Choosing a Rational Sample Size for the Underwater Inspection of Marine Structures

Underwater condition surveys have become common practice for marine structures as owners strive to ensure safety and protect their infrastructure investment. Due to limited visibility and the need to remove marine growth, it is impractical to inspect 100 percent of each component underwater. For this reason, it has long been a common practice on pile-supported structures to remove marine growth in three bands (waterline, mudline, and midpoint) on a "representative" sample of piles. A sample size of 10 percent is commonly used, although other guidelines are also used. The problem with this current industry practice is that the sample size has no reliable statistical basis. This has become a particular issue with large structures supported by several thousand piles, where the current industry practice may result in far more piles inspected than are necessary to reliably assess the condition of the structure. The cost impact in such cases can be significant. In order to improve the current industry practice, a methodology has been developed for the selection of sample size based on a statistical approach. It is recognized that any such method could get complex and onerous if it attempted to distinguish between levels of deterioration and the possible structural consequences for various structure types and construction materials. For this reason, a simplified approach was chosen which focuses on "reportable defects". Using this method, a pile will either exhibit a "reportable defect" or not, with corresponding numerical representation of 0 and 1, respectively. A reportable defect is defined as any defect that is significant enough to justify making a note of it in the inspection notes.