Integrity, reliability and safety of subsea structural systems transporting multiphase flows as shown below are major global concerns for offshore oil and gas developments. External impacts of environmental waves and currents on such systems are addressed at the design stage; however, fundamental problems of internal multiphase flow-induced vibration (MFIV) in a long-span flexible pipe are not well understood or properly addressed. Fatigue failures due to the hidden and complicated phenomena caused by multiphase fluid-body interactions can be catastrophic and result in costly production downtime and underwater repair. As hydrocarbon production and transportation increasingly involve reservoirs in remote or deep water locations which necessitate greater flow rates to maximise hydrocarbon recovery and greater flexibility of subsea pipes to accommodate the well fluids with time-varying pressure and temperature, MFIV is now recognised as one of the crucial factors to be considered during design, operation and maintenance.
Multiphase liquid and gas flows have a highly complex thermo-physical, hydraulic and hydrodynamic nature as the different mechanical properties of the deformable and compressible phases cause spatial and temporal variability in the combination and interaction of the interfaces. Subsea layout architecture, operational lifetime and environmental conditions can all affect the flow pattern of the multiphase mixture along the long containing pipe. From an industrial perspective, liquid-gas slug flows induced by the pipe geometry, seabed topography or hydrodynamic instability are common and problematical. The very nature of a slug flow leads to sudden variations in the fluid densities and velocities within the pipe: a main liquid slug body, containing small gas bubbles, is propelled along the pipe at a high speed by the pressure of a consecutive elongated gas bubble. This unstable flow generates fluctuating dynamic forces due to pressure, weight and momentum changes with excitation frequencies which can coincide with the pipe natural frequencies, creating multi-resonant MFIV that leads to cyclic stress, higher corrosion rate, and fatigue life reduction of the pipe. MFIV can weaken the flow assurance performance, disrupt production and trigger system failure, and can also pose an integrity risk. Despite the significance of these phenomena, reliable practical guidelines and systematic frameworks for the assessment of subsea structural response, stress and fatigue due to MFIV are lacking. Previous studies have mostly focused on the understanding of multiphase fluid mechanics and flow regimes in static and rigid pipes, rather than on elucidating MFIV and inherent fluid-structure interaction (FSI) mechanisms.
The MUFFINS team is developing the next generation of computationally-efficient tools and recommended guidelines which will support the industry in maximizing oil and gas recovery with the most efficient production flow rates, whilst maintaining the minimum level of MFIV-generated stress and fatigue for subsea production and transportation systems.