Offshore stainless steel pipes are subjected to potential chloride corrosion due to the high salt content in the atmosphere.
Such chloride corrosion is typically seen as very minor openings to the outer wall of the pipes. While they are externally visible as minor defects, the chloride corrosion causes very small pitting defects to grow internally in the mid-wall through to the inner wall of the pipes.
The microscopic view (see picture) shows a cut-out 316 stainless steel pipe from an offshore gas field in the North Sea with identified leaks and chloride corrosion defects in the wall areas.
Such defect types with very minor openings are not only difficult to visually identify in terms of the location but are also challenging to determine the extent or depth of such defects with conventional NDT (non-destructive testing) techniques.
As a Norwegian operator had recently identified leaking areas in the form of minor openings to the external pipe wall in its offshore installed stainless steel 316 pipes which appeared comparable to chloride corrosion defects, Innospection had been requested to perform a verification test to determine if and how well the MEC (magnetic eddy current) technique could detect the very minor defect sizes. A 500mm section of a stainless steel pipe from the offshore jobsite with OD (outside diameter) of one inch, WT (wall thickness) 3.38mm and a known leakage area was provided.
The MEC technique combines a DC magnetic field with multiple eddy current sensors that induce an eddy current field penetrating through the entire pipe wall.
Defects cause the eddy current field line configuration in the pipe wall to change which is then measured as an impedance change of the specific individual sensor.
The changes in the signal amplitude and signal phase are displayed and compared to the calibration for the defect size and wall loss analysis.
Utilising the MEC-P9 pipescanner with a permanent magnet typically used for carbon steel pipe inspection, the MEC technique was proven in the verification test to demonstrate a very high probability of detecting the small volumetric mid-wall pitting defects.
Distinction of the different defect locations – externally, internally or mid wall of the pipe material was also achieved by means of analysing the signal phase angle while the wall loss severity was determined by the signal amplitude.
Dedicated smaller and light MEC-pipescanners will be built to achieve the faster detection and mapping of chloride corrosion in the stainless steel pipes.
Andreas Boenisch is a co-owner of Innospection