Fatigue Design and Analysis for Welded Tubular Joints: Results from the API Offshore Tubular Joint Research Center
Authors: Mohr William  C. , Dimitrakis Stamati , Lawrence Frederick  V. , Tsai Chon , Tso Chin Ming , Kirk Mark 
Published: October 2008

The fatigue performance of tubular connections can limit the life of offshore constructions. Fatigue research for the API Offshore Tubular Joints Research Center focused in its first three projects on fatigue design. An examination of design rules in API RP2A and equivalentstandards as well as the basic data has defined several areas where API RP2A can be updated to include new design curves and explicit mess of determining hot spot stresses. In addition, the several suggestions have been made that would introduce new provisions, such as inclusion of other weld improvements besides grinding and a different method of including the effects of cathodic protection in sea water .

INTRODUCTION

The API established the Offshore Tubular Joint Research center to Contribute research result at ongoing efforts to improve the Provisions of API RP 2A'. The first three projects of the center involve the fatigue design and analysis, since the fatigue provisions of API RP 2A are significantly different from other national codes and standards. The projects addressed stress concentration factor determination, S-N results for tubular joints, thickness effect, weld profile effects are post-weld fatigue improvements. The projects collected and analyzed available experimental data to allow suggestions for fatigue design and analysis of tubular joints. These suggestions have been provided both to API and ISO working groups to define the basis for any changes in fatigue provisions as API RP 2A is converted to an ISO document. This paper summarizes the results and suggestions of the fatigue projects of the API OTJRC.

STRESS CONCENTRATION FACTORS (SCF'S) FOR TUBULAR CONNECTIONS

EWl, the University of Illinois, and the Ohio State University have Conducted a review of the stress concentration factors which are used to define the stress range applicable to tubular connections under fatigue loading.

A hot spot stress (HSS) is a fictitious stress derived by extrapolating the stresses away from the welded joint into the toe of the joint. The HSS method allows the geometrical stress concentration at the welded connection to be included in the stress, without including the focal effects of the stress concentration of the weld toe itself. These methods were refined for tubular joints by the ECSC groups working on fatigue of tubular joints in the late 1970's. The position a of strain gauges to be used in defining the hot spot stress are shown in Figure 1.

A stress concentration factor (SCF) is a multiplying factor on the brace nominal stress which turns the brace nominal stress into a hot spot stress. The hot spot stress is defined as occurring at either the brace or chord weld toe, so different SCFS will apply at these two locations. In addition, for simple loadings the crown and saddle positions n the weld will have different CFS.

Stress concentration factors are functions not only of the geometry of the tubular joint, but also of its mode of loading. For a T joint, three modes of loading can be introduced through the brace: axial tension, in-plane-bending, and out-of-plane-bending.