Motion analysis of FPSO in multidirectional seas : the West African offshore region

Abstract

The use of experiment remains the most accurate method in the prediction and evaluation of roll damping. Several models ranging from CFD to analytical and empirical techniques and tools have been developed over the years for this purpose. However, the issue of accurately capturing the adherent multilinear behaviour for hulls with sharp edges and bilge keels remains a challenge until date. The elaborate works of Oliveira and Fernandes (Oliveira and Fernandes ,2006,2010,2014) identified and characterized the existence of two regimes using the bilinear model, later modified to the hyperbolic model. Following their work, and identifying this gap, an enhancement in their formation lead to the introduction of a third damping term, which represented the transition between the large angle side and the small angle regions. A modified hyperbolic model has been proposed and tested against existing models with reasonable agreement in terms of regenerating the measured decay. The model’s capture of the transition region was validated using the rigorous procedure of the bilinear methodology. The relative uncertainty associated with the predictive model was evaluated to fall within 3.5% to 5.9% .The decay data were used to modify the regression model of Oliveira and Fernandes and the enhanced model reduced the predictive error in the model parameters from second to first order range. The extracted damping coefficient and model where implemented in a code to study the influence of directionality and spectrum type on the roll motion response of the freefloating unit (typical FPSO) in real sea environment. Interactive contour plot representation was used to capture the sensitivities of spectrum type, directionality and the multidirectional wave streams summation techniques developed prior to roll motion response simulation. A barred region for the number of regular waves to be used was established using the maximum spectra energy density and the estimated significant wave height as the indicators. A 6dof code was developed using simplified methods and techniques. The novel frequency-spectra weighted technique was proposed for the estimation of the excitation force components of the equation of motion in irregular short crested seas from regular wave formulations. The method was validated by running similar scenarios in HydroD and the irregular wave test on scaled model. The roll motion response from the proposed method compared favourably within first order error range against the HydroD simulations and the irregular wave experiment conducted for ii JONSWAP spectrum for the targeted significant wave heights. Similar error margins were also observed for the measured as well as the estimated wave elevations and all other motion modes. The interactive results from the contour plots when translated into roll motion was very evident in the estimated magnitudes in different sea state spectra combinations (type and directions). The use of the suggested spectra form (lognormal or triangular for the swell sea and JONSWAP-Glenn for the wind sea) for the West African region identified variations in the roll response of between 1-23% or more than 5.5o This goes to further show the need to use site or region specific spectrum for the determination of design and operational parameters for offshore structures and associated units and for personnel seakeeping comforts.