Pipeline inspection is vital to ensuring safe transfer of hydrocarbons and continued integrity of subsea pipelines. Environmental factors acting on pipelines can threaten integrity by moving and bending the lines, and waves can cause scouring that can create long free spans where the pipeline is not sufficiently supported. Additional damage can be caused by ship’s anchors being dropped and by trawling in areas where commercial fishing activities take place. Pipeline integrity must be carefully monitored to ensure that this sort of damage does not advance to the point of compromising safe transfer.

Historically, pipeline inspections have been performed primarily by acoustic sensors on towfish. The development of ROVs allowed inspections to move to another level and added the possibility of using cameras as inspection sensors. Today, both towfish and ROV-based inspection are common, but the industry has not stopped investigating ways to improve the inspection process. R&D efforts have gone into developing autonomous underwater vehicles (AUVs) that can be used as pipeline inspection tools.

The first 8-hour mission is indicated by the green line. (Images courtesy of Kongsberg Maritime)

In a recent inspection carried out in the Hjelte fjord near Bergen, Norway, Kongsberg Maritime carried out the world’s longest multisensor AUV pipeline survey.

AUV inspection

The milestone AUV pipeline inspection took place between February 9-11, 2011, offshore mid-Norway. The subject of the inspection was the Troll I and II pipelines, which move hydrocarbons produced offshore to the onshore Mongstad oil refinery.

The HUGIN 1000 AUV, operated from the Royal Norwegian Navy vessel HNoMS Måløy, was equipped with an advanced suite of Kongsberg imaging equipment including the high-resolution interferometric synthetic aperture sonar (HISAS) 1030 synthetic aperture sonar, the EM3002 multibeam echo sounder (MBES), and an optical camera with LED lighting. The instruments were used to inspect approximately 30 km (18.6 miles) of subsea pipeline in an 8-hour, two-pass mission.

Executing the survey

Several pipelines were present on the seafloor, sometimes crossing each other via buried sections. During the first pass in the inspection process, when the primary pipeline’s position is determined with high accuracy (to get close enough to use camera and high-res MBES on the second pass), the pipeline tracking software managed to re-acquire the correct pipeline after a buried crossing. This was possible because the tracking function keeps track of several pipelines and chooses the one best matching criteria.

This image is a single 3-megapixel camera image taken at an altitude of 4.6 m (15 ft) at 2 mm by 2 mm resolution.

HISAS was critical to the inspection. SAS systems are superior to conventional systems because the resolution of the images produced does not degrade with range. The HISAS 1030 typically produces better than 3 cm by 3 cm image resolution out to 300-m (984-ft) range. Interferometric technology sets it apart from other SAS systems in that it provides full-swath bathymetry and increased immunity to conditions that traditionally have been considered difficult for SAS, like crab angles and platform movement. The HISAS 1030 mounted on a HUGIN AUV achieves a full survey area coverage rate of more than 2 sq km/hr (0.8 sq mi/hr).

Typically a HUGIN AUV performs a detailed seabed survey in shallow or deep water with a standard geophysical survey suite. Commercial operations of this kind have been conducted since 1997. The pipeline inspection carried out on the Troll I and Troll II lines introduced a new way of fully autonomously achieving detailed digital still imagery of an underwater pipeline by using interferometric SAS, pipeline tracking software, an MBES, and a high-resolution still camera in a two-pass mission.

In the first pass, side-scan data from the HISAS 1030 sonar was used to detect and track the pipelines in real time using PipeTracker software, which was developed in a collaborative effort with the Norwegian Defence Research Establishment (FFI) in a project funded by the Norwegian Research Council. The software runs as a plug-in module in the standard HUGIN payload system and was used in this application for pipeline detection and for tracking extracted pipe-like features in the sonar images with a high degree of robustness toward false detections.

The HUGIN 1000 control system uses identified pipeline tracks to position the vehicle at an optimal range for HISAS imaging. The entire process is fully automated inside the AUV and requires no operator intervention.

In the second pass, the AUV followed the pipeline tracks identified in the first pass at low altitude and inspected the pipelines using the EM 3002 multibeam and the optical camera. After the mission, the recorded HISAS 1030 data was post-processed into high-resolution (4 cm x 4 cm) sonar images and bathymetry maps of the pipeline. The maps combined with the optical images and the multibeam data recorded in the second pass gave a detailed view of the pipeline surroundings as well as the pipeline itself. The complete procedure was repeated the next day over the second pipeline in another 8-hour, two-pass mission.

This HISAS image shows an area of 120 m by 90 m (394 ft by 295ft) with a range of 32 m to 152 m (105 ft to 499 ft) at 35 mm by 35 mm resolution.

Both pipelines were surveyed at a constant speed of 4 knots and at 4 m to 25 m (13 ft to 82 ft) altitude, depending on the sensor in use. Water depth ranged from 180 m to 560 m (590 ft to 1,837 ft).

The greater speed of the HUGIN 1000 compared to that of an ROV meant that 60 km (37 miles) of pipeline could be inspected in a little more than 16 hours during the two passes. The stability of the HUGIN platform and the ability to simultaneously operate both at high speed and at low altitude resulted in an efficient survey with clear images from the onboard optical camera.

Kongsberg’s REMUS family of AUVs also can perform visual pipeline inspections and pipeline surveys when equipped with high-frequency sidescan sonar, a multibeam, and camera. This means the REMUS 100 AUV, used for deployment in shallow water, and REMUS 600 AUV, used for operation in depths to 600 m (1,968.5 ft), are viable low-logistics tools for pipeline inspections. For pipeline inspection and surveys in depths greater than 600 m, HUGIN AUVs offer long endurance and swappable batteries, resulting in high operational efficiency with few launch and recovery cycles and fast topside turnaround time. In addition, the AUV can be equipped with the HISAS 1030 from Kongsberg, providing unmatched sonar image resolution throughout the entire sensor swath.

A raw bathymetry image from EM3002 is shown for a 120 m by 120 m (394 ft by 394 ft) area.

The road ahead

With this survey successfully completed, there are now opportunities for the technology to be used on pipeline inspection in and around oil and gas fields, for initial baseline pipeline inspection surveys and follow-on surveys after hurricanes, and for environmental monitoring pipeline inspection surveys.

Kongsberg, meanwhile, is aggressively pursuing enhancements to the technology, including hovering capability, subsea docking, increased onboard energy storage, and several other technologies that will further extend its application.