Nuclear Grout Monitoring on Offshore Platforms
Authors: Evans Gordon V. , Parsons Tony V. , Wallace Michael R.G. 
Published: December 2007

Over the last 4 years, the Atomic Energy Research Establishment (AERE) at Harwell, England, has developed nuclear instrumentation and tracer techniques to measure the density and movement of grout used in the installation of steel jackets to the seabed. These methods have been applied to a number of steel jackets in the North and Celtic seas where weather conditions can affect greatly the application of traditional methods of visual inspection.

Introduction

Radioisotope density gauges capable of indicating the presence and accurately measuring the density of cement grout during and immediately after grouting of sleeve/pile annuli have been developed by AERE. They have been used successfully on a number of structures in the North and Celtic seas and since 1979 have been produced and used by Wimpey Laboratories Ltd. under license. These instruments, of a type well-established in a wide variety of industries, have been developed for subsea operation at depths exceeding 1,640 ft (500 m). Nuclear gauges also have been used for control at the grout mixing plant, thus enabling more constant grout densities to plant, thus enabling more constant grout densities to be achieved during pumping operations.

Radioisotope tracers have been used for a considerable time for a number of types of investigations in oilwell drilling and logging. One such method has been used to monitor the movement of grout around the outside of insert piles of a structure. This method used powdered glass, size-matched to the cement and containing Scandium 46, which is introduced into the grout, and its progress up the outside of the pile is monitored by means of radiation detectors suspended within the insert pile.

Grout Density Measurement

Description of Gauges

The grout density gauges are based on measurement of the decrease in intensity of a beam of gamma radiation after passing through grout as it flows between a radioactive source and radiation detector. Typical gauge arrangements are shown in Figs. 1 and 2. Calibration of the gauge gives the following relationship for the density of grout.

I Pg =Pref -A 1n --,.................................(1) Pg =Pref -A 1n --,.................................(1) B

where I is the ratio of gamma radiations passing through grout and water, respectively, and A and B are calibration constants.

In practice, it is convenient to use seawater as the reference fluid since the measurement of count rate can be made in situ, with constant geometry of the gauge to the surrounding steel structure, and immediately before grouting commences. A typical calibration curve is shown in Fig. 3.

The gauges used on offshore platforms have included sources of either Caesium 137 [half-life of 30 years and gamma energy of 0.66 MeV (0.1 pJ)] or Iridium 192 [half-life of 74 days and gamma energies of 0.3 to 0.6 MeV (0.05 to 0.1 pJ)]. The latter isotope, although of a weaker and marginally more favorable gamma energy for this application, has the disadvantage of a relatively short half-life if major construction holdups delay float out or installation of the structure.

The radiation detector, a geiger counter specially constructed in a waterproof housing, has been tested to withstand shock loads of up to 300 g (3000 m/s 2), hydraulic pressure tests at more than 1,640 ft (500 m) of head for periods of several weeks, vibration tests over a wide frequency range at accelerations of up to 15 g (147 m/s2), and over a 40 degrees C temperature range.

JPT

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