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Risk-based maintenance for tunnel 495
Construction Management and Economics
ISSN 0144-6193 print/ISSN 1466-433X online 2003 Taylor & Francis Ltdhttp://www.tandf.co.uk/journals
DOI: 10.1080/0144619032000089616
*Author for correspondence. E-mail: [email protected]
of resources and planning of schedules for effective
preventive maintenance programmes are normally deter-
mined by the companys Engineering Department
according to the requirements set by the equipment
manufacturer or the experienced maintenance staff. The
failures and effects of equipment (risk factor) and the
corresponding preventive actions are not communicated
well between different departments in the company.
Moreover, there are increasing demands for tighter regu-
latory requirements, shorter allowable maintenancetimes and lower maintenance budget, etc., which have
increased the complexities and difficulties of mainte-
nance operations significantly. As such, new approaches
need to be considered that would help management
to choose the best course of actions for reducing or
eliminating the potential risks of equipment failures.
Tomic (1993) proposed the use of risk-focused
maintenance in improving system reliability or availability
through systematically identifying the applicable and
Introduction
Maintenance management for toll road/tunnel manage-
ment is not new in Hong Kong. The primary objectives
of a toll road/tunnel management company are to provide
reliable, safe, fast and cost effective journeys for tunnel
users. The failure of any of the critical equipment in the
systems, such as power supply systems, tunnel ventilation
systems, tunnel lighting systems, sump pump, traffic
control and surveillance systems may cause disasters orhazards to users and operators. Although a toll road/
tunnel management company may adopt an expensive
preventive maintenance programme to keep the equipment
and facilities in good working condition at all times,
there is no formal and consistent method currently used
for setting up preventive maintenance programmes in
tunnel operations. It should be noted that the allocation
A risk-based maintenance management model for toll
road/tunnel operations
M. F. NG1, V. M. RAO TUMMALA2* and RICHARD C. M. YAM3
1Engineering Department, Route 3 (CPS) Company Limited, NT, Hong Kong2College of Business, Eastern Michigan University, Ypsilanti, MI, USA3Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong,
Kowloon, Hong Kong
Received 16 May 2002; accepted 20 February 2003
Preventive maintenance (PM) has long been recognized as a method to increase equipment reliability andavailability. However, for equipment in complex plant installations like toll road/tunnel systems, to carry
out PM on all components may not be feasible, or, may end up with excessive maintenance costs. This
paper describes how a risk-based maintenance management model was formulated to systematically
prioritize PM activities. The model was based on the five core elements of the risk management process
(RMP): identification, measurement, assessment, valuation, and control and monitoring. This model was
applied to a toll road/tunnel company in Hong Kong to enhance the PM operations of its lighting system.
The improvements recommended in this case study show that the application of RMP in preventive main-
tenance could effectively identify and assess potential risks for equipment and facilities. The RMP results
provide quantified information for decision-makers to select the best course of actions for implementing a
more cost-effective risk-based PM system.
Keywords: Risk management process, preventive maintenance, toll road/tunnel, operations
Construction Management and Economics ( July 2003) 21, 495510
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effective course of action for each failure mode of a
system. The major advantage of employing a risk man-
agement approach is to provide a thorough assessment of
risk factors of equipment failures. On the other hand,
Vaughan (1997) defined the fundamental part of risk
management function as the design and implementation
of procedures to minimize the occurrence of loss or the
financial impact of the loss. According to him, the
objective of risk management is to reduce and eliminate
certain types of risks facing organizations by avoiding,
reducing, and transferring risks.
Similarly, several authors have developed different
risk management approaches based on different objec-
tives. For example, the approach adopted by the Engi-
neering Council (1994) is more on a general application
suitable for most kinds of engineering activities. The
European Community promotes a comprehensive risk
management methodology, RISKMAN, which provides
a more comprehensive framework to enumerate and
assess potential risk factors associated with a project.RISKMAN focuses on project management issues, and
emphasizes heavily towards the active management of
risks rather than the identification and assessment of
them (Carter et al., 1994). On the other hand, both
Raffia (1994) and Hayes et al. (1986) defined risk man-
agement as a process consisting of several steps, as
against what Hertz and Thomas (1984) referred to as
risk analysis. Charette (1989) defined risk engineering
consisting of two separated but interdependent concepts:
risk analysis and risk management. As described by
Cooper and Chapman (1987), risk management involves
a multi-phase risk analysis approach, which covers the
identification, evaluation, control and management of
risks from the perspective of social hazard management.
Rowes (1993) approach does not consider the phase of
controlling and monitoring. Thus, a lot of confusion
exists among practitioners in applying different risk
management approaches.
