The role of local joint flexibility (LJF) in the structural assessments of ageing offshore structures

Copyright © 2016 by the International Society of Offshore and Polar Engineers. The distribution of fixed steel offshore platforms around the world reveal a global fleet that has exceeded or is approaching the end of its design life. In many operating areas, there is an attraction to continue using these aging facilities due to continued production or as an adjoining structure to facilitate a new field development or expansion. To justify continued operations of the fixed offshore platform, various integrity assessment techniques are often used. One of the major techniques used is the phenomena of Local Joint Flexibility (LJF). The substructure of a fixed offshore platform is generally made up of steel tubular members. These tubular members are connected at joints by thickened sections called joint cans. These joint cans are designed as rigid tubular joint connections however in reality tubular joints exhibit some degree of flexibility. This local flexibility at the tubular joint allows a better redistribution of the moments and thus stresses to other members of the jacket truss structure compared to the rigid joint condition. This re-distribution of moments and stresses therefore allow the tubular joint to exhibit much longer fatigue lives and greater strength capacity than the design condition, assessed to 20-30 years previously.