Innospection completed its first mooring line inspection project on a FPSO in Norway in May 2014.
Two mooring lines were inspected at dedicated areas for possible zinc layer removal or damages at the steel wires due to seawater ingress as a result of the damaged plastic jacket.
Cracks and damages in the outer plastic jacket of mooring lines were detected during a visual wire rope inspection in 2012. This follow-up inspection was to detect the loss of the wire zinc cover and possible corrosion of the mooring line steel wires as a consequence of the damaged sheathing.
The inspection was performed utilising the customised MEC-Wire Scan inspection tool which incorporates the MEC (Magnetic Eddy Current) technique with a specific developed flexible Eddy Current sensor array system.
The MEC technique is a further development of the fast corrosion screening Saturation Low Frequency Eddy Current technique proven in the field to be highly sensitive and accurate in the detection of internal and external defects and wall loss caused mainly by corrosion. This technique has been used for the inspection of subsea risers with typical Neoprene coating, caissons, pipelines, structures as well as for flexible risers in the detection of cracks and corrosion at the armoured wires.
The flexible Eddy Current sensor array system was specifically developed for larger circumferential area coverage due to the thickness of the polyethylene outer sheath and to follow the mooring line diameter changes where a repair patch might be in place. Hence, the induced electromagnetic field of the customised sensor array system was able to overcome a standoff of 33mm.
The original MEC-Wire Scan inspection tool was designed to open and close around the mooring line with the opening of the two magnet halves being fail safe arranged. Although the scanner sensors and magnet poles ran at a consistent standoff to the inspection surface, only the polyethylene covered wheels were in contact with the mooring line. The field tool for this specific inspection was modified to overcome the mooring line patched area.
Deployed by ROV, the inspection data was transferred via the ROV fibre optic cables to the control room where the inspection engineers evaluated and analysed the findings immediately while the offshore team continued with the inspection simultaneously.