Strength and Stiffness of Tubular Joints for Assessment/Design Purposes
Authors: Dier A.F. , Lalani M. 
Published: October 2008

Tubular joints are integral components of offshore structures and, as such, many codes address their design. The strengths predicted by the codes often differ widely due to the adoption of different philosophies during code formulation and to differences in the underlying databases used during curve fitting and calibration exercises. In the assessment of existing structures, a hierarchy of analysis methods, from elastic analysis with improved code checks to nonlinear 3-D pushover analysis, may be employed. To avoid potentially unnecessary and expensive offshore strengthening work, it is important that these assessments are based on best estimates of joint strength (mean or characteristic, depending on application) and joint stiffness behavior.

This paper gives the background to and interim results of a major international joint industry project in this area. Based on the most extensive carefully screened database ever assembled, the robustness of present design formulations have been examined, and new formulations have been developed where present codes are found wanting. Other issues, such as chord load, can length and material yield to ultimate strength have also been dressed. For the first time, expressions have been developed for P-? and M-? joint stiffness response, as may be used in advanced analysis including the simulation of joint behavior during pushover analysis. The paper goes on to describe the expected future developments in this field of technology.

INTRODUCTION

The continuing need to demonstrate the structural integrity and safety of offshore steel platforms requires that they are subject to periodic structural analysis and assessment. Initially, a linear elastic analysis for extreme event loading is conducted for the jacket structure, modeling the actual condition of the platform (as-built plus all subsequent modifications). The joint components are subjected to code checking (to API RP2A(1) or HSE Guidance Notes(2), for example) or specific assessment depending on whether the joints are undamaged or damaged. In a number of instances, joint utilisation ratios may exceed unity, as a result of increased applied loads or code updates, for example. In these cases a hierarchy of analysis methods, of increasing complexity, can be adopted to assess with greater accuracy the integrity and adequacy of the structure under consideration. This hierarchy can be defined as follows:Elastic analysis with improved code checks for joints.Elastic analysis with modeling of joint rotation for the joints in question. Code check to API RP2A.As (b), but with improved code checks for joints.As (b), but with full modeling of joint nonlinear behavior, uncoupled and coupled.As (d), but with improved code checks for joints.System reliability, utilizing one of (a) to (e) above for characterizing joint and system resistance parameters.2-D pushover analysis (non-linear joint and member behavior).3-D pushover analysis.System reliability, utilizing (g) or (h) above for characterization.

The above hierarchy is nominal, and may be (and often is) adjusted depending on the structure under consideration and the findings from preceding analyses. This is particularly valid for options. Two specific technical issues become clear from the above analysis methods.