanical capabilities and utilisation parameters of the equipment. Production planning/schedule as a function of time. Organisational structure of the project. Mine investment analysis. Environmental impact issues. Lignite mining enterprises entail the development of both multidisciplinary and

Average calorific value

1300 kcal/kg

Density of the lignite

1.2 t/m3

Power plant capacity

1.6 GW

Mine life

37 years

long-term projects. The initial mine plans are usually reconsidered, due to the dynamic nature of the exploitation itself and uncertainties relating to electricity market changes, deposit characteristics, environmental constraints and the social infrastructure of the broader area. The main objectives of long and shortterm mine projects are to:5-7 Optimise schedule.

the

lignite

production

Ensure safe conditions for exploitation personnel and equipment. Maximise equipment use and minimise idle time. Minimise production costs.

Lignite mine exploitation model Mine geometry and technical characteristics For a simplified project management model, it can be assumed that a mine consists of four benches of overburden (three [A, B and C] with a thickness of 20 m, containing hard formations, and one [D] with a thickness of 15 m, consisting of loose material above the stratigraphic roof of lignite deposit) and three lignite deposit benches (E, F and G), each with a thickness of 15 m (interburden and lignite). Figure 1 schematically illustrates a

Note 1. Exploitation activities include preparatory loosening, excavation, transportation and dumping. 2. The exploitation period for each bench includes the set of sub-activities depicted in Figure 2 (i.e., A={A1,A2}, B={B1, B2, B3}, etc.)

Figure 3. Typical Gantt diagram for the project.

56


Outside dump reclamation. This dump is part reclaimed, since it continues to be developed horizontally. The spreader operates in deep dumping.

Figure 4. Enriched Gantt diagram in a matrix form for excavation and lignite production control.

typical longitudinal section of the mine development after the equipment has been installed in the last bench. Table 1 provides technical characteristics for an open-pit lignite mine (figures recorded at a Greek mine). Lignite production commences at the beginning of the fifth year of operation with the startup of the first lignite bench (E), and reaches 20 million tpa after full installation of the equipment in the last bench (G). Normally, an operating period of one year is necessary between the initial installation of the equipment in the two subsequent benches. This time can be reduced if the equipment is available and if there are no delays. At the end of the opening phase, bench G will therefore have been in operation for one year, while bench A will have already been operating for seven years.

Long-term project management For a project management framework, a complete cycle of the ETD model for each bench represents one phase of activity. The exploitation phases are considered as a subsystem with a chain structure which functions efficiently under continuous operation. So, in a project management model, a set of seven basic activities (A - G) is adopted. In

review technique) method should therefore outline more realistic results. The PERT method applies a probabilistic view of the project, as the completion time of each activity is a stochastic variable following a Beta distribution.8 Using this method, the (expected) mean completion time (ti) of the activity (i) is:

this phase, the development of dump benches is not taken into consideration. Optimum installation of the benches is necessary to maintain uninterrupted mine operation. They must advance simultaneously, forming, if possible, the safest excavation slopes achievable (usually 1:5 - 1:4). The mine then operates at the lower and more economical exploitation ratio, while the maximum space available for the inside dump is created. It is also supposed that no bottlenecks are caused as a result of insufficient equipment. In addition, waste dumping is assumed to be unobstructed. Figure 2 shows the project’s activity on arrow (AOA) network, and Figure 3 demonstrates its progress in the form of a Gantt chart. These two diagrams refer to the total operation lifetime of the mining project extended to 37 years (depletion time). In network analysis, the critical path method (CPM) was applied, taking into account the deterministic operation times of each activity. In this case, all paths are critical, as any delay in one bench can cause delays to the whole project. Mining projects are complicated and stochastic in nature because of the uncertainties experienced with such dynamic conditions. The combined CPM/PERT (project evaluation and

and the variance is:

where: Ai = the optimistic (minimum) completion time for the activity Mi = most likely time for the activity Bi = pessimistic (maximum) completion time for the activity The results of the CPM/PERT method are subject to evaluation by mining managers. Costs can initially be estimated and then later compared with the minimum time and cost calculations. Thus, CPM/PERT is recommended as a managerial support tool, on condition that reliable historical data for Ai and Bi parameters are available.

Notes 1.

Definitions:

ELTDi: Excavation, loading, transportation,

dumping (bucket wheel excavators and belt conveyors).

MCRi: Mechanical crushing (drilling).

BLSi: Blasting (preparatory loosening).

EMSi: Excavation by conventional excavators

and/or mining shovels, loading and transportation.

Figure 5. Typical annual activities network for each one of the benches A, B and C (months). 58


2.

