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Offshore ferrous link investigation

Following a chain failure on one of the North Sea's compliant offshore installations the integrity of similar chain, working in a similar environment and orientation, was called into question. TWI was asked by the operator to assess the evidence including identifying any degradation of the chain links and determining the extent of damage due to each degradation mechanism.

TWI has a track record of examining such links dating back nearly a decade. Failure in the past has been attributed to out-of-plane bending. The Industrial Member concerned undertook an assessment of other chain systems. Six further lengths of chain were removed from one of the previously assessed mooring systems and were forwarded to TWI for assessment.

A detailed experimental procedure was devised. First, from the six chain lengths the links were photographed and visually examined, to assess the surface condition and any apparent wear.

On the basis of the visual examination, one chain length and a single link from another line were singled out for non-destructive examination.

After wire brushing, these links were subjected to magnetic particle inspection (MPI), concentrating particularly on the interlink regions and the regions where failures due to out of plane bending had been observed in other systems.

In the absence of any significant MPI indications, one link was selected for sectioning. A transverse section was taken through the transition region between the crown and leg of the link where failures due to out of plane bending had previously been observed. Coincidentally this section also exhibited a region of mechanical damage.

A second section was taken through a region of mechanical damage adjacent to the weld. A third section, a plan section, longitudinal with respect to the axis of the original bar, was taken in the region of contact with the adjacent link at approximately mid-thickness. These sections were mounted and prepared using standard metallographic techniques, etched in 2% Nital and examined using optical microscopy. Photographs were taken to record the key observations.

A Vickers hardness survey with an indenting load of 10kg was carried out in accordance with BS EN ISO 6507-1:1998. This was complemented by a micro-hardness survey of the near surface region and into parent material. The hardness data were subsequently used to estimate the yield and tensile strengths of the chain link material. A further sample of link material was removed for chemical analysis using optical emission spectrometry.

Consistent with all TWI's work the findings of this project are client confidential and TWI was able to give the Industrial Member concerned with more than enough information to enable it to deduce a repair and maintenance strategy.

The findings were:

  • the degree of general corrosion, marine fouling and calcification which it found, consistent with service in a marine environment
  • the degree of pitting found
  • the existence and significance of any cracks in the contact zones where failure had previously been observed
  • the degree of mechanical or abrasive wear or hawser damage
  • the detailed metallographic examination of one link which could highlight prospective sites for crack initiation and preferential corrosion

To learn more about TWI's work on the structural and metallographic investigation of offshore installations contact us.

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