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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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This document is the coursework/project coversheet for all NAME classes conducted at University of
Strathclyde for academic year 2015-16. Please do the following when submitting your coursework:
Staple a completed printed copy of this form to every piece of coursework/project work
you submit for classes in the Department of Naval Architecture & Marine Engineering.
Avoid the use of document containers such as cardboard or plastic covers,
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personswork as though they were your own.
SUBMISSION
DETAILS
Please ensure that the details you give are accurate and completed to the best of your knowledge.
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
Declaration
I have read and understood the University of Strathclyde guidelines on plagiarism.http://www.strath.ac.uk/media/ps/cs/gmap/academicaffairs/policesandprocedures/student-
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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%
CrankPinBearing2 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%
CrankPinBearing1 5,56% 5,56% 5,56%
CrankPinBearing2 5,56% 5,56% 5,56%
CrankPinBearing3 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.htm7/25/2019 System Availability & Maintenance-Stais-Gkoumas CW2 Rev02(1)
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11. Appendix
Planned Main tenance System-Vessels Master List
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