Steering Gear Failure
One of the critical concerns of any naval architects, ship owners, and ship builders is steering gear. It is considered the most important in governing safe operations of the vessels. Safety of the crew, cargo, and operations in different weather conditions depends on the reliability of steering gear. In spite of compliance with regulations during manufacturing and operations, there are cases when steering gear malfunctions during normal ship operations. Among key causes of steering gears failure, includes a low level of hydraulic oil in the tank pump, which is caused by leaks in the hydraulic system. Moreover, hydraulic lock due to damage to the electro-hydraulic system is another major reason for failure. Along with this line, this study conducts failure analysis of steering gear by developing critical failure modes of different components of the system.
In the modern steering system, preventing failure of steering systems is achieved by making each control component and power of the system to be redundant. However, regardless of redundancy in steering systems, this study asserts that isolation process is manual and takes a lot of time, which causes failure.The research is informed by curiosity based on recent incidents involving ships such as Kraken in Vanuatu and Fisktrans in Norway that raises questions regarding the safety of steering gear. Further, as an engineer, I consider the failure of steering gear systems as an applied research topic that presents a problem that needs a solution.
As explained by He et al. (2015), many ship navigation accidents can be directly linked to faults in steering gears. One of the first steps in improving steering system reliability was introduced in 1981 through Amendments to International Convention (SOLAS-74) (He et al., 2015). In the modern steering system, this is achieved by making power and control components redundant. However, in almost all systems, fault finding or isolation process is manual, which takes a lot of time. Thus, the manual process increases risks concerning steering gear failure.
Just like in any other system, defects in steering gear may cause malfunctions that often occur at the worst time. Zhao and Su (2016) explain common defects that affect the steering gear and emphasize the need to conduct a thorough assessment during maintenance and operational checks. Su (2016) points out that the steering compartment may not be well ventilated a fact that may cause water and moisture to enter and cause corrosion. Over a period, corrosion cause parts to seize up, which may be hardly noticeable in the day-to-day checks. The steering compartments in most vessels are positioned above the propeller. The vibration from propeller tends to affect gearing systems, where nuts become loose, causing fractures and excessive wear.
The presence of air in steering gear tends to cause jerky or spongy operations. In effect, this can be felt at the steering wheel or observed in tiller arm and piston rod. Hydraulic oil is not compressible; thus, any movements of the steering wheel results in an immediate movement of the rudder. However, any air in the system will have to be compressedin order to overcome the load being experienced by the rudder before it moves, and this is a primary cause of failure. Zhao and Su (2016) note that excessive air may enter the steering gear system when there is an oil leak. Often, this occurs through helm pump, hydraulic ram, relief and by-pass valves, oil-line connections, and flexible hose connection.
The steering gear power for a particular ship is chosen in such that it satisfies ruder angular velocity at full ahead, which is not supposed to be less than 2.33 degree per second (Szlapczynska and Smierzchalski, 2007). Further, it is a requirement that the time of changing the rudder from 30 degrees on one side to 35 degrees on the other is not supposed to be more than 28 seconds (Delefortrie et al., 2007). As explained by Szlapczynska and Smierzchalski(2007), this prerequisite is derived from the requirement that safe passing of ships proceeds in the opposite direction. In such circumstances, a ship needs to obtain non-dimensional angular velocity equal to 0.2 after a ship has covered a distance equal to its length. Consequently, as a rule, the steering gear power selected need to guarantee appropriate rudder angular velocity ranging from 3 to 5 degrees per second. Chiefly, these parameters correspond to the power of disturbances that affect a ship at sea state.
Consequently, lack of equilibrium tends to be observed between disturbances and control power. More exactly, at low sea state, the average-yaw-angle amplitude tends to be minimal. On the other hand, while at high sea state, the steering gear does not allow effective compensation of the effect of sea disturbances such as waves, current, and wind. From this consideration, it follows that characteristic features of ship steering gear face the risk of overloading at high seas, in particular, when the vessel is operating in the auto pilot mode. Delefortrie et al. (2007) argue that such overloading cause damage to steering gear system.
Steering gears are equipped with overload alarms, where such signaling is used as required standards that tell when particular parameters are exceeded. Consequently, the independent variables adopted by this study include the signals that are activated due to exceeded average current of motors that are responsible for driving hydraulic pumps, the excess temperature that is recorded in the winding of motors, and excessive hydraulic oil temperature. Moreover, the steering gear system is equipped with an alarm system that indicates errors. In parallel, independent variables also include a signal for low tank level of hydraulic oil pressure increase or drop at oil filters, and detected electric supply errors in power and control system. The frequency characteristic of the rudder angle would be used as a dependent variable to illustrate the steering load variable.
- To elaborate the functional description of components of steering gear system using functional tree analysis
The functional tree analysis is a critical tool that can aid understanding of the system with the objective of proper analysis of reliability. The development will help to define behavior and interactions between different components of steering gear system with the aim of identifying failure that can interrupt the functioning of the steering gear system. The functional tree analysis will detail vertical orientation of the steering gear system.
- To identify the most critical components of steering gear system using Failure Tree Analysis
The failure tree analysis is a reliable and secure technique that can be applied to a complex and dynamic system with an objective of determining the cause of undesirable events. To define critical components of the steering gear system, the concept of item importance, which is defined as the ratio of probability sum of the minimal cut sets will be used.
- To develop Failure Mode and Effect Analysis (FMEA) for each critical component
The failure mode and effects analysis is an imperative tool for analyzing the reliability of any system. The tool is essential for identifying failure modes of different components and defining possible effects caused by a system (Zhao, 2012). FMEA is often applied in design process machinery, where its primary goal is to identify and limit operational risks within a given design.
Research Process (Gantt Chart)
|Initial project Outline|
|Revised project plan and literature review|
|Data collection and statistical analysis|
|Analysis and evaluation|
|Writing the final report|
|Final editing of the report|
|Submit the Final project Report|
|Presentation of project Finding|
Delefortrie, G., Vantorre, M., Verzhbitskaya, E., &Seynaeve, K. (2007). Evaluation of safety of
navigation in muddy areas through real-time maneuvering simulation. Journal of Waterway, Port, Coastal & Ocean Engineering, 133(2), 125-135.
He, W., Xiong, J., Liu, M., Chu, X., Liu, C., Shi, L., & Wang, L. (2015). Technology of
information collection and analysis about steering operation behavior of inland waterway sailing ship. Journal of Coastal Research, 73483-489.
Szlapczynska, J., &Smierzchalski, R. (2007). Adopted isochrone method improving ship safety
in weather routing with evolutionary approach. International Journal of Reliability, Quality & Safety Engineering, 14(6), 635-645.
Zhao, D., Guo, C., & Su, Y. (2016). Hydrodynamic performance of ichthyoid rudder at different
rudder angle settings. Journal of Coastal Research, 32(5), 1184-1195.
Zhao, G. (2012). The method of FMEA and FTA based on system fault behavior Model. Applied
Mechanics and Materials, 224, 77-81.
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