Simulation Experiment and Mechanism Research on the Change of Lateral Friction Against Surface Conductor in Deepwater Drilling

摘要

Lateral friction against surface conductor plays an important role in the stability of subsea wellhead. Itrnchanges during jetting process and increases with soaking time. But the change rule and mechanismrnremain unclear. Therefore, the purpose of this paper is to explore the change mechanism of lateral frictionrnduring and after conductor jetting process by field experiment and numerical simulation.rnFirstly, a field experiment was conducted to simulate actual surface conductor jetting process. Therntime- dependent change rule of radial pressure, pore water pressure and effective skeleton pressure werernobtained. Then, the stress states of soil bodies during this process were analyzed. Based on experiment andrnanalysis results, the primary reason that causes the change of lateral friction was found. After that, tornexplore the specific change features of bearing capacity of seabed soil for surface conductor, another fieldrnexperiment was conducted. Finally, finite element method was adopted to simulate the conductor jettingrnprocess and to verify the research conclusions.rnThe field simulation experiment showed that the sensor at the base of surface conductor suffersrnconspicuous higher value of radial pressure than other sensors above. Meanwhile, the effective pressurernof each layer has a tendency of declining as time increasing and then gradually adds after reaching arncertain value. These regularities matches the change rule of radial pressure on piles during driving process.rnBased on the stress analysis, it is certain that the soil compaction effect does exist in conductor jettingrnprocess, caused by which, the formation and dissipation of pore water pressure during jetting processrnwould be highly related to the change of lateral resistance against surface conductor. After jetting intornposition, soil lateral friction would increase with time non-linearly. The change rate would gradually risernat the first 10 hours. After 72 hours, the added values of friction are small and the change rate turns tornflat. According to the numerical modeling results, under the action of jetting force, on one hand, soilrnbodies that are closed to bit are cut into grains and flow out from wellbore with drilling fluid; on the other,rnsoil bodies that are farther from bit would be squeezed down and then pushed around.rnPrevious studies mainly considered the soaking time and bearing capacity of surface conductor,rnfocused on specific values. This research designed a system to simulate surface conductor jetting processrnto explore the change mechanism of lateral friction against surface conductor by combining the methodrnof numerical modeling and field experiment.