Requency domain techniques for health monitoring of steel structures

The dissertation refers only in steel structures and not concrete. The main idea is to write about different methods that can be used for crack detection. The structure i am writing for is a steel jacket offshore platform with tubular steel members welded together.

One method for the identification of offshore platform structural damage for example is called modal analysis technique. Your task is to find and write about other methods that could be used for detecting a damage in offshore structures including this one. I am planning to write about corrosion and welding problems that appear in these structures so dont write about these two only if it is important.

How can i upload a document in order to understand the basic idea?

P.S I dont need photos

Thank you