FreeStar Subsea Services and project partner Seekable have completed a subsea survey campaign for transmission system operator TenneT, mobilising multiple cable inspection and detection technologies aboard the vessel Navicula Star. The work took place in February 2026 near Eemshaven and focused on sections of the COBRA interconnector linking the Netherlands and Denmark, with the campaign designed to confirm cable location and assess burial depth in shallow coastal waters.

 

A Multi Technology Survey Spread Built Around Burial Depth

 

TenneT commissioned a 4K ultra-high-definition video survey to support a cable burial depth assessment, with FreeStar deploying ROV inspection capability from Navicula Star in water depths around five metres. In parallel, Seekable introduced its electro-magnetometer-based system to determine cable positioning and burial depth, enabling the survey to move beyond visual inspection and add an electromagnetic measurement layer suited to conditions where the cable may not be fully visible.

 

Three Complementary Methods Collected in a Single Campaign

 

The project integrated three different approaches for cable detection and inspection from one offshore platform. Vriezoo ROV Services supported the ROV element, while MAPPEM provided sensing systems that measure very small electric and magnetic fields associated with HVDC cables through both a mobile passive system and a seabed-based station. Seekable deployed its TN-1 cable tracking system, using an electro-magnetometer and model-based analytics to predict cable position even when the sensor track is offset from the cable’s true alignment, supporting more reliable localisation in nearshore settings.

 

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Why Model Based Electromagnetics Matter in Shallow Water

 

Seekable’s approach combines high-resolution magnetic field measurements with data modelling to deliver real-time tracking results, with the aim of reducing time spent re-running lines or manually reconciling sensor offsets. For operators, the practical value is faster confirmation of route and burial depth in a dynamic coastal environment where currents, sediment movement, and shallow-water constraints make survey time and vessel access more expensive than the water depth would suggest.

 

Mobilisation Speed Was the Defining Operational Feature

 

A central feature of the project was the rapid integration of partner systems into the vessel’s survey spread. FreeStar coordinated the collaboration and integrated the technologies within a few days, including installing an umbilical connection and configuring data acquisition for Seekable’s system within three working days. The emphasis on mobilisation speed reflects a market reality where inspection opportunities can be weather-driven and short, making the ability to assemble a full survey stack quickly a competitive advantage.

 

Navicula Star Positioned for Short Duration Coastal Campaigns

 

FreeStar highlights Navicula Star’s shallow draft and adaptable deck layout as key enablers for nearshore work, including bathymetric and geotechnical surveys, cable protection, and burial operations. By supporting ROV deployment and multiple survey systems from a compact platform, the vessel is being positioned as a flexible coastal operations base that can host specialist technologies and deliver high-quality datasets efficiently when clients need rapid verification rather than long mobilisation projects.