Abstract

The objectives of this research are: to analyze the effects of the catenary shape during the occurrence of severe slugging and to develop a simplified but accurate mathematical model to calculate the main parameters of severe slugging in a flowline - catenary riser system.

Offshore exploration is achieving new records of ultra deep water petroleum production in subsea oil fields with multiple production wells at different flow rates, multiphase flow, high pressure gradients, and a wide range of flowlines and risers systems spread over the irregular seabed topography, which can lead to the occurrence of severe slugging. Severe slugging is an undesirable phenomenon.

Catenary riser is the most used geometry in the Brazilian offshore deepwater production scenario, and the catenary shape influence over the severe slugging is unknown.

The proposed model, which is based on Taitel´s model (1986), has been modified in order to take into consideration the geometric configuration of the riser in catenary. Therefore, new formulations for the residual liquid fallback have also been proposed and incorporated in the original models for the vertical riser. The developed model has been validated with experimental data of three test cases: being the first one of a vertical riser with 38.1m of length (case 1), the second one of a group of nine combinations of superficial speed of gas and liquid obtained from a catenary riser system with 9.886m height (case 2), and finally the third one being a real offshore system in production of a flowline-riser in catenary, operating in a water depth of 1,250m (case 3).

The obtained conclusion is that the catenary riser geometry has direct influence over the severe slugging cycle, the slug formation pressure, the liquid level inside the system, and the liquid fallback. It was also possible to verify the severe slugging cycle model sensibility to the top side riser angle, separator pressure, riser height, and liquid and gas superficial velocities.

Based on the comparison between the simulation results and the experimental data, it could be concluded that the proposed modified model for the flowline-riser system in catenary may be applied on the simulation of severe slugging with rather accurate precision.

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