Introduction: Why Subsea Cables Matter More Than Ever
Subsea cables are the silent backbone of the modern world, carrying everything from renewable power to internet data beneath our oceans. These underwater arteries connect offshore wind farms, onshore grids and continents. They are a critical part of our energy and communication infrastructure. Global Net Zero targets and digital interconnectivity are increasing demand for subsea cables. Their engineering and geotechnical design must therefore become more resilient and efficient.
The length of subsea cables varies depending on their function and geographical reach, ranging from a few hundred metres to several thousand kilometres. For example, the Xlinks Morocco–UK Power Project is set to become the world’s longest subsea power transmission link. This ambitious project will lay four high-voltage direct current (HVDC) cables, each approximately 3,800 km long, to deliver renewable energy generated in Morocco directly to the UK.
In the realm of data transmission, systems such as SEA-ME-WE 6 (Southeast Asia–Middle East–Western Europe 6) demonstrate the scale and complexity of modern communication infrastructure. Stretching approximately 21,700 km, SEA-ME-WE 6 connects multiple countries across three continents, supporting high-capacity internet and telecommunications traffic between Southeast Asia, the Middle East, and Western Europe. The interactive TeleGeography submarine cable map shows just how extensive these cables are and how they literally wrap around our world! (https://www.submarinecablemap.com)
Geotechnical Challenges
Designing and installing subsea cables involves unique, complex geotechnical challenges. These cables often extend over hundreds or thousands of kilometres. They cross variable sediment types that are continuously eroded and redeposited. They may also pass through geohazard-prone areas, such as subsea landslides and migrating sand waves.
Poor understanding of ground conditions results in these hazards being missed resulting in an ineffective design, which is one of the top causes of project delays and cost overruns. During operations, an ineffective design may result in undesirable insurance claims. In fact, 75–80% of subsea cable insurance claims relate to failures such as anchor strikes, poor burial and overheating. These failures can be linked to poor understanding of ground conditions. Better site characterization can help mitigate these risks. (https://ore.catapult.org.uk/resource-hub/blog/subsea-cable-failuresdont).
The Ground Model: From Data to Engineering Decisions
A ground model is more than a collection of geological and geotechnical data. This is a structured interpretation of seabed and subsurface conditions along the cable route. It brings together geological understanding, geophysical observations, geotechnical test results and engineering judgement. It provides a consistent basis for understanding spatial variations in ground conditions. These variations can affect cable design and installation.
For subsea cable projects, the ground model should develop progressively as new information becomes available. A conceptual model is developed during the desk study. It provides an initial understanding of the expected geology, sedimentary processes and potential geohazards. Geophysical surveys can then establish the continuity of geological units and identify areas of particular interest. Targeted boreholes, sampling and in-situ testing provide direct information on ground conditions. Laboratory testing further refines the engineering parameters required for design.
An effective ground model should communicate uncertainty as well as interpreted ground conditions. Areas with limited data, unexpected geology or significant spatial variability can be identified. Further investigation can then be targeted where it provides the greatest reduction in project risk. This enables a more efficient investigation strategy rather than relying on uniform investigation spacing along the entire route.
Importantly, the ground model should remain a living model throughout the project. As new investigation and laboratory data become available, the interpretation can be updated. This allows teams to reassess burial requirements, trenchability, thermal conditions, geohazards and route-specific installation constraints. In this way, the ground model becomes the link between site investigation and engineering design, helping project teams make better-informed decisions while managing uncertainty.
The objective is therefore not simply to collect more data, but to collect the right data, in the right locations, to answer the engineering questions that matter most to cable design and installation.
Why a Ground Model-Centred Approach Matters
In practice, subsea cable investigations can still follow a relatively fixed approach. Boreholes, vibrocores and cone penetration tests (CPTs) may be specified at predetermined intervals along the route.
Systematic coverage has value. However, applying the same investigation density everywhere can result in repetitive data collection. At the same time, important uncertainties may remain unresolved.
A ground model-centred approach uses each investigation stage to progressively reduce the uncertainties that matter to design and installation.
The following elements form the key building blocks of this process:
The Desk Study
A desk-based review of available geological and geotechnical literature can identify hazards before the site investigation even begins! Some key considerations include:
Shallow bedrock that may be difficult to excavate, for example an igneous intrusion.
Active sediment environments where seabed mobility can lead to scour and exposure of cables or free spans.
Strata that may produce shallow gas or pock marks along the route.
Areas that are susceptible to submarine landslides.
