Abstract

Flow Induced Vibration (FIV) caused by interacting internal flow in the subsea spanning pipeline, which is one of the important phenomena that contribute to failure of pipelines, especially when the internal flow pattern forms slug flow. A "two-way coupling" fluid-structure interaction (FSI) analysis of piping vibration and response is recommended. By using the general finite element (FE) code ANSYS CFX and ANSYS Mechanical, this paper discusses the procedure for FSI analysis of the span pipeline model conveying gas-water two-phase flow with two fixed ends, analyzes the dynamic behavior of the vibration induced by the internal flow, in different velocities of the two-phase flow and different volume fraction values. In addition, this FSI study compares the natural frequencies of the pipelines with the structural system vibration frequencies and consequently obtains the stress range, in the meanwhile to assist to predict and prevent flow induced vibration in pipelines.

Introduction

Internal flow induced vibration (FIV) have observed in subsea pipelines during operation in subsea developments. Product flows in pipelines and risers in subsea fields are typically multiphase flows in nature. Among the internal flow, slug flow is one of the most complicated phenomena in subsea pipelines, jumpers, and risers. Due to the large difference between the densities of gas and liquid in the pipelines and risers, with the gravity effect, liquids settle at the bottom of the pipe, while the gases take the upper part of the pipe. Multiphase flows induce vibration in pipelines, jumpers, and risers, and potentially cause resonance or fatigue damage of the damage of the pipelines, jumpers, and risers. Internal production flow, other than multiphase flow, such as single phase flow, also can induce vibration to a less severe degree in pipelines, jumpers, and risers, particularly at pipes with curvatures, such as, at bends, and tees.

Internal two-phase FIV can be induced by various hydrodynamic phenomena. Depending on the geometrical configurations of flow channels and operating conditions, gas-liquid two-phase flow may create vibrations with various modes of amplitude and frequency (JSME, 2003). Two physical aspects should be considered as potential sources of flow induced vibration in slug/churn flow regimes. They are, (1) an agreement between the frequency of momentum/pressure fluctuations and the piping natural frequency, and (2) collision of liquid slug onto a structural surface transmitting a large excitation force (Shuichiro, 2015). When the internal liquid-gas two-phase flow forms slug flow, the frequency of passing liquid slugs depends on the length of Taylor bubbles and liquid slugs that are dependent on void fraction and volumetric flux. Such fluctuations are in the order of several Hz for internal two-phase flow. Usually, piping system in processing plants consists of the combinations of vertical, horizontal, and inclined pipes, and it is expected that frequency of passing slug flow is in the order of few Hz. Therefore, FIV in slug flow regime is expected to take place in relatively low frequency range, and collision of liquid slugs induces characteristic frequency vibration.

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