Coupled Motion Analysis of a Semisubmersible Platform in Campos Basin

Abstract

This work describes the generation of numerical models for the coupled analysis of a semisubmersible platform to be installed in Campos Basin, and presents the results, in terms of platform motions, of a set of analyses under fatigue environmental loading cases. The coupled model comprises the hull, the mooring lines, and all risers connected to the platform. The model includes also a FSHR (Free Standing Hybrid Riser) system to be connected to the platform; this system is composed by a rigid riser, an air can and a flexible jumper. A set of environmental loading cases appropriate for fatigue analyses is defined, in terms of current profile, irregular bi-directional seastate, and wind. For each loading case, nonlinear time-domain dynamic analyses are performed employing the Prosim program. This program is based in a coupled formulation that incorporates, in a single code and data structure, a hydrodynamic model to represent the hull, and a finite element model to rigorously represent the structural/hydrodynamic behavior of the lines. At every time step of the time integration of the equations of motion representing the hull, a series of nonlinear dynamic analysis of the lines is performed following a subcycling procedure. The results of the coupled analysis are expressed as time histories of platform motions. The next step is to proceed to the treatment of these time series, in order to determine, for each loading case: a) The frequency spectra of each motion component; and b) Representative statistical values in terms of static offsets, first and second-order motions (standard deviations and periods for low-frequency and wave-frequency motions). The platform motions that result from the coupled analyses presented in this work can be employed to perform the detailed structural analysis of the FSHR system, by employing an isolated model of the riser in a specialized program.