The taps tankers have demonstrated that fatigue fracture of tanker structure is a major factor concerning both pollution hazards and maintenance costs. Such costs and risks are not abated by the double hull regulation of OPA90. Fatigue damage in welded connections is proportional to cyclic stress, which in turn is the product of the stress concentration factor (SCF) and the general stress field in each structural member. The SCF is controlled at relatively late stages: detail design and weld monitoring. The stress field is determined earlier, in preliminary design, which establishes the member sizes. If fatigue is not addressed in preliminary design, then some stress fields can be too large to be compensated for by detail design and weld monitoring. Even in Phase I the proposed method will fulfill an urgent need by developing a prototype version of an efficient computer-based method for fatigue design of ships. The method uses wave data, unified hydrodynamic/structural analysis and structural optimization to automatically size members and control the cyclic stresses. Earlier work has verified that the basic methodology is sound, and that its fatigue predictions for the taps tankers agree well with reality.
