General

Degradation of offshore structures is mainly caused by corrosion and fatigue crack growth. The effects of corrosion are included in the design of these structures by incorporating a corrosion allowance or by use of a protection system by anodes and/or coating, enabling control of corrosion development also by possible replacement of anodes. In contrast, fatigue crack growth can be more critical, as cracks can result in a sudden rupture under conditions of large storm loads. Moreover, cracks are hard to detect because they are small for a significant part of crack growth time. Therefore the Fatigue Limit State is important for design of marine structures as well as during operation; see also Section I.4.

Defects much larger than those implicit in fatigue design curves are also of concern as some cracks found during inspection can be attributed to such defects. These defects are significantly larger than normal fabrication defects included in a probabilistic fatigue analysis, and are also sometimes denoted as gross errors, as explained in Section I.4. Therefore, the following safety principles should be implemented:

1. • Design for adequate fatigue life, including Design Fatigue Factors (DFFs) and a sound corrosion protection system.

2. • Design for robustness in relation to fatigue failure.

3. • Plan inspection of the structure during fabrication as well as during service life.

When inspection priorities are set, the potential for abnormal fabrication defects should also be considered. Since inspection after fabrication onshore can be performed much more cheaply and with higher reliability than during operation offshore, it is worthwhile emphasizing such inspections, particularly for components that are significant for the integrity of offshore structures.

As offshore structures possess different robustness with respect to fatigue cracking, and because inspection, repair, and failure costs vary significantly, different inspection strategies may be relevant for different types of structures.

Jackets with four or more legs show a larger reserve strength with X-type bracing than with K-type bracing which was frequently used in older jackets, but less so in new structures. The consequences of a fatigue crack will still depend on the position of the crack, type of loading such as amount of local bending stress over the plate thickness versus membrane stress at the hot spot, and the possibility of stress redistribution during crack growth.