This work presents methods for reducing computational costs to estimate fatigue damage in riser structures by simplifying the sea state matrix to be solved in time domain by finite element analyses. Two different methods are proposed: The Equivalent Damage Method (EDM), which is a lump block method with an innovative formulation using vessel linearized response to determine significant sea states; and the Response Surface Method (RSM), which uses results from a few sea states to estimate results for other intermediary scenarios required for the fatigue analysis. Basic assumptions and guidance on how to apply the proposed methods are explained through the text.
The methods are used to solve an example of a Steel Catenary Riser (SCR) connected to a Semi-Submersible vessel, resulting in a reduction of almost 80% of the required computing time. Results show accurate values for damage estimates at the top of the riser, and limitations at the touch down zone. Advantages and disadvantages of each method are discussed in detail, demonstrating that they can be used with discretion at early design stages to estimate fatigue damage of risers, umbilicals and mooring lines in general.