New Built FPSO is being classed with DNV, for unrestricted service to transit worldwide and to withstand the environmental loads during its operation at site located offshore UK Continental Shelf – in 85-95M Water Depth. To assess the vessel structural capacity towards the notation ULS, Global FE analysis is being performed for the operating and sailing conditions, at the maximum and minimum drafts considering the still water shear force and bending moment limit curves. Operating condition is based on Site specific met ocean data, ULS analysis being carried out for both the 100 year & 1,000 year return periods.
A finite element model of the whole ship is generated including all the structural elements contributing to longitudinal and transverse strength the vessel.
The entire mass distribution is modeled as structure self-weight, compartment load, equipment load and concentrated nodal masses as appropriate and a panel model is generated from the outer hull surface of the vessel.
Hydrodynamic analysis is carried out for sufficient number of wave time period to capture the accurate representation of transfer functions and structural responses. Hydrodynamic analysis is carried out using frequency domain approach, including forward speed and nonlinear wave loads, wherever applicable. After load transfer from hydrodynamic model to structure model, structural analysis is carried out for these loads using the structural analysis software (GeniE) and the respective structural transfer functions are calculated.
On account of the weather vaning capability of vessel, 0 deg, 15 deg, 30 deg, -15 deg and -30 deg heading (with respect to centreline of vessel) are considered for the operating conditions. For transit condition, 0 deg to 360 deg direction heading with an interval of 30 deg is considered for the analysis. Global hull analysis is carried out for both the long term distribution of seastate as well as the short term extreme seastates to ensure that geometric tuning effects are not overlooked.
The model extent comprises of two tank length model, i.e. one complete tank and two half tank on either side. Dynamic wave pressure is calculated from global analysis and applied over hold model to simulate the site specific wave loading scenario. Dynamic internal tank loads, inertia loads as well as still water loads are all mapped from the global hull model corresponding to various site specific and recommended transverse loading scenarios.
Loading conditions corresponding to the maximum mooring/riser loads iwo turret interface is selected as the governing loading conditions for the analysis. Analysis is carried out for the headings consistent with the global analysis at an interval of 15 deg.