2.1. Criteria for the Selection of the Research Object
To take into account the rational amount of information about the technical condition of the tested object, the following should be selected using the experiment planning methods [
1,
4]:
the research object
type of experiment
locations of measuring sensors
measured quantities and observation sites
measurement methods and devices
test conditions
diagnostic symptoms
frequency of measurements
inference methods
The coverage of the entire ship with diagnostic tests is unjustified economically considering the performed functions and varied probability of damage to its components during operation. It is recommended to choose a research object that affects reliability, operating costs and performed functions by sea-going vessels.
The basic criteria for selecting a diagnostic test object include:
the responsibility of the assembly (element) in realising a task chain
functionality on the ship
the object’s impact on safety (people, goods and the environment)
unreliability
maintenance costs in upstate
The required reliability of a ship is called its value in the interval (0, 1) and a given value of time (0, ∞), which results from the needs [
8]:
ensuring the safety of people, cargo and the environment
implementation of tasks
effective (optimal according to economic criteria) operation of a sea-going ship
Therefore, it is necessary to perform both a frequency analysis of events and an analysis of consequences. Every unintended event involving fatality or an injury, entire vessel loss or part failure of ship, other property loss or failure, or environmental damage which results in loss of ability or limitation of ability in any time interval to action may be considered a maritime accident [
9]. Ships, wharfs and people are subject to sea accidents. In general, marine accidents can be divided into navigational, technical and of mixed nature. It is possible to estimate entire objects or detect weak links in the reliability chain.
2.2. Research Object Selection Methods
The selection of the object of research on a ship, which is a complex over-system, is important. A system
O is a set of interrelated elements, referred to as called objects, related to each other and relationships among them
Re [
7]:
where:
O1—element of the system
O,
i = 1, 2, …,
n;
Rej—relationships among elements of the system,
j = 1, 2, …,
m.
The most important indicators are indicators of reliability and safety of the operation of vessels and equipment installed on them [
10,
11]. They have an impact on the profitability and ecological aspects. Functional reliability of a motor vessel is the probability of correct performance of specific tasks by the vessel in a given time and under certain conditions of exploitation [
8] (p. 15):
where:
Rcp(
τ)—reliability of the deck part of the ship,
Rmp(τ)—reliability of the machinery part,
τ—time.
The functional reliability of the deck part of a sea-going vessel can be specified by the following formula:
where:
Rnv(
τ)—reliability of the navigation system,
Rst(
τ)—reliability of the steering system,
Rh(
τ)—reliability of the hull system,
Rc(
τ)—reliability of the cargo system,
Rs(
τ)—reliability of the safety system,
Rma(
τ)—reliability of the mooring and anchoring system,
Rsh(
τ)—reliability of the social-hotel system.
In turn, the functional reliability of the machinery part
Rmp(
τ) can be determined by the formula:
where:
Rmd(
τ)—reliability of the main propulsion,
Rpp(
τ)—reliability of the power plant,
Ras(
τ)—reliability of the auxiliary steam system,
Rca(
τ)—reliability of the compressed air system,
Rsw(
τ)—reliability of the sea water system,
Rfw(
τ)—reliability of the fresh water system,
Rf(
τ)—reliability of the fuel system,
Ro(
τ)—reliability of the lubricating oil system,
Rog(
τ)—reliability of the outlet gases system,
Rb(
τ)—reliability of the bilge system,
Rba(
τ)—reliability of the ballast system,
Rco(
τ)—reliability of the cooling system,
Rac(
τ)—reliability of the air conditioning system. The risk level of each subsystem and element was estimated [
9].
The reliability and economic indexes were used to determine the weak link of the seagoing vessels. Information on adverse events was collected from more than 1100 cases of one of the largest ship-owner in the world, mainly operating the bulk carriers. Information on adverse events was collected on the basis documentation of the department of the insurances and breakdowns of the ship-owner as well as the verdicts of the Maritime Chamber from the period of 12 years. In the investigations of adverse events, those contained in ISO 3534-3 standard and literature [
1,
2,
6,
9] were followed. The basic reliability indexes of the renewable facilities are the readiness coefficient, meaning the probability that in a certain instant the object is in the state of disability [
2,
4]:
where:
E(τz)—the expected value of the duration of the state of ability enabling the execution of the task,
E(
τn)—the expected value of the duration of the disability to perform the task.
