PSI - Issue 57

Arne Fjeldstad et al. / Procedia Structural Integrity 57 (2024) 692–700 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Inspection philosophy Corrosion and fatigue crack growth are the main degradation mechanisms for offshore structures. Corrosion is normally designed for by increasing the dimensions (corrosion allowance) and/or a corrosion protection system. This may be achieved by applying coating and/or cathodic protection where applicable. The fatigue crack growth can be more critical because cracks can result in accelerated growth and ultimately sudden fracture. Moreover, cracks are hard to detect because they are small and difficult to detect for a significant part of the crack growth time. Different inspection strategies may be relevant for different types of offshore structures during the in-service life. This is because structures have different robustness with respect to fatigue cracking. Moreover, inspection, repair and failure costs vary significantly among different type of structures. The consequence of a fatigue crack is dependent on position of crack, type of loading, and possibility for redistribution of stresses during crack growth. For ship shaped floating production vessels there is significant residual strength with respect to fatigue cracks for typical details which normally makes it possible to detect cracks using leak-before-break detection; see e.g. Bjørheim (2006). However, it may be a challenge to document redundancy for long through thickness cracks in the main plates. When planning inspection, it is important to assess the consequence of a potential fatigue crack for each considered hot spot. A sketch of a procedure to establish an inspection plan fora structural detail excerpt from DNV RP-C210 is shown in Fig. 1. Cracks might have been detected during former inspections but have been assessed to not require a repair before another inspection is performed. If there are such cracks in the structure, this information should be used as a basis for the next inspection. All structural details should be evaluated in the process of developing an inspection plan. Each detail is considered based on calculated fatigue lives and the applied methodology. In addition, the probability of a gross error should be considered when planning for inspections. General visual inspection (GVI) has traditionally been recommended for purpose of control of gross errors. A general visual inspection may thus also have a positive effect on reliability with respect to fatigue. Probabilistic analyses are performed for selected details to account for uncertainties in cyclic loading and fatigue capacity. The acceptance criterion for time to next inspection is related to consequence of failure. In fatigue assessment of an existing structure, one will use the best available data and information about fatigue condition as derived from performed fatigue analyses. However, it should also be realized that it is not practical to assess all details by probabilistic analysis and some screening based on the fatigue analysis result should be performed. Due to the nature of fatigue and number of uncertain parameters involved, there will be uncertainty to when and where fatigue cracks will occur in a structure subjected to significant cyclic loading. The more information available, the better it is for predicting future behavior with respect to fatigue cracking.

Assess detail: target safety level as a consequence of fatigue failure, and inspection history

Perform fatigue life calculation

Measurement of crack size

Yes

Cracks detected

earlier?

No

Short

Long

Perform probabilistic analysis

Fatigue life?

No

NDT required?

Yes

High

Low

of gross error? Consequence

General visual inspection Leakage detection

Medium

NDT

Close visual inspection

Inspection plan

Fig. 1. Schematic development of inspection plan with respect to fatigue.