Through comparison of these several risk management
approaches, Tummala et al. (1994) developed the risk
management process (RMP) consisting of five core
elements. As shown in Figure 1, the five core elements
are: risk identification (finding and understanding risks);
risk measurement (measuring the severity of risks); risk
assessment (assessing the likelihood of occurrence of risks);
risk evaluation (determining or ranking the identified
risk factors according to the management objectives and
available resources, and implementing risk response
action plans); and risk control and monitoring (tracking
the progress made and the results achieved by the risk
response actions taken as a result of risk evaluation phase
and taking corrective actions). The RMP is a compre-hensive, detailed and easy to apply approach to manage
risks. There are several successful applications that prove
the viability of the RMP approach in the construction
and maintenance fields. Burchett and Tummala (1998)
studied the need and feasibility of employing the RMP to
assess risks in capital investment for extra-high voltage
(EHV) transmission line construction projects (Tummala
et al., 1999). On the other hand, Tummala and Lo
(forthcoming) and Tummala and Mak (2001) applied
the RMP in developing a risk management model for
improving electricity supply reliability and transmission
operation and maintenance, respectively. In addition, Yu
Figure 1 Risk management process framework
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Risk-based maintenance for tunnel 497
(1996) developed a knowledge-based system applying
RMP in tackling schedule risks in project management
for an EHV substation construction project. Similarly, a
knowledge-based expert system was developed by Leung
(1997), who used RMP and applied it to an EHV trans-
mission line construction project to identify, evaluate and
manage project cost (Leung et al.,1998). Another risk
management model was developed by Mok (1994) to
apply RMP in preparing cost estimates for building serv-
ices installation of the building construction projects
administered by the Building Services of Architectural
Services Department of the Hong Kong Government. In
the field of maintenance, Leung (1994) developed a
framework by integrating the system hazard analysis with
RMP to make it more applicable to assess safety and
reliability risks associated with the door system of a train
car for the Mass Transit Railway Corporation (MTRC)
in Hong Kong (Tummala et al., 1996).
It should be noted that either the RMP or other risk
management models can assist project managers/decisionmakers in identifying and assessing potential risk factors
to develop and implement the best course of actions in
eliminating or reducing the identified risk factors. Even
though they may not be able to identify all the potential
risk factors, they can still provide an effective means
to quantify and manage risks as opposed to other non-
quantifying approaches. Burchett et al. (1999)carried
out a worldwide survey within the context of electrical
power supply projects and confirmed that there is a drive
towards a more thorough assessment of risks. They also
pointed out that a formal risk management process
would meet the expectations of business growth and
project sponsors and ensure that all risks are activelymanaged throughout the life cycle of a project. However,
the issues of risks are not just technical, e.g. on hazard
or failure processes, they are concerned with decision
making and management support systems as well.
Understanding risks and their control processes may still
need further R&D, especially in some industries. Each
industry should therefore review its own situation
relating to the relevant experiences of the others and
develop its own appropriate risk management systems.
This paper aims to describe the development and
implementation of an effective risk-based maintenance
management model for a toll road/tunnel company to
eliminate or reduce risks of equipment failure. The
proposed model is developed to integrate RMP with the
generic maintenance processes planning, scheduling,
executing, analysing and improving (Figure 1). The
application of RMP in maintenance modelling overcomes
the deficiency of most of the maintenance models by
considering the consequences of faults, their likelihood
of occurrence and the cost of implementing risk response
actions in a meaningful fashion. Moreover, suitable
maintenance strategies can be determined based on the
identified risks. The formulated model was then applied to
a real case of a toll road/tunnel operations to examine its
applicability. The results obtained and effectiveness of
the proposed risk-based maintenance model is described
later in this paper.
The risk-based maintenance managementmodel (RBMMM)
A risk-based maintenance management model is formu-
lated using the RMP as shown in Figure 2. The model
begins with the identification of the strategic importance
of the project. The mission, aim and objectives of the
company are the driving forces behind the model leading
to the improvement of quality and effectiveness of
maintenance operations under different internal and
external factors facing the company.
The potential risk factors are identified for each critical
unit that may affect the success of the project. Subse-quently, the consequences of all identified risk factors are
determined and the magnitudes of the impact of their
consequences (consequence severity) are enumerated.
