This article analyzes the reasons for the overload of marine main diesel engines and examines the special and complex operating environment on board.
The variable and unpredictable working conditions it is subjected to are not the overload conditions that would occur in the fixed (on land) unit operation, but are very likely to occur on board.
There is a phenomenon of abnormality and normality reversal, and it mostly appears in an implicit form, with unpredictable characteristics.
Disastrous weather, harsh sea conditions, and waterway topography are the main causes of overload.
I. General Definition of Diesel Engine Overload
During the operation of the fixed diesel generator set, as long as it does not exceed the "power-speed characteristic curve" limit of the operating unit and the main technical parameters (mainly various temperature parameters; pressure parameters; rotational speed parameters), there will be no overload.
If these regulations are exceeded, the unit is considered overloaded. This is the case for fixed (land-based) units. However, the determination of overload for marine diesel engines during operation is far more complicated.
This is because marine diesel engines are determined by their propulsion characteristics.
In addition, they are also affected by the sea conditions.
Marine diesel engines are subject to external natural and non-human-induced environmental disturbances and constraints during operation.
This situation is abnormal for stationary units but normal for marine diesel engines during operation, thereby leading to the occurrence of various unpredictable (such as adverse weather; complex sea conditions; underwater obstacles, etc.) overloading conditions.
The variability of these loads caused by external factors is usually outside the scope of regulations or procedures and is a highly random event with great randomness and uncertainty.
This overloading can be loosely referred to as hidden overloading to distinguish it from fixed-unit overloading.
It is a more hazardous and dangerous condition.
The following will analyze the causes, environmental conditions, hazards, and prevention of various sudden conditions frequently encountered by marine diesel engines triggered by external factors.
II. The Relationship Between the Speciality, Complexity and Overload of the Operating Environment of Marine Diesel Engines
Due to the significant differences in operating environment, space, conditions and fixed units (on land) between marine diesel engines and fixed units, they are inevitably constrained by these factors during operation, and the unit load will also change accordingly.
The following points are highlighted for brief explanation to illustrate the extent of their impact on the operation of the units.
1. Severe sea conditions
It refers to the disastrous hydrological and meteorological phenomena in the sea caused by factors such as storms, snow, tsunamis, and typhoons.
The sea conditions change at any time due to various factors including space, time, weather, shipping routes, and geographical and topographical conditions, and the situation is extremely complex.
As an old saying goes: "There is a three-foot wave on the sea even without wind", which is an image description of the perilous weather of the ocean.
And waves are one of the main causes for the hidden overload of the machinery.
Figure 1 Changes in the operating point of the diesel engine under various navigation conditions
From Figure 1, it can be seen that if the diesel engine operates at the rated load speed characteristic, and works in conjunction with the propeller with the characteristic curve I, the operating point is a. At this time, the diesel engine emits the rated power Pe, and drives the propeller to rotate at the rated speed ne.
If the resistance of the ship increases, the propulsion characteristic curve of the propeller will become steeper, for example, it will become characteristic curve II. Thus, the operating point changes from point a to point b. At this time, although the power and speed of the diesel engine are both lower than the rated values, they are already working in the over-torque range, which is very unfavorable for the operation of the diesel engine.
Conversely, when the resistance of the ship decreases, the propulsion characteristic curve of the propeller will become flatter, for example, it will become characteristic curve III. Thus, the operating point changes from point a to point d. At this time, although the diesel engine does not operate in the over-torque range, the speed and power of the diesel engine both exceed the rated values ne and Pe. The diesel engine is also in an overloaded state, which is very unfavorable for the operation of the diesel engine.
The resistance of ship navigation will be affected by the following uncontrollable factors:
Factors related to storms, snow and sea conditions; factors related to the direction of ocean currents; factors related to the topography of the waterway; factors related to the sediment at the underwater part of the hull; factors related to grounding and running aground; factors related to the entanglement of the propeller with foreign objects; factors related to the increase in dragging (towing); factors related to the impact of the propeller on floating objects, etc.
The controllable factors include:
Sharp turns during navigation; Starting; Narrow waterways; Mooring; Acceleration; Reverse navigation; Changing direction; Loading, etc.
These factors, either acting alone or in combination, can cause the actual propulsion curve (or torque characteristic curve) of the unit to deviate from the original designed propulsion characteristic curve (or torque limit characteristic curve). Any movement of the actual propulsion to the left or right will lead to overload of the unit's power, torque or speed.
