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The Vibration Analysis Process

Sea trials are conducted to evaluate the overall performance, safety, and reliability of vessels at sea. A crucial part of this is the vibration analysis process, designed to provide a wealth of invaluable, actionable data. The process starts with the preparation of a database within specialized software. This lays the foundation for the whole process as it establishes measurement parameters, locations, and hierarchy in a way that facilitates accurate interpretation later on. Once the database is created and uploaded into the analyzers, the vessel is taken out to sea. It's here that the rubber meets the road. Vibration measurements are taken, in accordance with SNAME and Class Society guidelines, at different data points based on the vessel’s machinery arrangement and hull characteristics whilst underway. These guidelines state the vibration survey takes place at a constant speed and course, in water depths five times the vessel’s draft, in sea state conditions not greater than 3. This on-site data collection is critical in assessing the vessel's comfort and machinery conditions. The collected data is then post-processed and analyzed in the software by reviewing the frequency, amplitudes, locations, and direction of the vibration. Advanced analytics and algorithms make light work of this complex task, identifying outliers and patterns that might indicate potential issues. Finally, a comprehensive report is prepared based on this analysis, serving as a baseline for future trending and maintenance purposes. This report documents the vessel’s current state, provides valuable insights for captains and chief engineers, facilitates preventative maintenance, and averts costly breakdowns and subsequent repairs. Vibration analysis surveys during sea trials serve as a critical function in ensuring operational efficiency, safety, and longevity of a vessel.

The creation of a vibration analysis database requires specialized software designed to store a plethora of data points, each corresponding to different parts or components of the vessel's structure and systems. Preparing this database is a meticulous task, demanding comprehensive knowledge of the vessel’s structure and machinery components. The database is prepared by inputting information such as structural design, propulsion system machinery, and specific areas of concern – Accommodation areas, vicinity of excitation, local structural elements, etc. Measurement tasks are created to include factors such as the frequency range of interest, similar to the size of a television being watched; lines of resolution which can be compared to the resolution of said television – more lines of resolution result in a clearer picture of the measurement data; and filters – similar to adjusting brightness and contrast in the television picture. If this information is set up incorrectly, one runs the risk of missing the root cause of vibration altogether.

The second phase of a vibration analysis is data collection with the vessel underway. At this point, a portable vibration analyzer is utilized to measure vibration at each predetermined data point. Raw vibration data is recorded in the form of a “Time Waveform,” the analyzer runs this data through a mathematical function called a “Fast Fourier Transform” or “FFT” to convert the vibration from the time domain to the frequency domain – this will help in the next phase, analysis. This data is collected and validated through multiple methods, including numerical, physical, and historical analyses. To ensure accurate and reliable data collection, great care must be taken with sensor placement and testing conditions – if the sensor is not firmly attached, or if operating conditions change during a measurement, the data collected will be unusable. This meticulous attention to detail yields the most accurate depiction of the vessel’s overall health, allowing for enhanced operational safety, efficient performance, better habitability, and most importantly, smooth sailing. To ensure optimal results, it is also important to measure vibration data at all locations, even if work performed or area of concern is isolated to one area of the vessel, or one piece of machinery. Each measurement adds a piece to the puzzle, which when complete, a more clear and accurate analysis can be performed.

Upon returning to shore, the collected vibration data is downloaded into the analysis software to carefully scrutinize for any anomalies or discrepancies. This is performed by looking at the aforementioned “Time Waveform” and “Spectral” (FFT) plots to identify specific frequencies with the greatest amplitudes (severity) of vibration. Mechanical issues are often identified by their “fault frequencies,” which can occur at low or high-frequency ranges (hence the importance of database creation). For example, misalignment often presents itself at 1 or 2 times running speed, which is considered low frequency; whereas bearing faults can be detected at specific frequencies determined by the geometry of their internal components, often in the high frequency range between 30K-120K cycles per minute (cpm). These discrepancies help identify potential maintenance needs that may have otherwise gone unnoticed. The process of analyzing vibration data in the time domain, which involves mapping vibration amplitude over time, can be intricate. Key parameters such as the amplitude and plot shapes are observed to identify potential impacting of bearings or gear teeth. Understanding these parameters help to quantify the strength of a vibration profile and are vital in identifying potential machinery failures in the vessel. 

Upon completion of thorough processing and analysis for any discrepancies, all conclusions and findings are detailed in a formal report. This report serves as a baseline for future trending and maintenance purposes and should include a comprehensive account of the collected data, the analysis process, specific ISO and Class Society guidelines that are applicable, any identified discrepancies, and any recommendations to rectify the issues identified. This baseline report is an invaluable tool for the vessel’s ongoing maintenance, as it provides a reference point for comparison of future analyses and potential troubleshooting. For example, if a vessel has a vibration issue and performs corrections, a pre-vs.-post works vibration analysis survey will allow contractors to prove that their work helped improve the overall operating condition; conversely, if work takes place and vibration gets worse or a new issue presents itself, vessel owners and operators are empowered to hold contractors accountable. Ultimately, careful analysis of vibration data from sea trials significantly improves the reliability, efficiency, and safety of a vessel, pre-emptively addressing problems before causing costly damage and downtime; ensuring longevity of vessel operations.

Looking to schedule a sea trial? Visit us in Fort Lauderdale, the yachting capital, for all your mechanical needs, vibration and alignment included. Contact us today!

 


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