Managing Pipeline Integrity and Dynamic Free Spans on Mobile Seabed in the Southern North Sea

This paper describes the continuous improvement efforts to manage the integrity status of the Southern North Sea subsea pipeline system in the context of free spanning. The dynamic free-spanning threat is typically attributed to a mobile seabed. Current and wave action are constantly moving and eroding sediment by means of sand wave migration and scouring. It can lead to a fluctuation in span characteristics with respect to span length, span height and location over time. It makes pipeline integrity demonstration and spans remediation challenges. Focus areas include (1) identifying regions where operational pipelines are susceptible to critical span formation (2) understanding the broader context of seabed mobility, supported by several years of multibeam echo sound and met ocean data (3) risk-ranking & criticality of span formation (4) developing simplified calculation tool that allows fatigue damage to be estimated and accumulated for every location along the pipeline, conservatively (5) optimising and incorporating risk/event-based survey requirements (6) identification of suitable remediation solutions and developing a decision flow chart to facilitate selection of fit for purpose remediation solutions, with respect to span configuration and the surrounding seabed features. The outcome has improved the robustness of span management, reduced “reactive” span remediation activities, and allowed application of sound technical theory to allocate pipeline traffic light integrity status regarding the observed free spans.