VIV Analysis Of Deep-water Top Tensioned Riser And Its Fatigue Damage Influences

With the increasing exploitation of deep-water oil and gas, topicsabout vortex-induced vibration have aroused attention from academic andindustrial areas. When a fluid flows about a cylinder, vortices generate,develop, and shed in the wakes behind. Because of the periodic sheddingof the vortices, a periodic force is exerted on the cylinder, causing it tovibrate. The phenomenon of vortex-induced vibration will reduce thefatigue life of mooring cables, risers and submarine pipelines. Anaccurate prediction of fatigue damage is quite important for deep-waterriser design.When a riser is subjected to flow, vortex-induced vibration may occurboth in cross-flow direction and in-line direction. And at present mostresearch work are aimed at riser vibration in cross flow direction.However, experiments and in-situ observations indicated that in-linefatigue damage cannot be neglected. Although the vibration amplitude incross-flow direction is larger, the excited mode’s number and vibrationfrequency of in-line VIV is higher than the counterpart. The forecastmethod in cross-flow direction is further improved in this paper, andprediction model for in-line VIV is established based on it. Considering the shear rate of flow velocity, top tension and the material of riser,vibration amplitude and damage rate in both direction is analyzed, whichcan provide reference for riser design and relative standards.Further study of deep-water top tensioned riser is conducted based onfrequency domain theory, and the main contents of this paper includes:1. VIV analysis model in cross-flow direction is created based on thefinite element method and the principle of energy conservation, usinghydrodynamic parameters which are derived from forced vibrationexperiments. As the steady-state analysis procedure of SHEAR7isdifferent from that of VIVANA, comparison of results from these twoways are presented, which will provide theoretical foundation for theprediction of amplitude response and fatigue damage.2. Response amplitude is calculated in different frequency interval andsuperposed, considering the influence of cross-flow vibration andhydrodynamic parameters which are derived from forced vibrationexperiments. A damage calculation method which is easily applied toengineering application is recommended. Compared with measuringresults from two famous experiments abroad, the prediction method inthe paper gives satisfactory results.3. Considering the shear rate of flow velocity, the density of inner fluid,top tension and the material of riser, vibration amplitude and damagerate in both direction is analyzed. The result shows that the shear rate of flow velocity counts, and a precise forecast of velocity is necessaryfor damage rate evaluation. In addition, the distribution of tension isrelated to the stiffness of riser, and fatigue damage can be reduced bya reasonable top tension. At last, the material of the riser model isreplaced, then the variation of amplitude, stress and fatigue damage iscontrasted, which can provide reference for further study.