Session: 02-05-01 Extreme Loads and Responses

Submission Number: 157052

Investigation on the Effect of Bimodal Sea States on Mooring Systems' Extreme Response Offshore Brazil 

Due to second-order effects, low frequency wave drift forces may excite resonant responses for moored FPSOs in horizontal motions, resulting in mooring responses in both low and wave-range frequencies. Time-domain analyses are usually performed since frequency-domain analyses fail to properly represent the system’s nonlinear effects. Furthermore, most accurate approaches to determine extreme load effects are based on the long-term statistics of the response. Meanwhile, the typical sea state in offshore Southeast Brazil is comprised of wind sea and swell. The separate wave description is expected to be relevant in response calculations of turret-moored floating units, which are sensible to directional loading, and are expected to lead to less conservative tension estimates in spread-moored floating units. Despite this, it is common practice to combine wind and swell waves into a simplified unimodal model, where a total significant wave height that combines each wave energies is described.  In this study, a sensitivity analysis is carried out to investigate the response characteristics of a spread and a turret-moored FPSO when subjected to the bimodal sea state comprised of wind sea and swell and to the combined unimodal sea state. Extreme responses of the investigated mooring systems are obtained through an efficient full long-term integration approach, using Importance Sampling Monte Carlo Simulation (ISMCS).

Presenting Author: Marina Leivas Simão Federal University of Rio de Janeiro

Presenting Author Biography: Marina is a structural civil engineer with over 6 years of experience in offshore structures analysis. She works as a researcher at the Laboratory of Analysis and Reliability of Offshore Structures (LACEO), at COPPE/UFRJ Civil Engineering Program, where she obtained her Ph.D. on the analysis and reliability of mooring systems for floating production units.