Modeling the Effects of Steel Catenary Riser (SCR) Wall Thickness on Its Structural and Fatigue Performances for Wave-Induced Load

Abstract

The Steel Catenary Riser (SCR) exhibits significant bending and oscillation owing to undue vessel (which the riser is attached to) movements. The prevalent hydrodynamic loads (wave, current, hydrostatic pressure) are the leading causes of this structural behavior. These initiate a motion - related fatigue phenomenon. This study evaluates the feasibility of increasing the SCR Wall Thickness (WT) for better fatigue performance and structural responses when subjected to the dominant wave spectrum (Ochi Hubble) in deep water West Africa. Relevant data were acquired from an offshore facility in the study area and managed empirically through the use of marine structural analysis software- ORCAFLEX®. Following Det Norske Veritas (DNV, 2010) methodology, 4 SCRs of uniform Internal Diameters (ID) were designated (SCR1, SCR2, SCR3, SCR4), modeled with different WTs (0.0626ft, 0.1876ft, 0.3126ft, 0.4376ft) and analyzed under eight wave load cases. Before the fatigue assessments, the systems were analyzed for static and dynamic responses. Results showed that increasing the WT increases the Fatigue Life (FL) only along each riser arc length - from top to bottom. It was also observed from the systems’ structural response analyses that the effective tension at the Hang off Point (HOP) increases with increase in WT and a compromise in the capacity of the SCRs to flex alongside the hydrodynamic loads due to cumulative rise in the weight of the system. This eventually lowered the FL at the HOP below the DNV’s minimum FL for design purposes – 200 years.

Keywords: Steel Catenary Riser, Wall thickness, Fatigue Performance, Ochi-Hubble, Structural response.

DOI: 10.7176/ISDE/10-6-03

Publication date:July 31st 2019