Sea vessels may appear in one of the distinguished or technical states:
where:
St1—the state of full ability,
St2—the state of partial ability,
St3—the state of task disability,
St4—the state of disability.
The technical readiness coefficient does not provide information on the impact of the disability state of the objects on the performance of the tasks by the object in the operating system. For data stored in the deck and engine room documents on the operation of piston marine engines, it is difficult to use them to estimate the readiness coefficient because the fault time of the combustion engines is not always recorded. Another possible use is the participation coefficient of undesirable events in the
i-th subsystem of the ship (engine)
Kes:
where:
Nes—the number of undesirable events of the
i-th subsystem,
Neo—the number of undesirable events of the object.
The results of the estimation of participation coefficient of the adverse events of the examined seagoing vessels are presented in
Figure 1. From
Figure 1a, it appears that most often the adverse events occurred in the navigation and cargo systems of the deck part. In the machine part, which the author represents, generally events occurred mainly in the main propulsion and power plant. However, when analysing the costs incurred due to the occurrence of adverse events, they were definitely related to the main propulsion engines, while those in the auxiliary engines, located in the engine room, come third in terms of costs. Unit cost indicator for an undesirable event of the ship
kjz has been calculated according to the following formula:
Cis—the costs of an undesirable event
i-th system,
Ceo—the total costs of all adverse events.
The greatest damage to the navigation system was dominated by losses caused by collisions of ships with other floating objects and by collisions with quays or a sluice on the entrance to the shoal. A loading system was predominantly affected by failures which occurred during cargo handling and the main propulsion by failures of reciprocating main engines.
The losses incurred due to adverse events initiated in the main propulsion system of the ship were the highest considering the prices of spare parts and failure repairs (
Figure 1b). The most common adverse events in the main propulsion were generally the failures to piston combustion engines and their lack of readiness to operate. The requirement to ensure a high level of reliability of marine combustion engines results from the safety of shipping as a stoppage of the ship is associated with take out from use and reduction of profit, possible loss of cargo or collision, as well as the possibility of loss of human life or health [
9]. Similar results include literature data where the reliability of the main propulsion engine had a principal influence on the ship’s technical readiness and the share of failure amounted to 40% of the undesirable events on the ship [
8,
9,
10,
11]. The drive of today’s operated ships is more than 90% of piston combustion engines and in the total costs of operating the ship, the costs of the engine room are at least 30% [
12].
Considering the above, the reciprocating internal combustion engine was selected as a research object which has a significant impact on reliability, operating costs and functions performed by sea-going vessels. The main object of a sea-going vessel is therefore the self-ignition engine. Research into adverse events involving internal combustion engines has demonstrated the need for its upgraded diagnostics. Ensuring safety is not possible without the use of appropriate diagnostics.
2.3. Selection of the Location of Measurement Places
The marine combustion engine is a complicated object in respect to dynamics and kinematics. To determine the locations and areas of measurement of diagnostic signals, the methods that were used were those proposed in the literature [
3,
4,
13]:
about the highest level of noise signal generated
about the highest level of vibration signal generated
about the highest number of harmonics in the frequency spectrum
about the expected location of the damage signal generation
of places independent of each other
of reliability analysis
of preliminary test results
The selection of measurement points of diagnostic signals was made using reliability indexes. The tests were carried out for main propulsion engines and auxiliary marine engines of the same rotational speed and were fed with distillate and residual fuels. The purpose of these investigations was to assess the reliability of marine combustion engines by determining the values of reliability indicators for the engine as an object as well as for its basic assemblies, subassemblies and components. The reliability functions were estimated depending on the working time, however it has been shown that it is not a univocal relationship.
