Flooded Member Detection

Introducing Flooded Member Detection

Despite sounding like a title for a porn film, Flooded Member Detection (FMD) is actually an inspection process that can determine the presence of water in a supposedly sealed structural member, water that has probably entered via a through-wall crack or weld failure. A sub sea member with a crack will fill with water (or more correctly the high-pressure air will escape and the vacuum will be filled with the surrounding water), so by looking for flooded member you're effectively searching for cracks.

Cracks in members indicate a weakness in the structure, and that's a Bad Thing when that's what's preventing you from choosing between a fiery death or plunging into the freezing / shark infested waters below[1].

So finding cracks is important. Once member flooding is detected all of the welds associated with that member are suspect which leads to a more detailed examination. 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 up to half a meter of soft algae and 10cm of hard coral, with cracks that aren't likely to be visible to the naked eye a fast alternative for finding cracks is very valuable.

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 requiring the destruction of the item being tested.

Flooded Member Detection Techniques

There are several alternative methods used to determine member flooding, each has its own advantages and limitations. The most popular are as follows:

Through-Gamma Radiography

The most recently popular 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 passes. 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 detected.

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 you to the presence of water. Unfortunately, it does require extensive surface cleaning and accurate, diver placement of the ultrasonic probe.

Neutron Backscatter

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.

Radiographic Photography

Takes an 'x-ray' photograph of the member, but uses gamma radiation instead of x radiation. The results are unambiguous, but they require topside processing and deployment is a serious pain.

* Adapted from my earlier post at Everything2.