Depending upon the probability distributions of all
identified risk factors, the likelihood of occurrence of
consequences is assessed. Checklists, event tree analysis,
fault tree analysis, Failure Mode and Effects Analysis
(FMEA), HazOp analysis and Cause-and-Effect (C-E)
Diagrams are some of the well known and widely used
techniques to identify potential risk factors (risk identi-
fication) (Sundararajan, 1991; Tummala et al., 1994;
FMEA, 1995). The System Hazard Analysis technique
along with the FMEA is useful in enumerating andassessing the consequences of the identified risk factors
(risk measurement) (Military Standard, 1993). The
System Hazard Analysis technique is also suitable in
assessing the severity of consequences and risk probability
levels through qualitative analyses (risk assessment).
Several cases have been reported on the successful appli-
cation of the System Hazard Analysis technique (Leung,
1994; Tummala et al., 1994, 2001). Monte Carlo Simu-
lation is another popular simulation technique used to
generate probability distributions for project success
factors by observing the probability distributions of all
risk factors affecting them (Hammersley and Handscombe,
1967; Schmidt and Taylor, 1970). Other tools, such as
five-point estimation and probability encoding can also
be used if data are not sufficient. If sufficient data are
available, one can use the Bestfit software to determine
the best fitted distribution (@Risk, 1992; BestFit, 1993).
All these techniques are complementary to each other. In
selecting the suitable techniques for risk identification,
measurement and assessment, the following factors
should be considered: the objectives of the study, the
nature of the problem, the complexity of the process, the
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data requirements of the study, the resources available
for the study and the level of expertise required in the
use of these techniques (IEEE Spectrum, 1989). After
reviewing these factors, the System Hazard Analysis
technique (Military Standard, 1993) and FMEA (1995)
were selected in this model for risk identification, risk
measurement and risk assessment.
The risk evaluation phase is to rank and prioritize the
identified risk factors and to determine the risk accept-
ance levels according to the aim, objectives and available
resources of the project. The risk severity and probability
levels generated in risk identification, risk measurement
and risk assessment phases can be used to calculate the
risk exposure values (risk severity risk probability) for
each, or each group of, risk factor(s). All such information
could be used to determine the acceptable risk exposure
levels, the appropriate preventive maintenance programmes
and the risk control actions. The Hazard Totem Pole
(HTP) approach proposed by Grose (1987) can be used
to systematically evaluate the identified risk factors and
to integrate the severity, likelihood of occurrence and
cost of preventive action into a format for easy decision-
making by management. The advantage of HTP is
that it simultaneously assesses the three fundamental
management concerns: performance, schedule and cost.
When the three variables are known, a HTP diagram can
be plotted out. Finally, the cut-off points or risk acceptance
levels can be determined based on the identified risks, and
Figure 2 Risk-based maintenance management model for toll road/tunnel operations
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Risk-based maintenance for tunnel 499
the aims, objectives and available resources of the project
and suitable maintenance activities can be determined.
The risk identification, measurement, assessment and
evaluation are repeat processes so that when a new situa-
tion occurs, such as change of government regulation or
decrease in performance level resulting from system or
component failure or malfunction, the HTP analysis will
indicate the risk levels of respective risk factors to alert
management. Based on such information, management
can then revise the existing acceptance levels and formu-
late appropriate maintenance strategies to improve the
performance to meet the revised acceptable levels.
The execution phase is the actual implementation of
the preventive maintenance tasks according to the
planned schedule. Suitable check sheets should be used
for a proper control and monitoring system. During the
execution phase, appropriate feedback channels should
be established to report the deviations from the planned
activities or changes in environmental factors. The risk
control and monitoring phase reviews the progress of theproject continuously and recommends necessary correc-
tive actions to management for accomplishing the project
objectives. Moreover, it serves to ensure that the training
of staff, the auditing of risk management activities and
the established emergency plans are properly executed
and coordinated among various parties in an effective
and efficient manner. It is useful to generate information
regarding major events/milestones, project status and
project summary reports throughout the lifetime of the
project to facilitate information distribution to staff and
management.
Finally, as shown in Figure 2, the risk-based mainte-
nance management process should be supported by a
computerized maintenance information system (MIS).
The MIS includes information storage, data processing
and analysis and report generation. Basically, the MIS
system consists of several databases to keep track of all
maintenance activities. This maintenance information is
useful for future risk measurement, assessment and the
determination of the best courses of actions for reducing
or eliminating the identified risk factors. It is also useful
for planning the contingency measures and training of
staff in an organization.
