Updated Free Span Response Near the Seabed

Abstract Most offshore pipelines, umbilical’s, and cables installed exposed on the seabed may exhibit free spans. These slender circular cylinder structures, when subjected to external flow, may experience vortex-induced vibrations (VIV), potentially leading to significant fatigue damage. To better understand the seabed’s influence on the VIV response of free spans, the Free Spans in Scour Trenches Joint Industry Project (FIST JIP) was initiated. The first phase of the JIP documented that VIV, both in-line and cross-flow, can occur regardless of the gap between the seabed and the free span. Compared to the VIV behaviour of an isolated circular cylinder, without seabed influence, the in-line VIV is generally reduced; however, the initiation of cross-flow VIV occurs at lower flow velocity, which will lead to cross-flow induced in-line vibrations. Due to these conflicting behaviours, the proximity of the seabed may not necessarily reduce the fatigue damage on a free span. A second phase of the FIST JIP involved modifying the cycle counting approach of DNV-RP-F105. The findings from both phases of FIST JIP will be incorporated into the upcoming revision of DNV-RP-F105. This paper presents application examples of the FIST JIP findings, illustrating the expected effects on fatigue damage for various free span scenarios and highlighting changes from the current version of the recommended practice.