Free Span Rectifications in Submarine Pipeline Projects: A Case Study

Seabed features along a subsea pipeline route are highly stochastic. Free spans may be created in the pipelines due to seabed irregularities, subsequent scouring, and horizontal movements of pipeline during operation. It is quite common to encounter free spanning sections along the pipeline route from the very start till the end. Spanning of subsea pipelines is a primary area of concern not only in the detailed design and installation stage but also during the operation stage. For ensuring the pipeline safety during operation, underwater surveys must be conducted at suitable intervals. The frequency of such pipeline free spanning surveys depends on the operators’ interest and the statutory requirements. The static and dynamic characteristic of the pipeline spans should be investigated to ensure that the pipeline can be operated within acceptable safety levels. The unsupported spans that incur static as well as dynamic loads on the pipeline, may lead to vortex-induced vibrations and ultimately fatigue, and thus affecting the pipeline serviceability and design life. Vortex induced vibrations are not allowed to occur in the operation life as far as the conventional design is considered but DNV - RP - F105 allows the onset of vortex induced vibrations provided that the fatigue damage due to vortex induced vibrations doesn’t exceed the allowable values. Pipe soil interaction has a huge impact on the pipeline design as well as the pipeline service life. Analysis of the existing conditions and stress levels based on the site-specific surveys and environmental data needs to be carefully carried out for determining the acceptability of spans and the effective intervention works if required. Hydrological studies and numerical modeling may also need to be carried out for sediment transportation analysis and for proper assessment & quantification of sea bed erosion, trenching and backfilling requirements. In the present work, the acceptable criteria in terms of static and dynamic stresses and fatigue damage limits due to vortex induced vibrations as per DNV - RP - F105 have been discussed. Further comprehensive analysis philosophy and the criticalities in the design analysis for free spanning of subsea pipeline are presented. A case study based on an offshore project in western India has been presented involving the major project issues. The main areas of concerns & challenges faced are examined in detail. Further study has been conducted for the other available strategic solutions in the VIV mitigation and rectification of free spanning sections.