Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow Waters

Abstract

In-line and Transverse Vortex Induced Vibrations (VIV) pose potential Fatigue damage threat to Vertical Rigid Risers (VRR) even in the less volatile Nigeria Shallow Waters. In this paper, a typical VRR of 31.031m length clamped to a fixed jacket platform in 18.29m water depth was used while relevant metOcean data were used to simulate the environmental conditions. The process was statically and dynamically simulated using different wave spectra on Orcaflex platform. The results from JONSWAP spectra showed a fatigue damage value of 102.5 x 10-5 due to in-line VIV which is greater than 29.1 x 10-5 due to transverse VIV. The results from the Ochi-Hubble spectra indicate a fatigue damage value of 96.2 x 10-5 due to in-line VIV which is greater than 7.03 x 10-5 due to transverse VIV. Also, the in-line vortex force (VF) analysis on the VRR for the JONSWAP spectra showed VF range, beginning from the touchdown point (TDP) to the unstraked region (UR), of 0.16 - 0.34kN/m; 0.01 - 0.53kN/m; and 0 - 0.85kN/m at End A (0m); 8.74m and 21.00m, respectively. Whereas, for the Ochi-Hubble spectra, VF range, from TDP to the UR, of 0.16 - 0.302kN/m; 0.018 - 0.56kN/m; 0 - 1.42kN/m at End A (0m); 8.74m and 21.00m, respectively, were obtained. The results for both spectra showed zero in-line VF on the VRR at 23.69m, 26.21m and End B (31.031m). Hence, fatigue damage and VF due to in-line VIV is important from the TDP to most parts of the UR on the VRR irrespective of the wave spectra and requires proper analysis in riser designs.