Study On Vortex - Induced Vibration Mechanism And Its Suppression Of Deep Water Drilling

Deep water oil and gas resources will become the main battlefield of hydrocarbon exploration and exploitation in the near future. Higher requirements on offshore drilling equipment have been expressed under the complex deep water environment. Therefore, as the only channel connecting drilling platform and underwater wellhead, marine riser is of crucial importance to deep water oil and gas exploitation. Because marine riser is under complex ocean current environment, fatigue damage tends to happen due to vortex-induced vibration, which has a strong impact on the safety issue of marine riser. As a consequence, the study on the mechanisms and inhibition methods of marine riser vortex-induced vibration has great theoretical meaning and practical engineering significance. The main contents and results of this paper are as follows.(1) Based on massive literature research about marine loads, the CFD model is established to study the flow field around a deep water riser body. The flow field, lift and drag coefficient and vortex shedding frequency of different riser sizes are studied. The results reveal the mechanisms of vortex-induced vibration, and obtain the relationship between lift coefficient, drag coefficient, vortex shedding frequency and flow rate, pipe diameter. Then the mechanisms of vortex-induced vibration are revealed and recognized.(2) The inherent frequency model of deep water riser is established with consideration of deep water riser’s structure system. The influence of tension, platform migration, buoyancy modules, water depth, pipe diameter and thickness on the inherent frequency of riser is analyzed. The obtained results can provide theoretical basis for vibration control and elimination of deep water riser.(3) The mechanical model of deep water riser under ocean wave and current is established. The vortex-induced responses under ordinary sea conditions and extreme sea conditions are analyzed and compared. By use of proposed method, the "lock-in" phenomenon is successfully captured. The analysis results can be used to determine the maximum deformation position and maximum force bearing position. These results can provide basis for the safe use of riser.(4) After the analysis of the influence of environmental factors, platform migration, tension and buoyancy modules on vortex-induced vibration, it can be concluded that the bottom low speed region is more likely to bear vortex-induced vibration with the increase of flow velocity of sea surface. Platform migration can aggravate the’vortex-induced vibration and damage of riser. The use of buoyancy modules and increased tension can effectively control the upward vibration, lower the mode and order of transverse vibration and decrease the transverse force applied on deep water riser.(5) Three-dimensional fluid-solid coupling model of marine riser and deep water shear flow is established using FEA software. The actual size of the model is1500m (length-diameter ratio is2800). The study of vortex-induced vibration under different tension is carried out. Meanwhile, the effect of tension on inhibiting is analyzed based on the flow field features and motion characteristics of riser. This study is of guiding significance to strength design, life prediction and field application of deep water rise.