Motion behavior in a turret-moored FPSO caused by piston mode effects in moonpool

Abstract

The water trapped in a moonpool area by a turret-moored Floating Production, Storage and Offloading unit (FPSO) in some cases can give rise to resonant periods due to piston or sloshing modes that cause unstable behaviors in heave and pitch. In this work, experimental tests have been conducted in order to analyze the dynamic effects of water trapped in moonpools and to determine how they affect the global performance of a turret-moored FPSO. Two series of experimental test have been conducted in two different configurations: one using a turret that prevents water inflow, and the other using a turret that allows water inflow between the moonpool and the internal cylinder. The obtained results show that this so-called piston mode appears in the turret model with moonpool area, while at the same time instabilities in dynamic response are observed in FPSO. The experimental results seem to indicate that the piston effect has some contributions in this behavior related to the yaw motion instabilities at regular wave period of 10.0 s.