Testing the waters

Elaine Maslin reports on Subsea 7’s new autonomous inspection vehicle and an AUV specification that has been put out for tender by French major Total.

Subsea 7’s AIV. Images from Subsea 7.

Life-of-field autonomous underwater vehicles (AUV), able to remain subsea for long durations and perform inspection, and potentially intervention operations, has been a dream for the offshore oil and gas industry.

Two such dreams are nearing reality in Europe, where an autonomous inspection vehicles (AIV) has been put through its first offshore trials on a North Sea field, and an AUV specification has been put out for tender by French major Total.

The AIV is the work of Subsea 7. A life-of-field concept, Subsea 7’s aim is for the AIV to be deployed from a basket-based launch system, which could be positioned on the seabed, a platform, or vessel, from which it could be sent on inspection missions without the need for a tether or support vessel, as is the case for ROVs. Its difference from an AUV, says Subsea 7, is that it has intelligent decision-making capabilities, using development partner SeeByte’s complex navigation software, which, using sensors, can recognize subsea infrastructure configurations. By comparing what it sees to its world model, the vehicle knows where it is and complete its mission. It can also hover on station, making it useful for riser and pipeline analysis.

It is a project that has been in development for about six years, with tank testing and then trials in Loch Linnhe, and offshore Peterhead, Scotland. July saw the system successfully complete its first offshore mission, in the North Sea for Shell, from Subsea 7’s Normand Subsea.

“The field operational tests were part of an ongoing proving and acceptance of the Subsea 7 AIV as part of Shell’s ‘new technology integration process,’ and the trials anticipate the development of future AIV applications with which to support the inspection of Shell’s subsea facilities,” Subsea 7 said.

For the purposes of the trial, the AIV system was incorporated into the vessel’s deployment systems and launched through the vessel’s moonpool. The trials were conducted in parallel with planned inspection tasks being conducted by Subsea 7 ROVs, which were also deployed from the vessel.

The trial included a series of deployment and recovery missions, with autonomous subsea launch and basket dockings, fully autonomous navigation and inspection activities.

“Data was also obtained and will be used to supplement and support final further development activities within the ongoing AIV program,” Subsea 7 said.

The AIV has been designed using computational fluid dynamics and has a unique station-keeping system and remote decision-making software. The MK-1 specification was rated to 3000m water depth, and could run for 24 hours, or a 40,000m excursion, depending on conditions. A future development of the AIV could see intervention equipment added, increasing its capabilities and potential applications.

The Normand Subsea

Pipeline inspection AUV

Total E&P, which has long been working on AUV technology under the Deep Offshore Project, with Chevron and others, has developed a full specification for an AUV system capable of performing a complete pipeline inspection, according to a presentation at this year’s Underwater Technology Conference (UTC) in Bergen.

It is a key requirement for Total and many other operators with subsea infrastructure. Total’s Subsea Intervention Specialist Amin Nasr, said at UTC in June, that Total has more than 3000km of offshore pipelines, all requiring inspection services as part of the firm’s asset integrity management.

Total’s vision is an AUV, deployable from a number of platforms, with a mission specific sensor payload gathering critical data. A pipeline inspection AUV specification has been put out to tender, Nasr told UTC, and a feasibility study is underway to incorporate the future developed system into a wider scope project, as a “resident AUV system” in one of Total’s new developments offshore West Africa.

“We have developed a specification; we have launched a tender for system development between major AUV service providers. According to our planning, the system should be ready early 2016,” Nasr said.

Inspection requirements and challenges

Screenshot composite photo taken during the AIV trials, showing the AIV docking into its basket autonomously, sonar imagery and a still from the monitoring work class ROV.

According to Total’s specification, the pipeline inspection AUV would need to perform pipeline integrity inspections and would need to perform buried pipeline detection, free span detection and measurement, seabed condition and buckling surveys, pipeline movement detection, damage assessments, debris identification, and corrosion surveys, Nasr said. The AUV’s capabilities would need to include subsea navigation, visual inspection, sonar inspection, pipeline tracking and pipeline feature identification.

“The advantages of an AUV is being faster, at 3.4-5knots, and better data quality,” Nasr said. “They could operate in a bigger weather window, and most importantly reduce the size of and even eliminate the need for a support vessel. To do this, we need to close the technology gaps and develop new philosophies for pipeline inspection. The technology gaps are in navigation, visual inspection, free span detection, and corrosion surveys, which have never been done by AUV before.”

