These and many other examples illustrate structural vibration and resonance problems and methods used to avoid structural vibration (contact Wood for details):įigure 2: Field testing for vibration on skid and structural beamsįigure 3: Evaluating storm loading is one example of a Quasi-Static Analysis.
Structural resonance is a common problem, but avoidable. Structural resonance is best avoided by a properly conducted Structural Vibration and Dynamic Design Analysis 3 Examples of Structural Resonance They also occur with smaller reciprocating pumps or compressors, as the examples presented below clearly illustrate. Structural resonance problems are not limited to larger horsepower equipment. Refer to Wood's (BETA Machinery Analysis) Training Tools module 1 (video)for a more detailed explanation of resonance. At resonance, the forces are amplified up to 20 times, and cause deck beams to vibrate above safe operating limits. The term “resonance” occurs when dynamic forces coincide with the natural frequencies of the supporting structure. The vibration is due to the structure being mechanically resonant. This vibration leads to piping failures, poor equipment reliability, and safety concerns. Structural vibration occurs when dynamic forces generated by compressors, pumps, and engines cause the deck beams to vibrate. 2 Structural Vibration and Resonanceįigure 1: Resonance and vibration on offshore structures is a common problem This is a critical consideration which when overlooked considerably reduces the reliability of the machine and might even cause safety concerns. They will interact with the foundation, the platform or the FPSO and the only way to know the magnitude of this interaction is to conduct a structural dynamic analysis that includes the foundation.
What makes these analyses even more challenging is the fact that the machinery, its equipment and the mounting skid cannot be seen as black boxes. Such loadings cannot be replaced by quasi-static equivalents. In such a case, only a dynamic analysis will accurately predict the amplification of the response of the structure. This is the case for most machinery (compressors, pumps, engines, etc.) which produce loadings whose frequency content overlaps the natural frequencies of the structure on which they are mounted (platform, FPSO, etc.). When the frequency content of a dynamic loading and the natural frequencies of a structure are in the same range, then this approximation is no longer valid.
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