In ship and offshore operations, machinery systems have associated operational hazard because of the prevailing harsh environment. Therefore, the need for an overall evaluation of the associated risk and failures of these systems, such as the marine steam boiler , is crucial to the industry. The concept of probability risk model is used to model the failure mode considering the overall risk associated with the system as a whole. The rate of occurrence of the failure that described the basic events as represented by the fault tree was developed to model the marine steam system. This specific event was implemented and evaluated to estimate the failure frequencies of the overall systems, based on the available failure rate in core literatures. A risk model which is hazard severity weight with its failure frequencies, and the time of operation was applied in the analysis. The probability of failure of the boiler system was estimated at 0.323225 at 35 , 040 operating hours with hazard severity weight of catastrophic if it occurs. The associated failure frequency calculated for the period is 1.114 × 10-5. The over failure frequency of the marine steam system for the period of consideration is conditioned on the pre-defined minimum cut sets of the top event. This therefore agreed with the fact that the basic events with their failure frequencies will lead to the catastrophic failure of the entire system within the period if the maintenance plan is not proactive.
Marine steam systems are units that generate steam for electric power generation or process heating and operation onboard and other offshore operations. On board ship and in process plant, steam is used for personal uses such as producing heat and hot water. Marine steam system such as boilers contains two basic systems [
A ship machinery space or engine room is the compartment of the ship where the main engine(s), generator(s), compressor(s), pump(s), boiler plants and other major machineries are located [
Risk-based analysis is used as a monitoring object (condition monitoring, diagnostics and servicing) that integrates technical, economic and safety issues together to provide solution to system problem [
In reference to offshore vehicle as a safe maritime facility, different operation strategies and maintenance methods can be applied to the sub-system to sustain reliability and performance [
This research seeks to investigate associated risk and failure mode of a steam system that form major part of a marine propulsion system and also for process offshore operations. There exists little research in the area of modeling boiler system risk in the maritime industry and none have applied a probabilistic based technique to predict the trend of the system performance within its design and off-design envelopes. Hence, we proposed the application of a predictive probabilistic tool to model the entire boiler system with the aim to ascertain its failure rate, frequency and consequences for a predefined duration. This provide a novelty application of the probabilistic model in predicting failure characteristic of a marine steam system (boiler plant) which previous literatures did not provide a holistic illustration as is done in this research.
The boiler system is configured with feed water drum and the water utilizes the heat energy released by the burning fuel. This energy gained converted the water into the form of steam with very high temperature and pressure. The system has a combustion chamber for the pressurization of the fuel to high temperature and air is supplied to this combustion chamber through a separate arrangement to enhance combustion [
Heat exchange occurred from the hot gas to the water through the boiler drum wall of a given large surface area, which enables the highest rate of energy transfer [
・ Steam production for process,
・ Steam superheating for power generation.
We have two main types of boiler. These two are the basic, all other boiler are different versions of them.
• Water tube boiler,
• Fire tube boiler.
The water tube boiler is a shell and tube heat exchanging system where the exhaust gases are the product of combustion passes over the tubes containing flowing water. Research shown that boiler tubes are made of materials that typically withstand higher internal pressure compare to large chamber shell in a fire tube boiler. For higher temperature application, water tube boilers are mostly used with high steam pressures (as high as 3000 psi) are required. Water tube boilers performance also show high efficiencies and can generate saturated or superheated steam as the need arises. The merit of water tube boiler to generate high pressure steam (superheated steam) makes it attractive in applications for steam turbine power generation [
In Fire tube boiler, heat is transferred from the combustion gases pass inside boiler tubes, to the water on the shell side. Fire tube boilers are described by their number of passes configuration. This describes the number of times the combustion (or flue) gases flow the length of the system as they transfer heat to the water [
Over the year risks associated with boiler plant operations has been drawn from the following hazardous situations within the boiler as outline below.
• Control system malfunction;
• Fire;
• Fire side explosion;
• Loss of power supply;
• Loss of water;
• Overpressure;
• Overheating (overheating as a result of low water is the most common cause of boiler damage or explosions, usually a result of the malfunction of the automatic controls);
• Unauthorized access;
• Unauthorized modifications and repairs.
The overall operation and maintenance of boiler plant gives rise to a high level of risk, basically the super heater, boiler water level, boiler furnace fuel and air supply, boiler safety valves and more will be analyzed in this research work. Safety and risk are related. The safer operation is defined as a case of fewer risks. Property damage is considered a risk that might cause injury or loss of life [
• Melt Down
Melting down occur when the heating surface metal reach its melting point, which is temperature dependent. This occurs mostly when boiler operating at a very low water level. Although it not results to boiler explosion in itself, but it effects causes major damage to the boiler and create a dangerous situation which could lead to an explosion.