Geophysical site investigations
High-resolution geophysical surveys are the next building block:
Bathymetry & Side-Scan Sonar reveal seabed features (boulders, wrecks, pockmarks, etc…).
Seismic Reflection: This data allows for subsurface geological interpretation. It helps extrapolate geological units between boreholes and understand the continuity of geological layers.. Given that cables are typically buried only a few metres below the seabed, these surveys must be high-resolution and specifically tailored to capture surface / near-surface features with precision.
The outcomes of geophysical survey are continuous plots along the cable route showing the above features and information.
Geotechnical Boreholes
Boreholes complement geophysical data by giving direct access to subsurface materials and producing samples for laboratory testing and examination. As the number of boreholes is limited these should be targeted to high-risk areas.
Vibrocores: Fast and low-cost but the recovered samples are likely disturbed especially for fragile sediments with weak cementation (e.g. carbonate sands). This method struggles to penetrate competent rock or stiff clays.
Drop Cores: Similar to vibrocores these are fast and low cost. The sample quality is usually acceptable, but the method is only useful for soft sediments.
Push Sampling: Great for retrieving high quality clay samples, but it can produce limited recovery in other soils.
Rotary Coring: More expensive but provides high-quality samples in mixed or hard geology.
In Situ Testing
Testing ground conditions in place is essential to reduce uncertainty:
CPTs (Cone Penetration Tests): Measure tip resistance, sleeve friction, and pore water pressure. This allows the soil type and geotechnical parameters to be estimated.
Thermal CPTs help evaluate thermal conductivity, which is important for power cable design. Thermal conductivity, also known as thermal resistivity, is an active area of research. New tools and data-processing methods are being developed to improve the accuracy of ground condition measurements.
Shear Vane Tests: Accurately characterises the undrained shear strength of clay soils.
Ball Penetrometers: Measure strength properties of soils and is especially effective in soft clay soils.
Jet Lance Tests: An unconventional method that uses divers to shoot pressurised water at the seabed to assess its erodibility. This inexpensive test provides information on sediment composition and the appropriate excavation technique. It can also be carried out by local contractors, potentially reducing supply chain issues! Careful planning of diving operations is critical, and safety is always of the highest priority.
Laboratory Testing
High-quality lab testing on samples retrieved from geotechnical boreholes give some of the most detailed information. Soil classification, strength and density tests are routinely carried out for trenchability assessments, trench stability and volume calculations.
Specialised tests such as thermal conductivity are essential to model the heat dissipation from a power cable.
Managing Laboratory Dataflow
For cable routes that extend over tens or hundreds of kilometres, the laboratory testing programme can quickly become difficult to control. Hundreds of samples may be generated across different geological units and burial risk zones. They may also cover thermal conditions, trenchability considerations and ground model uncertainties. Each sample may require classification, strength, density, thermal or specialist testing, and the results often need to be reviewed progressively rather than only at the end of the campaign.
This is where structured laboratory dataflow becomes important. Platforms such as LabHarmony can support the management of laboratory testing by linking samples, assigned tests, test conditions, approvals, results, comments, costs and chain of custody in one controlled workflow. This helps project teams understand what has been tested and what is still outstanding. It also highlights delays or unsuitable samples and shows whether the laboratory programme remains aligned with the ground model and design objectives.
For subsea cable projects, this level of visibility is particularly valuable because laboratory results directly influence burial assessment, trenchability, thermal design and route risk decisions. A well-managed digital workflow can reduce manual reconciliation between spreadsheets, emails and reports, improve traceability, and allow engineers to act earlier when data gaps or inconsistencies are identified.
Results & Impact: A Smarter Route, A Safer Cable
By progressively integrating desk study findings, geophysical interpretation, in-situ testing and laboratory results, the ground model becomes an increasingly reliable basis for engineering decisions. As uncertainties are reduced, project teams can refine route alignment, burial requirements, trenchability assessments and installation methods based on the conditions expected along the route. This helps focus further investigation where it adds the greatest value and reduces the risk of unexpected ground conditions during installation. The result is a more informed, efficient and resilient cable design.
Key benefits include:
• Reduced risk of unforeseen ground conditions and geohazards.
• Optimised route alignment and burial requirements.
• Better-informed trenchability and installation planning.
• More targeted site investigation, reducing unnecessary data collection.
• Improved cost and programme certainty.
Resource : https://ore.catapult.org.uk/resource-hub/blog/subsea-cable-failuresdont