Understanding of cable stability in deep, wide trenches. Partners sought.

AOE & UWA will be launching a new joint industry project in 2025. This project will advance the understanding of cable stability in deep, wide trenches. Existing industry guidance in this area not useful for wider, deep trenches.

Stability of offshore wind turbine cables in deep & wide trenches

Offshore wind is reliable source of renewable energy. Wind turbine subsea power cables are often required to be installed into pre-cut trenches, especially in stiff clays, clays with embedded rocks & boulders, seabeds subject to natural backfilling, & cemented carbonate soils. Hydrodynamics & stability of the cables within these trenches fall outside of the parameters developed for oil & gas pipelines. This project will explore the flow characteristics within trenches, hydrodynamic forces on the cables, & cable-trench interactions. Through experiments & numerical modelling; design approaches will be developed, which will improve reliability & integrity of the cables & reduce installation & maintenance costs.

 Pre-dredged shore-crossing trenches

To increase the on-bottom stability of the cables & aide in the post-installation trenching & burial of cables through the shore-crossing, it is not uncommon for trenches to be pre-cut into the seabed. A previous AOE project example is shown below, varying from around 2 m to about 5 m below LAT. In this location, tidal levels vary by ~ 5 m. The marine environment in this location is characterised by periodic severe storms which result in breaking waves across this shore crossing, with the most onerous headings roughly orthogonal to the trenches. There exists a long-shore current of up to 0.5 m/s. The trenches varied between 2.2 m & 1.7 m deep & 5 m to 3 m wide at the base. These dimensions ignore the adjacent spoil berms & any backfilling of the trenches.

Post Lay Cable Stability

It is common for cable corridors to be pre-trenched, to prior to cable lay. E.g., this is common in stiff clay seabeds, where post-lay burials may be problematic. The trench may be subject to natural or constructed backfill, or if the external cable risk is low, the cable may be left exposed in the base of the trench. In the former cases, the interim stability of the cable requires assessment, most likely for a 10-year return period event, prior to trench back-filling. Where the trench may not be backfilled, the cable must be stable for the operational condition, most likely a 100-year return period event.  If the seabed material contains rocky material, then this needs to be taken into account. AOE demonstrated the presence of rocks in a precut trench can be beneficial to cable stability.

Existing Guidance

The globally recommended practice for on-bottom stability analysis of cables, small diameter pipes & conventional hydrocarbon pipelines is DNV-RP-F109 (2021). The latest revision specifically claims the guidance to be relevant & applicable to seabed cables & umbilicals, albeit without formal demonstration of this applicability. The approach adopted is to define the hydrodynamic forces of a pipeline as though it was at rest & placed flush on the ‘smooth’ surface of the seabed. These hydrodynamic forces are then factored to account for the presence of a trench as long as:

• The trench sidewalls have slope between 5 & 45⁰

• The depth of the trench is taken at a maximum of 3D away from the pipe

• It is not explicitly stated, but implied that the maximum ratio of trench depth to pipe diameter is one.

The guidance is based on research programs looking at the benefit to on-bottom stability due to the placement of pipelines into pre- or post-lay trenches. However, the range of trench profiles considered in the literature do not encompass or provide sufficient guidance for the subsea cables industry under the project conditions described herein. What is apparent is the type of trenches used for cables are far deeper & wider than those considered in existing industry guidance and that these types of trenches used for cables may provide substantial hydrodynamic shielding for the cable, with greatly improved associated on-bottom stability.

Cables on Rocky seabeds

The deep wide trench research may also be applied to assessing the improved stability of cables laid on rocky seabeds. In this instance cables are found in naturally occurring deep/wide trenches when they are installed across rocky seabeds. This can occur as in the first example below where the route has preferentially targeted the cable installation to benefit from lay into a natural ravine which runs roughly orthogonal to the governing tidal currents, or below where the cable has moved from its as-laid position due to localised instability & has been found to be located partially spanning within a natural deep/wide ravine.