• Thermal Shock
This is a condition where low water causes the heating surfaces to become overheated and then cooler water is added. The water then flashes to steam which expands 1600 times its volume as water and causes the explosion because there is not enough room for the steam to expand.
• Combustion Explosions
These can be a result of gases which build up and an ignition source ignites the gases. This can happen inside the boiler or outside. There are safety devices in place to avoid these situations and we will discuss these in the following slides.
• Steam Pressure
Excessive steam builds up which exceeds the design pressures of the vessel. There is also safety device to prevent this.
Power system functionality and effectiveness form a strong hold on the seaworthiness of all oceans going vehicle. Ship power system management involves planning and decision making, organizing, managing and control. This system management from the operational strategy which defines the methods of maintaining the technical condition of an assumed level as in the required time of their operation, often estimated with key indication of effectiveness [
Operating maintenance management of ship power system should be carried out in compliance with regulation of the ISM Code, classification solution and the applied operating strategy guaranteeing that the power system will perform the task facing it. The operational approach is justified by the creations of maintenance strategies of technical conditions of ship equipment based on optimizing economic resources on technology and safety [
• Fuel dripping inside the furnace of the boiler (blowback and even explosion),
• Misfiring,
• Overheating of boiler due to loss of water circulation,
• No pre- and post-purging,
• Exhaust gas boiler fire.
Many safety assessment approaches, such as probabilistic risk assessment method, have been widely used but do have some challenges such as reliance on the failure rate data, which may not be available. The fault tree analysis (FTA) is a productive hazard analysis technique widely used in the maritime industry [
Wang and Trbojevic [
Risk = consequences × likelihood (1)
= Hazardseverity ( s ) × failureprobability ( p ) (2)
= Hazarsseverity ’ sweight ( S w ) × failureprobability ( p ) (3)
In the risk assessment of the marine boiler, failure probability intends to define and follow an exponential distribution path, such that;
P = 1 − e − λ t (4)
λ = − ln ( 1 − P ) t (5)
Therefore
R = S w × ( 1 − e − λ t ) (6)
where
1 − e − λ t is the exponential distribution formulas,
P is the failure probability,
λ is the failure rate or frequencies,
t is the time of interest.
The basic risk level (basic events) of the whole boiler system can be determined by the sum of the risk associated with its systems.
R T = R subsystem ( 1 ) + R subsystem ( 2 ) + ⋯ + R subsystem ( n ) (7)
where
RT is the total risk of boiler system,
R System ( i ) = risk of the boiler subsystem i, i = 1 , 2 , ⋯ , n or ( i ∈ n ).
Therefore, by substitution we have the following
R T = S w 1 × ( 1 − e − λ 1 t 1 ) + S w 2 × ( 1 − e − λ 2 t 2 ) + ⋯ + S w n × ( 1 − e − λ n t n ) (8)
where
S w ( i ) is the hazard severity’s weight of the boiler subsystems i,
λ i is the failure rate of the boiler subsystem i,
t i is the time interest of the boiler subsystem i,
i = 1 , 2 , 3 , ⋯ , n or ( i ∈ n ).
The associated risk which defined the top event of an FTA modeling of boiler system is evaluated using the level/consequences pathway.
Qualitative risk analysis is used identified hazard that can be categorized to be catastrophic, critical, marginal and negligible categories as shown in
Severity Index | Description | Equipment | Personnel |
---|---|---|---|
4 | Catastrophic | System loss | Death |
3 | Critical | Major system damage | Severe injury/illness |
2 | Marginal | Minor system damage | Minor injury/illness |
1 | Negligible | Non-significant damage | Non-significant injury/illness |
Level | Description | Equipment |
---|---|---|
A | Frequent | Likely to happen |
B | Probable | Several times during lifetime |
C | Occasional | Likely to happen once |
D | Remote | Unlikely but possible during lifetime |
Hazard Severity | Weight | A (Frequent) | B (Probable) | C (Occasional) | D (Remote) |
---|---|---|---|---|---|
Catastrophic | 1000 | A-01 | B-01 | C-01 | D-01 |
Critical | 100 | A-02 | B-02 | C-02 | D-02 |
Marginal | 10 | A-03 | B-03 | C-03 | D-03 |
Negligible | 1 | No significant hazards |
• Adequate design and operational actions are required to eliminate or control hazards classification as A-01; A-02; A-03, B-01, B-02 and C-01;
• Hazard consequences must be controlled for classification B-03; C-03, and D-01;
• Hazard control is desirable if cost effective for hazard classified as C-03 and D-02;
• Hazard control is not cost effective for hazards classified as D-03.
