Title: Chapter 2: Maintenance Forecasting and Capacity Planning
1Chapter 2 Maintenance Forecasting and Capacity
Planning
- Overview
- Maintenance load forecasting
- Qualitative Forecasting Techniques
- Quantitative Forecasting Techniques
- Error Analysis
- Forecasting Maintenance Work
- Maintenance Capacity Planning
- Deterministic Techniques for Maintenance
Capacity Planning - Stochastic Techniques for Maintenance Capacity
Planning - Contract Maintenance
2Maintenance Load Forecasting
- Definitions
- Maintenance load forecasting is the process by
which the maintenance load is predicted - Maintenance load f(age of equipment, rate of
equip. use, maintenance quality, climatic
factors, skills of maintenance workers, etc.)
and varies randomly - Models
- Qualitative forecasting expertise, experience
and judgment (historical analogy, surveys, the
Delphi method) - Quantitative forecasting mathematical models
uses historical data to estimate future trends (
time series-based methods moving averages,
exponential smoothing structure methods
regression models)
3 Role of Maintenance Load Forecasting in a
Maintenance System
4Maintenance Load Forecasting
- Quality of forecasting models
- Accuracy
- Simplicity
- Flexibility
- Factors to select the forecasting approach
- The purpose of forecast
- The time horizon for the forecast
- The availability of the data
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6Qualitative Forecasting Techniques
- Extract information by questionnaires or
interviews identify variables - Identify the relationships among the variables
cause-and-effect or the Delphi techniques - Identify which variables influence the forecast
and the impact of each one - Estimate the magnitude of the variables reduce
variation by interactive process
7Quantitative Forecasting Techniques
- Simple Moving Average
- Weighted Moving Average
- Exponential Smoothing
- Seasonal Forecasting
- Regression Analysis
8Simple Moving Average
- Given n observations x1,x2,,xn that xt follows
xtb?t and equally important ?t1,2,..,n .
Where b is expected demand in any period, ?t is a
random error with mean 0 and variance ?2? - Requirement estimate the parameter b for the
future value - Solution Use least square method to minimize the
sum of squared error
9Weighted Moving Average
Where w1w2wn 1
10Exponential Smoothing
- Assign weights to observations of previous
periods in an inverse proportion to their age -
- Where forecast for period t and
all future periods in the case of a constant
model x(t-1) Actual demand at period t-1
Forecasted value for t-1 ?
Smoothing constant, 0lt ?lt1
11Exponential Smoothing
- The ES ? estimate the parameters for a constant
model (simple exponential smoothing), linear
model (double exponential smoothing), and any
polynomial model. - Constant model x(t)b?t where b is expected
demand. Corresponds to - Linear model x(t)abt?t where expected
demand at time t is E(x(t)t)abt. We have
12Exponential Smoothing
- Apply exponential smoothing again (double
smoothing), we have - at each period t-1, the
values of a and b are updated as follows -
- Initial conditions to start the process are
13Regression Analysis
- A regression model expresses the relationship
between a dependent variable (characteristic)
with some independent variables. The mathematical
form is yf(t,x1,x2, )
?(t,x1,x2,). Where coefficients of f are
needed to determine - A regression analysis refers to the process of
estimating the model parameters using the least
square method. - Least square minimize
14Error Analysis
- Objectives
- Check the effectiveness of a forecasting mode
- Evaluate and select forecasting model
- Measurements
- Sum of errors
- Mean absolute deviation (MAD)
- Mean squared error (MSE)
- Mean absolute percentage error (MAPE)
- Mean squared percentage error (MSPE)
15Forecasting Maintenance Work
- Forecast maintenance load - Emergency
maintenance workload( random and can be minimized
by a well designed planned maintenance)
historical workload forecasting techniques
and/or management experiences. - Preventive
maintenance workload historical records
preventive maintenance programs routine
inspection and lubrications - Deferred
corrective maintenance historical records
future plans - Forecast for
overhaul-removed items fabrication historical
records future plans for improvement -
Shutdown, turnarounds, and design modifications
historical records future maintenance schedule
16Forecasting Maintenance Work
- Backlog errors in forecasting and job standards
- Load forecasting for the coming weeks or
months work forecasting by examining the
maintenance backlog.
