TWI Bulletin, May - June 2008

Case study: 1980s material - a question of fracture toughness

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 original1980s 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 steelsand welds associated with padeyes in the 1980s generation of offshore platform topsides. The answers were to be based on a review of contemporary available data.

A literature search was carried out aimed at obtaining data, from the 1970s through to the early 1990s, for use in a statistical analysis. This included data generated by TWI on steels and welds used in the construction of North Seaplatforms. Crack tip opening displacement (CTOD) 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 estimatelower 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 thepresent day decommissioning requirements.