Effect of Mathieu instability on motion response of Spar hull with heave damping plate

摘要

The use of Spar hulls in floating solutions for offshore oil and gas application is well established. However, the studies on hydrodynamic responses in near-resonant region of loading, especially in harsh met-ocean conditions are limited. The phenomenon of Mathieu instability and reducing its effect on hydrodynamic response using heave damping plate configurations has been investigated using experimental studies and numerical simulation. This near-resonant-coupled response has been investigated in this study by tuning the pitch and heave natural frequency of the Spar such that the ratio becomes half. This may lead to increased response due to Mathieu instability and hence this study will provide insight into the response characteristics. The experimental investigations were conducted in a laboratory wave basin using a scale model (1:150) Spar hull with four different configurations of heave damping plate in regular waves. The hydrodynamic response of the proposed configurations was measured and simulated for different wave periods and heights. The Spar hull models were moored using catenary mooring line. The responses of the system are presented in the form of response amplitude operators (RAOs). Numerical simulation of the hydrodynamic response has been carried out by solving pitch/heave-coupled non-linear response equation. The results obtained from experimental studies are compared with numerical results and are found to be in good agreement. The increased pitch response coupled with the heave response for all four configurations are investigated and it is observed that the Spar with heave damping plates near the water line shows reduction in response. Mathieu stability diagrams are prepared for all four Spar configurations.