Vibration Control of Multi-Modular VLFS in Random Sea Based on Stiffness-Adjustable Connectors

The response control of a very large floating structure (VLFS) is a crucial issue that affects the stability and safety of the structure. This paper presents a vibration control method for stabilizing the motion of a multi-modular VLFS by using a set of stiffness-adjustable connectors. The proposed connector consists of a cylindrical spring with an embedded actuator, making its stiffness adjustable. In a case study, a layout for such connectors is suggested to reduce the surge, heave, pitch, and yaw motions of the VLFS in random seas. To control the vibration responses of the VLFS, a mathematical model of the floating structure with the proposed connectors is established. A state feedback control scheme is developed using Sequential Quadratic Programming, which is able to adapt to varying wave conditions. Numerical studies indicate that the control method based on the stiffness-adjustable connectors was able to greatly reduce the responses of the modules when compared to flexible connectors and was also able to reduce the connector loads when compared to hinged connectors. Most importantly, this control method enables the elimination of resonant responses by changing the system stiffness.