The case study
Reliability of a tunnel lighting system is crucial for tunnel
users, and its continuous operation without interruption
must be assured. As illustrated in Figure 3, the tunnel
Figure 3 Tunnel lighting configuration for one tunnel tube
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Nget al.500
lighting configuration can be divided into three sections
entrance, interior and exit in a tunnel tube. The
entrance section is the most critical area, because without
sufficient portal brightness, the entrance will appear to
the approaching drivers as a black hole. The most severe
visual task is not when the driver is passing through the
plane of the portal shadow, but when he or she is outside
of it and is trying to see within the portal shadow. The
entrance section comprises the threshold and transition
zones installed to provide sufficient reinforcement lighting
to reduce the black hole effect by gradually decreasing
the luminance level so as to finally match the basic lighting
in the tube section. The interior section simply provides
an adequate luminance level for safety driving. In order
to ensure reliable tunnel lighting, the power supply of the
basic lighting is provided by two independent uninter-
rupted power sources connecting from the two ends of
the tunnel. The odd number lighting sets are connected
to one power supply and the even number lighting sets
are connected to another. In case of failure or poweroutage of a single power supply system, it will not cause
a total or a sectional black out of the tunnel lighting that
would endanger the drivers in the tunnel. The exit
section on the other hand appears as a bright hole to the
motorists. Usually, all obstacles will be discernible by
silhouette against the bright exit and thus they will be
clearly visible. However, in order to comfort the eyes of the
drivers, reinforcement lighting similar to the entrance
section is also provided. The reinforcement lighting at
the exit section is also designed for bi-directional traffic
condition as well. The reinforcement and basic lighting
are divided into six control stages. Depending on the
photometer reading, an appropriate lighting set up will
be selected by the central monitoring and control system
(CMCS) or manually by the operator in the central
control centre. Figure 4 shows the basic control circuit
schematic (Ng, 1998).
Driver process
As shown in Figure 2, the model begins with the driver
process. In line with the vision, mission and the overall
corporate business strategy of the company, the driver
process identifies the strategic importance of the projectunder different internal and external environmental
factors. The purpose of the driver process is to translate
the aims and objectives of the project into several project
success factors that can be used as guidelines for and
Figure 4 Basic control circuit diagram of tunnel lighting control
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Risk-based maintenance for tunnel 501
understood by the project team. This process also enables
top management to recognize the importance of the
project so as to obtain their commitment and involvement
in supporting the project. The internal factors are influ-
enced by two external factors: government and customer
requirements. Government requirements are concerned
mainly with the changes in standard or ordinance, while
customer requirements emphasize more on service quality,
safety and cost. For the toll road/tunnel company, the
corporate business plan and the toll road/tunnel manage-
ment plan are the two major internal factors for developing
the mission, aims and objectives of the operations.
The aim of this case study was to apply the formulated
risk-based maintenance management model to the toll
road/tunnel company for selecting the best course of actions
in improving its existing preventive maintenance activities
(Ng, 1998). In order to achieve this, the following
objectives were established:
(1) to reduce the breakdown duration and frequency
of the tunnel lighting system; and
(2) to minimize hazards to drivers in case of break-
downs of the tunnel lighting system.
These objectives were in line with the aims and objectives
of the toll road/tunnel company. The outcome of the case
study was to propose an action list for the decision by the
management of the toll road/tunnel company. The action
list should include the priority of preventive actions and
improvement works that would eliminate or reduce the
identified risks in the tunnel lighting system so as to
achieve a more cost effective maintenance operation.
System decomposition
Before identifying potential risk factors, the tunnel lighting
system was decomposed into a controllable hierarchy.
The system decomposition involved the categorization of
the equipment and the identification of the objectives and
performance criteria of maintenance for each unit in the
hierarchy. All the correspondence, manuals, drawings
and schematics were collected at this stage to form the
detailed equipment information database for the tunnel
lighting system. The hierarchical/top-down techniques
were used to illustrate the construction of the component
list. The power-supply system, the central monitoring
and control system (CMCS) and the dimming control
system were the three major sub-systems of the tunnel
lighting system (Ng, 1998). All the units of these sub-
systems were grouped together and listed out in different
functional parts as shown in Table 1.
Risk identification
From the component list created in the system decom-
position stage, the potential risk factors for equipment
failure of the tunnel lighting system were identified.
According to McAndrew and OSullivan (1993), failure
mode and effects analysis (FMEA) is a simple technique
used to identify potential risks and it is also suitable for
service industries such as toll road/tunnel operations. In
addition to FMEA, the following tools and techniques
were also used in assisting the risk identification process:
the instrumentation diagram, schematic and blockdiagrams, logic diagram, process flow diagram, installation
drawing, inventory parts list, manufacturers manual,
flow charts, etc. The possible failure modes, their symp-
toms and the possible causes were identified and filled
in the FMEA check sheet as shown in Table 2 for the
three functional parts: the power supply, system control
and field equipment. Subsequently, two different kinds
of failure effects the hazards to drivers and traffic
blockage were listed out. The detection of the failure
and the kind of actions recommended preventing the
re-occurrence of the breakdown or failure are shown in
the FMEA check sheet (Ng, 1998).
