Study On The Vertical Bearing Capacity Of The Wellhead Structural Casing

The subsea wellhead structural casing is jetted to design depth to support the BOPassembly weight. It also supports subsequent casing strings that are run in the well. The keyduring designing the bearing capacity of the structural casing is to accurately predict thestrength and the increasing rule of the soil around the structural casing. If the soil strengthwas chosen larger than reality, the designed length and the flexible stiffness of the structuralcasing will be small, which will result in wellhead sink under external vertical loads,evenworse, the well will be abandoned. This thesis is based on the past research. By the methodof theoretical analysis and numerical simulation, the structural casing's bearing capacity andits time-eficiency in the saturated clay has been detailedly analyzed in the paper, theinfluencing factors have been studied in detail.(1) Based on the water jet theory, this paper investigates the hydraulic parameters andsoil properties and presents a method to calculate the flow rates in structural casing jettingoperations. The influencing factors have been studied. Some conclusions are got. With theincrease of the bit size and the angle between the centre line of the nozzle and bottom and thedecrease of the nozzle number, the flow rates will decrease.With the increase of the shearstreng the flow rates will increase when other conditions remain unchanged.(2) Based on the Henkel Expressions calculating the pore water pressure, and combinedwith the Circular Hole Expand Theory, the author deduced the calculatingly formulas of theinitial pore water pressure. We will find that the initial pore water pressure in the radialdirection is a special case of the logarithmic function distribution.The influencing factors ofthe initial pore water pressure have been studied in detail.Some conclusions are got. With theincrease of initial bore diameter ratio R0/Ru, hyper-static pore water pressure of the sameradial distance and the action radius of hyper-static pore water pressure will decrease. With the increase of shear modulus G, the hyper-static pore water pressure will raise.When thewall thickness and the compacting factor is given, with the increase of the structure casing'sdiameter, the pore water pressure will gradually decrease at the surface of the structurecasing. The initial bore diameter ratio R0/Ru and compacting factorαhave no influence onthe relation betweenΔu/Cu and r/r0.(3) ABAQUS software is used to analysis the set-up effect on ultimate bearing capacityof structural casing. The influencing factors have been studied. The results indicate that theeffective stress increases in the soil adjacent to the structural casing with the excess porepressure dissipation. So bearing capacity of the structural casing increases. The permeabilityis bigger, the excess pore pressure dissipates more quickly, the radial effective stress andultimate bearing of pile increase more rapidly. The compacting factorαand the diameter ofthe structural casing is bigger, the ultimate bearing of structure casing is greater.