A steel lazy wave riser for turret vs. Spread-moored FPSO in extreme and wave-induced fatigue conditions
American Society of Mechanical Engineers (ASME)
School of Engineering
Here, the performance of a steel lazy-wave riser (SLWR) was numerically investigated and compared to a conventional steel catenary riser (SCR) for an internal turret and spread-moored FPSO in extreme 100-yr return and ambient metocean conditions. As expected, the SLWR demonstrated favourable characteristics by the elimination of local dynamic buckling in the touch down zone (TDZ) via a wave configuration that decouples surface-motion, hence significantly reducing the maximum stress and fatigue damage. Non-collinear waves and current in the oblique direction relative to the vessel heading showed amplified stress concentrations in the sag and hog sections as a response to the increased compressive (heave and pitch) vessel motions. Correspondingly, in the ultimate limit state, a SLWR showed similar stresses for both turret and spread-moored FPSOs. However, the turret-mounted SLWR demonstrated a less superior fatigue life compared to its midship-mounted, spread-moored counterpart, owing to riser hangoff location differences and correspondingly varied motions experienced by the riser.