Introduction

As we discussed in Chapters 3 and 5, limit states are classified into four categories: serviceability limit states (SLS), ultimate limit states (ULS), fatigue limit states (FLS), and accidental limit states (ALS). This chapter presents FLS design principles and criteria together with selected engineering practices applicable for the structure of ship-shaped offshore units.

Under the action of repeated loading, fatigue cracks may in time be initiated in the stress concentration areas of ship-shaped offshore structures, and indeed have been reported by Hoogeland et al. (2003) and Newport et al. (2004), among others. In general, the fatigue damage at a crack initiation site is affected by many factors, such as material properties (e.g., elastic modulus, ultimate tensile stress); high local stresses (e.g., stress concentration, residual stresses); size of components; nature of stress variation (e.g., stress variation during the loading and off-take cycles, number of wave-induced stress range cycles); and environmental and operational factors including corrosion and performance of coatings. Potential flaws (e.g., poor materials, porosity, slag inclusions, undercuts, lack of fusion, incomplete weld root penetration) and misalignments can also significantly increase stress concentration and initial defects at welds.

To achieve greater fatigue durability in a structure, therefore, stress concentrations, flaws, and structural degradation, including corrosion and fatigue effects, must either be avoided or minimized or, more commonly, their levels and effects either in design, construction, and/or service must be monitored and effectively controlled to acceptable levels.