Risk in stability evaluation for floating offshore units

During the useful life of a vessel it undergoes various changes that lead to uncertainties in determining the displacement and center of gravity. Besides upgrading work, the very operation of the platform itself involving constant ballast and oil loading and unloading or the use of maintenance equipment can generates uncertainties concerning the displacement and center of gravity. These, therefore, are the parameters that present uncertainties inherent to the vessel's operations. Taking this into account, one example of the application of uncertainties on the displacement and transversal and vertical center of gravity positions will be presented in the analysis of the static stability of a converted FPSO tanker. Another question raised in this work is the deterministic treatment of certain criteria for the parameters that depend on random factors, such as wind, currents and waves. This paper will present a probabilistic approach for the calculation of the roll angle according to resolution IMO A.562 (environment criteria). The two chosen criteria for this analysis were resolutions IMO A.167 (general criteria) and A.562 (weather criterion).