ABSTRACT

We experimentally investigated the flow characteristics of severe slugging (SS) in a flexible catenary riser and the excited vibration response. The severe slugging possesses three stages in one cycle namely: slug formation (SF), slug production (SP) and fast liquid production stages (FLP). The dynamic behavior varies with the flow stage, presenting spatial-temporal differences. The correlation between the responses in the two directions of the curvature plane also exhibits the spatial variation.

INTRODUCTION

Oil and gas resources extracted from marine reserves are lifted by pipeline-riser system to floating platforms in offshore engineering. Variable gas-liquid flow regimes are formed in the riser, depending on the flow rate and gas-to-liquid ratio. Among them, severe slugging is a destructive one owing to the frequent alteration of mass distribution and the violent fluctuation of pressure, leading to the dynamic response of the riser and overload of the platform separator. With the aim of the guarantee of structural integrity, it is urgently called for the understanding of severe slugging characteristics and the associated flow-induced vibration of marine risers.

Severe slugging was firstly described by Yocum (1973). Schmidt et al. (1980, 1981, 1985) experimentally and theoretically investigated the air-kerosene flow characteristics in an inclined pipeline-to-vertical riser system and firstly proposed the severe slugging I (SSI) and severe slugging II (SSII). For the first type, the length of liquid slug ranges from one to even several height of the riser. Luo et al. (2011) broadened the range of flow velocity and incline angle of the upstream pipeline to examine the related influences. Apart from SSI and SSII, another type of severe slugging, characterized by shorter aerated liquid slugs, was observed and termed as SSIII. Moreover, severe slugging was even found in the riser with a horizontal upstream pipeline. Wang and Guo (2006) and Malekzadeh et al. (2012a) experimentally demonstrated this phenomenon. Malekzadeh et al. (2012a, 2012b, 2012c) carefully described the SSI, SSII and SSIII with more detailed stages. In the transition case between two types, a dual-frequency severe slugging was observed with the higher one related to the severe slugging cycle and the lower one associated with the transition between two metastable states. Although the divided stages are different in aforementioned literature, the characteristics of severe slugging can be concluded as follows: for SSI, the length of purely liquid slug is longer than the riser height and the slug production stage occurs before gas blowout stage; for SSII, the length of liquid slug is usually shorter than the riser height with liquid slightly aerated; the length of liquid slug becomes much shorter for SSIII, containing more gas bubbles.

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