From the FMEA analysis, the failure effects of the
dimming controller, dimming output control unit, dim-
ming input module and electronic control ballast were
found having no impact and hazard to drivers. Moreover,
failure of these components would not cause serious or
total breakdown to the tunnel lighting system. These
components were, therefore, eliminated from the subse-
quent analysis. Table 3 lists out the potential risk factors
that cause the traffic blockage and hazard to drivers. For
easy reference, an identification code was assigned to
Table 1 System decomposition for the tunnel lighting
system
Component name
Control/protection relay
Isolator
ContactorBooster transformer
MCCB
Dimming controller
Dimming output control unit
Dimming input module
CMCS central computer
CMCS field control unit
CMCS programmable logic
controller
Basic lighting fittings fluorescent
tube
Reinforcement lightingfittings sun lamp
Photometer
Electronic control ballast
Item
1. Power supply
1.1
1.2
1.31.4
1.5
2. System control
2.1
2.2
2.3
2.4
2.5
2.6
3. Field equipment
3.1
3.2
3.3
3.4
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Nget al.504
each potential risk factor as shown in the last column of
Table 3. The data generated in the risk identification
phase were also stored in the maintenance information
system (MIS) for analysis at a later stage.
Risk measurement
Risk measurement involves the enumeration of the
consequences and the magnitude of impacts for all
identified potential risk factors generated in the risk
identification phase. The four-severity category scale
recommended by the US Military Standard 882C was
used for assessing the level of severity of consequences.
By reviewing the specific requirements of the toll road/
tunnel operations, an additional severity category called
significant was added in between the original severity
categories of critical and marginal. As such, a five-
severity category scale catastrophic, critical, signifi-
cant, marginal and negligible was formed to assess the
severity levels of the consequences for the hazard todrivers and the duration of traffic blockage failure effects
(see Table 4).
The failure effects reported in the FMEA analysis
were used to determine the severity level of the conse-
quences. For example, by referring to Table 2, the
failure effects of the control/protection relay breakdown
(CP) would cause the tunnel illumination decreasing to
an uncomfortable level to drivers; hence, the conse-
quence severity level 2 on the hazard to drivers was
assigned to CP (x symbol in Table 5). Similarly, in
consultation with experienced operations staff, the same
failure would also cause an outage of less than 50 m basic
lighting, which would slightly affect the traffic. As such,
the consequence severity level 2 on the duration of traffic
blockage was assigned to CP (# symbol in Table 5).
Consider another illustrative example, namely the
booster transformer breakdown (BT). As shown in Table 2,
the failure in BT might cause a major accident to occur
which could be critical; therefore, the consequence
severity level 4 on the hazard to drivers was assigned to
BT ( symbol Table 5). The same failure would also
lead to the closure of the affected tunnel tube and theother tube would have to be operated in single-tube
two-way traffic causing a critical traffic jam, and hence
1
2
3
4
5
6
78
9
10
Control/protection relay breakdown
Isolator/contactor breakdown
Booster transformer breakdown
MCCB breakdown
CMCS central computer
CMCS field control unit
CMCS programmable logic controllerBasic lighting fittings fluorescent tube
Reinforcement lighting fittings sun lamp
Photometer
CP
IC
BT
MC
CC
FC
PLBL
RL
PH
Table 3 Potential risk factors
Item Risk factor Identification code
Table 4 Severity categories for hazard to drivers and duration of traffic blockage
Consequence severity
categories
Hazard to drivers Duration of traffic blockage Assigned
Index
Catastrophic
Critical
Significant
Marginal
Negligible
Serious traffic accident
Major traffic accident
Minor traffic accident
Illumination in tunnel decreases
to an uncomfortable level,
very difficult to see objects
The eyes feel twinkle
Both tunnel tubes lighting outage
Traffic stopped, more than 45 min. delay in travelling
time
Less than 500 m of basic lighting outage or one entrance
portal reinforcement lighting outageTraffic jam, 1545 min. delay in travelling time
Less than 200 m or either odd or even no. of basic
lighting outage or more than two stages of
reinforcement lighting outage
Traffic slowed, 515 min. delay in travelling time
Less than 50 m of basic lighting outage or one stage of
reinforcement lighting outage
Traffic flow slightly affected, less than 5 min. delay in
travelling time
Minor effect to traffic flow
5
4
3
2
1
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the consequence severity level 4 on the duration of traffic
blockage was assigned (# symbol in Table 5). Table 5
shows the different consequence severity levels for hazard
to drivers and duration of traffic blockage for all the
other identified risk factors (Ng, 1998).
