Method and device for lowering casing string

The invention discloses a method and device for lowering a casing string. The method includes the steps that stress analysis is carried out on the casing string to obtain a casing string friction calculation equation; based on the casing string friction calculation equation, hook loads corresponding to different floating lengths are calculated; on the basis of meeting a preset load condition, the floating length corresponding to the hook load with the largest numerical value is taken as the optimal floating length; a floating coupling is arranged at the position, corresponding to the optimal floating length, of the casing string, and casing string lowering operation is executed. By applying the floating casing pipe lowering principle, stress analysis is carried out on the casing string, a floating casing pipe lowering model is set up, and an optimized scheme of calculating the casing pipe floating length is formed, so that the optimal floating section length is determined, the floating coupling is placed at t