Fatigue Design and Performance Verification of Deepwater Risers

Abstract

With the advent of the development of deepwater projects, ExxonMobil developed and successfully implemented a fatigue design and verification protocol for fracture-critical components, such as risers and tendons, to ensure design performance and reliability. This protocol has now become an industry practice. This paper discusses the analytical, fabrication, and testing aspects of the design process. The linkage among actual weld performance, welding procedures and inspection reliability is addressed. From the design implementation standpoint, reliability of the fabrication inspection is the key issue. Practical methodologies were developed to conduct and interpret the fatigue tests. In particular, specimen design, instrumentation, testing protocols, and postmortem examination are discussed. Data generated by testing 56 full-scale risers of various sizes and welded by different procedures are also presented. These data, including tests past 100 million cycles, show that (1) actual riser fatigue performance can be substantially better than that recommended by codes, (2) failures can occur in the long-life regime, and (3) fatigue performance varies with riser size and thickness. However, as a matter of practice, analyses, fabrication and testing are required for particular designs.