It is assumed that all appropriate equipment is available.

3.

Dummy activities: - - - - - - - - - - -

Plantations in outside dumps. Soil material from the excavation side is deposited as the upper layer.

Figure 6. Gantt diagram of annual excavation activities for benches A, B and C. (The duration of the activities corresponds to the typical annual activities network in Figure 5.)

Exploitation production scheduling For the control of lignite production, the mine is separated into sectors. For each sector, quantitative/qualitative data are obtained through either geostatistical or conventional methods. Excavation and lignite production time schedules are better controlled using the diagram shown in Figure 4. This diagram simulates an enriched Gantt chart showing annual excavations and annual lignite production per bench and per sector. In practice, the matrix is structured according to the following rules: Each bench enters a sector following the completion of the previous one. In order for a bench to complete operation in one sector, the previous bench must have completed its operation in the same sector, or two benches need to have completed the sector simultaneously. The maximum available space for inside dumping and the minimum exploitation ratio are obtained when all benches finish each sector at the same time. The same methodology should be applied in considering the sectors of the

mine and evaluating all natural, technical, technological and financial factors affecting the duration of every activity. In Figure 4, the number of operating benches is 1, the mine consists of m sectors and mine lifetime is n years. Bench one operates in sector one for the first two years, while bench p enters the first sector in the year k. The deterministic mathematical analysis of the production scheduling includes the following figures. The total annual excavations of year k are expressed by: l

TAEk = Σ Qi,k

(3)

i=l

where l represents the number of benches. The annual lignite production (ALP) of the mine is calculated by the following equation:

The exploitation ratio in the benches where there are time delays.

1

ALP = Σ Li

where: Qi = the annual number of excavations of the bench i (m3) Si = the exploitation ratio (volume of overburden and interburden/weight of lignite) in the corresponding sector of the bench i (m3/t) Di = the lignite density (t/m3) l = number of a specific bench Assuming that the exploitation ratio Si is constant, the lignite production from bench i is proportional to the annual excavations Qi. Thus, any delay to the progress of bench i results in fewer excavations and consequently reduced lignite production. The annual loss in lignite production caused by the time delay in the network diagram is a function of the following parameters:

(4)

i=l

The time delay period corresponding to the annual losses in excavations is:

where Li = ALP (t) of the bench i. Li is calculated using the following equation:

l

Σ Qlos,i i=l

Note SMCRi: Mechanical crushing by subcontractors. SBLSi: Blasting by subcontractors. SEMSi: Excavation with conventional and/or mining shovels by subcontractors. ADSRi: Additional (geological) research. EQDFi: Equipment definition. INQRi: Inquiry execution. PTREi: Purchasing, transportation, receipt of equipment. TTIEi: Testing, training and incorporation of equipment.

Figure 7. Typical networking for each one of the benches A, B and C, in cases of hard formation volumes greater than those estimated (months). 59


Reclamation in an excavated area, following mine depletion. The final void represents the volume of the Figure 8. Typical Gantt diagram illustrating the excavation activities for each of the benches A, B and C in cases of hard formations interference.

where Qlos,i (m3) expresses the excavation losses in bench i. The corresponding lignite production losses are calculated using equations 4 and 5. The above model is simplified but representative. However, mining projects present a dynamic development, with time variations in mining conditions related to the deposit characteristics. The deterministic model should be supplemented by stochastic models, introducing uncertainty into the mine planning parameters.

Exploitation problems due to deposit characteristics Short-term project management As a case study for the application of short-term project management, the authors shall assume that hard formations are excavated by bucket wheel excavators with pre-loosening. The exploitation process in each bench is divided into the following sub-activities: Excavation (by bucket wheel excavators), loading, transport and dumping. Drilling. Blasting. Excavations by auxiliary equipment (in small areas). Figure 5 shows an AOA network diagram of the annual operation of each of the benches A, B and C, with the assumption that hard formation volumes can be handled using pre-loosening techniques. Figure 6 shows the corresponding Gantt diagram. The duration of the activities is

outside dumps plus the volume of the lignite. Normally, where possible, small lakes are created in the area of the final void, especially if there are rivers nearby.

related to the typical annual activities network in Figure 5.

