The Use of API Simplified Fatigue Design Methodology for Gulf of Mexico Structures
Authors: Luyties W.H. , Stoebner A.M. 
Published: October 2008

Abstract
The results of a calibration study of the API RP2A simplified fatigue design procedure for tripods and other structures in the Gulf of Mexico are presented. A wide variety of tripod waterline geometry and natural periods were examined for a structure in 202.5 feet of water. Results initially appeared to show that these structures were more susceptible to fatigue than their 4- or 8-leg counterparts. Further analyses with 4-leg geometry were performed in order to validate this. However, results showed that there is no significant difference between tripod and 4-leg jacket response. The controlling factor in determining fatigue sensitivity is natural period, with waterline geometry a secondary parameter. Data from the previous API calibration studies were reexamined in light of these results and found to be consistent. Based upon these findings, the recommendation is made to eliminate the current restrictions in API RP2A on using the simplified fatigue design procedure for tripods. In addition, more restrictive fatigue requirements are proposed for all structures.

Introduction
In 1983 API created a task group to revise the fatigue analysis and design recommendations in RP2A. One of the products from the task group was a simplified design procedure for fixed jackets in less than 400 ft. of water and having natural periods less than 3 seconds. The procedure was calibrated by analyzing several 4- and 8-leg structures and was documented by Luyties and Geyer (ref. I). It was included in the 17th edition of RP2A along with an extensive commentary on the method and its applicability. In 1992 several members of the task group were reunited to update the simplified fatigue design procedure to be compatible with changes in design wave loading proposed for the 20th edition of RP2A. The original platforms were reanalyzed, plus a caisson structure was investigated to examine the limiting case of waterline geometry. This work was documented by Luyties (ref. 2) and included in the 20th edition of RP2A. Also in 1992 a different API task group prepared recommendations for minimum structures, including tripods. In the 20th edition of RP2A wording was added that has been interpreted to mean that tripods, as minimum structures, cannot be designed using the simplified procedure, but must be designed using a complete detailed fatigue analysis. Six years later, there are still differences of opinion within the industry on how to interpret the API recommendations. One interpretation of the available data is to assume that if I-leg (i.e. caisson), 4-leg, and 8-leg structures were analyzed and used to calibrate the procedure, then a 3-leg (tripod) structure should fit within the range of data. Another, more conservative, interpretation would be to argue that if a tripod had not been explicitly analyzed, it should not be included, particularly in light of some industry concern over the general robustness of tripods. The current study was devised to fill this gap in data and directly answer the question on applicability of the simplified fatigue design procedure to tripod structures. This paper presents the results of the study, including an evaluation of the effect of waterline geometry on the response of 3- and 4-leg structures.