Digital free span assessment method for subsea pipeline integrity
Proceedings Publication Date
Dr. Linlin Jiao
Linlin Jiao, Kjetil Bergseng
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Free spans on subsea pipelines develop due to various reasons, e.g., seabed unevenness, scouring, installation of artificial supports, crossing and end terminations, etc. Free spanning of subsea pipeline is a typical concern for pipeline integrity with respect to fatigue and local buckling (ULS) capacity. The fatigue herein is mainly induced by dynamic environmental loads due to Vortex Induced Vibration (VIV) and direct wave. The onset of VIV depends on the characteristics of spans, e.g., span length and gap. The span lengths are thus required to be monitored regularly and limited within design criteria. The widely used free span assessment method in industry is still a manual processing, going from simple screening criteria through to advanced calculations for each critical span configuration. Recently, DNV has developed an automated (digital) model for free span assessments which assesses the fatigue damage per KP location along the entire pipe. The advancements in data storage and processing supports automatically accounting for multi-span interactions and changes in free span configurations within entire survey history. So far more than 60 subsea pipelines have been assessed by DNV applying this digital model. This paper introduces the theory background of fatigue assessment in accordance with recommended practice DNV-RP-F105 Free spanning pipelines, outlines the application of digital free span assessment method in pipeline integrity management work, and presents the relevant fatigue results of two representative pipeline cases.

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