Model Tests and Time-Domain Coupled Analysis of a Dicas-Moored FPSO in Deep Water

Abstract

Model tests of a DICAS moored FPSO have been carried out through 1:90 scaled experiments. The vessel motions and the mooring line tensions are investigated. The full-scale depth is 1240m. The model tests are performed with a mooring and riser system model truncated at 450m water depth. Extreme metocean conditions for Brazilian waters are simulated, including waves, current and wind in collinear configuration. A time-domain global hydrodynamic analysis of the truncated system is also carried out. Initially, wave loads on the hull are modelled by use of linear diffraction analysis (WAMIT), including mean loads and wave-frequency excitation. Low frequency excitation is included by use of Newman's approximation. In addition, viscous drag loads on the hull due to relative motions in waves and current and wind loads are included through quadratic current- and wind coefficients. The vessel motions are dynamically coupled in the time domain to the top-end mooring line and riser tensions, through detailed FEM (Finite-Element-Modelling) assuming slender-body hydrodynamics of this part of the system. The RIFLEX-C software is used. Irregular wave records from the experiments are used as input to the simulations, making direct comparison of response time series and spectra possible. Results from numerical simulations are compared with model test results. Based on the comparisons, hydrodynamic parameters of the numerical model are adjusted to match the measurements. In particular, drift force coefficients and thus slow-drift excitation, as well as damping coefficients, are empirically modified. Wind and current coefficients are calibrated by comparisons to experiments where the FPSO is exposed to pure wind and current conditions, respectively. Resulting simulations using the calibrated numerical model agree well with experimental results, and represent significant enhancements relative to the initial model. Using the calibrated hydrodynamic vessel coefficients, the numerical model can be extrapolated to full depth. Accordingly, numerical simulation of the full depth system can be performed. In the present paper, the focus is on the model tests and the calibration of the numerical model. The extrapolation phase, which is more straightforward, is not emphasized here.