Identification of Interacting Free Spans in a Subsea Pipeline

Abstract

Abstract Free span in pipelines can occur intentionally either due to the seabed topography during laying, or it can be caused by scouring of the seabed during operation. A free span section can either be on its own which is known as ‘single span’ or multiple spans can be in close vicinity and form a ‘multi-span’. The unsupported section (free span) of the pipeline in presence of currents has a potential of causing vortex induced vibration (VIV). In such a scenario, the pipeline can vibrate along the flow direction (inline) or perpendicular to it (crossflow). This is detrimental to the pipeline especially to the welds, as this leads to the accumulation of fatigue damage leading to a reduction in fatigue life. The recommended practice DNV-RP-F105 ‘Free Spanning Pipelines’ [1] lays out the methodology to assess fatigue in free-spanning pipelines. In a multi-span scenario, the spans in close vicinity may interact with each other. The morphological classification plot (release 2006 Figure 1-2) in the DNV-RP-F105 provides means to decide whether the span is interacting based on relative lengths of the spans and intermediate shoulder. In a recent update of the DNV-RP-F105 (2017), this classification plot was called out as inaccurate. Additional guidance was added describing an algorithm to assess whether a span is a single span or an interacting multi-span. In this work the new guidance was used to classify the spans as isolated or interacting for an existing pipeline. The free spans were identified through a survey of the pipeline using multibeam echosounder and sidescan sonar. The multi-spans were identified from the free span listing. For each multi-span multiple finite element (FE) models were developed to compare the modal response of the multi-span against each span member. A comparison was also made with the morphological classification plot to check its validity.