Experimental Study On Vortex-Induced Vibration Of Steel Catenary Risers

Marine riser is the main connector between the floating platform and the mouthof a well on the seabed which is generally used for production and intervention of theoil or natural gas. There is high pressure internal fluid(oil or natural gas) transportingin the risers and combination of external currents, waves, earthquakes, etc. are exertedon the riser. On the seabed, there is the interaction between riser and soil. The forceson the riser are severely complex. At certain velocities currents flowing through theriser will result in the shedding of vortices which can cause the vortex-inducedvibration(VIV). When the vortex shedding frequency fvand the structures’ freevibration frequency fnare closing, the oscillation amplitude will increase distinctlyand the phenomenon of “lock-in” occurs, which can result in great fatigue damage ofthe riser.Vortex-induced vibration is a complex phenomenon coupling vibration of fluidand structure, which is a hot research topic both at domestic and abroad. Steelcatenary riser (SCR) is a flexible riser with large slenderness ratio, whosevortex-induced vibration is very complicated. And that due to many factors effectingthe vortex-induced vibration (e.g., characteristics of riser, internal flow and toptension, etc.), especially considering with the complex ocean environmental loads, thedynamic response analysis and numerical simulation on vortex-induced vibration ofsteel catenary riser become very difficulty. This thesis presents the test onvortex-induced vibration of the catenary riser with larger slenderness ratio (L/D=390)and property of large deformation, and the seabed is simulated with clay, sand andmixed soil, which is carried out in large2D random wave pool coupling with waveand flow in marine engineering, in order to explore the mechanism of vortex-inducedvibration and the interaction between seabed and soil. And the main research contentsare as follows. (1)In order to study the laws of vortex-induced vibration of steel catenary riser,the experiment of catenary riser is carried out in the large pool which is imposedexternal flow at the top1.2m. The results show that the amplitude of vortex-inducedvibration gradually increase when the velocity of external flow increases. Thecross-flow vibration amplitude is larger than the in-line direction.(2)The experiment is carried out to explore the effect on the vortex-inducedvibration of catenary riser under different seabed. The result shows that the stresses ofriser firstly increase and then decrease with velocity of external flow increasing whenthe seabed is simulated with clay. The stresses of riser gradually increase withvelocity of external flow increasing under seabed of sand. The stresses on towedsection increase with velocity of external flow increasing, so is the cross-flow stresseson catenary section, and the in-line stresses firstly increase and then decrease oncatenary section, when the velocity of external flow increases.(3)Finally there will be an obvious trench due to the interactions between towedsection and seabed under external flow. And the forms of the trench are differentbecause of the properties of seabed. The trench forms and develops as the timechanges, and the force that steel catenary riser endures decreases gradually.