System Availability & Maintenance-Stais-Gkoumas CW2 Rev02(1)

7/25/2019 System Availability & Maintenance-Stais-Gkoumas CW2 Rev02(1)

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NAVAL ARCHITECTURE OCEAN AND MARINE ENGINEERING

NM916

Systems Availability & Maintenance

Dr I. Lazakis

Coursework 2

Fault Tree Analysis (FTA) Application

Gkoumas Dimitrios

Reg. No. 201580394

Stais Giorgos

Reg. No. 201582379


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2Stais Giorgos|Gkoumas Dimitrios

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Registration Number :201580394 Name : Gkoumas Dimitrios

Registration Number:201582379 Name:Stais Giorgos

Class Code :NM916 Coursework Title: Fault Tree Analysis (FTA)

Lecturer: Dr . I. Lazakis

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Table of Contents

1. Introduction 4

2. Analysis Procedure 4

3. FTA Symbols Description 5

4. Systems Fault Tree Analysis 7

5. Overall Results 11

6. Cut Sets 14

7. Importance Measurements 16

8. Systems Improvement 21

9. Conclusion 22

10. References 23

11. Appendix 24


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1.Introduction

The fault tree analysis (FTA) is a well-known reliability tool used in various research

studies and industries since its original introduction in reliability analysis in the 60s

and 70s. It is a deductive procedure used to define the various combinations of

hardware and software failures and human errors that could cause undesired events at

the system level. Fault tree analysis maps the relationship between faults, subsystems,

and redundant safety design elements by creating a logic diagram of the overall system.

The analysis begins with a general conclusion, then attempts to determine the specific

causes of the conclusion by constructing a logic diagram called a fault tree. This is also

known as taking a top-down approach. The main purpose of the fault tree analysis is to

help identify potential causes of system failures before the failures actually occur. It

can also be used to evaluate the probability of the top event using analytical or statistical

methods. These calculations involve system quantitative reliability and maintainability

information, such as failure probability, failure rate and repair rate. After completing

an FTA, you can focus your efforts on improving system safety and reliability.

2.Analysis Procedure

Many different approaches can be used to model a FTA, but the most common way

can be summarized in a few steps. A single fault tree is used to analyze only one

undesired event or top event, which may be subsequently fed into another fault tree as

a basic event. FTA analysis can be defined in five steps:

1. The Definition of the undesired event. This can be very hard to find, although

some of the events are very easy and obvious to observe. Someone with an

engineering background and knowledge is the best person who can help define

and number the undesired events.

2. Understanding the problem. Once the undesired event is selected, all causes

with probabilities of affecting the undesired event of 0 or more are studied and

analyzed. Getting exact numbers for the probabilities leading to the event is

usually impossible for the reason that it may be very costly and time consuming

to do so.


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3. Fault Tree Construction. After the selection of undesired event and having

analyzed the system so that we know all the causing effects we can now

construct the fault tree. Fault tree is based on AND and OR gates which define

the major characteristics of the fault tree.

4. Fault Tree Evaluation. After the fault tree has been assembled for a specific

undesired event, it is evaluated and analyzed for any possible improvement.

5. Control of systems hazards. This step is very specific and differs largely from

one system to another, but the main point will always be that after identifying

the hazards all possible methods are pursued to decrease the probability of

occurrence.

3.FTA Symbols Description

The FT structure consists of a number of gates and events. The most

commonly used are described below

AND gate

The output occurs if and only if all the input parts of the gate occur.

The input parts can be intermediate gates, basic ivents or a

combination of the two. The output can be the top event or any

intermediate event

OR gate

The output occurs if and only if any of the inputs parts of the gate

occur. The input parts can be intermediate gates, basic events or a

combination of the two. The output can be the top event or anyintermediate event. An OR gate should have at least two inputs

VOTING gate

By using this gate, the output occurs when m out of n input events occur.

When m is equal to n, the gate reacts like an OR gate. The input parts

can be intermediate gates, basic events or a combination of the two. The

number of inputs needs to be higher or equal to three. The output can

be the top event of the FT or any intermediate event.


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Sequence Enforcing (SEQ) gate

Is used if all the input events occur in a specific order

(from left to right) as presented in the FT structure.

This means that the left most event occurs followed by

the one next to it and so on.

Transfer Gate

This gate is used as a connector between different parts

of the FT structure. In case of FT being too big in size

the transfer gate is used to represent part of the FT as

a single gate, thus minimizing the graphical size of the

whole FT structure

Basic event

Basic events describe the final stage of a FT structure

or branch of a FT and define the end of the analytical

structure.


