We regularly perform flow induced vibration (FIV) assessments of piping systems to understand whether any vibration induced by internal flow is within acceptable levels.
The internal flow of production fluids can give rise to low frequency excitation of piping, caused by turbulence generated by the flow disturbances or multiphase flow induced forces at bends. For example, slugging flow can cause dynamic loading and a change in self-weight for subsea jumpers with fluctuating bending stresses and fatigue damage.
At Xodus, initial screening is based on the Energy Institute’s ’Guidelines for the avoidance of vibration induced fatigue failure in process pipework’ assessment method. This considers different operating scenarios and gives a likelihood of failure (LOF) score. The LOF is a form of scoring used for screening purposes and is not an absolute probability of failure nor an absolute measure of failure. The calculations are based on simplified models to ensure ease of application and are necessarily conservative. In the case of FIV screening for internal pipe flow the calculations are based on the pipe diameter, pipe wall thickness and the internal fluid mixture ρU2 value. When the LOF is greater than one, the Energy Institute recommends several actions. One of which is for more detailed analysis, and this is an area of work that Xodus routinely undertakes on behalf of clients.
More detailed analysis is performed by employing a combination of finite element (FE) methods together with computational fluid dynamics (CFD) and/or empirical forcing functions. A fatigue life estimate is calculated using a probability density function of stress ranges based on the two slope method and S-N data for a particular class of weld.
Typically, subsea jumpers transport multiple fluid phases such as gas, water and oil. The flow regimes and subsequent forcing functions for these are complex and there is limited understanding of their behaviour. In 2014, a multiphase FIV joint industry project (JIP) was set up by Xodus Group and TNO. Sponsored by Aker Solutions, BP, FMC, Lundin, Shell, Statoil, Suncor, and Total, the aim of the JIP was to investigate and understand the forces induced by multiphase flow on piping systems, and generate validation data for multiphase CFD to model and analyse its occurrence on industrial scale six inch piping. The JIP took high quality two-phase flow induced force measurements on different bend configurations (providing empirical forcing functions for a range of flow regimes and a strong validation suite for numerical simulation) and carried out numerical simulation using CFD which compared favourably with the measurements.
An FIV assessment was performed on a typical subsea flexible jumper (as illustrated) with a six inch diameter pipe. One end of the jumper is connected to an oil producing well via a Xmas tree and the other end to a Flowline End Termination. Different operating conditions were assessed over the production lifetime. The ends of the jumper were assumed to be fully fixed, and entrained mass (including the insulation of the jumper and surrounding water) was included in the FE model.
Initial screening results for this six inch jumper gave an LOF score of greater than 1.0 for certain production years, and the worst LOF score for any year was 2.0. Following the Energy Institute guidelines, a more detailed FIV analysis was carried out.
More detailed FIV analysis of this jumper involves building an FE model, and performing a modal analysis to assess the frequency response of the structure. The deflected shapes of the first four modes are illustrated below.
The full FIV analysis showed that the lowest fatigue life is at the welds close to the end connectors, where the maximum predicted peak-to-peak stress is, was 35 MPa. The lowest fatigue life for any production year was 160 years. The predicted stress spectral density and PSD at the fixed end weld location is shown below, where the dominant frequency is at 1.2 Hz. When all the production years were accumulated, the fatigue life of the jumper exceeded the design life by an order of magnitude and so no further action was required. Although initial screening is a useful tool to highlight potential FIV issues, it should be followed up by more detailed assessment using a technique such as the one described above.
For more information on vibration, contact: Sandy Hutchison at firstname.lastname@example.org or +44 141 299 0000