TOI drillship, first drillship of DSME, was designed to have more than 25 years' fatigue life against given environmental conditions. Fatigue damage in hull and hull-topside interface structures was assessed based on stochastic method. D-SAFS, developed by DSME, generates stress RAO based on ‘Component Based Method’. Structural analyses were carried out for each load component. Resultant stress was combined with wave load analysis results. Whole ship FE models having fine mesh in fatigue sensitive area were used in structural analysis. According to the agreement with Owner the vessel is assumed to spend 20% of her life in North Atlantic and 80% in On-site (Gulf of Mexico or Shetland in Summer Season). Some hot spots did not satisfy design fatigue life of 25 years for given sea states. Through geometry modification, structural reinforcement or improving fabrication method such as grinding, all structural details could satisfy design fatigue life of 25 years.
To verify hull strength of TOI drillship, first drillship of DSME, various kinds of analysis were carried out in DSME according to DNV Offshore Standards and Ship Rules. Global and local design of TOI drillship was determined based on direct calculation for DNV rule and direct wave loads. Fatigue damage was also assessed for hull & hull-topside interface structure using wave load analysis results based on stochastic method. WADAM developed by DNV was used in wave load calculation. In traditional stochastic analysis, calculated wave loads are directly transferred to whole ship model. In this method, force equilibrium is automatically obtained if FE model has exact mass distribution. In this project, D-SFAS, which was developed by DSME and already applied to several past projects, was used in stress RAO generation and damage assessment. Stress RAOs were made based on ‘Component Based Method’.
