Cost Effective Free Span Rectification for Offshore Pipelines

Abstract

Offshore pipelines often experience free spans due to uneven seabed, local scour or a storm event. When free span lengths are beyond acceptable limits, vortex induced vibrations (VIV) can cause pipelines to undergo fatigue damage and severely reduce the pipeline’s design life. Therefore, surface laid offshore pipelines are periodically surveyed to ensure that there are no free spans that are beyond the acceptable limit. When such free spans are identified, they are rectified immediately, often by grout bags. While grout bag use is a standard solution for free span rectification, it does not always provide a long-term remedy to free spans. Often lines 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. This paper presents a cost effective long-term free span rectification method known as “Pipeline Lowering.” This unique solution will ensure the free span rectification is a long-term fix compared to grout bags that may be affected by scour and wave loadings. This paper provides vital technical knowledge for pipeline engineers, contractors and operators who need to ensure free spans are rectified safely and efficiently.