17Maintenance Capacity Planning
- Maintenance capacity planning determines the
optimal level of resources (workers, skills,
spares, inventory equipment, and tools) required
to meet the forecasted maintenance load (future
load forecast and backlog) - Procedure
- Determine total maintenance load
- Estimate the required spares material to meet
the load - Determine the equipment tools that are
necessary for all types of maintenance work
18Maintenance Capacity Planning
- Determine the skills the number of workers from
each skill. Special attention should be given to
multi-skill crafts - Provide special plans for highly computerized
equipment, if required - Problems use maintenance load, standard times,
job arrival times to determine the optimal mix of
skills of crafts from the sources available and
the optimal capacity that meet the required load.
Objectives cost, availability, reliability. - Techniques - Deterministic approach
heuristic, LP - Stochastic approach queuing
models, simulation
19Deterministic Techniques for Maintenance
Capacity Planning
- Heuristic Tableau Method
- Problem Determine craft mix, minimize cost
- Principle use common sense guidelines and
principles to solve - Provide sufficient in-house workers for
high-priority work - Measurement ratio between overtime work and
regular-time work, and a fixed healthy backlog - Examples (text book)
20Linear and Integer Programming Techniques
- Decision variables the number of hours from
different skill levels and trades made available
through regular in-house, overtime or contract
maintenance - Objectives maximize resource utilization,
minimize total cost - Constraints overtime hours, capacity of sources,
etc. - Examples (text book)
21Stochastic Techniques for Maintenance Capacity
Planning
- Queuing theory has been used to determine
maintenance staffing, to evaluate the performance
of the system under steady-state conditions.
However, queuing theory do not reflect the
transient system behavior - Simulation is applied for the complex system and
consideration of transient situation.
22Queuing models
- Description Customer ?arrive?Server choose
(FIFO, LIFO, etc.)?leave - Elements of a queuing model
- Arrival distribution
- Service time distribution
- Design of service facility
- Service discipline
- Customer population (size of the queue and
customer population - Human behavior
23Queuing models
- Output information (under steady-state
condition) - Ls expected number of customers in the system
- Lq expected queuing length
- Ws expected waiting time in system
- Wq expected waiting time in the queue
- Input information (a/b/c), (d/e/f)
- a arrival distribution
- b service time distribution
- c number of parallel servers
- d service discipline
- e maximum number allowed in the system
- f size of customer population
24Example 1 of Queuing Model
- (M/M/C),(GD/?/?). If C servers have an
exponential distribution with parameter ? and if
we let ??? then we have - where P0 is the probability of
zero customers in the queue,
25Example 2 of Queuing Model
- (M/M/R),(GD/K/K), RltK find a value of R
repairmen (servers) for K machines that minimizes
the total expected costs, which consist of the
cost of failure and the cost of service. If ? is
the rate of breakdown per machine and there are n
broken machines, given the arrival rate - and the service rate is
- Pn is the probability of n
machines in the system. The steady-state results
Where expected number of idle repairmen
26Simulation
- Stochastic simulation is the process of
representing a system on the computer and then
employing well-designed experiments (scenarios)
to evaluate its performance. Using this process,
systems can be analyzed, planned, and designed - Procedure to implement a simulation for
maintenance capacity planning - Study company maintenance requirements to
determine the types of maintenance workers
crews required, the types criticality of
equipment repaired, the failure mechanism for
each piece of equipment, the effect of a
failure on production or service provided by the
organization
27Simulation
- Forecast maintenance workload and divide it
according to priority - Outline the existing work order system and define
the logic of work assignments - Setup the relevant machine servicing model after
determining the failure rate of each machine, the
service rate, and the cost of each machines
being out of service - Develop simulation software application and
verify and validate the model - Perform production runs and, on the basis of all
measures of performance, find the optimal
staffing levels
28Contract Maintenance
- Does contracting maintenance build or diminish
the competitive advantage? - Contracting maintenance during peak periods is
more effective in most working environment when
finite projects or tasks can be estimated and
outsourced as a work package. (capital expansion,
modification projects)