Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Dynamic behavior of a flexible catenary riser excited by severe slugging with periodically fluctuating flow
characteristics
Hongjun Zhu*, Jie Hu, Yue Gao, Honglei Zhao, Wenli Liu, Meijing Zhang, Hong Liu
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University
Chengdu, Sichuan Province, P.R. ChinaABSTRACT
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 theresponses in the two directions of the curvature plane also exhibits thespatial variation.

KEY WORDS: severe slugging, flexible catenary riser, flow-inducedvibration, flow characteristics, response correlation.

INTRODUCTION
Oil and gas resources extracted from marine reserves are lifted bypipeline-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 andthe violent fluctuation of pressure, leading to the dynamic response of
the riser and overload of the platform separator. With the aim of theguarantee of structural integrity, it is urgently called for theunderstanding of severe slugging characteristics and the associated flowinduced vibration of marine risers.
Severe slugging was firstly described by Yocum (1973). Schmidt etal. (1980, 1981, 1985) experimentally and theoretically investigated theair-kerosene flow characteristics in an inclined pipeline-to-vertical riser
system and firstly proposed the severe slugging I (SSI) and severeslugging II (SSII). For the first type, the length of liquid slug ranges fromone to even several height of the riser. Luo et al. (2011) broadened therange of flow velocity and incline angle of the upstream pipeline toexamine the related influences. Apart from SSI and SSII, another type ofsevere slugging, characterized by shorter aerated liquid slugs, wasobserved and termed as SSIII. Moreover, severe slugging was evenfound in the riser with a horizontal upstream pipeline. Wang and Guo(2006) and Malekzadeh et al. (2012a) experimentally demonstrated thisphenomenon. Malekzadeh et al. (2012a, 2012b, 2012c) carefully
described the SSI, SSII and SSIII with more detailed stages. In thetransition case between two types, a dual-frequency severe slugging wasobserved with the higher one related to the severe slugging cycle and thelower 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: forSSI, the length of purely liquid slug is longer than the riser height andthe slug production stage occurs before gas blowout stage; for SSII, thelength of liquid slug is usually shorter than the riser height with liquidslightly aerated; the length of liquid slug becomes much shorter for SSIII,
containing more gas bubbles.
As submarine risers are usually deployed in catenary configuration,
Wordsworth et al. (1998) experimentally investigated the behavior ofsevere slugging in an inclined pipeline-to-catenary riser system. SSI wasfound to be similar as that in vertical riser. In contrast, gas intermittentlypenetrated into the riser when SSII emerged. For SSIII, gas penetration
occurs continuously at the riser base. Balino et al. (2010, 2013) andEhinmowo et al. (2016) validated these findings theoretically.
The afore-stated literature focused on the flow characteristics.
Nevertheless, the rapid modification of mass distribution and violent
fluctuation of pressure along the riser may excite the dynamic response.
Wang et al. (2018) and Onuoha et al. (2018) numerically investigated thesevere slugging induced vibration (SSIV) of a vertical riser with aninclined upstream pipeline. It was found that the dynamic response wasclosely related to the severe slugging characteristics. The response
amplitude was proportional to the slug velocity within a considered range.
However, the constructed severe slugging perfectly reoccurred in a fixed
cycle with stable flow features in their model. The associatedexperimental studies are scanty, to the knowledge of authors, as thiscomplicated fluid-structure interaction issue involves in both the spatialtemporal variation of dynamic response and flow characteristics. It is themain objective to conduct this experimental work, concerning the SSIVof a flexible catenary riser as well as the variation of flow characteristics.

EXPERIMENTAL SET-UP
As shown in Fig. 1, the experimental apparatus mainly consists of awater and air supply system and a recirculation loop. The water supply
system comprises of a water pump, a valve and a flowmeter in order,
corresponding to the liquid flow path, while air is compressed by an airpump, stabilized by a buffer tank and then transported through a valve anda flowmeter. The flexible catenary riser made of silica gel tube was marked
with 30 black rings evenly distributed along the span, and was fix-fix
supported in both ends. The main parameters of the riser model aresummarized in Table 1. In comparison with the flexible riser model1839