NDT Inspection of Mooring Chain Links

TWI has worked with a Member company to inspect steel mooring chain links for fine surface-breaking defects. TWI used a multi-disciplinary approach, combining both non-destructive testing (NDT) techniques and metallurgical theory, to thoroughly inspect the chain links. The consultants carried out some of the work on-site.

TWI’s client was procuring a set of mooring chains for an upcoming project. Shallow, crack-like indications were discovered on the crown region of two of the links, which were believed to have originated from the steel manufacture. The client wished to use an impartial company, such as TWI, to inspect the links, review the root cause analysis and report the findings.

Objectives

To implement a multi-disciplinary approach to inspect the mooring chain links, gaining information regarding the type, frequency, size, origin and risk level of the potential defects
For TWI to review the failure investigation, applying their impartial and expert opinion
For TWI to produce a report of their findings

The eddy current array T-flex probe on a chain link.

Solution

The manufacturer provided a selection of samples for TWI to use for trials, some of which were suspected to contain shallow surface cracking. Having reviewed the suppliers’ root cause analysis report, TWI postulated the potential type of flaw that could be present and considered the advantages and limitations of each of the main NDT methods. TWI concluded that eddy current array (ECA) would be the most suitable NDT technique to use, supported by magnetic particle inspection (MPI) for defect confirmation.

Initially, MPI inspection was carried out using an alternating current (AC) electromagnetic yoke, with colour contrast (black) MPI ink, and a wet continuous method of application. After the MPI inspection, ECA was implemented. The ECA equipment manufacturers were consulted, and they advised the use of an I-flex probe with small coils to provide greater sensitivity.

The combination of ECA and MPI worked well together. The two methods were equally sensitive, and both had a good probability of detection, detecting surface cracks as small as 2mm long and 0.6mm deep. However, on occasion the ECA picked up some crack like indications, which MPI did not, and vice versa.

During initial on-site inspections, it became apparent that the I-flex probe was unsuitable due to access constraints. TWI contacted the ECA manufacturer, who created a bespoke T-flex probe that was specifically tailored to have the required sensitivity for this project.

Conclusion

TWI successfully completed the on-site inspections. The ECA/MPI inspection programme found no additional indications. Therefore, TWI successfully used their impartial and qualitative judgment to assist their client in deciding that the chain was acceptable.

Eddy current array equipment on the mooring chain link.

Malcolm Spicer NDT Consultant

Malcolm joined TWI in 1998 as an NDT lecturer. In 2003 he assumed responsibility for the co-ordination of NDT training courses and at the beginning of 2005 he became the Chief Examiner. Since February 2007, Malcolm has been working on various projects in the NDT section. Before joining TWI, Malcolm had 24 years of experience in aerospace engineering and NDT in the Royal Air Force.

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