The sub sea component of most offshore structures -- like oil and gas drilling and production platforms -- are generally composed of a series of tubular members, much like giant scaffolding underwater. Members can range from small pencil sized tubes, to giant pipes you could drive your oversized SUV through.
Despite sounding like a porn film title, Flooded Member Detection (FMD) is actually a process that determines the presence of water in a previously sealed member, which has entered (usually) via a through wall crack. A sub sea member with a crack will fill with water as it is under pressure. So by inspecting a member for flooding you are effectively inspecting it for cracks.
Cracks in members indicate a weakness in the structure, which is the last thing you want when that's all that's preventing you from plunging to your fiery death in the freezing, shark infested waters below.
So finding cracks is important, and once member flooding is detected, the member, and all of its associated welds, are suspect and are then more closely examined. Using FMD means we can skip the detailed inspection for members we know don't have cracks.
Seeing as the members being inspected are up to 80m underwater, covered in algae and coral, and the cracks may not be visible to the naked eye regardless, a fast alternative method of finding cracks is very desireable.
Flooded Member Detection belongs to a group of inspection techniques described as non-destructive testing or NDT. As the name suggests, these techniques allow for quality and integrity testing without inherently requiring the destruction of the item being tested.
Flooded Member Detection Techniques
There are several alternative methods used to determine member flooding, each has inherent advantages and limitations. The most popular are as follows:
The most recent FMD innovation. Requires no cleaning or preparation of the member being tested. The amount of radiation absorbed as a stream of radiation passes between a known radioactive source and its detector is related directly, and predictably, to the amount of mass through which it must pass. Essentially, water will act as a shield to the radiation, so we'll get a lower radiation count if the member is full of water, plus it's predictable, so we should be able to tell how much water is in the member based on the radiation count we get.
Low Frequency Ultrasonics
Uses an analysis of the echo returned from the far wall of the member when a low frequency pulse is created at one side. The time taken for a response will alert the user to the presence of water. Unfortunately, it does require extensive surface cleaning and accurate, diver placement of the ultrasonic probe.
Makes use of the property of the fast neutron, which emits an easily detectable, slow neutron backscatter when it hits a hydrogenous material (like water). An empty member will emit a far lower backscatter component than a full one. The technique requires extensive surface cleaning to eliminate a water gap, and a wall thickness of any reasonable size (> 25mm) will cause an undetectable result be blocking the backscatter completely.
Takes an 'x-ray' photograph, but using gamma radiation instead of x radiation, of the member. The results are unambiguous, but they require topside processing and deployment is a serious pain.