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4.Systems Fault Tree Analysis Diagram

In order to construct our system, the primary goal we have in mind is to

understand the working principles of the system and which are the elementswhich fail first and how their reliability can be improved. Practically the latter

can be reached by applying condition monitoring to the most critical elements

so as to extend their operation life. Also the ships crew can achieve the increase

of the elements lifetime by performing regular inspection to the critical

equipment.

The system which we are going to examine is the Main Engine. Our Planned

Maintenance System includes the engines sixcylinder items. The cylinderscover, liner, piston and stuffing box are incorporated as well as all the crosshead

and crankpin bearings. The crankshaft component is consider as a separate unit

along with the thrust and main bearings. Apart from those units which need

either overhauling or dismantling and inspection, also some samples and

deflection measurements for some items are needed. Those items are composed

in a third separate category. Below, the overall layout of the system is

illustrated.

In order our diagram to be accurate, so as the results to be acceptable and

logical we have to select the appropriate gates. For the top gate a Voting gate

is used, so as to determine that in case two out of three units fail, the whole Main

Engine will stop working, as the first subsystem is measurements and samples


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will always fail much earlier than the rest two subsystems. Therefore, if any of

the two other subsystems will fail in addition with the first, this will occur a

complete malfunction of the Main Engine.

At this point, transfer gates are used to handle each item as a subsystem for

convenience reasons. Each category are described in detail below.

The first subsystem is the Measurements & Samples. An OR gate is placed at the top

of the subsystem leading to a failure of the engine if only one of the following four

basic events fails. This is the less critical subsystem because in case engine crew forget

to take any of them, the Main Engine could still working without any problem. On the

other hand, the results of the above measurements and samples are very valuable as we

can evaluate the current condition of the Main Engine and prevent future major

components damages.

The second subsystem is the Cylinder Unit Failure. Our Main Engine consists of 6

Cylinders as a result we divide them separately. We chose a Voting gate for this

subsystem where in case two out of six cylinders will breakdown the top gate will

become out of order. We made this selection because as per classification requirements


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all vessels should carry onboard a minimum quantity of major spare parts. As a result,

in case any item of cylinder unit breakdown then engine crew could easily replace it

from spare parts. But if an additional unit will fail then the top gate will stop working.

Each cylinder unit is sub-divided in two systems which are the Cover-Piston-Liner

Failure and ConRod - PistonRod Failure. In the first system is included the Piston, the

Cylinder Cover and the Cylinder Liner. In the second system is included the Stuffing

Box, the CrossHead Bearing and the CrankPin Bearing. All basic events are connected

with an OR gate with the top event due to the fact that all of them are extremely

critical, thus failure of any of them will occur complete failure of the unit.

The third subsystem is the Crankshaft Failure which is divided in two parts. The first

is Main Bearings and the second is the Thrust Bearing. We used an OR gate to

connect them because crankshaft is very sensitive to any possible damage of the

bearings, thats why all engines are equipped with an Oil Mist Detector whichautomatically slow-down the engine in case of bearings failure.


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Finally the Main Bearings subsystem consists of eight basic events which represents

each individual Main Bearing. As we mentioned before, every vessel is obliged to carryonboard some spare parts and one of them is one set of Main Bearings. Consequently,

we chose a Voting Gate to connect the basic events with the top one which will fail

only in case two out of eight main bearings run out of order.


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5.Overall Results

Availability & Reliability Calculations

In this stage of our coursework we have to calculate the overall results. We start

calculating the FTA results from the M/E for 43800 hours correspond to 5years of

operation. The time steps for this simulation time are 21. We select the exact

calculations method. This method calculates the reliability and availability, the cut sets

with their probability and normalized probability as well as their roots and the

Importance measures.

The reliability represents a components ability to work at a certain capacity. The

reliability can be again to its initial condition after the proper maintenance is applied.The availability shows the ability of a component to be accessible and ready to work

the amount of hours set by the manufacturer. This value also decreases after several

working hours and can be brought back up to approximately 100 percent after the

adequate maintenance.

