Subsea pipelines are susceptible to free span development due to the natural variability and continual movement of seabed topography. Over a pipeline's operational life, seabed conditions can change as a result of sediment transport, scour, shifting sand waves or seabed subsidence. These processes can gradually create or increase unsupported sections of pipe. Areas where pipelines cross uneven terrain, mobile seabed features or regions exposed to strong hydrodynamic forces are particularly prone to free spanning.
Excessive span lengths can lead to increased bending stresses, vortex-induced vibrations (VIV) and, ultimately, structural fatigue. Routine monitoring and assessment are therefore essential to ensure spans remain within the pipeline's maximum allowable limits and to maintain long-term integrity.
The client engaged Jee to undertake a detailed maximum allowable span analysis of a known set of interacting spans. The objective was to evaluate the pipeline's mechanical behaviour, understand how the spans influence one another and determine whether the existing configuration remained within acceptable design and operational limits. The assessment would provide a basis for informed integrity management decisions and any necessary mitigation measures.
Solution
Finite element analysis (FEA) was used to model individual and interacting spans along the pipeline route. This enabled the natural frequencies of the spans to be determined, providing a more accurate understanding of their dynamic behaviour under vortex-induced vibration (VIV). The analysis was then used alongside calculations in accordance with DNV-RP-F105 to establish the maximum allowable span lengths.
The assessment identified several areas of unsupported pipeline where seabed movement and hydrodynamic loading had contributed to spans exceeding recommended limits. If left unaddressed, these sections could experience increased stresses and a greater risk of fatigue damage over time.
While a small number of spans were found to be acceptable, most required intervention to safeguard the pipeline's long-term integrity. Jee recommended introducing additional seabed support, such as rock placement or other stabilisation methods, beneath the affected sections to reduce unsupported lengths and improve stability.
For locations where immediate remediation was not possible, ongoing inspection and monitoring were also recommended to identify any further deterioration and ensure continued pipeline integrity.
For more information, visit www.jee.co.uk/integrity-management
To contact Jee’s Technical Authority, Graham Wilson, email Graham.Wilson@jee.co.uk.