Strain-Life Fatigue Analysis for HPHT Equipment: Theory to Validation

Subsea equipment covered by the API Spec 17 subcommittee has had limited focus on assessing fatigue life because of external environmental loads using traditional analysis methods. With the current trend of high-pressure, high-temperature (HPHT) development, the industry is migrating to an era of modern analysis methods with complex material testing programs to assess potential fatigue life impacts due to such high-pressure and -temperature exposures as well. This paper presents an approach and an example of a multiaxial strain-life analysis method that meets the provided HPHT design guidelines of API Technical Report 17TR8. The paper bridges the gap between theory and practicality in strain-life-based fatigue analysis and presents a robust process developed for HPHT nickel alloy components, which are part of the subsea 20,000-psi vertical monobore subsea tree. The endeavor includes strategizing for required material tests in environment, actual material testing, followed by material data processing, which includes statistical corrections and extraction of parameters necessary for efficient fatigue analysis. The components are then analyzed in finite-element analysis (FEA) with typical loading sequences as seen in its life of field. Finally, the FEA results are postprocessed using the critical plane approach for all nodes in the model. The governing equations are presented throughout the analysis to enable readers to develop their own results. The 20,000-psi vertical monobore tree fatigue analysis depends on the operations forecasted for its life cycle. Using the expected load histogram, a series of pressure and thermal analyses were executed to produce cycles to failure. Implementing the Palmgren–Miner's rule enabled obtaining the total damage produced by factory acceptance tests total field life shut-ins, and flow-in pressure cycles. This not only serves as verification that the required safety factor is met per API Technical Report 17TR8 but also enables making engineering assessments of "what-if" operations. In this sense, a change or addition of an operation will lead to a simple recalculation of fatigue damage without requiring performing the analysis from the ground up. The method also allows for computation of cycles to failure for a pressure range when the other pressure ranges and conditions don't change. In addition to the life cycle calculation, the method evaluates the damage of all nodes, which produces full-contour plots. The contour plots, in addition to displaying the hot-spot locations, when used with structural analysis results, enable the engineer to assess areas of improvement and product optimization. The method proposed gives an effective way to communicate and recommend the design life capabilities of a product to the operator to predict life expectancy for combinations of expected load scenarios.