Abstract

Since the early 1990s, API sponsored a series of research projects to develop advanced formulation for design of non-overlapping tubular T, Double T (DT-X), and K offshore platform joints (API RP2A Upgrade Plan, 1990). The University of Illinois at Urbana-Champaign, sponsored by API, developed nonlinear finite element models and verified them against available test results. These models were then used to simulate a wide variety of geometries and static loading conditions, establishing a broader database than available test results. Advanced closed form parametric formulations were developed and verified against these analytical results to reduce scatter and generate more reliable formulation than provided in the API RP 2A WSD 14th through 21st Editions. One significant feature of the new static design formulations is better representation of the chord load and geometry effects and the boundary conditions that are not possible with physical testing (Pecknold 2000, 01, 02, 05).

The Task Group also evaluated and upgraded the tubular joint fatigue design procedures. The welded joint X and X' fatigue design curves in the RP 2A Editions 11 thru 21 have been replaced by a basic SN curve with a slope m=3 that changes to 5 at ten million cycles. Fatigue life correction factors for seawater, thickness, and use of weld profile control, grinding, and peening have also been introduced. In addition, the comprehensive Efthymiou 1988 equations replaced the Alpha Kellogg Stress Concentration Equations. The Simplified Fatigue Design Procedure has been maintained, with the allowable peak hot spot stresses rechecked for the new SN curve. SN curves and fatigue design requirements for cast nodes and grouted joints have also been introduced.

Introduction

The API tubular joint static design technology has been under continuous development since the first edition of the API RP 2A in 1969. In the 3rd edition of API RP2A, issued in 1972, some simple recommendations were introduced based on punching shear principles (Marshall, 1974). In the 4th Edition, factors were introduced to allow for the presence of load in the chord and the brace-to-chord diameter ratio (Beta =รณ). In the 9th edition, issued in 1977, differentiation was introduced in the allowable stress formulations for the joint and loading configuration i.e. T/Y, X, and K. In the 14th Edition, the punching shear stress formulations were considerably modified and included a more realistic expression to account for the effect of chord loads as well as providing an interaction equation for the combined effect of brace axial and bending stresses. Also introduced in the 14th Edition was the alternative nominal load approach, which gives equivalent results to the punching shear method (Yura, 1980). The static strength guidance then essentially remained unchanged for all editions up to the 21st, although further recommendations were added on load transfer through the chord in the RPRA Edition 20 issued in 1993. No tubular joint revisions were made since then, until now.

Much further knowledge, including both experimental data and numerical studies, has been gained on the behavior of joints since the API RP 2A 14th Edition.

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