Through a number of recent projects, and in line with a growing need to understand the effect of fatigue on cable service life, Mojo Maritime have successfully developed and used a number of cable analysis techniques. This includes:
- Dynamic Cable Analysis
- Static Load Analysis
- Seabed Stability Analysis (for energetic sites – wave and tide)
- Route analysis and planning
Dynamic Analysis of Free Spans
A detailed model of the cable span was created in Rhino. This was then imported into ANSYS Mechanical and a finite element structural analysis was then performed on the maximum dynamic span to assess the likelihood of the primary failure mode and provide an indicative service life. The different cases for hydrodynamic loading were determined from the wave and tidal data for the installation site, with the three cases combining an increased likelihood of occurrence and most detrimental forces to the model. Once all three load cases had been analysed, a cumulative damage assessment as per DNV was performed and a service life estimated.
Free Hanging Cable Analysis 1
This work used a collection of detailed environmental data to determine local flow conditions around an unsupported section of high voltage, subsea cable. A detailed structural analysis was subsequently carried out to assess the effect of the hydrodynamic forces on the free hanging cable span, principally with respect to fatigue failure of the lead alloy sheath and subsequent water ingress which could impair cable performance. The deflection of the cable under load, and the associate stresses and strains were used to assess the fatigue life of the cable and using DNV regulations cumulative damage was evaluated. Ultimately a fatigue damage rate and predict a service fatigue life for the lead sheath of the cable was generated.
Free Hanging Cable Analysis 2
In this situation a high voltage export cable had two considerable spans that had become exposed due to scour action. The length of exposed cable alters year upon year, and as such several scenarios required assessing for potential damage and overall predicted service life. The cable was located in shallow water and a strong tidal regime. The metocean data was analysed and the greatest forces calculated. These loads were applied to a range of free spans covering the extremes of the exposure noted in recent years and a fatigue damage rate and predict a service fatigue life for the lead sheath of the cable at each location. Assessment of whether remedial action, such as rock dumping, would be required was also considered.
Subsea Cable Stability Analysis
A new method of analyzing the stability of subsea cables was developed for assessment of cables being deployed in a strong tidal race. The method coupled fluid dynamic analysis with structural cable assessment to provide a holistic picture of the loads, and resulting displacements, for subsea cables in aggressive conditions. The method takes into account the sheltering effects of the natural bathymetry as well as the mechanical properties of the cable/seabed. The factor of safety is calculated along the cable route and compared to that prescribed by DNV. Any necessary remedial actions can then be assessed and quantified under extreme wave conditions in order to provide protection of the cable.
Subsea Cable Installation Planning and Routing
This work involved a cable route desktop study for various project phases which studied to categorise different areas that would create different levels of shielding from waves and current along the cable route determined utilising the bathymetric features. A route was determined following the local features providing sheltering with regard to the tidal velocities observed on the cable.