The Development of Allowable Fatigue Stresses in API RP2A

ABSTRACT The results of a calibration study of the new simplified fatigue design procedure adopted in API RP2A are presented. Detailed fatigue analyses were performed on seven structures with different geometries, natural periods, and water depths ranging from 58 feet to 314 feet. The calibration results along with realistic descriptions of the long-term wave climate for different water depths in the Gulf of Mexico were used to develop curves of allowable hot spot stress. These curves will be included in the 17th Edition of API RP2A as new recommended design values. INTRODUCTION The 16th edition of API RP2A (Reference 1) states that template type structures in the Gulf of Mexico with natural periods less than three seconds may be designed for fatigue without a rigorous fatigue analysis. The fatigue design method specified limits the peak nominal brace stress due to the "design environmental loading (wind, wave, etc.)" to 20 ksi and the corresponding cyclic punching shear to 10 ksi. Alternatively, if stress concentration factors are known, the peak hot spot stress should be limited to 60 ksi. These criteria can be traced back to research and development carried out in the late 1960's, with the limit on brace nominal stress first appearing as a design guideline in the 3rd Edition of API RP2A (1971). This fatigue design method is an attempt to predict fatigue behavior using the design wave event. The accuracy depends upon how well the particular platform characteristics and force levels match those of the structures used to calibrate the method. Given the operator's choice in the selection of wave height, wind, and current, there could easily be a factor of two difference in total force level from that used in the calibration. In fact, the original guidelines were developed when the deepwater design wave in the Gulf of Mexico was believed to be 60 feet. Marshall and Luyties (Reference 2) showed that a comparable hot spot stress for a 70-foot wave, which is consistent with the current API reference level wave height, would be 75 ksi. This method has several other shortcomings. It does not distinguish between platforms in different water depths where the relationship between design wave and fatigue wave climate varies, nor between different members within a structure that might be more fatigue sensitive. In addition, the method implicitly assumes an S-N curve and fatigue life, and doesn't allow the operator to select appropriate values. There is also no guidance given for structures located outside the Gulf of Mexico. In 1983, the API Subcommittee on Fixed Structures organized a task group to study the problem of fatigue and propose updated guidelines. This task group, which was comprised of representatives from major oil companies and design consulting firms, accomplished several goals.