Risk assessment
Risk assessment involves the determination of the
likelihood of occurrence (probability) of each identified
risk factor. Occurrence (frequency) is the rating value
corresponding to the estimated expected frequencies
or cumulative number of failures that would occur for
a given cause over the lifetime of the equipment.
Depending on the available information, the likelihood
of occurrence may be expressed either in qualitative
or quantitative terms. The US Military Standard 882C
five-level risk occurrence category frequent, probable,
occasional, remote and improbable was used. Table 6
shows the qualitative and quantitative descriptions of the
risk occurrence probability categories (failure rates) for
component failures. Similar to the consequence severity
category levels, a severity level was also assigned to each
risk occurrence category as shown in Table 6. At the
time of conducting the risk assessment, the equipment
had been operating for less than one year, hence
sufficient failure data were not available. Therefore, the
qualitative approach suggested in Military Standard
882c was adopted and its risk probabilities were
determined as shown in Table 7 (Ng, 1998).
Risk evaluation
The risk evaluation process begins first with determining
the risk exposure values (Grose, 1987).
Risk exposure value
The risk exposure value for each identified risk factor is
calculated as follows:
Risk Exposure Value =Consequence Severity LevelRisk
Probability Level (Table 5) (Table 7)
The risk exposure values for risk factors on hazard to
drivers () and duration of traffic blockage (
#) were
Table 5 Consequence severity levels for hazard to drivers () and duration of traffic blockage (#)
Consequence severity Identification code of risk factors
Categories
Catastrophic
Critical
Significant
MarginalNegligible
Level
5
4
3
21
CP
#
IC
#
BT
#
MC
#
CC
#
FC
#
PL
#
BL
#
RL
#
PH
#
Table 6 Probability categories
Frequent
Probable
Occasional
Remote
Improbable
Likely to occur frequently
Will occur several times in the life of an item
Likely to occur some time in the life of an item
Unlikely but possible to occur in the life of an item
So unlikely, it can be assumed occurrence may
not be experienced
The probability is greater than 0.1
The probability is between 0.1 and 0.01
The probability is between 0.01 to 0.001
The probability is between 0.001 to
0.000001
The probability is less than 0.000001
5
4
3
2
1
Risk probability
categories
Qualitative description Quantitative description Level
Table 7 Risk probabilities on tunnel lighting component breakdown
Risk probability Identification code of risk factors
Categories
Frequent
Probable
Occasional
Remote
Improbable
Level
5
4
3
2
1
CP
+
IC
+
BT
+
MC
+
CC
+
FC
+
PL
+
BL
+
PH
+
RL
+
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Nget al.506
calculated using the consequence severity levels of Table 5
and the corresponding risk probability levels shown in
Table 7 and were tabulated as shown in Table 8. For
simplification and easy reference, the risk exposure
values were grouped into four risk exposure classes with
designated class codes and risk exposure levels respec-
tively as shown in Table 9. The FMEA check sheet
shown in Table 2 have already listed out the possible
preventive actions to eliminate or reduce the identified
risk factors. The costs for each of the preventive actions
for the toll road/tunnel operations were calculated and
described in Table 10 with designated cost category, cost
level and cost class code. Subsequently, the total cost of
preventive actions for each identified risk factor could be
determined as shown in Table 11 (Ng, 1998).
With all these costs and risk information, the next step
is to determine the risks that are to be acceptable, toler-
able or unacceptable. The hazard (or class) codes and the
numerical level numbers for individual risk factors are
tabulated as shown in Table 12 for the three variables i.e.
