Annual subsea inspections of Murchison's major welded joints have indicated that a significant proportion of the welds have suffered from preferential corrosion of their heat affected zones..

Only a limited number of joints can economically be inspected each year, and the programme is normally biased towards structurally important welds with relatively low predicted fatigue lives. The presence of corrosion grooves modifies considerably the weld SCFs and thus Impacts the fatigue rankings and choice of welds for inspection.

With only limited inspection data, there was a need to identify the extent of the corrosion problem. This paper discusses the parameters that Conoco studied in an attempt to correlate the spread and severity of the corrosion problem with variances in geometry, metallurgy and cathodic protection. Correlations were found which have enabled the successful prediction of location of further corrosion grooves and have allowed a new optimum inspection programme based on both fatigue prediction and weld corrosion history.

The presence of the corrosion grooves in many cases give spurious crack-like indications with MPI inspection. This paper also discusses how ACPD inspection techniques together with profile moulding techniques were used to eliminate spurious readings and to characterise the grooving shape for detailed fracture mechanics assessment.

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