Api Provisions For Scf, S-N, And Size-Profile Effects
Authors: Marshall P.W. 
Published: October 2008

Paper describes basis and use of API provisions for stress concentration factors, S-N fatigue design curves, and size/profile effects, as well as ongoing research efforts. These provisions app[y to either fixed platform or compliant tower design, per either WSD of LRFD versions of API RP 2A. They also provide a point of departure for further AP1-sponsored research on fatigue.

Alpha Kellogg remains a reasonable approximation for classical planar joints, and provides for the multiplanar cases which inevitably arise in design. Existing AWS/APl fatigue design curves have been re-affirmed, when corrected for size effect, following six years of industry-sponsored testing.

INTRODUCTION

Perhaps to a larger degree than with other structural forms, welded tubular connections require an integrated approach to structural integrity. Architecture, design, materials, fabrication, welding, and inspection, must all be considered -- and they are inter-related.

Responsible design includes using more than canned computer programs to dimension the main structural elements. Especially in the connections, the designer must understand the demands he implicitly places on the. materials to be used; and he must anticipate the methods of fabrication and welding, their limitations, and their effects on performance.

Reference 1 covers these issues in more detail than can be done here. However, it is useful to review the several scales at which the analysis of an offshore structure should be considered (see Figure 1):A global stress analysis of the overall structure resolves service loads, wind, wave, and current into nominal axial and bending stresses in the various members. Atypical level of nominal stress for tubular jacket bracing would be 20-ksi (140 Mpa).If we now focus on the tubular connection as a structure, we seethe joint-can experiencing significant shell stresses as it transfers load from one brace to another -- punching shears around the brace footprint on the order of 7 to 10 ksi (50 to 70 MPa), and hot spot stresses (peak shell bending plus membrane stresses at the toe of the weld) of say 50 to 80 ksi (350 to 560 MPa) -- even in well-designed connections.Finally, if we zoom in on the weld for even more localized stress analysis, we find notch effects arising from the weld profile and from weld-toe defects. Although these are responsible for the reduced fatigue Performance of as welded hardware, they are typically not calculated by the designer, but rather built into the empirical S-N curves he uses. However, it is in this region where the size effect arises.

Many design codes treat fatigue design in terms of nominal member stress, and AWS D1.1 also provides criteria in this format. Cyclic punching shear criteria are also given -- one step closer to appropriate understanding of tubular connections. However, hot spot stress is the most useful criterion, bringing many diverse connection geometries into a common design basis. Stress concentration factors (SCF), derivable from finite element analysis or strain gages, provide the designer the means to calculate hot spot stress.