Main Engines Reliability & Availability

Months Hours Unreliability Unavailability Reliability Availability0 0 0 0 100,000% 100,000%

3 2190 0,865934 0,865934 13,407% 13,407%

6 4380 0,984126 0,984126 1,587% 1,587%

9 6570 0,998242 0,998242 0,176% 0,176%

12 8760 0,999813 0,999813 0,019% 0,019%

15 10950 0,999981 0,999981 0,002% 0,002%

18 13140 0,999998 0,999998 0,000% 0,000%

21 15330 1,000000 1,000000 0,000% 0,000%

24 17520 1,000000 1,000000 0,000% 0,000%

27 19710 1,000000 1,000000 0,000% 0,000%30 21900 1,000000 1,000000 0,000% 0,000%

33 24090 1,000000 1,000000 0,000% 0,000%

37 26280 1,000000 1,000000 0,000% 0,000%

40 28470 1,000000 1,000000 0,000% 0,000%

43 30660 1,000000 1,000000 0,000% 0,000%

46 32850 1,000000 1,000000 0,000% 0,000%

49 35040 1,000000 1,000000 0,000% 0,000%

52 37230 1,000000 1,000000 0,000% 0,000%

55 39420 1,000000 1,000000 0,000% 0,000%

58 41610 1,000000 1,000000 0,000% 0,000%

61 43800 1,000000 1,000000 0,000% 0,000%


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Crankshafts Reliability & Availability

Months Time Unreliability Unavailability Reliability Availability

0 0 0 0 100% 100%3 2190 0,159943 0,159943 84% 84%

6 4380 0,37676 0,37676 62% 62%

9 6570 0,567512 0,567512 43% 43%

12 8760 0,712154 0,712154 29% 29%

15 10950 0,813808 0,813808 19% 19%

18 13140 0,882027 0,882027 12% 12%

21 15330 0,926412 0,926412 7% 7%

24 17520 0,954656 0,954656 5% 5%

27 19710 0,972331 0,972331 3% 3%

30 21900 0,983252 0,983252 2% 2%33 24090 0,989929 0,989929 1% 1%

37 26280 0,993978 0,993978 1% 1%

40 28470 0,996416 0,996416 0% 0%

43 30660 0,997876 0,997876 0% 0%

46 32850 0,998746 0,998746 0% 0%

49 35040 0,999261 0,999261 0% 0%

52 37230 0,999566 0,999566 0% 0%

55 39420 0,999746 0,999746 0% 0%

58 41610 0,999852 0,999852 0% 0%

61 43800 0,999913 0,999913 0% 0%


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Measurements & Samples

Reliability & Availability

Months Time Unreliability Unavailability Reliabilty Availability

0 0 0 0 100,00% 100,00%

3 2190 0,840409 0,840409 15,96% 15,96%

6 4380 0,974531 0,974531 2,55% 2,55%

9 6570 0,995935 0,995935 0,41% 0,41%

12 8760 0,999351 0,999351 0,06% 0,06%

15 10950 0,999896 0,999896 0,01% 0,01%

18 13140 0,999983 0,999983 0,00% 0,00%

21 15330 0,999997 0,999997 0,00% 0,00%

24 17520 1,000000 1,000000 0,00% 0,00%

27 19710 1,000000 1,000000 0,00% 0,00%

30 21900 1,000000 1,000000 0,00% 0,00%

33 24090 1,000000 1,000000 0,00% 0,00%

37 26280 1,000000 1,000000 0,00% 0,00%

40 28470 1,000000 1,000000 0,00% 0,00%

43 30660 1,000000 1,000000 0,00% 0,00%

46 32850 1,000000 1,000000 0,00% 0,00%

49 35040 1,000000 1,000000 0,00% 0,00%

52 37230 1,000000 1,000000 0,00% 0,00%

55 39420 1,000000 1,000000 0,00% 0,00%

58 41610 1,000000 1,000000 0,00% 0,00%

61 43800 1,000000 1,000000 0,00% 0,00%


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6.Cut Sets

A cut set identifies which unique combination of component failures and/or

events can cause an undesired event to occur. A minimal cut set is the smallest

set of events, which, if they all occur, cause the top event to occur. The Cut

Sets show the weak parts of a system and the ways of having a break down

with the less amount of non-operational elements.


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7.Importance Measures

Importance measures are the systemsmost critical components, which will define

the failure of the system. They are important in order to identify which components

should be maintained first and help us to prioritize the overall maintenance work. The

importance measures are expressed in percentage of importance. The highest the

percentage, the highest the importance of an element.

We can use several methods to identify the importance measures. The software

calculates those values by using the Criticality and the Fussell-Vesely method. Both of

them obtaining the same results. Below, the top 10 most important measures of each

subsystem are presented.


