How can the suspected corrosion on the external surface of a 24’ casing located inside an offshore 30’ conductor pipe, with inaccessibility to the gap for the deployment of the inspection tool, be determined?
Innospection was presented with this inspection task by an offshore operator in the UAE who suspected that external corrosion existed at various and unpredictable areas above and below the splash zone on the 24’ casing.
However, the challenge was the limited space between the casing and conductor pipe, respectively partly filled gap, which restricted the deployment of the inspection tool onto the casing.
SLOFEC is a dynamic electromagnetic inspection technique. By superimposing the DC magnetisation onto the Eddy Current field lines, the depth of penetration is increased such that internal corrosion attack can be detected from the external wall. This technique allows the detection of internal and external defects with wall loss from 10% onwards.
Based on the operator’s decision to assess the internal corrosion condition of the conductor pipe to conclude the external wall condition of the casing, an external combined SLOFEC and Ultrasonic inspection using the marinised MEC-MPS200+ Scanner was performed above and below the splash zone.
As the inspection system provides a colour mapping image of the internal and external wall condition either separately or combined, the more interesting internal condition of the conductor pipe was well assessed. This inspection job was completed to the satisfaction of the operator.
To determine the best NDT method for the integrity management of the well conductors, the operator had also engaged an independent institute to perform a comparison of the proposed NDT methods, namely, SLOFEC, Pulsed Eddy Current, Long Range Ultrasonic and Ultrasonic.
It was concluded that SLOFEC provides the best results out of the four proposed NDT techniques for the inspection of the well conductors.
With medium accuracy (+/- 10%) and low surface preparation, SLOFEC scored high for rapid inspection (<2 days per conductor), 100% coverage within a short time, ability to cover topside, splash zone and subsea area as well as the differentiation between internal and external wall loss.