A subsea cable stability, seabed interaction, and free span integrity assessment was undertaken for a new fibre optic cable installed in shallow to mid-water depths offshore in the Middle East. The cable forms a critical telecommunications link between onshore infrastructure and multiple offshore installations, replacing an existing system that no longer met current and future operational requirements.
The objective of the study was to assess the on-bottom stability, predict free span development, vortex-induced vibration (VIV), and fatigue performance of the cable under both temporary and permanent conditions. The assessment was required to support installation strategy, define burial and stabilisation requirements, and demonstrate long-term integrity of the cable system in compliance with project-specific acceptance criteria.
The scope of work included the following engineering analyses:
The study considered multiple cable routes, including a long nearshore-to-offshore trunk section and two shorter branches connecting to offshore facilities.
On-bottom stability
On-bottom stability was assessed using specialised cable stability software in accordance with DNV-RP-F109. Both absolute stability and generalised stability (10D displacement) criteria were applied to screen cable performance and identify regions where more detailed dynamic assessment or stabilisation measures may be required.
For the nearshore section, the analyses demonstrated that the cable was unstable in water depths shallower than approximately 50 m when laid directly on the seabed. The cable weight required to achieve absolute stability was significantly greater than the as-designed cable weight, indicating a high likelihood of excessive lateral displacement. Dynamic analysis carried out for selected nearshore regions confirmed that predicted movements exceeded project acceptance criteria.
Based on these findings, burial of the cable was identified as the most robust and operationally efficient solution. It was recommended that the cable be buried continuously along the nearshore section using simultaneous lay and burial where possible, supplemented by post-lay burial in regions with unsuitable soil conditions. Beyond the nearshore region, the cable was shown to satisfy the generalised stability criteria, with more stringent displacement limits applied in the vicinity of offshore facilities. Localised mattress installation was recommended near connection points to ensure minimal movement at tie-ins and to provide additional conservatism.
On-bottom roughness, freespan, and fatigue analysis
The first step was to calculate the acceptable span length, where spans shorter than this length were known to be safe from damage due to VIV. Next, finite element analysis was carried out to predict the length and location of spans along the full cable route. The analysis found that long spans were only predicted to form in the regions where the cable would be buried immediately after installation and therefore failure due to fatigue was not a concern.
The analysis also identified a number of shorter spans that were close to the allowable limit and were in regions where the cable would not be buried. Jee carried out more detailed fatigue analysis that accounted for local environmental conditions and was able to show that these spans were acceptable, saving our client from needing to implement costly post-lay span rectification.
The spans forming between the j-tube exit and seabed touchdown where subjected to detailed fatigue analysis using finite element analysis techniques to quantify the level of fatigue damage in the armour wires. This analysis showed that fatigue damage was low and the cable would easily last the 25 year design life.
Soil analysis
The cable-soil interaction assessment was based on available route-specific geotechnical and geophysical survey data. The seabed was generally flat, with localised gradients associated with seabed features but the soil type was variable along the route.
Shallow soils along the route were characterised predominantly by carbonate sands, with localised clay layers at the seabed in certain sections and rocky outcrops in others. Where route-specific geotechnical data were limited, both drained (sand) and undrained (clay) soil behaviours were considered to ensure a conservative assessment.
Cable embedment, axial and lateral resistance, and vertical stiffness parameters were derived and applied consistently across the stability and free span analyses. Conservative parameter selection was recommended, with the governing soil behaviour used for design and installation checks.
The on-bottom stability assessment demonstrated that burial of the cable along the nearshore section was essential to achieve acceptable performance, with simultaneous lay and burial identified as the preferred installation method. Post-lay burial was shown to be suitable for regions with challenging soil conditions or crossing constraints.
Beyond the nearshore region, the cable was shown to be stable under operational conditions, with only localised stabilisation measures required near facility tie-ins to meet stringent displacement limits. Seabed roughness and free span analyses confirmed that no unacceptable spans would remain during operation and that temporary spans prior to burial were within allowable structural limits. Fatigue assessments demonstrated acceptable fatigue performance at free spans and tie-in locations over the design life.
Overall, the study confirmed that the proposed installation, burial, and local stabilisation strategy provides a robust and practical solution, ensuring the long-term structural integrity and operational reliability of the subsea cable system.
For more information, visit www.jee.co.uk/design
To contact our Head of Design, John French, email john.french@jee.co.uk, or call +44 (0)1732 371 371.