Fatigue failure of intrusive fittings such as erosion probes, sample quills and thermowells, has occurred in Woodside’s offshore and onshore operations. The mechanism of failure is generally thought to be resonant vibration caused by vortex shedding. The consequence of these failures can be severe, in particular for thermowells, as they form part of the pressure envelope, and hydrocarbon release can result. Thermowells on the Goodwyn Alpha platform flow lines were designed to withstand the maximum normal operating flow velocities. During production train pressurisation, however, the thermowells can experience velocities higher than the design limit, albeit for a limited time. These start up flow velocities are likely to cause vortex shedding frequencies exceeding flowline thermowell resonant frequencies. If a vortex shedding frequency occurs that is close to a thermowell natural frequency, a vortex “lock on” resonance can occur, resulting in large amplitude thermowell vibration transverse to the flow direction. In order to determine if thermowell replacement was warranted, a study was undertaken to measure the thermowells in situ. The specific aims were: to determine if vortex “lock on” was occurring; and to determine what cyclic stresses are present. To do this, a novel vibration measurement probe was developed and commissioned. The probe is capable of measuring bidirectional acceleration in thermowells without the need for the thermowell to be taken offline. This paper presents the development of the probe and the results of the measurements during flowline pressurisation.

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