Improved LJF equations for the uni-planar gapped K-type tubular joints of ageing fixed steel offshore platforms

Abstract

The distribution of fixed steel offshore platforms around the world reveals a global fleet that has exceeded or is approaching the end of the design life of the facility. In many operating areas, there is an attraction to continue using these ageing facilities due to continued production or as an adjoining structure to facilitate a new field development or expansion. To justify continued life extension of the fixed platform, various integrity assessment techniques are often used. One of the major techniques used is based on the phenomenon of local joint flexibility (LJF). While the phenomenon of LJF has been well known in the offshore industry since the early 1980s, there has been little experimental data available. In 1983, Amoco conducted an experimental study primarily to determine stress concentration factors associated with gapped K-type steel tubular joints. The LJFs calculated were based on the effects of in-plane bending, out-of-plane bending and axial compression and tension. The derivations of the existing LJF equations have evolved in many ways including use of finite element (FE) methods to predict the joint behavior. There has been no benchmarking exercise to large-scale experimental data. This paper provides an improvement on existing LJF equations by benchmarking the Amoco K-joints test results to a FE model and through a detailed parametric study. Improved formulations are provided for local joint flexibilities for gapped uni-planar K-type tubular steel joints.