Since pipelines and transported substances are high value assets, continuous and smooth operation is of utmost importance. In the early days inspection and maintenance was focused on securing functionality from a supply and economic point of view. Accidents occurred, but rarely induced new procedures or regulations.
There have been occasions, over the years, where fishing vessels have damaged submerged natural gas pipelines. The escaping gas has ignited, killing a number of crew members. Investigations have, in more than one case, showed that the accidents were caused by the pipelines not remaining at the burial depth to which they were initially installed. Based on the findings, authorities have issued safety recommendations, where frequent pipeline inspection is a vital component. Now inspections are also addressing safety related issues. In recent years the threat of deliberate damage and sabotage to underwater installations further accentuates the need for inspections.
Regular inspection of all pipelines deployed to this day (more than 12 500km in the North Sea alone) is a daunting task.
Pipeline inspection can be categorised by the method; internal inspection and external inspection.
Internal inspection is performed using dedicated sensors (eg, for imaging and ultrasonic/magnetic sounding) mounted in Pipeline Inspection Gauges ­(PIGs). PIGs are inserted in one end of the pipeline and can, propelled by the transported substance, cover long distances in limited times. This is a straightforward and well proven method.
External inspection involves a combination of divers and unmanned vehicles to inspect a number of pipeline characteristics, such as:
l State of corrosion.
l Structural integrity (checking for leaks).
l Location on the seabed.
l Tendency of undermining.
l Burial depth in the sediments.
l Presence of unidentified objects (bombs).
The depth below surface can be substantial and the pipeline can be partly or entirely covered by sediments.

To carry out external inspection, with a similar level of automation as when using PIGs, an Autonomous Underwater Vehicle (AUV) can be used. Equipped with a suitable set of sensors, the AUV can autonomously track and inspect a pipeline.
Like a PIG, the AUV is launched at one end of the pipeline and travels without operator interaction to the far end. Here it is recovered and sensor data is uploaded for analysis.
The AUV has to be equipped with on-board systems for: pipeline tracking; pipeline inspection; vehicle control; long distance propulsion; geographical tracking
There is also need for: mission planning systems; maintenance systems; mission evaluation systems.
Pipeline tracking can be based on seabed penetrating hydro acoustic sensors and magnetic field sensors. These sensors can also be used for the actual pipeline inspection by using:
l Hydro acoustic sensors to check the pipeline burial depth and look for structural failures and tendencies of undermining.
l Magnetic sensors to look for anomalies due to structural failures, cracks and fatigue..
Visual imaging using a camera and floodlight illumination is also possible.

High level commands

The AUV control system has to be capable of executing high level commands such as ‘Track and follow a detected pipeline’. The
control system must also be capable of dealing with unforeseen events, such as obstructed paths.
Since missions can last for many hours, and imply covering many miles of pipelines, the propulsion system has to be of good capacity. In some cases it might be preferable to launch and recover the AUV from the same position, meaning that the AUV has to travel the pipeline distance back and forth.
The AUV62 system, which has been in operation since 2002, is a modular and easily configurable AUV, equipped with sensors for reconnaissance and surveillance, under as well as on and above the surface. The system is designed for long range autonomous reconnaissance over vast areas, to be used in various scenarios ranging from sea bed mapping to search operations.

Off-shore support

Equipped with suitable sensors, the AUV62 can be used to support off-shore operations such as:
l Mapping an area prior to deploying pipelines and other seabed installations.
l Detect, track and inspect various underwater installations.
The basic module, used for all AUV configurations, is equipped with an electrical propulsion system including systems for navigation, communication and vehicle control.
For geographical location tracking the vehicle utilises multiple information sources, such as Inertial Measurement Systems, GPS, Doppler Velocity Log and terrain aided navigation.
AUV62 is assembled with interchangeable, mission specific, payload modules. The modules contain all mission specific equipment such as sensors used to detect, track and survey pipelines and other submerged installations. Typical sensor systems are:
l Side Looking Sonar System for objects on the seabed and in the water volume
l Seabed Penetrating Sonar for objects buried in the sediments
l Magnetic Field Sensors for ferromagnetic objects on the seabed, in the water volume and buried in the sediments.
The on-board computers facilitates advanced sensor signal processing, resulting in high resolution images of the seabed and detected objects.
Sensor information is recorded for later evaluation and can, in parallel, be transmitted by radio.
The AUV62 system is highly mobile and the vehicle is easily launched and recovered by the use of dedicated equipment. The system is complete with tools for mission planning, preparation and command/control.l

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Thomas Petersson is Senior System Designer, Saab Underwater Systems AB, Motala, Sweden. www.saabgroup.com