An Update On Allowable Fatigue Stresses In Api Rp2A

ABSTRACT The results of a recalibration of the simplified fatigue design procedure for tubular joints using the new design wave hydrodynamic criteria in the 20th edition of API RP2A are presented. The four structures used in the original calibration study are examined with the new criteria and revised curves of allowable peak hotspot stress for the Gulf of Mexico are derived. As a first step in potentially expanding the simplified design procedure to other oceanographic climates, results of a calibration study are presented for a structure designed for a wave climate outside the Gulf of Mexico. Also, to provide justification for new API requirements for minimum structures, calibration results are presented for a self-standing caisson structure in the Gulf of Mexico. INTRODUCTION The 17th edition of API RP2A (reference 1) provided curves of allowable hotspot stress versus water depth to be used in simplified design of tubular joints for fatigue. Given a platform water depth, design fatigue life, location of member within the structure, and S-N curve, an engineer could determine the appropriate allowable hotspot stress. This allowable stress would be used in conjunction with the peak hotspot stress from the fatigue design wave to design joints and would result in generally conservative fatigue lives when compared to those determined from a full detailed fatigue analysis. The fatigue design wave was defined as the API reference level wave applied with tide and marine growth, but with no gravity load effects, wind, or current. A key factor in the accuracy of the simplified design method is the magnitude of wave force generated by the fatigue design wave. In the 20th edition of API RP2A (reference 2) the design wave force criteria were upgraded to incorporate new data on wave heights, currents, drag coefficients, current blockage, and wave directionality. In general, the entire "recipe" used to define the design wave changed. It was recognized that such a change might alter the relationship between design wave and allowable hotspot stress. Four courses of action were available to API:do nothing and accept an unknown effect on simplified design,leave the 17th edition reference wave height in the new edition as the fatigue design wave,abandon the simplified method within RP2A, orrepeat the calibration effort to determine new allowable hotspot stress curves to use with the new wave recipe. Option 1 was deemed unacceptable. Option 2 would result in two design wave criteria within the same document and was judged to be undesirable and potentially confusing. Option 3 would require detailed fatigue analyses of all structures, which was felt to be an unnecessary added cost and effort. Therefore, in 1991 a task group was formed to recalibrate the simplified fatigue method and propose new allowable hotspot stress curves for inclusion in the 20th edition of API RP2A