A Steel Lazy Wave Riser Installed on Converted and Purpose-Built FPSOs in Omni-Directional and Prevailing Metocean Conditions

Abstract In this paper, we present a numerical investigation into the performance of a steel lazy wave riser (SLWR) in extreme 100-year return and ambient conditions connected to a turret vs. spread-moored system for both purpose-built and converted FPSOs. The aim was to determine the sensitivity of the strength and fatigue life of a SLWR to wind and wave non-collinearity for these two types of FPSOs and two mooring systems. The performance characteristics of several risers (with varied distance between buoyancy modules) were obtained with the aid of the Orcaflex software. For the ambient metocean conditions, there were no real differences in riser’s maximum von Mises stresses and fatigue life between the turret-moored converted and purpose-built FPSOs. The vessel’s heave motions caused the most detrimental compressive loading onto the riser, and these motions were similar for both vessels. The roll motions were greater for the converted vessel, but they did not significantly affect the riser because of its turret connection point on the vessel’s centreline. When the vessels were in a spread-moored configuration, the purpose-built FPSO had an advantage of decreased roll motions, which resulted in greatly reduced riser stresses and fatigue life compared to the converted vessel.