Cable and Pipeline Inspection

What a route survey sets out to answer

A route survey answers a few practical questions: whether the line is where the as-laid records put it, whether it is still covered where burial was specified, and whether anything has happened to it since the last visit. Free spans can open where currents scour sediment from under a pipe, and fishing gear or dragged anchors leave marks that a camera or sonar can pick up. A review in Ocean Engineering by Adetunji and colleagues at Heriot-Watt University surveys the ways cables and pipelines are detected, tracked and inspected from outside, setting out the advantages and drawbacks of each. Use it to judge which methods suit your route before you write a tender.

Survey AUVs and inspection ROVs together

Long routes are usually split between vehicles. An AUV can run many kilometres of corridor with sonar and cameras, flagging places that look wrong, while an ROV is sent to those spots for close video, cleaning and contact measurements. For each section, decide what decision the data must support: confirming burial, measuring a span, checking a repair or proving nothing has changed. That decision sets the sensors, survey speed and line spacing. Ask contractors to show how they reference every finding to distance along the route, so engineers can return to the same feature on the next survey and compare like with like.

Following a cable you cannot see

Buried power and telecoms cables are hard to inspect by camera, so vehicles often sense them instead. In a 2016 paper in Sensors, Xiang and colleagues described an AUV that uses two tri-axial magnetometers to detect the electromagnetic field induced in a cable and estimate its heading deviation, horizontal offset and buried depth, then steers to follow the route. Their results came from numerical simulation, so treat the method as a research demonstration rather than proof of field performance. When you assess a contractor's tracking system, ask whether it needs the cable to be energised or carrying a signal, and how accuracy changes as burial gets deeper.

Records a pipeline regulator expects

Some regulators spell out inspection duties in law. In US federal offshore waters, 30 CFR 250.1005, Inspection requirements for DOI pipelines, says pipeline routes must be inspected for indication of leakage at intervals and by methods the Regional Supervisor prescribes, with results kept for at least 2 years and available on request. Where the life expectancy of a cathodic protection system cannot be calculated, or is calculated at less than 20 years, the same section requires annual measurements of pipe-to-electrolyte potential. Outside the US, those rules still make a useful model: agree the inspection method in writing, store raw data with the reports and plan protection surveys around anode life.

Sources and further reading

• Tracking and inspection of subsea pipelines and cables using underwater vehicles: a comprehensive review — Ocean Engineering (Adetunji et al.), via Heriot-Watt University Research Portal.
  2026 review of methods for detecting, tracking and inspecting subsea cables and pipelines from outside.
• Subsea cable tracking by autonomous underwater vehicle with magnetic sensing guidance — Sensors (Xiang et al.), via PubMed Central.
  Simulation study of an AUV that follows a buried cable using two tri-axial magnetometers.
• 30 CFR 250.1005 Inspection requirements for DOI pipelines — Electronic Code of Federal Regulations (US National Archives).
  US rule on inspecting offshore pipeline routes for leaks, keeping records and checking cathodic protection.

Common questions

How often are subsea cables inspected?

There is no single interval. Owners set survey frequency from the route's risks, such as fishing activity, shipping lanes, a mobile seabed and the age of the asset, and from any licence or regulator conditions. Higher-risk sections are often surveyed more often than the rest.

What is a free span on a subsea pipeline?

A free span is a length of pipe left unsupported where the seabed beneath it has been washed away or was uneven when the pipe was laid. Long spans can vibrate in current and stress the pipe, so surveys measure them and engineers decide whether support is needed.