2.9.1 Function. Flex and ball joints are used to allow angular misalignment between the riser and the BOP stack, thereby reducing the bending moment on the riser. They Eire also used at the top of the riser to allow for the motion of the rig. In some instances they may also be installed at some intermediate level in the riser string below the telescopic joint to reduce stresses in the riser. The rotational stiffness of flex joints makes them more effective than ball joints in controlling riser angles. Typically, the rotational stillness of a flex joint is a nonlinear function of angle and ranges from 10,000 to 30,000 foot-pounds per degree of rotation. Rotational stiffness may also vary with temperature.
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