Dynasim—A Time Domain Simulator of Anchored FPSO

Abstract

The prediction of the dynamic behavior of moored tankers like FPSO system is very important to the mooring systems design. There are several types of mooring systems, such as Turret, Spread Mooring System (SMS) and Single Point Mooring (SPM), for which the specification of number and dimensions of the lines depends on the dynamic of the entire system composed by tanker and mooring and riser lines. Although several moored systems were already installed as a positioning system of FPSO, there are still some unknowns that should be investigated to provide more safety and lower cost systems. The drag/damping terms on the ship hull and the mooring and riser lines damping are one of the most important terms in the dynamics of the moored tanker that should be investigated. The hydrodynamic model due to the interaction between hull and fluid has been very studied due to the complexity of the phenomenon. The authors have studied the models of several researchers and three of them were picked up and implemented in the DYNASIM, a moored ship dynamic simulator. These models will be discussed and compared. In deep water, the total length of the mooring lines and risers becomes large and consequently the forces acting on these elements become more important. However, in many cases these important forces acting on the lines have not been considered. There are some works that point out the magnitude of the damping forces of the lines. Recently, a full scale decay test using a 30kDWT moored tanker was realized in Brazil offshore. The results showed the significance of the line damping, in particular on surge motion. Another important phenomenon that has been discussed is the wave drift damping term. There are several research studies concern’s this force resulting in several estimation methodologies. However, only surge direction has been considered important and there is little research about the damping in others directions. The time simulation methodology will be used to analyze and compare the effects of each term in the dynamics of the system. Finally, the comparison between simulated results and model test data will be performed in order to validate the model proposed. In this study, the experimental data of DICAS system (Differentiated Compliant Anchoring System) will be used.