Gulf of Mexico Hurricane Single Event Fatigue Method for Riser Analysis

Abstract

Deepwater steel catenary risers (SCRs) are designed considering strength under extreme loads and long-term fatigue of the riser from installation through the full range of conditions anticipated over the service life of the riser. Short-term single event fatigue is also checked to ensure that a single extreme storm or current event does not consume an excessive amount of fatigue capacity. For Gulf of Mexico SCRs, single event fatigue can be a governing case for the riser hang-off. The requirement imposes a relatively modest number of higher stress range cycles and may bias the selection of the hang-off option (titanium, steel, and flex joints) since the three common choices have widely different sensitivity to this case. Assessing a single event hurricane is problematic because the evolution of storms which generate an extreme 100-year significant wave height at a site can vary considerably from storm-to-storm. Simplistic representations of the extreme hurricane event could lead to either non-optimal hang-off option or a unconservative design. This study documents a response-based analysis simulating fatigue damage accumulated in differing risers over a broad set of extreme hurricanes. The extreme hurricanes are gathered from hundreds of years of hindcast storms to develop a statistical assessment of the amount of damage accumulated per event. Since the response-based analysis would be cumbersome in the design process, a simplified method benchmarked to the response-based analysis is proposed for design. The simplified method consists of accumulating fatigue damage from multiple seastate cases that are already developed to assess riser strength.