For weight optimised deep-water structures as well as subsea equipment and piping, designing for dynamic loading from e.g. ocean waves, rotating and pulsating equipment is a challenge. A special case is the acoustic vibrations experienced in the steel piping in either end of flexible risers. Excessive vibrations have been experienced both on topside and on subsea equipment. To the authors knowledge, fatigue failure in gas containing pipes has been the result in at least two known cases, one due to acoustic vibrations, and another caused by a traditional piston compressor. During the design process it is generally a problem to predict the inherent level of damping in the structures or the equipment in order to estimate the response as accurately as possible. Much effort has been spent trying to predict the inherent damping. However, little has been done to deliberately increase structural damping in order to reduce the dynamic response significantly. Controlled application of structural damping is an alternative to changing stiffness or inertia characteristics of the structure to avoid resonance, and is often the only solution for broadband loading where resonance cannot be avoided. This paper describes solutions for two types of frequently occurring resonance problems on offshore installations and discusses general possibilities for the use of designed damping. One solution, applicable for high frequency acoustically induced vibrations in piping, is successfully applied on a full-scale mock-up of a pipe segment with a blind flange and a flange with valve, representing two real world problem details. The applied damping solution is a tailored design. Another example shows use of standard industrial dampers for vibration control of a piston compressor dtid. The latter is implemented offshore and by visual control vibrations were significantly reduced. On site measurements will be conducted later. The paper will cover design and construction of the actual vibration dampers including selection of damping material. Selection of damping material depends on the occurring frequency and temperature range. The dampers should be designed to obtain maximum damping effect given the stiffness, inertia, excitation and response amplitudes of the structure. Avoiding resonance by designing natural frequencies away from excitation frequencies is sometimes close to impossible. Therefore, the deliberate addition of damping to substructures at which high stress is expected at high frequency and localized vibration is probable, can be a fatigue and risk reducing design measure that by far exceeds anything that can be achieved through other means.
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