Study On The Dynamic Response Of Marine Riser Under Multiple Loads

Marine risers are the key equipments to connect the submarine wellhead to the offshore platform,which is mainly used for drilling or conveying fluid.Since the riser is a slender and cylindrical structure,when the fluid passes through,there will be regular vortices shedding behind it and the riser will vibrate periodically,which may eventually lead to structural fatigue failure.Jack-up drilling platform is a kind of the offshore fixed platform,which is widely used for oil and gas exploration and development in the continental shelf(with water depth within 200 meters).Unlike the deep-water riser,which is mainly affected by ocean currents,the riser used in the jack-up drilling platform will also be affected by wave loads.Especially in the South China Sea,typhoons are frequent in summer,which can cause huge waves with the wave height up to tens of meters.In addition,during operation,the jack-up drilling platform will pierce the water surface to a certain height,so the relative water depth will also have an impact in the vibration response of the riser.In order to increase the stability of the riser,top tension is usually applied on the riser to keep its stability from vibration.Therefore,under the combined action of ocean current,wave,top tension and variable water depth,the dynamic response of riser is a complicated fluid-structure interaction problem.In this thesis,model tests and numerical simulations are carried out to study the dynamic responses of a riser under multiple loads,which is of significance for the design and maintenance of marine riser,and also has a scientific value of researches on responses of a slender vertical cylinder under multiple loads.The main contents and conclusions are as follows:(1)Based on the similarity theory and dimension analysis,the model test was carried out in a wave-current tank.The plexiglass tube was used as the model of the riser.The bottom of the riser was fixed and the top of the riser was pinned.The ratio of length-to-diameter of the riser was 150.The dynamic strain response is measured by FBG strain sensor,and the vibration displacement is obtained by the modal analysis.The vibration characteristics of the riser under the conditions of current,wave,top tension and variable water depth were studied,including the spatial distributions of displacement standard deviation,vibration displacements in cross-flow direction and in-line direction,vibration frequencies and trajectories of characteristic points.Finally,numerical simulations and analyses were carried out on the wake vortex shedding mode and force characteristics of the riser with the open-sourced CFD software OpenFOAM,and the flow mechanism of the riser vibration response under the actions of waves and currents was revealed.(2)The experimental results indicate that under the combination of wave and current,the vibration amplitudes of the riser in both cross-flow and in-line directions are in the same order of magnitude,which is different from that of the riser subjected to the wave only or current-only.Under the combination of wave and current,the vibration in in-line direction is dominated by wave force,and the vibration in crossflow direction is caused by vortex force.Multi-frequency phenomenon appears in both in-line and cross-flow directions,which is dominated by both the wave frequency and lock-in frequency.For the first time,it is found that there are multiple "8" shapes in the trajectories of the riser.The riser trajectory becomes more complex,combined with the top tension.(3)The numerical analysis shows that the vortex shedding behind the riser are regular and strong near the water surface,while it remains stable in the middle and the bottom position of the water depth,which results in that the interference of waves with the riser is mainly concentrated near the water surface.There is also a multi-frequency phenomenon in frequency response with one wave frequency difference between the dominant frequency and the secondary frequency.The numerical results show that the time-variations of the forces on the riser in both in-line and cross-flow directions are consistent with those of the dynamic strains of the riser in the experiments.