Current practice in global structural dynamic analysis of topside/hull systems involves executing a global finite element model for wave pressure inputs at specific periods. The main purpose of this global model is to generate loads for detailed local models to compute stresses; the global model itself lacks the fidelity and detailed modeling necessary for direct stress computation. The main driver behind executing separate global and local analyses is a computational constraint - each local model can be several million degrees of freedom making an upfront direct integration of a more accurate combined global model for dynamic analyses infeasible. The known issues with this traditional process include:
global model lacking the fidelity required for accurate computation of system modal properties (especially for shorter wave period regime), impacting the loads and stresses,
local analysis boundary conditions and loads mapping effecting the stress accuracy, and
long turnaround schedules preventing sensitivity and trade-off studies for risk/cost reduction and design optimization.
This paper introduces a dynamically substructured global structural dynamic solution which is a significant improvement over the traditional solution in terms of accuracy, computational speeds, and schedules. The improved solution enables a significantly higher fidelity global model, computes its structural dynamic response for the entire wave regime, and directly computes stresses as a function of wave period - all in a few minutes of computation. The improved process eliminates the lower fidelity global loads model utilized in the traditional process. Therefore, the inaccuracies in the lower fidelity structural dynamic loads analysis are removed. Furthermore, the improved process eliminates the local analyses and associated schedules/costs. This eliminates inaccuracies due to local model boundary conditions and loads mapping. In addition, the entire wave spectrum is analyzed in a single analysis, generating complete high resolution dynamic stress spectra as a function of wave period. This ensures that all wave periods important to the stress response are automatically accounted.
Finally, the very fast computation allow for trade-off and sensitivity studies to optimize structure and reduce overall risks, schedules and costs. Therefore, with dynamic substructuring solution, all of the deficiencies in the traditional global analysis are removed. Given the accuracy and extreme computation speeds afforded by the dynamically substructured solution, the next step will be the utilizing the irregular wave pressures as forcing functions. This more realistic representation of the forcing functions will result in the most realistic stresses possible. A proprietary nonlinear dynamic substructuring (NDS) solver, FLEXAS, is utilized to demonstrate this capability on a topside/hull FEM.