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Main Engines Important Measures

Crankshafts Important Measures

Event Criticality Fussell-Vesely Fussell-VeselyAlternative

MainBearing1 23,73% 23,73% 23,73%

MainBearing2 20,77% 20,77% 20,77%

MainBearing3 24,41% 24,41% 24,41%

MainBearing4 23,13% 23,13% 23,13%

MainBearing5 20,77% 20,77% 20,77%

MainBearing6 21,86% 21,86% 21,86%MainBearing7 20,77% 20,77% 20,77%

MainBearing8 23,50% 23,50% 23,50%

Thrust Bearing 10,52% 10,52% 10,52%

Event Criticality Fussell-Vesely Fussell-Vesely

AlternativeBearings Clearance 12,1% 12,1% 12,1%

CorssheadBearing3 5,5% 5,5% 5,5%

Cover1 5,5% 5,5% 5,5%

Cover2 5,5% 5,5% 5,5%

Cover3 5,5% 5,5% 5,5%

Cover4 5,5% 5,5% 5,5%

Cover5 5,5% 5,5% 5,5%Cover6 5,5% 5,5% 5,5%

CrankPinBearing1 5,5% 5,5% 5,5%



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Cylinders Important Measures

Measurements and Samples

Important Measures

Event Criticality Fussell-Vesely Fussell-Vesely Alternative

Bearings Clearance23,4% 23,4% 23,4%

CrankShaft Deflection23,4% 23,4% 23,4%

CrossHead Guide & Shoes

Clearance23,4% 23,4% 23,4%

Lub Oil Analysis29,9% 29,9% 29,9%

Event Criticality Fussell-Vesely Fussell-Vesely Alternative

CorssheadBearing3 5,56% 5,56% 5,56%

Cover1 5,56% 5,56% 5,56%

Cover2 5,56% 5,56% 5,56%

Cover3 5,56% 5,56% 5,56%

Cover4 5,56%

5,56% 5,56%

Cover5 5,56% 5,56% 5,56%

Cover6 5,56% 5,56% 5,56%





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8.Systems Improvement

There are various measures could be taken in order to improve the overall system/sub-

system reliability such as:

i. All Maintenance jobs should be carried out prior reaching the maximum

limit as per manufacturer data in order to achieve highest performance of

our equipment.

ii. All similar components of our system like all Liners or all Pistons should

have the same maintenance intervals in order to avoid continuous stoppages

for repairs and better engine performance as all the similar items will have

the same running hours

iii. As it is known, classification inspections take place on annually, 2.5 and 5

years interims therefore major repairs works should take place based on

above periodicity in order to eliminate off-hire periods.

iv. Additionally, in order to minimize the repair costs some items that effect

one each other, it is preferably to have the same maintenance intervals. For

example, in order to dismantle the piston, it is compulsory to dismantle the

cylinder cover as a result it is an opportunity to perform all

inspections/repairs on it.

v. As this vessel has only one Main Engine, additional spare parts for critical

items should be kept onboard in order crew to have the ability to replace

them in case of malfunction.


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9.Conclusion

The Main Engine of the vessel is one of the most complicated machinery item

onboard consisting of a lot of parts. On this coursework, we checked only few majoritems of the main engine. This is the reason our results not represent exactly the real

situation but very close of it. To conclude, through the PTC WINDCHILL QUALITY

SOLUTIONS Software we can obtain valuable results which will assist us to prepare

an effective and cost-competitive maintenance plan.


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10.References

1.

NM523 / NM916 Systems availability and maintenanceLecture

Notes, Dr. Iraklis Lazakis, University of Strathclyde, 2014.

2.

Fault Tree Analysis as a toolfor modeling the marine main

engine reliability structure, Rafal Laskowski, Scientific Journals

of the maritime university of Szczecin, 2015

3.

Operational Information of The Two Stroke Crosshead Engine-

The Piston,

http://www.marinediesels.info/2_stroke_engine_parts/piston.htm

4.

Operational Information of The Two Stroke Crosshead Engine-

The Cylinder Liner,

http://www.marinediesels.info/2_stroke_engine_parts/liner.htm

5.

PTC Windchill FTA Data sheet, 2014
http://www.marinediesels.info/2_stroke_engine_parts/piston.htmhttp://www.marinediesels.info/2_stroke_engine_parts/piston.htmhttp://www.marinediesels.info/2_stroke_engine_parts/liner.htmhttp://www.marinediesels.info/2_stroke_engine_parts/liner.htmhttp://www.marinediesels.info/2_stroke_engine_parts/liner.htmhttp://www.marinediesels.info/2_stroke_engine_parts/piston.htm


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11. Appendix

Planned Main tenance System-Vessels Master List


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System Availability & Maintenance-Stais-Gkoumas CW2 Rev02(1)

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