Reliability of inspection for fatigue cracks in offshore structures

Offshore structures are subject to wind and wave loading, which-eventually causes fatigue crack growth. The rate of growth of cracks depends on the load history, environment, design of joint and structure. In order to check for this growth the structure is periodically inspected with the inspection schedule designed to detect cracks before they are a danger to the structural integrity. The capability of an inspection method to detect cracks of a certain size is therefore an important aspect in establishing the schedule and maintaining the structural integrity. Also important is the capability of equipment to size the cracks, as this will determine the actions which may be taken in the event of a crack detection. The types of inspection reliability information required therefore includes probability of detection (POD), probability of false calls (spurious) and accuracy of sizing. Inspection reliability information can be obtained in a number of ways. These include anecdotal (inspection experience with particular equipment), manufacturers information, experimental determination and modelling. Each of these has its advantages and disadvantages. The offshore industry decided to explore an experimental inspection programme which led to the ICON project. The programme and the ICON project are briefly described