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.
This paper presents the mathematical model of the real-time ship simulator for low-speed maneuvering developed by the University of São Paulo, Transpetro and Petrobras, with the technical collaboration of Brazilian Pilots Association (CONAPRA). The software is based on the TPN (Numerical Offshore Tank) numerical code which had several modifications, in order to perform real-time simulations. After the complete description of the mathematical model, some illustrative results of simulations executed with pilots are exposed.
PETROBRAS has developed a new mooring system for production and storage tankers moored in offshore Brazil. The system is denoted DICAS (Differentiated Compliance Anchoring System), and is basically a spread mooring system with different stiffness at the bow and stem of the ship. This difference in stiffness allows the ship to weather vane partially under environmental conditions. Due to the characteristics of this system, the design has to take into account the varying direction of the weather and the best layout of the production risers and mooring lines. Since DICAS is a partial weather vaning system, it makes the production swivels and turrets unnecessary. Therefore, the cost for production systems based on existing tankers will be drastically reduced, so it shall be very useful for low production and marginal fields in mild environments. This paper presents some results from experiments and simulations, and discussions on the design criteria for such systems. Important factors to be included in the design are the damping coefficients, namely the wave drift damping, the mooring line damping and the linear and quadratic viscous damping. The effect of interaction between current and waves is also important.
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