| Severe slugging is a transient and unsteady flow regime that occurs in offshore flowlines-riser production systems,most prevalently on aged oil fields which are proned to heavier oil deposits,low flow rates and the peculiar nature of the unevenness of the seabed topography.This phenomenon is characterized with pressure fluctuations and instabilities of the flow system.The behaviour of this flow regime poses a threat on the structural integrity,reliability and safety of the offshore production facilities due to its fatigue loading behaviour and damage of the material structure.Few researchers in China and abroad have made contributions towards describing the dynamic behavior of pipeline-riser structures induced by severe slug flows.However,due to the complicated nature of severe slugging,no study has fully understood the dynamic behavior of steel lazy-wave risers induced by a full-staged severe slug flow which this research draws its originality from.It is therefore in the interest of the offshore oil and gas industry to improve and optimize the fatigue design of deepwater riser structures by including the fatigue loads from severe slugging.This study presents the numerical modelling and simulation of the interaction between the full-staged severe slug flow and the dynamic behavior and stress impact of a steel lazy-wave riser.The fluid analysis,structural analysis and the FSI coupling were carried out using FLUENT,ABAQUS and Mp CCI software programs,respectively.The two-phase flow system is modelled with a comingling flow of water and air with liquid and gas velocities of 0.382m/s and 1.37m/s respectively.The 2D steel lazy-wave riser was modelled with material properties of API Grade X-65 steel with an inner diameter of 0.05 m,outer diameter of 0.075 m,pipe length of 1.8m and a pipe inclination of-5 degrees.Quadrilateral uniform meshes with wall refinement were used to generate15,599 mesh elements for the fluid domain.The volume of fluid(VOF)model,kepsilon turbulence model and the pressure-based solver were used to simulate the transient flow using the implicit formulation.The flow geometry of the lazy-wave riser exhibited the occurrence of severe slugging after the total flow time of 2000 s at a fixed time step of 0.02 s.Mathematical models based on conservation laws and the modified Euler-Bernoulli beam theory were proposed to model the steel lazy-wave riser subjected to severe slugging using numerical methods.The results obtained showed that with a liquid velocity of 0.382m/s and gas velocity of 1.37m/s,the downcomer at an inclination of-5 degrees is the critical section of the lazy-wave riser with a high stress value of 1905 k Pa during the gas blow out stage.The dynamic response of the lazy-wave riser is observed and seen to be critical during the gas blowout and liquid fall back stages.A maximum amplitude of 0.0237 m is reached due to the vigorous pushing of liquid out of the riser during the gas blow out while a minimum amplitude of-0.0344 m is reached due to the fast system depressurization during liquid fall back.The pressure and liquid holdup profile of severe slugging before and after FSI coupling is seen to differ which describes the effect of the two-way FSI coupling.During the liquid fall back,the liquid hold up before the FSI coupling dropped to 0.55 signifying the presence of 55% of liquid.However,after the FSI coupling simulation,the liquid hold up further dropped to 0.4 signifying the presence of 40% of liquid.Similarly,the liquid fall back pressure before FSI coupling was observed to be 5242 Pa while after the FSI coupling it further dropped to 4321 Pa.The results obtained will help to improve in the fatigue design of deepwater riser structures and subsea production systems so as to achieve increased fatigue life,operational and personnel safety,and structural integrity and reliability. |
PDF Full Text Request