Surge Response Control of FPSO Using Nonlinear MTLDs

Abstract

Floating Production Storage and Off-loading (FPSO) systems are being utilized world widely by offshore industries. The surge response of FPSO subjected to severe sea states needs to be suppressed in order to keep a particular station for operational purposes. The oil cargo tanks of FPSO can be used as passive dampers for minimizing the surge displacement response of FPSO. These cargo tanks can act as Tuned Liquid Dampers (TLDs) if the natural frequency of the liquid oscillation in containers is tuned to the surge natural frequency of FPSO or tuned to the excitation frequency. The containers can be partitioned into several tanks (as Multiple Tuned Liquid Dampers, MTLDs) with different lengths and liquid depths. The natural frequencies of MTLDs can also be distributed over a range around the natural frequency of FPSO. The present study attempts to comprehend the response control of FPSO under surge motion and so the FPSO is modeled as a single degree of freedom under random sea state. The containers can be modeled as either linear TLDs or nonlinear TLDs. Each linear TLD can be modeled by an equivalent Tuned Mass Damper (TMD) analogy based on linear wave theory. In order to account the nonlinear liquid motion in TLD, an impact damper analogy is used along with the TMD analogy. The study includes the efficiency and robustness of MTLD and it compares the response control obtained by using linear MTLDs and nonlinear MTLDs.