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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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Nget al.496

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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(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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Nget al.498

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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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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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

+


12/16

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)


13/16


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


14/16

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


15/16


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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