Operability Improvement of Deepsea Riser by Active Control

Abstract

This paper presents a research on how to keep the riser angle of iclination within the admissible extent in the deep sea drilling operation, from the viewpoint of the positioning of the drill ship and the effects of using actuators attached along the riser.In the operation of deep sea drilling where the water depth is deeper than 3000m, it is very important to keep the top and the bottom angles of the riser inclination smaller than 4 degrees to prevent the drill strings inside the riser from touching and breaking the riser. To satisfy this criteria and to reduce the payload, the riser is tensioned at the top with a tensioner, the weight in water is reduced by attaching the buoyancy module, and the ship position is controlled by using the DPS against the disturbance resulted from current or wave. However, because of the huge mass and the very low rigidity, the dynamic response of the riser is generally so weak that the riser cannot follow the ship motion well, and that it resulted in decreases of availability of the system.Based on the above circumstances, the equations of coupled motion of the ship and the 4000m riser are formulated. With these equations, the optimal control is formulated. Then simulation results by Newmark-β method are shown for a set of parameters of 750tf, 850tf top tensions and 1kt, 2kt currents. From this study it becomes clear that an actuator with about 270kgf thrust can effecitively improve the response of the riser bottom and top inclination angles by the ship motion. And the inclination angles of the riser can be manipulated by the ship positioning within the permissible range. Furthermore, it is clarified that the two actuators with maximum thrust of 500kgf can effectively control the dynamic responses of the riser, and that positioning the ship with 850tf riser top tension, the angles of inclination at the riser top and bottom can be kept in the desirable range of less than 2 degrees even when the current speed is 2kt.