Free Span Rectification by Pipeline Lowering (PL) Method

Surface laid pipelines experience free spans due to various reasons such as uneven seabed, local scour or a storm event. If free span lengths are beyond acceptable limits, vortex induced vibrations (VIV) can cause the pipeline to undergo fatigue damage and severely reduce the pipeline's design life. Thus pipelines need to be periodically monitored to ensure that there are no free spans that are beyond the acceptable length. When such free spans are identified, these need to be rectified immediately to avoid fatigue damage to the pipeline. Rectifying unacceptable free spans in existing pipelines are a constant battle for operators around the world. Free spans on existing pipelines are commonly rectified by grout bags. While this method is considered to be a standard and cheap solution, it often does not provide a long term solution to free spans. It is often the case that the pipeline whose free spans are rectified by grout bags needs further free span rectification the following year. The onset of more free spans and the need for rectification cycle often continues annually when grout bags are used to rectify free spans. This paper presents a new approach named the "Pipeline lowering (PL) method" for free span rectification. This unique solution will ensure the free span rectification is a long term solution compared to grout bags that may be affected by scour and wave loadings. The solution involves lowering the crests of the free spans such that the pipeline follows the natural seabed profile while ensuring the pipeline integrity is not compromised at any stage. The lowering operation is carried out solely by fluidizing the seabed soil by a mass flow excavator and the pipeline lowers under its self-weight. A laboratory demonstration and successful field implementation of PL method for free span rectification are presented in this paper. This paper provides vital technical knowledge on free span rectification by PL methodology for pipeline engineers, contractors and operators who need to ensure free spans are rectified safely and efficiently. This new methodology is a cost effective and elegant solution.