Published on 28 January 2013
The complex task of topside decommissioning took on a new dimension recently when a leading North Sea operator approached TWI about one of its structures from the eighties.
Replacement lifting padeyes were to be installed at the same locations as their original counterparts, which were removed after the structure's installation in 1982. The replacements were planned to be attached to both the original 1980s steel plate and remains of original submerged arc weld metal and their associated heat affected zones.
But, the client asked of TWI, what was the fracture toughness of the original material? More specifically it asked TWI's structural integrity experts to recommend appropriate lower bound estimates of fracture toughness for steels and welds associated with padeyes in the 1980s generation of offshore platform topsides. Its answers were to be based on a review of contemporary available data.
A literature search was carried out aimed at obtaining data, from as early as 1972, for use in a statistical analysis. Crack tip opening displacement values were obtained for the heat affected zone material, the weld metal and the parent metal. The data for each material was then fitted to a suitable distribution and used to estimate lower bound values of CTOD for the HAZ material, weld metal and parent metal.
Armed with this information from TWI the client was able to design, manufacture and install appropriate replacement padeyes in the knowledge that they were compatible with the 1982 originals and easily capable of servicing the present day decommissioning requirements.
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