Sensor power and payload requirements, control, maneuverability, and complexity of operating fully autonomously, are also challenges, he said.

Navigation has traditionally been vessel-based acoustic positioning, with a pre-determined route and ROV visual piloting. The challenge with this approach is ability to perform real-time feature-based navigation to pipeline features and not being able to operate without a surface vessel, Nasr said. An alternative, AUV-based approach, is to use pipeline recognition and subsea features positioning, using an onboard-stored map on the AUV. SeeByte in Edinburgh has already been working on such a system, which is used in commercially available ROVs, including SMD’s MKII Quasar. FMC Schilling Robotics uses its own Station Keep visual recognition technology.

Challenges with such a system include “blind spots,” where there are a lack of seabed features, which would make the auto tracking difficult. A solution could be tridents installed on the seabed to aid navigation and this is seen as being future standard practice, Nasr said.

Another challenge is visual inspection. This is usually carried out using a video streams, using three video cameras, two located on booms, to show the pipeline to seabed interface. This uses a lot of data and power for continuous footage, as well as lighting and the need to be close to the pipeline. Nasr said an alternative is to use HD stills cameras and strobe lighting synchronized to each photograph. The images would then be mosaicked to create a continuous, geo-referenced image of the pipeline.

For free span detection, mechanical profilers (or dual beam multibeam echosounders) are used, and sonar processing. But there is a need for faster surveys, Nasr said. He suggests using laser scanners, creating a 3D rendering of the pipeline and seabed relationship, or high frequency sonar (multibeam) offset from the pipeline, with bathymetry and sonar backscatter processing. The benefit of a laser scanner would be not having problem with frequency management. The challenge is understanding laser scanners underwater, Nasr said.

For corrosion surveys, a direct stab of the cathodic protection anode is currently used, which has autonomous identification, stab accuracy and reference cell location challenges, Nasr said. An AUV-based option would be contactless cathodic protection using data harvesting—with a sensor transmitting data which can be harvested by the passing AUV.

An outline specification

Total’s outline specification incorporates using HD stills cameras to visually inspect the full length of the pipeline, in conjunction with a strobe light, with high frequency multibeam sonar, taken at a fixed offset and consistent aspect angle reference from the seabed. These would be used to identify damage or anomalies, debris, and pipeline features, through post-processing of laser or multibeam data.

Pipeline tracking would be via visual recognition and or laser/multibeam data in real-time. The AUV would need to perform buried pipeline detection, and free span detection and measurement, using laser scanners or multibeam echosounder. It would also need to detect global and local buckling, and pipeline movement, using post processing of imagery, AUV navigation and laser/multibeam cross sectional profiles, Nasr said.

Cathodic protection surveys would be a requirement, to assess coating damage and anode condition, including anode current output. The AUV would need to perform seabed conditions using post processed laser or multibeam cross sectional profiles.

Finally, Total has an outline specification for mission reporting. It says the AUV’s system would need to include providing a comprehensive inspection report, standard ROC and sonar inspection type deliverables, as well as mosaicked, geo-referenced HD photographic data, geo-referenced sonar mosaic, 3D rendering of laser or multibeam echosounder cross sectional data, and GIS format delivery.

Nasr says such a system should be able to travel a minimum 100km in one mission. “The sensors and technology are available, but the challenges are to accommodate them to meet AUV pipeline inspection needs,” he said. He hopes the outline specification will provide guidance on operator requirements to the AUV industry, but that it could need to be modified to meet improvements in sensor capability, and changing integrity management requirements from operators or regulators.

Future systems

“For future systems, we could have a docking station for communication and power, linked to a host platform (a field resident system). The AUV will send its data for evaluation and collect new mission details,” Nasr said.

Optic transceivers could also be incorporated into subsea processing systems and downloaded at short range at 10-50mb/s data rates. This would also allow the AUV to be controlled real time, wirelessly.

A key function will be hydrocarbon leak detection, he says. If it detects a leak it will abort its mission and either return to look at it again to better characterize the leak, or resurface or go to a docking station to report the incident. Riser inspection could also be a future addition.

The next step would be simple tooling, he said. “If we are able to control it, you could have assembly tooling to do assembly tasks. The ultimate goal is to reduce the work of the support vessel.”

Click Here to see a video about Subsea 7’s new AIV technology.