Active control of axial dynamic response of deepwater risers with Linear Quadratic Gaussian controllers

Active control of axial dynamic stress response of deepwater risers with Linear Quadratic Gaussian controllers is presented in this paper. The risers are installed between the subsea wellhead and the Tension Leg Deck located in the middle of the moon-pool in the Floating Production, Drilling, Storage and Offloading unit. The inevitable heave motion of the floating hull causes a time-varying axial tension in the risers. As a result, the frequent occurrence of axial dynamic stress response of relatively large amplitude may cause fatigue damage to steel risers. This paper thus explores the possibility of using a winch to reduce axial dynamic stress response of deepwater risers. The equations of motion of actively controlled risers against waves are first established. Axial dynamic stress response of the system is then determined in terms of the complex dynamic characteristics and the pseudo-excitation method. Based on the derived formulae, the performance of the winch for reducing axial dynamic stress response of risers is numerically assessed. The investigations show that the winch not only provides great flexibility for selecting system parameters but also significantly reduces the axial dynamic stress response of risers under wave excitation.