Study on fatigue damage optimization mechanism of deepwater lazy wave risers based on multiple waveform serial arrangement

Abstract

Lazy wave risers can mitigate the kinematic coupling between floating structure and riser touchdown zone (TDZ), so as to improve its fatigue life. However, due to the introduction of buoyancy blocks, nonlinear dynamic response and fatigue damage prediction will be more complicated. In this paper, a concept of “multiple lazy wave riser”, namely, deepwater lazy wave riser configuration based on multiple waveform serial arrangement, is proposed. According to the improved slender rod theory and equivalent optimization of buoyancy sections, three riser configurations with different heave excitations and currents are structured. The dynamic response characteristics, excitation motion transfer paths and fatigue damage hotspots are well compared. Moreover, the influence law of riser fatigue damage and decoupling mechanism of riser TDZ movements are also explored. The results indicate that: by constructing lazy wave configuration, deepwater risers based on lazy wave configuration can attenuate the kinematic coupled response of floating structure in the first sag bend section, and reduce the normal motion response in the riser TDZ, so as to optimize its anti-fatigue performance; compared with lazy wave riser, multiple lazy wave riser has some competitiveness in the anti-fatigue performance for the riser TDZ. For flexible risers, this competitiveness will become even more prominent.