This kind of situation occurs repeatedly and alternately during navigation in adverse weather and sea conditions.
When the ship is at the crest of the wave, the propeller is exposed to the water surface, resulting in zero-load idling, a sudden increase in the unit speed, and the main engine overspeeding (overrunning).
When the ship is at the trough of the wave, the volume of the hull submerged suddenly increases, the friction resistance suddenly increases, and the main engine crankshaft (including the shafting) is overloaded. At the same time, the thrust and torque of the propeller increase.
This adverse sea condition causes longitudinal and lateral swaying of the ship's hull, which will result in both of the above two types of overloads (actually accompanied by overheat load).
And these two types of overloads are the most dangerous for marine diesel engines.
Other various factors also have similar harmful effects on the unit, but the severity varies.
2. Ship Grounding or Running Aground
Among the numerous marine accidents, grounding or running aground (including running into the reef) are both serious marine disasters.
When such an accident occurs, the ship's speed suddenly drops from the normal speed to zero speed or nearly zero speed instantaneously, but the engine continues to drive the propeller at its original condition.
From the "Propeller Performance Characteristics Curve", it can be known that at this time, the thrust and torque of the propeller will rise from the minimum value to the maximum value.
The working principle of the propeller tells us: "As the speed gradually increases, the thrust of the propeller continuously decreases. When the speed reaches a certain value (generally slightly greater than the H value), the thrust of the propeller is zero. This is commonly referred to as zero thrust."
Conversely, when the speed changes from the maximum value to zero, the attack angle of the propeller reaches its maximum value. At this time, the propeller, shafting, and the unit will all suffer severe overload due to the propeller generating more power and torque than the designed propulsion power and torque.
After a ship experiences the above accidents, in order to save itself and escape, it often reverses at a high load in the zero-speed condition of the ship, artificially causing the unit to overwork.
3. Foreign objects getting entangled in the propeller or stern shaft
When this fault occurs, the load on the propeller suddenly increases, the exhaust smoke from the engine turns black, the rotational speed drops, and if it persists for a long time, it will cause the temperature of the cooling water and lubricating oil to rise, the combustion to deteriorate, and the shafting and crankshaft to be overloaded.
4. Dragging
This kind of overloading condition mostly occurs in trawler fishing vessels.
The main reason is that when the fishing net enters a large group of fish or gets caught on an object (such as a sunken ship fragment, small rock, etc.), the original working condition will change, the process will decrease, and the attack angle of the propeller will change, resulting in an increase in thrust and torque.
This situation will cause prolonged overloading operation.
5. Propeller Impact on Floating Objects
In such cases, the propeller blades will deform, the propeller's performance will deteriorate, especially the pitch will increase, the dynamic balance will be disrupted, and the vibration will intensify. This will cause an increase in the load on various bearings, leading to damage.
6. Increased frictional resistance due to fouling on the hull
As the vessel operates over time, marine organisms (algae, shellfish, etc.) attach more to the hull surface, causing increased corrosion.
The frictional resistance of the hull gradually increases day by day. According to relevant data, the frictional force of the hull increases at a rate of 2% per year, which inevitably increases the additional load on the unit. The "propeller propulsion characteristic curve" shifts to the left, and the unit speed decreases.
If the original set speed is maintained at this time, the unit is actually operating at an excessive power level.
This situation can easily lead to the unit operating at excessive power for a long time, causing great harm.
7. Ship Turning
As is well known, each ship has its own specific turning radius.
This characteristic has little impact on ships with a single engine, but it is significant for ships with multiple engines.
The larger and longer the ship is, the greater its turning radius (generally about three times the length of the ship), and the longer the turning time. The impact on the engines is more obvious.
Because the inner and outer engines are at different radii during the ship's turning, the distances and courses they experience are different, and the processes experienced by the propellers on the inner and outer sides of the turning circle are also different. Consequently, the loads (rotational speeds) of the inner and outer engines are also different.
This can be directly identified from the changes in the exhaust smoke of the unit. If the load (rotational speed) of the unit is not adjusted in advance at this time, the inner unit will enter a severely overloaded state.
8. Navigation in Narrow Channels (Shallow Waters)
When ships enter rivers (or canals) from the ocean or move from deep water areas to shallow water areas, the so-called "ship suction (shore suction or bottom suction) phenomenon" occurs. The most direct consequence of this phenomenon is an increase in navigation resistance and a decrease in speed.
If the speed is blindly maintained without any change, the engine will operate under an overloaded condition.
