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Pipelines and power cables undergo thermal expansion of roughly 10–20 mm per km…

2026-07-30 · 08:37 UTC ·@Object_Zero_ ·1 min read

Brief

Subsea trenching protects pipelines and power interconnectors from damage (e.g., submarines, fishing) while accommodating thermal expansion: steel lengthens ~10–20 mm/km (≈1–2 m over 100 km), necessitating snaking routes. Giant ~500-tonne trenchers — in service 30+ years and painted yellow/white — plough, lay and rebury in one pass; trenched lines like Nord Stream demand multimillion-dollar search and intervention gear.

Why it matters

Pipelines and power cables undergo thermal expansion of roughly 10–20 mm per km, so over 100 km they gain about 1–2 m of length; offshore wind cables with frequent on/off cycles need snaking/winding routes to absorb that movement.

Key details

  • Subsea trenchers are massive machines (~500 tonnes) that plough a trench, lay the cable/pipeline into it and rebury in one pass; they’ve existed for 30+ years and are typically painted yellow or white for visibility.
  • Valuable assets (pipelines, power interconnectors) are usually trenched or rock-buried (telecoms less so); Nord Stream was trenched, meaning locating and intervening on such a pipeline requires many millions of dollars of equipment.
Source evidence

Subsea Trencher

Underwater pipelines and cables are often trenched (buried under the sea bed), this protects them from naughty Russian submarines and over enthusiastic fishermen.

Pipelines and cables are particularly interesting, because they heat up and cool down in usage cycles they can snake around on the seabed when they expand and contract. This is a big issue for offshore wind farms where the cables have lots of on/off cycles and therefore lots of thermal cycles.

Steel expands around 10-20mm per km, over 100km that’s 1-2 meters and that extra length has to go somewhere.

As a result you can’t lay these things in a straight line, you have to install snaking winding paths that can comply with the expansion / contraction cycles.

But you also want to trench the thing if it is anything valuable. Telecom cables are cheap and rarely trenched, but pipelines and power interconnectors are usually trenched or rock buried.

The subsea trenchers are giant robots that plough a trench in the seabed, fold the cable / pipeline into the trench, and then rebury the whole thing, all in one clean pass.

First image gives you some context of the scale.

These machines are ~500 tonnes of steel and they’re always yellow or white so you can find them on a camera when they’re down on the bottom.

These machines have been a thing for 30 years or more.

Interestingly Nordstream was trenched, which means merely finding the pipeline location at the seabed requires many millions of dollars of equipment, and that’s before you dive on it or do any “intervention”.