Flooded Member Detection by Gamma Ray Technique
Authors: Spencer Jon 
Published: October 2008

Following API's recommended procedures (Draft 17) for the assessment of existing platforms, inspection personnel and platform maintenance engineers need to determine the structural safety and physical integrity of the platform. To perform testing of a platform's subsea supports, there is now a tool hat allows evaluation on short notice without extensive member preparation. The expansion of topside facilities can be undertaken with confidence, and the sale or purchase of the platform is enhanced when proof of the structural integrity is presented.

With advances in the use of nucleonics, flooded member testing and evaluation is performed in 5 seconds, successfully and economically. Gamma Ray Flooded Member Detection performed by qualified Tracerco personnel is approved by Lioyds Register recertification in the North Sea area and is widely used. With today's emphasis on safe long term use of existing facilities, this easy, proven method of inspectionshould be the forefront of options available.

This paper presents the theory and practical aspects of the use of Gamma Ray radiation to test submerged pipes, legs or structural supports.

SURVEY OF PROBLEMS

Cross member testing with ultrasonics requires the member to be cleaned down to the bare metal for proper readings and the equipment calibrations are critical. The use of divers for member cleaning and ultrasonic testing requires working in a time sensitive hazardous environment with the deepest members often untested. Flexibility in operational changes and emergencies are at the mercy of the diver's down time.

ADVANTAGES OF GAMMA RAY FMD

Gamma rays have been widely used in many different applications for testing and evaluation. Their use for FMD has a short but effective history and points the way for future improvements in platform use and development. Benefits of the system include:Will unambiguously detect full, empty, or partially filled members, normally in five seconds of positioning.Accurate positioning of the instrumentation, and cleaning of the member surfaces is not required.FMD testing can also be combined with other ROV operations during the same dive.A totally mechanical system eliminates the hazards to human life.

The use of a low strength sealed radiation source, proper handling procedures, and trained personnel make Gain-maRay FMD testing the safest method of subsurface inspection.

DESCRIPTION OF THEORY

The Gamma Ray Flooded Member Detection system for the detection of water in subsea platform members is based on the gamma ray transmission technique for the examination of pipelines. The detection of flooded members is accomplished by calculations based upon the physics of gamma radiation in a narrow beam geometry. By measuring the gamma radiation intensity transmitted through the member, an accurate determination of the material, (air or water), inside the member is quickly made.

The determination of the member's status as flooded or dry is calculated based on the intensity of the gamma radiation received at the detector. The intensity of the transmitted radiation is an inversely promotional function of the average density of the material between the gamma radiation source and detector. High intensity readings indicate low density material, such as air.