Mechanical vibration in Subsea Production Systems (SPS) can be induced by internal or external fluid structure interaction (FSI) through a complex process driven by numerous factors such as the piping geometry, flow conditions and fluid properties. The mechanical response of the SPS piping and components can lead to production interruption or fatigue failures, which have in turn significant economical and possibly environmental implications . The existing data of failures and fatigue issues due to Flow Induced Vibration (FIV) in subsea installations is limited compared to topside. However, there have been a number of incidents with high level FIV recorded which resulted in production rate limitations and/or costly hardware solutions . Subsequently, FIV has become a key requirement in the subsea design process in recent years – this is also driven by the increase of the production flow rates and the push towards SPS designs with reduced weight, size and cost.
Dedicated predictive models and tools for FIV are essential to enable a more sophisticated understanding of the production system mechanical response in advance of the actual product design and manifacturing. In the early phase of the SPS design, the FIV analysis includes screening and characterizing of potential vibration sources in the system and calculating the expected range of excitation frequency and source strengths. The characteristics of the sources will be used to ultimately predict the mechanical response associated with the flow induced excitation if the screening indicates FIV risks.
The screening approach used for subsea is based on the Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework (AVIFF) , which was initially developed for topside applications. This approach is conservative and in most cases it indicates high FIV risks and recommends a time consuming and costly Mechanical Response Analysis (MRA) to verify the risk. However, at present, there are no dedicated FIV standards for subsea installations apart from AVIFF.
The objective of this paper is to review the state-of-the-art for flow induced vibrations in subsea systems. The paper provides an overview of the basic physics of typical FIV sources, describes the screening and Likelihood Of Failure (LOF) approachs, and discusses the industrial standards used for SPS as well as the limitations of the existing analysis methodologies.
Note that the intention of this paper is neither to provide a best practice guide line for FIV risk assessment nor to provide an analysis methodology. Rather, it shall raise awareness of FIV in SPS installations, with the focus on the importance of developing tools and models to enable accurate evaluations and assessments of the risks with the goal of SPS CAPEX and OPEX reduction.