Structural integrity management and improved joint flexibility equations for uni-planar k-type tubular joints of fixed offshore structures

Abstract

The distribution of fixed steel offshore platforms around the world reveal a global fleet that has exceeded or is approaching the end of the 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 life extension of the fixed platform, various integrity assessment techniques are often used. One of the major techniques incorporated is the phenomena of Local Joint Flexibility (LJF). The derivations of existing LJF equations have evolved in many ways, including use of finite element methods to predict the joint behaviour. There has been insufficient credible benchmarking to large scale experimental test data. In the early 1980s, AMOCO performed the only large scale test results of LJF which, prior to this research, has not been in the public domain. A major objective of this research is to develop a suite of improved LJF equations that have been appropriately benchmarked to large scale tests. In addition, with the issue of the API RP 2SIM (2014) 1st Edition and the development of the ISO 19901-09 SIM (DIS), this research also provides a basis for further Asset Life Extension (ALE) of an aging fixed offshore platform in terms of ultimate strength by using an improved suite of LJF equations. Furthermore, the research puts the structural assessments such as LJF in the context of a structural integrity management framework, which enables operators to manage their facilities holistically rather than isolated processes. The research within this thesis critically examined the suitability of the existing LJF equations, reviewed the guidance provided in the existing studies and described their limitations for gapped K-type tubular joints. A comparison study and benchmarking study demonstrated that a proposed finite element model provides a good fit with large scale experimental data (AMOCO) and was used to develop a suite of improved LJF equations for gapped K-type tubular joints. The LJF equations derived from this research were validated against the BOMEL large scale structural frame tests in terms of ultimate strength and demonstrated an improvement on the current MSL-1SO equations for uni-planar K-type tubular joints in the ISO 19902.2007 Fixed Offshore Structures code of practice. This research also provides a basis to update current offshore structures codes and standards for uni-planar gapped K-joints and also provide a standardized methodology for the derivation of LJFs from credible large scale test data for other tubular joint configurations including multiplanar K-joints, T-joints, Y-joints and X-joints. The LJF equations developed in this research will have high impact in terms of the structural integrity management of fixed offshore structures for OGPs globally, as they provide an improvement to the current MSL-ISO joint equations, for gapped uni-planar joints. Offshore structures are now able to operate more safely without compromising structural integrity and incurring costly underwater repairs and inspections as before. OGPs are now able to prioritize limited resources to other areas of concerns based on ALARP principles.