Abstract
Statistics tells us that 21% of all topside pipework failure in the UK sector is due to vibration-induced fatigue, whilst erosion and corrosion stands for 13% (Figure 1). This is not directly comparable with the subsea world. Still, we now know that aging subsea pipework and structures suffer from fatigue, in many cases caused by vibration. The number one cause of vibration-induced fatigue is flow line induced vibration (FIV). Advanced simulation and modelling are used when designing subsea structures and subsea pipework to ensure that the design minimizes the risk of harmful vibration occurring. Simulation and modelling does not fully reflect real conditions due to the complexity of the pipework in combination with the unpredictability of multiphase flow regimes. Therefore, these models are usually very conservative, with a significant safety margin built in. In some cases, this may lead to a "permitted production rate" that is less than is safely possible. Installation of permanent or temporary vibration monitoring is the only way to be sure what the actual state is. Data from instrumentation is useful for several reasons. Firstly, because it provides actual displacement/acceleration data at any given frequency and this is directly related to the rate of fatigue. By post processing raw data together with modelling, it is possible to estimate what the vibration is at locations downstream or upstream of the instrument. Secondly, by further analysis of the raw data the models can be "calibrated" to give more accurate fatigue estimates.