A hierarchical decomposition of the tested internal combustion engine at functional systems and elements included in its composition have been performed. As a complex object, the piston combustion engine consists of a structure of functional systems. The functional system in the reciprocating internal combustion engine accomplishes a partial objective, conditioning the achievement of a superior goal. The division of the engine into functional systems is the first level of its division into elements. The zero level represents the engine as the complex object. The next level is the functional systems of the engine, which realise partial objectives that enable the achievement of the objectives of the entire engine. Individual functional systems are further divided into subsystems which constitute the next level of hierarchical decomposition of the engine and accomplish partial objectives of the functional systems.
The engine was decomposed into layers of functional systems and assemblies, as shown in
Figure 2, taking into account the division presented in the literature [
7,
13,
14]. As the literature does not specify the limit of the engine, the author decided to include the collaborating installations. In this work, it was limited to the decomposition of engines up to three levels: engine, functional system, assembly. Each of the investigated engines was one of three with one non-working reserve and one working or two non-working reserves, depending on the load and operational situation.
This and their elements have been numbered according to the numbering system contained in the code book. The exception is components not covered by the code numbering system due to the fact that parts have been manufactured by other manufacturers than the engine manufacturer. The first digit means the functional system, e.g., 4—the fuel feed system; the next digit represents the assembly and its components, which are part of the functional system, 10—the injection pump as an assembly. The last part of the numeric block is the code number of the part specified by the engine manufacturer and included in the code book, H55 000—injection pump.
The reliability investigations carried out are aimed at estimating the values of reliability indicators of systems and components of tested engines in the operation phase. The operation phase, from the hand-over of the object to the user to the withdrawal from use, includes: awaiting the start of use, use, on-demand duty, stoppage between consecutive usage states, maintenance and awaiting maintenance.
The damage to the piston combustion engine was considered as random events because its occurrence is influenced by many factors: complex energy transformation processes, the impact of the environment and various operators. For reliability tests, information on the operation of 18 medium-speed engines by two ship-owners was collected according to the selected test plan. Reliability analysis was conducted on the basis of a passive experiment: data was obtained from machine documentation, computers’ databases, observation of conditions and work parameters during ship stops in ports and shipyards. The reliability assessment was carried out according to a specified plan, and in relation to the tests of the systems and engine components, it was carried out according to the plan (
Noe,
Ra,
θk) [
2]. This means that the observations were subject to the
Noe = 18 engines. Combustion engines damaged in the analysed period were repaired (
Ra) and tests of a particular engine were conducted from the beginning of operation and ended after three periodic maintenance (
θk) sessions for classification of the ship. The observation period for one engine was on average about 34,000 h of work.
The article presents the results of auxiliary tests of marine engines of the same rotational speed and fuelled by distillate fuels due to restrictions on the use of residual fuels in many regions. Various reliability indicators can be used for reliability investigations [
2,
8,
13,
15,
16,
17] and this work used the indicator of the individual damages of individual functional subsystems and components of
:
where:
—number of damages of the
i-th functional system,
—number of damages in all functional systems of the investigated sample of the internal combustion engines.
Figure 3a shows that the most unreliable functional system in the quantitative terms is the fuel feed system of engines fuelled by distillate fuel; the second most unreliable is the working medium exchange system. The reliability analysis carried out for the functional systems of the engines showed that the most failing system of engines fuelled by residual fuel is the lubrication system [
18]. Analogous estimates were made for the third level of decomposition of the investigated sample of combustion engines of one type and the indicator of the share of damages of particular elements was used
:
where:
—number of damages of the
i-th functional system,
—number of damages in all functional systems of the investigated internal combustion engines.
A similar analysis was carried out in order to select the most unreliable element of the internal combustion engine with the addition of the injection subsystem and the control valves grouped together (
Figure 3b). The most unreliable component of this system as well as the entire engine is at the injector valve.
Therefore, the injection subsystem of internal combustion engines was selected as the study object. In the preliminary phase, this looked at the influence of the location of measuring sensors from the injection pump through the injection pipe of a length of 460 to 860 mm to the injector on the values of diagnostic parameters and as presented in works [
3,
19].