Page 498 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

18.3 Flooded Member Detection (FMD) 493
removed from the coil, the protons then “precess” back to their natural state. As the protons pre- cess, a small yet precise electrical current is induced into the same coil that is directly proportional to the local magnetic field intensity. The rate of precession is thus measured, scaled, and outputted on a sampled basis. The typical PPM is highly accurate, but due to the time delay between the acti- vation/deactivation of the coil for the precession process to snap between coil orientation and Earth’s orientation the sampling rate (over time) is low. However, the PPM is typically simple to operate and inexpensive and requires little in the way of calibration.
The coil-based PPM has more recently been replaced by the Overhauser effect PPM with the noted advantage (over the coil-based PPM) of lower power consumption, higher sampling rates, and higher accuracy. With this method, a type of proton-rich fluid containing chemicals with free radicals is added. A constant high-frequency RF signal (approximately 60 MHz) is applied to the fluid generating the “snap” magnetic field at a much higher frequency. This allows a .2 Hz sampling rate with much higher theoretical accuracy over the solenoid method.
With the Optically Pumped Alkali Vapor Magnetometer (OPM), light is pumped through a series of gas vapor-filled chambers (typically nonradioactive cesium, but rubidium, potassium, and even helium are sometimes used), allowing the use of the electron energy and spin proper- ties of the gas in the magnetic field measurement. A narrowly defined wavelength of light is radiated through a gas-filled cell onto a photocell. An RF signal/field is applied to the gas cell so that the properties of the light’s interaction with the electrons of the excited gas will form a measurable output pattern on the photocell proportionate to the ambient magnetic field. The OPM obtains its higher performance over its PPM counterpart due to its much higher oscillat- ing signal (between 70 and 350 KHz for the OPM versus 0.94.5 KHz for the PPM), allowing for much more information carrying capacity. The advantages of the OPM over the PPM is its higher sampling rate with a corresponding disadvantage of higher cost, required calibration, and lower absolute accuracy.
The field of magnetic anomaly detection is quite fascinating. Many excellent technical papers explaining the technology are freely available for further research into the subject.
18.3 Flooded Member Detection (FMD)
Fixed industrial structures, for example offshore oil and gas production platforms, attached to the seafloor are typically fabricated from welded tubular steel. Once the jacket legs are sealed at the fabrication yard, the air-filled chamber within the tubular structure remains air-filled, allowing for buoyancy offset to the often extreme weight exerted on the structure’s base on the seafloor. Should the air-filled chambers of the jacket become flooded, a situation would evolve whereby some of the tubes would be flooded, destroying the buoyancy on a portion of the jacket. This would place additional stress on the structure’s base in an asymmetrical load. As part of a typical IRM program, FMD is performed on selected portions of the platform to verify structural integ- rity of the entire platform system. This method is also used for the detection of flooded pipelines.
FMD is performed either acoustically or radiographically, depending upon several factors including member size, metal thickness, degree of marine growth, and cost.