Potential hazards associated with the boiler system can be identified by experts in the maritime industry. Wang and Trbojevic [
A system failure may occur unexpectedly and care is needed for supercritical equipment like the marine steam system. The boiler system failure may be rooted in structural defects, corrosion, stress rupture, fatigue, erosion, and lack of quality control [
Major Events | Basic Events | Coding |
---|---|---|
Stress Cracks | J1 | |
Thermal Overheating | J11 | |
Temperature Fluctuation | J12 | |
Sensor Failure | J111 | |
Relief Valve Failure | J121 | |
Stress Rupture (J) | Creep Failure | J2 |
Overheating | J21 | |
Long-Term Overheating | J211 | |
Short-Term Overheating | J212 | |
Pressure Relief Valve Failure | J2111 | |
Temperature Sensor Failure | J2121 | |
Vibration | K1 | |
Defects | K11 | |
Thermal Stress | K2 | |
Fatigue (K) | Temperature Sensor Failure | J2121 |
Corrosion Effect | K3 | |
Material/Structural Defect | K31 | |
Fireside Corrosion | X1 | |
Fuel Ash | X11 | |
Personnel Faults | X111 | |
Embrittlement | X12 | |
Sulfidation | X13 | |
General Impurities | X131 | |
Corrosion (X) | Nucleate Boiling | X14 |
Material Impurities | X141 | |
Carbide Graphitization | X15 | |
Waterside Corrosion | X2 | |
Dew Point | X21 | |
Oxidation | X22 | |
Formation of Sigma Phase | X23 | |
Pitting | X24 | |
Intergranular | X25 | |
Fly Ash | Y1 | |
Erosion (Y) | Falling Slag | Y2 |
Soot Blowers | Y3 | |
Fuel Ash | Y4 | |
Oxidation | X22 | |
Lack of Quality Control (Z) | Chemical Excursion | Z1 |
Weld Defects | Z2 | |
Material/Structural Defects | K31 |
the categorization of the prevailing failure mode associated with the case study.
For the quantification of the top event, the marine boiler has five major events with different basic events and their failure consequences. Although the overall basic events are grouped into eight for the purpose of this research and their assigned frequency of failure and probability is shown in
For the entire marine boiler system, the failure probability gives
P ( Marineboilersystem ) = P ( A ) + P ( B ) + P ( C ) + P ( D ) + P ( E ) + P ( F ) + P ( G ) + P ( H ) → P ( A ) + P ( B ) + P ( C ) + P ( D ) + P ( E ) + P ( F ) + P ( G ) + P ( H ) − P ( A ) ⋅ P ( B ) − P ( A ) ⋅ P ( B ) − P ( A ) ⋅ P ( C ) − P ( A ) ⋅ P ( D ) − P ( A ) ⋅ P ( E ) − P ( A ) ⋅ P ( F ) − P ( A ) ⋅ P ( G ) − P ( A ) ⋅ P (H)
− P ( B ) ⋅ P ( C ) − P ( B ) ⋅ P ( D ) − P ( B ) ⋅ P ( E ) − P ( B ) ⋅ P ( G ) − P ( B ) ⋅ P ( H ) − P ( C ) ⋅ P ( D ) − P ( C ) ⋅ P ( E ) − P ( C ) ⋅ P ( F ) − P ( C ) ⋅ P ( G ) − P ( C ) ⋅ P ( H ) − P ( D ) ⋅ P ( E ) − P ( D ) ⋅ P ( F ) − P ( D ) ⋅ P ( G ) − P ( D ) ⋅ P ( H ) − P ( E ) ⋅ P ( F ) − P ( E ) ⋅ P ( G ) − P ( E ) ⋅ P ( H ) − P ( F ) ⋅ P ( G ) − P ( F ) ⋅ P ( H ) − P ( G ) ⋅ P (H)
And if the boiler is estimated for a 20 years period and is subjected to major failure analysis every four (4) years period, we have
At t = 35,040 h,
Basic Event | Failure Frequency | Failure Probability [ |
---|---|---|
Structural defects | 2.31e−006/h [ | P[A] |
Corrosion effects | 1.115e−006/h [ | P[B] |
Pressure relief system | 2.12e−005/h [ | P[C] |
Fire and explosion | 1.78e−006/h [ | P[D] |
Overpressure | 0.01/h [ | P[E] |
Material defect | 11.15e−06/h [ | P[F] |
Sensor failure | 0.03/h [ | P[G] |
Overheating (rupture) | 2.96e−010/h [ | P[H] |
P ( A ) = 1 − e − λ t where λ = 2.31 e − 006
P ( A ) = 1 − e − 2.31 e − 006 × 35040 = 1 − 0.922 = 0.078
P ( B ) = 1 − e − 1.115 e − 006 × 35040 = 1 − 0.962 = 0.038
