The assessment of the risk arising from multiphase flow induced vibrations in subsea production systems is strongly affected by the accuracy of data and modelling for the production conditions, flow induced loading and estimate of the damping of the structure, both structural and flow induced. These uncertainties in the flow conditions and fluid structure interaction analyses are further compounded by uncertainties in the capacity of the structure to withstand vibration and fatigue damage.
As most calculation codes rely on deterministic analysis, these uncertainties are often addressed by their characteristic input and through safety factors, which are intended to ensure sufficient safety level. Subjective judgements, with unknown degree of conservatism, may be difficult to avoid. A need to address the associated uncertainties and provide a prediction of the accumulated probability of fatigue failure, using a consistent and sound methodology, has been expressed throughout the industry.
Structural Reliability Analysis (SRA) offers a framework to take into account, based on the same modelling basis as the deterministic approach, the uncertainties in all relevant parameters. Moreover, the framework allows for simultaneous consideration of these identified uncertainties when modelled by probability distributions and enables calculation of the probability of fatigue failure.
This article describes a general approach to implement reliability principles in the analysis of multiphase flow induced vibrations, discusses the use of the methodology, both in design and in operations. It also concludes on the feasibility of using Bayesian analysis, based on the results of subsea inspection, to update the probability assessment and provide updated input to the risk evaluations for future operations. The methodology may also be used to determine operational limitations for future production.