Managerial rearrangements In some mining projects, the presence of hard rocks is not completely detectable in the initial stages. If hard rocks are confined to specific horizons, they can be excavated by auxiliary equipment. However, when they are distributed to all overburden benches, they cannot be excavated sufficiently by bucket wheel excavators, even if they are drilled and blasted. The advancement of those benches will therefore be delayed, affecting the whole mining development. Consequently, deviations from the initial mine plan will occur. An extended techno-economical study is required to select the optimum method for hard rock removal, taking into account all parameters affecting the operation of the exploitation system. The operations research methods are usually applied, emphasising the parameters of cost, time and quality.9 By definition, project management is the process of conceiving, preparing, driving and controlling transformations from an initial to a new state, meeting the targets of the project in the most convenient conditions.10 This process is carried out in terms of an organisational framework, a set of resources and a methodological approach. After the appearance of hard rocks and before the new equipment is installed, the removal of hard rock material may be partially carried out by subcontractors. At the same time, additional research into the deposit characteristics can take place (including face mapping), as well as the 60


necessary actions for the procurement of new equipment. These activities are presented in Figures 7 and 8. Until new equipment has been acquired, subcontracting activities and resources utilisation should be optimised by the mine managers. After testing and installing the new equipment, the mine plan should be systematically reviewed, ensuring efficient operation independent of the excavation equipment in each bench. Usually, relocation of excavation equipment from one bench to another is required to improve production. The time factor is more complicated, as the duration of a full cycle for the excavation machines is a stochastic parameter.6 In this case only a small amount of exploration data is available. The application of probability theory is therefore necessary. The probability distribution function for hard rock formations, P (H ≤ Q), expresses the likelihood of finding hard formation quantities less than the volume of Q. The expected completion time tij of the excavation sector j of bench i (activity = ij) can be expressed by the following equation: tij = P (Hij ≤ Qcr) t1 + P(Hij > Qcr) t2

(6)

where: Hij = quantity (volume) of hard formations not excavated by bucket wheel excavators. Qcr = critical amount of hard formations. Values exceeding this number necessitate the installation of additional equipment. t1, t2 = expected excavation times under the conditions that Hij ≤ Qcr and Hij ≥ Qcr

respectively. These time values are variable, related to the values of equation 1 and follow a beta distribution. However, the values of Ai, Mi and Bi are calculated using conditional probabilities. Thus, to calculate the distribution function of tij, in addition to the mean value and the variance, conditional simulation techniques can be applied. Finally, there is a stochastic correlation between duration and resource requirements.10

Organisational restructuring In order to efficiently handle extended hard rock excavations, special management activities and modifications to the project’s organisational structure are required. A new operational branch in the organisation chart should be created, which exclusively deals with hard rock removal. Human resources from the exploitation sector should be reallocated, and new personnel should be assigned, forming a matrix organisational structure. Thereby, a qualified project/mining specialist team will be created and will include a project manager, process/engineering manager, project/planning scheduler, contract administrator, procurement manager, construction manager, and quality and safety engineers.

Conclusion Mining projects are complicated, as a number of factors can affect their progress. Due to the geological/lithological formations, which present spatial variability and complexity, a revision of the regular mine plan is usually required. In addition, there are market uncertainties related to electricity demand, which can also influence progress. Based on the geometry of a TMM model, a project management methodology such as the CPM or the combined CPM/PERT can be adopted, contributing to improved management and production control. The application of the CPM method demonstrates that, in long-term planning, the progress of the mining activities on each bench is critical and, when delays occur, the whole project mine life is also affected. Uncertainties over geological deposits can cause serious delays to lignite production. In such cases, planning and management should be adjusted to meet the new conditions and thus to achieve good lignite production rates. In the case of hard formations, short-term project management methods can bring about significant changes to the project

framework. These include subcontracting and procurement of improved equipment, the extent of the geological/geotechnical investigation and the reallocation of the equipment within the current mining system. Revisions to project networking and scheduling have to be carried out. Excess costs also have to be accounted for, since the new activities increase investment cost. The organisation charts have to be revised, so that the hard formations excavation can be efficiently managed. Lignite mining projects require long-term project management. Therefore, reorganisation of the project, the optimised use of resources, as well as procurement of higher capacity equipment, may be required. For a Stacker dumping at a short-term period, the should have enough small delays and production. restricted availability of resources are critical for the final outcome. The CPM/PERT methodology should be applied in all project stages to overcome uncertainties, reduce downtime and meet the objectives of the mining project.

Acknowledgements The authors kindly acknowledge the contributions of Dr Dimitrios Filippou, fellow editor of the journal Mining and Metallurgical Annals; Dr Leontidis M., Head of Mine Planning Section in Mines Development Dpt., PPC S.A.; Dr Dellis A., Deputy Professor, Dpt. of Informatics, University of Athens; and Dr Paravantis J., Dpt. of Technology Education, University of Piraeus, Greece.

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