The influence of wind-wave energy spreading on the riser system response of a spread-moored FPSO

Floating unit, mooring lines and risers comprise an integrated dynamic system that responds to environmental loading due to wind, waves and currents in a complex way. The riser system response of a spread-moored FPSO in deepwaters is greatly influenced by first-order wave motions, where the consideration of wind-wave energy spreading may have an important impact on the integrity assessment. A 5-year database generated by a numerical wave model (available from NOAA) provides a proper representation of the wind-wave spreading factor distribution for Santos Basin, offshore Brazil. Based on these data and considering a cos2 s formulation for the spreading function, a short-term sensitivity study is performed to evaluate the spreading parameter s influence on the top tension response when the system is subjected to beam seas. The numerical simulations are carried out employing a simultaneous dynamic analysis of the vessel and slender structure system. Second-order loads, which may also be important by inducing mean and slow drift motions, are taken into account with a modification of Newman’s method, where the drift force coefficients are averaged with the squared surface elevation. The results show a significant influence of s on the riser response statistics. © 2015, Sociedade Brasileira de Engenharia Naval.