the duration of traffic blockage and the hazard to drivers
(Table 9) and the cost of preventive actions (Table 11),
respectively. According to the Hazard Totem Pole
(HTP) algorithm, priority is given to high severity, high
likelihood, and low cost (Grose, 1987). The hazard
index (HTP score) is determined as the sum of the
numerical level numbers of the three variables. More
preventive maintenance actions should be carried out for
those risk factors with higher hazard index values (HTP
scores). For easy reference, Table 12 shows the prioritized
Table 9 Risk exposure value classification
Hazard to
drivers
Duration of
traffic
blockage
Risk exposure
class
Risk exposure
level
Risk exposure
value
Risk factor
identificationcode
Number of
risk factors
Cumulative
number of riskfactors
J
K
L
M
A
B
C
D
4
3
2
1
4
3
2
1
1625
915
48
13
1625
915
48
13
IC
CC
FC
PL
BT
PH
BL
RL
CP
MC
IC
CC
BT
MC
FC
PL
PH
CP
BL
RL
0
4
6
0
1
1
8
0
0
4
10
10
1
2
10
10
Table 8 Risk exposure values for risk factors on hazard to drivers () and duration of traffic blockage (#)
1
5
5
1
5
5
2
4
8
2
4
8
1
5
5
1
5
5
3
4
12
2
4
8
3
4
12
2
4
8
1
4
4
2
4
8
4
3
12
3
3
9
4
2
8
4
2
8
2
3
6
2
3
6
3
4
12
4
4
16
CP IC BT MC CC FC PL BL RL PH # # # # # # # # # #
Consequence
severity (A)
Risk probability
(B)
Risk exposure
value (AB)
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Risk-based maintenance for tunnel 507
hazard index values (HTP scores) in descending order.
Figure 5 shows the HTP diagram constructed from
Table 12 (Ng, 1998). Figure 5 is easy to interpret and
ready to use for management to make decisions on main-
tenance activities. It helps management to re-arrange and
re-schedule existing maintenance tasks according to the
objectives of the organization. All the data and informa-
tion generated in this risk evaluation stage should be
stored in the maintenance information system (MIS) for
monitoring purposes.
Table 10 Cost categories on preventive actions
Substantial
High
Low
Trivial
Spare parts for booster transformer
Adding ventilation fan
Spare parts for CMCS central computer
Spare parts for CMCS field control unitDeveloping predictive algorithm
Providing training to operations and
maintenance staff
Spare parts for CMCS programmable
logic controller
Spare parts for photometer
Carrying out power supply loading test
Spare parts for control/protection relay
Spare parts for isolator/contactor
Spare parts for MCCB
Improving operating procedures
$250 000
$160 000
$150 000
$125 000$120 000
$70 000
$60 000
$50 000
$45 000
$9000
$6000
$3000
$2000
> $200 000
Between $100 000
and $200 000
Between $10 000
and 100 000
< $10 000
1
2
3
4
S
R
Q
P
Cost categories Preventive
actions
Preventive action
cost
Cost range Cost level CostClass
code
Table 11 Summary of cost of preventive actions
CostClass code
Q
Q
S
Q
S
R
R
R
R
R
Risk factor
identification code
CP
IC
BT
MC
CC
FC
PL
BL
RL
PH
Total cost of preventive actions
$45 000 +$2000 +$9000 =$56 000
$45 000 +$2000 +$6000 =$53 000
$45 000 +$160 000 +$250 000 =$455 000
$45 000 +$2000 +$3000 =$50 000
$2000 +$70 000 +$150 000 =$222 000
$2000 +$70 000 +$125 000 =$197 000
$2000 +$70 000 +$60 000 =$132 000
$120 000
$120 000
$2000 +$70 000 +$50 000 =$122 000
Cost level
3
3
1
3
1
2
2
2
2
2
Table 12 Prioritized hazard index of risk factors
Priority Risk factor
identification code
HTP score Cost of preventive
actions
$53 000
$56 000
$50 000
$222 000
$197 000
$132 000
$120 000$120 000
$122 000
$455 000
1
2
3
4
5
6
78
9
10
IC
CP
MC
CC
FC
PL
BLRL
PH
BT
10
7
7
7
7
7
66
6
5
Numerical
level no.
4
2
2
3
2
2
22
2
2
3
2
2
3
3
3
22
2
2
3
3
3
1
2
2
22
2
1
Hazard code
(class code)
A
C
C
B
C
C
CC
C
C
K
L
L
K
K
K
LL
L
L
Q
Q
Q
S
R
R
RR
R
S
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Nget al.508
Maintenance activities execution
According to the outcomes generated in the risk evalu-
ation stage, appropriate preventive maintenance activities
could be recommended. Figure 5 shows that it would be
most cost effective to conduct preventive actions for the
isolator/contactor of the power-supply (IC). However,
the available resources from management should decidethe determining factor for the cut-off point. If, for example,
HK$800 000 were allocated to implement the improve-
ment works, the first six preventive actions listed in
Figure 5 could be carried out to eliminate or to reduce the
corresponding risks. On the other hand, the consequence
severity level of the boost transformer (BT) was found to
be critical for both the risk factors for hazards to drivers
and the duration of traffic blockage (see Table 5).