III. Hazards of Overload and Prevention
Overload occurs during the operation of the unit, and the causes can only be identified and analyzed through the operation process.
However, in general, the causes can be attributed to either subjective factors or external objective factors.
External adverse factors can be mitigated through subjective actions and behaviors. It is particularly important to emphasize the need to deepen one's understanding of the objective world, such as the formulation of each flight plan, the analysis of weather forecasts, the understanding of the technical condition of the crew, the response measures, etc. Appropriate operational techniques should be adopted to cope with the objective operating conditions.
Overload occurs when there is an imbalance among the engine, the propeller and the ship.
The relative balance among the three is the guarantee to prevent overload from happening.
The establishment of this balance is random (especially related to the sea conditions) and is based on mutual dependence.
To prevent the occurrence of overload for the unit, the possession of comprehensive knowledge by personnel becomes very important.
Especially in modern ships with the integrated machine-pilot system, both the discipline of engine management and the driving profession should possess the basic knowledge of relevant cross-disciplinary fields. Any single subject knowledge is insufficient today.
In modern ship operations, it is unimaginable for the crew members who are not familiar with the "propulsion characteristic curve" to scientifically control the ship's operation.
And it is impossible for the crew members who do not know the performance of the ship to adjust the machinery to adapt to different navigation conditions.
Therefore, how to improve the quality of the crew should be the prerequisite for preventive work, while various regulations, procedures and systems come second.
Without high-quality personnel, even the best things cannot be implemented properly.
Preventing overloading of marine diesel engines is often difficult to achieve.
1) Because changes in sea conditions are unpredictable at the micro level, the most optimistic we can do is to make predictions about their macroscopic changes, which is of little significance for on-site technical operations.
Therefore, the changes in the overload of the engine units are also unpredictable. Prediction cannot come first, and prevention cannot be discussed.
2) Coordination and cooperation between the engine department and the steering department.
During actual operation, due to their different perspectives on the same matter,
each department has its own priorities. The steering department considers the safety of the entire ship, while the engine department considers the safety of the machinery. However, in critical sea conditions that may lead to various overload situations, it is necessary for the engine department to follow the instructions of the steering department, and this cannot be refused.
Therefore, overloading is inevitable, and the engine department can only request to minimize the extent of damage.
At this time, it is inappropriate to overly emphasize "regulations" or "procedures".
3) In cases of extremely dangerous ship conditions, such as running aground or getting stranded, in order to ensure the safety of the vessel, personnel and cargo, it is inevitable to sacrifice the safety of some of the main engines. This is also a form of prevention.
4) In case of sudden accidents, such as navigation in fog, in narrow waterways, or in complex channels, full-power reverse gear operation or sharp turns, and evasive maneuvers that may occur for avoiding collisions, all must be carried out under overloaded conditions.
5) As a last resort, the navigation department should consider deploying a "sea anchor" to protect the main engine. However, at this point, the maneuverability of the vessel should be carefully evaluated. The situation of a powerless vessel is extremely challenging.
6) For marine main diesel engines, excessive preventive measures should not be taken under the following navigation conditions:
① When encountering unavoidable adverse weather conditions (such as storms, snowstorms, tsunamis, etc.) during navigation;
② When the weather suddenly changes in busy shipping lanes;
③ When navigating in narrow waterways with favorable wind and current conditions and high flow rates;
④ When there is a risk of collision and emergency avoidance measures need to be taken.
In these several situations, although there is a risk of overloading for the engine, it is still necessary to do so for the safety of the entire ship.
The minimum allowable operating condition is to maintain sufficient steering effectiveness (commonly known as steering force) for the ship's navigation.
If the propulsion function of the engine causes the loss of necessary steering effectiveness, the ship will be at risk of capsizing.
Especially in disastrous weather conditions or when there is a risk of collision, more serious consequences than engine overload can occur, such as ship destruction and loss of life.
This is a necessary contingency plan that the ship management department cannot do without in advance.
The harm of engine overload for marine diesel engines is self-evident.
In mild cases, it accelerates component wear (especially moving parts), shortens the service life of the machine, reduces economic benefits, and damages the operational efficiency of the ship.
In severe cases, it may cause a tragic accident where the machine is destroyed and people are killed, and seriously threaten the safety of operation.
Therefore, preventing excessive operation should be taken as the key point of accident prevention.
Especially for situations where operation failure leads to engine overload, an alarm bell should be rung frequently and it should be eliminated.