ABSTRACT

Severe slugging is a flow assurance problem that threatens the integrity of offshore production systems due to its cyclic behaviour and a source for fatigue damage in the material structure. Due to the complicated nature of the physics of severe slugging and its interactions with the pipe structure, no study has fully understood the dynamic behaviour of steel lazy-wave risers induced by the 4 stages of severe slugging and of which this research draws its originality from. In this paper, a 2-way full coupled FSI numerical model is simulated with ANSYS-FLUENT, ABAQUS and MpCCI, respectively, for the numerical investigation of the interaction between the complete flow stages of severe slugging and the dynamic response & stress impact on a steel lazy-wave riser. A comingled two-phase flow and 2D pipe with 0.05 m ID, 0.075 m OD and −5° pipe inclination was modelled. MpCCI serves as the coupling interface server for real-time pressure and structural deformation data exchanges. Results show that the dynamic response is critical at the downcomer during the gas blowout and liquid fallback stages. Findings from this study will help to improve on the fatigue design of deepwater risers for structural reliability, safety and enhanced fatigue life.

INTRODUCTION

Severe slugging also referred to as terrain-induced slugging can be defined as a two-phase flow phenomenon which is peculiar in subsea production systems and occur mostly in aged oil fields which are susceptible to heavier oil deposits, low flow rates, pressure fluctuations, and the unevenness in the seabed topography. It is a critical condition that results in fatigue loading and material damage in the pipeline riser system, and as a result, poses a threat on the structural integrity, reliability and safety of offshore production facilities.

Yocum (1973) was the first to report the phenomenon of severe slug flows which has a prone occurrence in petroleum offshore facilities, where usually there are downward sections (pipelines) and upward sections (risers). In the history of experimental studies on severe slugging, Schmidt, James, and Beggs, (1980) were the first to describe the occurrence of severe slug flows in pipeline riser systems. Their research majorly classified severe slugging into four stages: slug formation, slug production, gas blowout and liquid fallback. Studies such as (Boe, 1981; Balino, Burr, and Lovate, 2007; Fabre, Peresson, Corteville, Odello, and Bourgeois, 1990; Schmidt, James, and Beggs, 1980; Taitel and Barnea, 1990) have highlighted several severe slugging problems such as high back pressure at the wellhead, loss in production due to separator process failure caused by stream surges, instability in control systems of the separator, reservoir flow oscillations, platform trips and equipment failure due to fatigue loads.

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