Because of the low occurrence probability (see Table 7)
and high preventive maintenance costs (see Table 11),
the priority for BT was determined to be the lowest in
the HTP diagram. If such low priority risk must be
eliminated, top management must allocate extra resources
to carry out the required preventive actions, which might
not be cost-effective. Alternatively, a contingency plan can
be implemented and the concerned staff can be trained
beforehand to cater for such high risk factors with limited
resource situations. Furthermore, the HTP diagram also
indicated that the basic and reinforcement lighting
fittings and photometer were not that important to affect
the normal operations of the tunnel. The preventive
maintenance frequency for these items, therefore, should
be reduced. The simple HTP diagram, which consolidates
all the results of the risk-based preventive maintenance
management model, is a simple tool helping management
in making effective decisions more easily.
It should be noted that the risk profiles and the related
information generated by the proposed model are useful
for understanding the impact of equipment failures.More importantly, such information should be shared
within the organization through proper training so that
the maintenance activities can be implemented effectively
and efficiently.
Risk control and monitoring
The risk control and monitoring processes continuously
review the effectiveness and the degree of compliance of
the maintenance activities through periodic checks or
audits. These control mechanisms provide feedback to
management for taking corrective actions and signals for
staff and the public regarding the effectiveness of the
implementation of the risk-based maintenance management
system. The risk control and monitoring processes must
be perceived by staff as means to determine possible
preventive measures and to provide guidelines for further
improvement, rather than a search for a scapegoat. In the
control and monitoring stage, deviation from specifications
or requirements, abnormal cases and accidents that
occurred are all reported. For example, if the reduction
of maintenance frequency of the basic and reinforcement
Figure 5 HTP diagram for risk evaluation of the tunnel lighting system
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Risk-based maintenance for tunnel 509
lighting fittings creates lighting blackouts, the maintenance
frequency must to be revised. For the isolator/contactor,
if the increased frequency of preventive maintenance
creates an unacceptable workload, additional manpower
needs to be provided. As such, the purpose of risk control
and monitoring is to check the quality of the works
performed and to take appropriate corrective actions, if
necessary.
Maintenance information system
From the system decomposition stage to the risk control
and monitoring stage of the risk-based maintenance
management cycle, a lot of information are required to
be processed, shared and stored in different processes. As
shown in Figure 2, the maintenance information system
(MIS) consisting of five different modules is designed to
facilitate information processing in the maintenance
management system. The system/equipment risk data-
base module in the MIS is one such module and isdeveloped to support the implementation of the risk-
based maintenance management model. The risk infor-
mation related to following are stored in this module and
updated as needed:
the identified risk factors;
the consequence severity levels;
the risk probabilities; and
the Hazard Totem Pole.
The other four modules that comprise the MIS include
the document module, maintenance record module,
work order system module and the material and labour
resource module. The computerized MIS supports vari-ous processes of the risk-based maintenance management
system. It is useful to build up a comprehensive failure
rate database for the implementation of a quantitative
and objective risk-based analysis. A proper MIS system
can also generate useful management reports for control,
monitoring and auditing purposes.
Conclusion
A risk-based maintenance management model has been
developed and applied to a real life case in a toll road/
tunnel company for enhancing preventive maintenance
activities. The advantage of the model is that it helps
operators to establish and determine suitable mainte-
nance strategies for selecting the best courses of action
in managing identified risks. The model also requires
the participation of different departments of the company
to determine the failure modes and effects of equipment
and the corresponding preventive actions. Therefore, it
improves the understanding on the impact of equipment
failures (risk factors) between different departments. The
model starts with identifying all potential risk factors due
to equipment failures (risk identification). Then, all the
possible consequences and their magnitude are enumer-
ated (risk measurement). Subsequently, the probability
of occurrence for each of the identified equipment failure
modes is assessed (risk assessment). Afterwards, the
identified risk factors are ranked according to their
exposure values and costs of preventive actions. By
combining the quantified data of the variables, a priority
table and the corresponding HTP diagram can be created
for management to decide on the best courses of action
to contain and manage the identified risks (risk evalua-
tion). The results of the case study clearly indicates that
the formulated model can be applied effectively in imple-
menting appropriate risk-based maintenance strategies
to reduce the risks due to equipment failures. More
importantly, it is easy to understand and apply for similar
kinds of maintenance improvement projects.
The application of RMP in maintenance modelling
overcomes the deficiency of most of the maintenancemodels by considering the consequences of faults, their
likelihood of occurrences and the costs of implementing
risk response actions in a meaningful fashion. Moreover,
if the risk-based maintenance model is repeatedly used,
it will generate a rich risk profile of each component of
the system. Based on this information, contingency
measures and training for staff can be implemented much
more effectively.
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