Experimental and numerical study on hydrodynamics of coupled FPSO and mooring/riser systems in deepwater GOM

Abstract

Floating production storage and offloading (FPSO) systems have been proposed for the development of recent oil discoveries in the southern region of the Gulf of Mexico (GOM) in water depths ranging from 1000 to 2000 metres. It is increasingly challenging to predict the environmental forces and the global responses of FPSO systems and the associated dynamic behaviour of their mooring lines and risers at these depths. The combination of an appropriate FPSO scale model with a suitable level of equivalent effect reduced depth for the mooring lines and risers using a hybrid passive truncated experimental methodology is a feasible approach. This thesis provides an effective technique to study the equivalent hydrodynamic behaviour of floating systems to validate numerical models and to predict full water depth behaviour, based on a hybrid passive truncated experimental method and a non-linear time domain coupled numerical analysis. The extreme motion response of an FPSO and the dynamics of the mooring lines and risers in the context of prevailing environmental conditions are investigated. Several important findings are obtained based on the present experimental and the numerical study: The linear motion transfer functions are sensitive to the direction of the incident waves, and the differences between the Full and the Ballast load conditions were found to be insignificant, except for the roll motion, which showed slight differences in resonant frequency responses and the maximum peak motion amplitudes. Further, the spectra analysis revealed that the risers have a great influence on low-frequency damping, particularly in the surge direction, whereas the damping mainly contributes to roll in the wave frequency motion response. The main horizontal motion response of the FPSO (surge) under the non-collinear environmental loading condition was found to be slightly higher than that obtained for the collinear loading condition and the numerical simulation identified that the Newman’s approximation is reliable to predict the extreme motion response. The dynamic tension responses of the risers are sensitive to the environment force directions and the mooring lines/risers dynamics do not affect significantly the wave frequency motion responses in heave and pitch. Additionally, the mean motion of the FPSO and the mean tensions in the mooring lines are dominant in deep-water and ultra-deepwater installations and conversely the dynamic components of the tension responses of the mooring lines decreases as the water depth increases. The findings are not only improve the understanding of the complex hydrodynamics, the extensive experimental and numerical results will also provide welcome benchmarks for ii future study of the FPSO with its mooring lines and risers under both collinear and non-collinear environment loading conditions.