P ( C ) = 1 − e − 2.12 e − 005 × 35040 = 1 − 0.928 = 0.072
P ( D ) = 1 − e − 1.78 e − 006 × 35040 = 1 − 0.940 = 0.061
P ( E ) = 1 − e − 0.01 × 35040 = 1 − 0 = 1
P ( F ) = 1 − e − 11.15 e − 06 × 35040 = 1 − 0.677 = 0.323
P ( G ) = 1 − e − 0.03 × 35040 = 1 − 0 = 1
P ( H ) = 1 − e − 2.92 e − 010 × 35040 = 1 − 0.999 = 0.001
P ( Marineboilersystem ) = 0.078 + 0.038 + 0.072 + 0.061 + 1 + 0.323 + 1 + 0.001 − ( 0.078 × 0.038 ) − ( 0.078 × 0.072 ) − ( 0.078 × 0.072 ) − ( 0.078 × 0.061 ) − ( 0.078 × 1 ) − ( 0.078 × 0.323 ) − ( 0.078 × 1 ) − ( 0.078 × 0.001 ) − ( 0.038 × 0.072 ) − ( 0.038 × 0.061 ) − ( 0.038 × 1 ) − ( 0.038 × 0.323 ) − ( 0.038 × 1 )
− ( 0.038 × 0.323 ) − ( 0.038 × 1 ) − ( 0.038 × 0.001 ) − ( 0.072 × 0.061 ) − ( 0.072 × 1 ) − ( 0.072 × 0.323 ) − ( 0.072 × 1 ) − ( 0.072 × 0.01 ) − ( 0.061 × 1 ) − ( 0.061 × 0.323 ) − ( 0.061 × 1 ) − ( 0.061 × 0.001 ) − ( 1 × 0.323 ) − ( 1 × 1 ) − ( 1 × 0.001 ) − ( 0.323 × 1 ) − ( 0.323 × 0.001 ) − ( 1 × 0.001 ) = 0.323225
P ( Marineboilersystem ) = 1 − e − λ 1 × 35040
e − λ 1 × 35040 = 1 − 0.323225
− λ 1 × 35040 × ln e = ln 0.6768 = − 0.39038
λ 1 = 1.114 × 10 − 5
The probability of failure of the boiler system is 0.323225 at 35,040 operating hours with hazard severity weight of catastrophic if it occurs. The associated failure frequency λ1 calculated for the period is 1.114 × 10 − 5 .
The over failure frequency of the marine steam system for the period of consideration is conditioned on the minimum cut sets of the top event. This therefore agreed with the fact that when the basic events occur with their failure frequencies, it will lead to the catastrophic failure of the entire system if the maintenance plan is not proactive.
The process of assessing the associated failure trend in a marine steam system is crucial for every offshore operation. This assessment provides knowledge for improving the level of safety (reduction of risk) in boiler operation. The research focuses on the areas of high risk in steam supply and the major causative events such as stress rupture, fatigue, corrosion, erosion and poor-quality control.
The total risk of the system was analysed mathematically from the probabilistic model. The model estimates the various safety levels of the failure mode by setting up severity and consequences based on the prevailing events. The captured safety level of the subsystems was integrated into the model and the risks were ranked using the prevailing operating condition of the system. The sub-events for the purpose of this research were integrated under the five major events with their failure frequencies from existing literatures. The result shows that the adopted probabilistic model can be used for modeling the failure and risk associated with marine steam systems. This research did not consider the cost implication for the maintenance of the failed subsystems of the overall steam system. This research is not exhaustive. Further work can be done by employing fuzzy based model and evidential reasoning to analyse the professional perfectives on the relative severity classification and modeling.
We sincerely acknowledged Mr. Ezenwoke Anwurike Roy of the Department of Marine Engineering on his contribution to the improvement of this research work.
Adumene, S. and Nitonye, S. (2018) Application of Probabilistic Model for Marine Steam System Failure Analysis under Uncertainty. Open Journal of Safety Science and Technology, 8, 21-34. https://doi.org/10.4236/ojsst.2018.82003